SpaceX Bets on Full-Flow Methane Engine to Slash Reusable Launch Costs

SpaceX押注全流量甲烷发动机以大幅削减可重复使用发射成本 SpaceX选用液态甲烷全流量分级燃烧循环并非为了极致效率，而是为了平衡高室压、火星燃料合成以及快速周转能力，这可能将单次飞行成本降至每千牛20美元。 蒂姆·多德通过阐述火箭推进物理学，并对比开放式燃气发生器循环、分级燃烧循环和全流量分级燃烧循环，对SpaceX的“猛禽”（Raptor）发动机进行了评估。他详细解释了SpaceX舍弃RP-1煤油和液氢而选择液态甲烷的原因，指出其在密度、沸点、低积碳以及利用萨巴捷反应实现火星原位资源利用方面具备平衡优势。多德在推力、室压、推重比、比冲、制造成本及单次飞行分摊成本等维度，将猛禽发动机与五款传奇火箭发动机进行了基准对比。他最终得出结论，尽管猛禽发动机并非在每一项单项指标上都格外突出，但其创纪录的室压、紧凑的结构布局以及极高的可重复使用潜力，使其成为一款“恰到好处的发动机”，堪称星舰运行架构的核心之王。 SpaceX的猛禽发动机采用液态甲烷和液态氧的全流量分级燃烧循环，这种配置此前从未在执行过任务的轨道火箭上飞行过。 (0:00) 在研发测试期间，猛禽发动机的燃烧室压力达到了270巴，打破了此前由俄罗斯RD-180保持的室压纪录（约265巴），其最终目标为300巴。 (18:59) 甲烷为深空探索提供了战略平衡：其密度远高于液氢，燃烧清洁以避免积碳结焦，并且能够通过萨巴捷反应和电解水在火星上就地制取。 (29:04) 如果SpaceX实现单台发动机以极少翻修飞行50次的目标，猛禽发动机单次飞行的分摊成本可能降至每千牛约20美元。 (40:36)

Tim Dodd of Everyday Astronaut evaluates SpaceX's Raptor engine, arguing that while it does not lead in any single performance metric, its combination of full-flow staged combustion, high chamber pressure, and liquid methane fuel makes it exceptionally tailored for the rapid reusability and operational requirements of Starship. Combustion Cycles and Full-Flow Mechanics (15:45) Liquid rocket engines span multiple architectures, from simple pressure-fed systems limited by heavy tank walls to turbopump-driven open and closed cycles. Open cycles, such as the gas generator powering the Merlin and Apollo's F-1, discard turbine exhaust overboard, trading efficiency for mechanical simplicity. Traditional staged-combustion closed cycles recapture that exhaust into the main chamber for higher performance, but encounter severe chemical hurdles: kerosene causes destructive soot build-up (coking) in fuel-rich preburners, while oxygen-rich preburners subject turbine metals to extreme oxidation—a problem solved historically by Soviet metallurgical superalloys on engines like the RD-180. The Raptor utilizes a full-flow staged combustion cycle (FFSCC), previously only tested on test stands via the Soviet RD-270 and the US Integrated Powerhead Demonstrator. By running separate fuel-rich and oxygen-rich preburners, all propellant passes through the turbines as hot gas before entering the combustion chamber, enabling gas-gas mixing and cooler operating temperatures. SpaceX addresses the oxidizer-rich environment using a proprietary superalloy, SX500, designed to withstand over 800 bar of hot oxygen gas, while eliminating the hazardous dynamic interpropellant seals required on engines like the RS-25. Propellant Tradeoffs and Martian ISRU (20:00) SpaceX bypassed RP-1 and liquid hydrogen in favor of liquid methane (methylox). RP-1 provides high density (813 g/L) but suffers from heavy coking, high combustion temperatures (~3,670 K), and lower theoretical specific impulse (~370 s). Hydrogen delivers superior efficiency (~532 s theoretical) and clean exhaust, but its very low density (70 g/L) demands massive tanks, and its deep cryogenic boiling point (20 K) causes severe boil-off and tank insulation penalties. Methane balances these extremes with a density of 422 g/L, an operating oxidizer-to-fuel ratio of 3.7:1 (requiring a fuel tank only 1.4 times larger than an RP-1 tank), and a 111 K boiling point close to liquid oxygen (90 K), which allows tanks to share common bulkheads. Methane also burns relatively soot-free and runs at cooler combustion temperatures (~3,550 K) than RP-1. Operationally, methane serves Starship's Mars architecture through in situ resource utilization (ISRU), allowing return propellant to be synthesized from Martian atmospheric carbon dioxide and water ice via the Sabatier reaction and electrolysis. Comparative Performance and Chamber Pressure (32:58) Across core mechanical metrics, Raptor occupies a middle ground rather than setting universal records. In sea-level thrust, Raptor's test rating of 2.0 MN sits below the RD-180 (3.83 MN) and F-1 (6.77 MN), though above the Merlin 1D (0.84 MN) and RS-25 (1.86 MN). For thrust-to-weight, Raptor achieves approximately 107:1, surpassing the RS-25 (73:1) and RD-180 (78:1), but trailing the open-cycle Merlin 1D's 198:1. In vacuum specific impulse, Raptor's planned 380 seconds outclasses kerosene engines but cannot match the hydrogen-fueled RS-25 (up to 452 seconds). Raptor sets its clearest technical benchmark in combustion chamber pressure. Reaching 270 bar during testing and targeting 300 bar, it surpassed the RD-180's operational record of roughly 257 to 265 bar. This high pressure packs substantial thrust into a compact physical envelope, enabling SpaceX to cluster dozens of booster engines on a single stage. Economic Metrics and Flight Reuse Potential (39:18) Evaluating propulsion costs through raw production numbers and thrust-adjusted economics highlights stark programmatic differences. Expendable or high-cost engines present massive costs per unit of thrust, with the RS-25 reaching $26,881 per kilonewton and the expendable RD-180 at $6,527 per kilonewton. SpaceX's Merlin 1D reduced this to $1,170 per kilonewton, while Raptor is projected to target roughly $1,000 per kilonewton based on an estimated unit cost near $2 million. Factoring in reusability dramatically alters flight economics. Blue Origin targets 25 reuses for the BE-4, yielding an estimated $133 per kilonewton per flight, while Merlin sits at roughly $117. If SpaceX meets its target of 50 flights per Raptor without major overhauls, Dodd estimates its amortized cost could drop to roughly $20 per kilonewton per flight. However, while engines like the RS-25 and Merlin possess demonstrated flight reliability (over 99.5% and 99.9% respectively across dozens of missions), Raptor and BE-4 lacked operational orbital flight heritage at the time of evaluation. Dodd concludes that while Raptor is not the most powerful, efficient, or lightest engine in isolation, it functions as an ideal compromise across thrust density, reusability, and propellant logistics for Starship's specific mission profile. Its ultimate standing depends on translating developmental test metrics into orbital flight reliability and achieving its targeted 50-flight refurbishment cycle.

科普创作者 Tim Dodd（Everyday Astronaut）指出，SpaceX 的猛禽（Raptor）发动机在单项孤立指标上并非各项第一，但通过全流量分级燃烧循环与液态甲烷燃料的结合，在动力密度、复用寿命与生产成本之间取得了高度平衡，是专为 Starship 深空运输体系量身定制的动力基石。 全流量分级燃烧循环与冶金突破 (15:45) 传统开式燃气发生器循环（如 Merlin 和 F-1）会将驱动涡轮的废气直接排放，造成推进剂浪费；闭式分级燃烧循环虽能将废气注入主燃烧室以提升效率，却面临严苛工况。以煤油为燃料富燃运行时极易产生严重积碳（Coking）堵塞喷嘴，苏联通过开发耐高温高压纯氧腐蚀的特种合金，开创了富氧分级燃烧循环（如 RD-180）；美国则在航天飞机主发动机（RS-25）上采用液氢与富燃分级循环，但氢的强渗透性迫使其设计复杂的氦气动密封，以防氢氧混合引发灾难。 猛禽发动机采用的人类历史上首次迈向实飞的全流量分级燃烧循环（FFSCC），彻底融合了上述两条路线：一台富氧预燃室驱动液氧泵，一台富燃预燃室驱动燃料泵。该架构消除了跨推进剂旋转密封的灾难风险，且由于两路推进剂均100%流经预燃室，偏离化学计量比的极度富氧与极度富燃状态反而降低了涡轮工作温度，从而大幅延长部件寿命。SpaceX 为此研制出能承受超过 800 bar 热高压气态氧的专属合金 SX500，攻克了此前苏联 RD-270 与美国 IPD 演示机未能逾越的工程障碍。 推进剂权衡与火星原位资源利用 (20:00) 在推进剂选择上，液态甲烷填补了煤油与液氢之间的性能空白。液氢密度极低（70 g/L），即便利于 6:1 的氧燃比配平，其储罐容积仍达到煤油级的约 5 倍；而甲烷密度为 422 g/L，配合 3.7:1 的运行氧燃比，其所需储罐容积仅比煤油（813 g/L）大约 40%，大幅减轻了箭体干重与绝热外壳的负担。热力学特性上，甲烷沸点为 111 K，与液氧（90 K）非常接近，允许两座储罐共用非绝热共底（Common Bulkhead），避免了液氢（20 K）极易导致液氧冻结或自身剧烈蒸发的窘境。 更关键的战略意图在于深空后勤与复用性。甲烷燃烧温度约 3,550 K，低于煤油的 3,670 K，且燃烧产物为二氧化碳与水蒸气，杜绝了游离碳黑沉积对涡轮和管路的磨损。在任务架构层面，火星大气富含二氧化碳，结合地表水冰资源，通过电解水与萨巴捷（Sabatier）反应即可在火星本地合成液态甲烷与液氧推进剂（ISRU）。这项特性免除了从地球携带返程燃料的沉重代价，成为 SpaceX 选择甲烷的核心驱动力。 核心性能对比：推力比冲与极致室压 (35:27) 在各项传统性能排名中，猛禽均处于中间偏上的均衡区间：在海平面推力上，土星五号的 F-1 以 6.77 MN 位居榜首，RD-180 达 3.83 MN，猛禽当前测试状态约为 2.0 MN，高于 RS-25 的 1.86 MN 与 Merlin 1D 的 0.84 MN；在推重比（TWR）方面，架构极为精简的 Merlin 1D 达到创纪录的 198:1，猛禽约为 107:1，同样超越了 RS-25（73:1）与 RD-180（78:1）；在理论比冲上，氢氧动力的 RS-25 凭借高能量排气达到 366 至 452 秒，而猛禽在 330 至 350 秒之间。 猛禽最具突破性的技术指标是燃烧室压力（Chamber Pressure）。极高室压能将相同推力的发动机物理尺寸急剧缩小，为一级助推器密集并联数十台发动机创造空间。此前保持实飞室压世界纪录的是俄制 RD-180（约 257 至 265 bar），猛禽在地面测试中已录得 270 bar，并正向 300 bar 设计目标推进，其内部承受的物理挤压相当于水下三千米深海的环境压力。 制造与复用经济性：每千牛飞行成本 (39:18) 从财务与工业化制造角度分析，单纯比较单台发动机造价会忽视其输出动力规模。将采购成本平摊至推力单位计算，单台超过 5,000 万美元的 RS-25 耗费达每千牛 26,881 美元，RD-180 约为每千牛 6,527 美元，而量产优化后的 Merlin 1D 仅为每千牛 1,170 美元。Elon Musk 预计猛禽在流水线成熟后单台成本可压低至约 200 万美元，对应每千牛推力建造成本仅约 1,000 美元。 若进一步计入复用折旧，经济效益对比更为悬殊。RD-180 与 F-1 为一次性耗损引擎；RS-25 虽可复用多达 19 次，但单次飞行间需要漫长昂贵的拆解检修，折算后单次飞行推力成本仍高达 1,414 美元/千牛。Merlin 1D 在无需大修设计下摊薄至 117 美元/千牛/次；若猛禽能达成设计预期的 50 次免维护复用目标，其每千牛每飞行成本有望降至惊人的 20 美元。SpaceX 通过在 McGregor 基地对单台发动机实施多次全工况长程试车，已初步验证其动力组件经受多任务寿命周期的耐久潜力。 Tim Dodd 总结认为，猛禽发动机虽然在绝对比冲、单机峰值推力或极限推重比上并未垄断榜首，但其兼顾了高室压动力密度、全流量燃烧效率、适中的储罐空间代价以及极低的气态复用磨损。对于 Starship 追求超重型密集并联、火星就地补给与高频次廉价发射的极端工程需求而言，猛禽在综合匹配度上堪称该细分体系下的最优解；其在轨可靠性与量产低成本预期的最终兑现，仍需依赖后续大规模轨道飞行的实际检验。

## Background 该视频由知名航天科普频道 Everyday Astronaut（Tim Dodd）于 2019 年 5 月 25 日在 YouTube 平台发布。视频围绕 SpaceX 为星舰（Starship）研发的新一代猛禽（Raptor）发动机展开，详细拆解了全流量分级燃烧循环（FFSC）与液态甲烷燃料的技术原理，并将其与航天史上多款经典火箭发动机进行全方位横向对比。 ## TL;DR 视频系统性梳理了挤压循环、开式燃气发生器循环、美苏闭式分级燃烧循环及全流量分级燃烧循环的工程演进逻辑，深入剖析了 SpaceX 选择液态甲烷作为推进剂的物理与火星原位资源利用（ISRU）考量。通过将 Raptor 与 Merlin、RS-25、RD-180、BE-4 及 F-1 等顶尖发动机在推力、推重比、比冲、室压、制造成本及复用寿命等多维度的横向对决，揭示了 Raptor 虽非单项指标最强，却是一台在综合性能上近乎无可挑剔的“金发姑娘”发动机。最终得出结论：对于星舰实现快速完全复用及火星拓荒的宏大目标而言，Raptor 是最具颠覆性且量身定制的动力之王。 ## Atmosphere & style 整部影片洋溢着极度硬核又富有感染力的极客热情，Tim Dodd 以亲和力十足的幽默口吻，将枯燥艰深的热力学与航天冶金学拆解得生动透彻。从德州博卡奇卡发射场的壮阔实景到精密细致的动态流体图表，叙事层层递进、逻辑极其严密。观众在观看过程中不仅能真切体会到人类挑战航天工程技术极限的窒息感与宏大感，更会被那种“将深奥航天带入普通人生活”的纯粹探索激情所打动。 ## Bulletin - [18:59](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=1139s) SpaceX 的猛禽（Raptor）发动机是航天史上首款成功飞天的全流量分级燃烧循环（FFSC）发动机，地面测试室压达到 270 bar，打破了俄罗斯 RD-180 保持的约 265 bar 原纪录，并向 300 bar 目标挺进。 - [29:04](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=1744s) 猛禽选用液态甲烷作为燃料，在密度、比冲、燃烧温度与储箱体积上达成平衡，且几乎不结焦积碳，还能利用火星大气中的二氧化碳与水通过萨巴捷反应就地合成推进剂（ISRU）。 - [15:53](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=953s) 全流量分级燃烧架构让气态燃料与气态氧化剂直接在主燃烧室内高效混燃，彻底消除了 RS-25 发动机上危险的轴间跨介质密封难题，并大幅降低了涡轮泵叶片的工作温升。 - [39:18](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=2358s) 在成本效益模型中，猛禽发动机初始建造成本预计可降至约 1,000 美元/kN，完全实现 50 次设计复用后单次飞行推力成本有望压低至约 20 美元/kN/次，具备显著的经济颠覆性。 ## Key takeaways - [02:55](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=175s) 挤压循环受制于流体物理规律无法使室压高于贮箱压力，导致贮箱死重随运力急剧上升；涡轮泵架构则是通过微型燃气做功突破储箱增压瓶颈的关键手段。 - [06:12](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=372s) 开式燃气发生器循环虽系统相对简洁，但因涡轮废气直接外排而损失比冲，且在富燃煤油工况下涡轮排气会析出大量黑色碳烟。 - [10:31](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=631s) 煤油在富燃闭式循环中会出现严重的结焦堵塞，苏联凭借攻克耐高温纯氧腐蚀的特种合金，成功走出富氧分级燃烧的技术路线并诞生了 RD-180。 - [12:42](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=762s) 美国航天飞机主发动机 RS-25 选用富燃氢氧分级燃烧规避了积碳，但为防范极寒液氧与高温氢气在同轴窜流，不得不引入极度精密的氦气吹扫密封结构。 - [15:53](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=953s) 全流量分级燃烧循环（FFSC）让两个预燃室分别在极端富燃和极端富氧状态下运行，实现全气相高效混合燃烧，同时将转轴两侧介质统一化，消除异种流体泄漏风险。 - [15:53](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=953s) SpaceX 自主研发了能耐受超过 800 bar 高温富氧燃气的 SX500 特种高温合金，突破了全流量发动机富氧预燃室的核心材料瓶颈。 - [20:00](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=1200s) 在 3.7:1 的实用氧燃比加持下，甲烷储箱体积仅比同质量 RP-1 储箱大约 40%，远优于储箱膨胀至约 5 倍大的液氢系统。 - [24:37](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=1477s) 液氧与液态甲烷的沸点极为接近（90 K 与 111 K），使得火箭内部贮箱可以直接使用共用底壁隔离，大幅削减箭体结构死重与绝热层负担。 - [29:04](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=1744s) 选择甲烷的核心战略逻辑是火星原位资源利用（ISRU），飞船无需从地球携带返程燃料，即可在火星就地取材补满推进剂。 - [32:58](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=1978s) 在主流发动机横向对比中，梅林（Merlin）凭借开式构型的轻便性以 198:1 摘得推重比桂冠，而氢氧发动机 RS-25 则以 452 秒的真空比冲展现出无可动摇的效率优势。 - [32:58](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=1978s) 超高燃烧室压力允许发动机在同等推力下大幅缩小推力室体积，为星舰尾部多发高密度并联布置提供了物理可能性。 - [36:44](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=2204s) SpaceX 采取激进的工业化品控策略，每台发动机在真正执行轨道入轨任务前，其累计点火时长已相当于经受了三次全时发射任务的验证。 - [44:08](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=2648s) 猛禽发动机在单项推力、最高比冲或单机推重比上均未拔得头筹，但其在复用性、燃烧洁净度、储箱体积、制造成本与星际自制潜力上达成了极其出色的系统级平衡。 ## Detailed walkthrough ### [00:00](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=0s) 走进博卡奇卡与猛禽发动机的探索序幕 Tim Dodd（即 Everyday Astronaut）站在 SpaceX 位于德克萨斯州博卡奇卡（Boca Chica）的崭新发射场，准备揭开被称为火箭发动机“圣杯”的猛禽（Raptor）发动机的面纱。在此之前，人类历史上从未有任何一枚火箭将这种发动机真正送入太空。猛禽发动机不仅采用了极度复杂的全流量分级燃烧循环（Full-flow staged combustion cycle），还使用液态甲烷作为推进剂燃料，这也是轨道级运载火箭此前从未涉足的领域。 面对这样一台复杂度惊人的精密庞然大物，普通人往往望而生畏。为了厘清猛禽发动机所处的坐标体系，Tim Dodd 将其与人类航天史上的诸多经典动力系统进行横向对比：包括 SpaceX 当下的主力发动机梅林（Merlin）、航天飞机主发动机 RS-25、阿波罗计划土星5号所使用的 F-1 巨型发动机、经典的俄罗斯 RD-180，以及蓝色起源（Blue Origin）同样以甲烷为燃料的 BE-4。针对猛禽的特质，Tim Dodd 直言：“这台发动机其实在任何单项指标上都不是最拔尖的。它不是推力最强劲的，不是推重比最高的，甚至不是效率最高的，但它在方方面面都做得极其出色。” - 猛禽发动机的核心技术标签：甲烷燃料、全流量分级燃烧循环、首度应用于轨道级航天器。 - 对比标的：Merlin（SpaceX 现役主力）、RS-25（航天飞机主发动机）、F-1（土星5号动力）、RD-180（高压补燃典范）、BE-4（蓝色起源甲烷动力）。 - 猛禽的核心竞争策略：并非单项性能绝对称霸，而是多项关键属性高度平衡并达到极致实用。 ### [02:55](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=175s) 火箭发动机的物理基础与能量瓶颈 为了彻底理解猛禽发动机的精妙，必须回归火箭运行的底层物理规律。剥离复杂的工程外壳后，火箭本质上就是一个由外壳包裹的大型推进剂储存罐，尾部装有一个能以极高速度向后喷出推进剂的装置；简单来说，将推进剂喷射得越快，火箭的推进效果就越好。实现这一目标最原始且直接的方法是挤压循环（Pressure-fed rocket engine）：把推进剂储存在超高压贮箱中，打开阀门让推进剂流经喷管膨胀加速，从而产生有效推力。由于无需复杂的机械泵驱动，冷气系统、单组元或双组元挤压循环发动机凭借极高的高可靠性与闪电般的反应速度，广泛见于反推姿控系统（RCS）。 但挤压循环存在致命的物理枷锁：流体永远只能从高压流向低压，这意味着燃烧室内部的压力绝不可能高过推进剂贮箱自身的压力。想要提升燃烧室压力，贮箱壁就必须做得越来越厚、越来越重。即便借助耐压接近 10,000 PSI（约 700 bar）的碳纤维复合材料缠绕压力容器（COPV），其所能容纳的介质总量和系统压力依然有限，随着火箭规模扩大，死重惩罚会迅速扼杀入轨运力。火箭科学家很快意识到，要打造极致轻量化的火箭，核心出路在于提升“焓”（Enthalpy）——即体积、压力与温度的内在关联。燃烧室温度和压力越高，做功能力与效率就越高；单位时间内推入发动机的推进剂质量越大，产生的推力就越强。面对“拼命加厚贮箱”与“用超高功率机泵强行注料”这两条技术路线，后者显然是更优解。但每秒吞吐数百升推进剂需要浩瀚的动力，科学家将一具微型火箭发动机喷出的高温燃气直接对准涡轮，将推进剂的部分化学能转化为驱动涡轮泵的高速动能，就此拉开了涡轮泵与分级燃烧循环的时代大幕。 ```mermaid flowchart TD accTitle: 挤压循环与涡轮泵驱动对比 accDescr: 展示了从高压贮箱挤压到利用涡轮泵增压注入燃烧室的能量与压力流动架构。 subgraph PF["挤压循环架构 (Pressure-Fed)"] T1["高压推进剂贮箱 (极厚/极重)"] -->|"高压流动 (受制于贮箱强度)"| V1["主控阀门"] V1 --> CC1["低压燃烧室与喷管"] end subgraph TP["涡轮泵动力架构 (Turbopump)"] T2["常压/低压轻质贮箱"] --> P1["涡轮高压泵"] PB["微型预燃室燃气"] -->|"冲击高速旋转"| TB["驱动涡轮"] TB -.->|"机械轴带动"| P1 P1 -->|"超高压注入"| CC2["高压燃烧室 (高焓增益)"] end ``` - 物理流动铁律：“压力永远从高压流向低压，因此发动机燃烧室压力绝不能高于推进剂贮箱。” - 焓的实用价值：高压高温燃烧室显著改善燃气喷射膨胀效率；单位时间推进剂吞吐量直接决定发动机绝对推力。 - 工程取舍：挤压系统省去泵体却付出了贮箱重量几何级暴增的代价；涡轮泵系统大幅降低贮箱壁厚，但必须在极高转速、剧烈温差与结构熔毁风险之间寻找微薄的工程裕度。 ### [06:12](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=372s) 开式循环：燃气发生器循环的运作逻辑与权衡 在现代火箭发动机的演进谱系中，燃气发生器循环（Gas generator cycle，俗称开式循环 Open Cycle）是轨道级液体火箭应用最广泛的构型之一。尽管比起挤压循环增加了诸多管线与阀门，但相较闭式分级循环，其逻辑相对明朗。燃气发生器循环的核心是被称为“动力包”（Power pack）的涡轮泵总成，它由一座微型预燃室（燃气发生器）、同轴涡轮以及负责将燃料与氧化剂加压注入主燃烧室的泵体组成。其关键特征在于“开式”二字：驱动涡轮做功后的预燃室废气不再流入主燃烧室，而是直接排入外部大气。由于这部分推进剂消耗没有完全参与主级膨胀喷射，因此在整体燃料利用效率（比冲）上存在天然的固有损耗。 开式循环在点火启动阶段面临着如同“先有鸡还是先有蛋”的悖论困境：预燃室必须依靠高压推进剂才能正常点火工作，但推进剂高压却必须依赖运转的涡轮泵来建立；换言之，涡轮泵需要预燃室喷出燃气才能旋转，而预燃室又需要涡轮泵旋转泵入高压液体才能启动。此外，依据“流体必然从高压流向低压”的定律，涡轮泵出口及通向预燃室的入口管线承受着整个发动机系统的最高绝对压力，越向下游，压力逐级递减。另一个极度危险的现实是，推进剂以理论完美化学配比燃烧时虽然能量最大、效率最高，但释放的几千摄氏度地狱般的高温会瞬间融毁涡轮叶片。为了保护涡轮结构，工程师必须故意让预燃室偏离最佳燃烧配比，采用极度“富燃”（Fuel-rich）或“富氧”（Oxidizer-rich）的状态运行以抑制运行温度。以使用 RP-1 煤油与液氧的 SpaceX 梅林发动机为例，其涡轮排气管喷出的滚滚黑烟就是典型富燃燃烧时煤油未完全裂解生成的浓重碳黑沉积。 - 系统压力梯级：预燃室入口处是全发动机系统压力最高的极值点，所有下游组件（主燃烧室、喷嘴、排气管）的压力均依次降低。 - 涡轮热防护策略：“按理论完美化学配比混合燃烧虽然效率最高、释放能量最多，但同时会产生疯狂的超高温度。” - 启动循环困局：涡轮泵转动与预燃室起压相互依赖，形成苛刻的点火建压循环逻辑。 - 排气表征分析：Merlin 发动机涡轮废气呈现明显的黑烟，即是为了将涡轮工作温度控制在金属耐受极限以内而故意采用极度富燃配比导致的游离碳析出。 ### [09:20](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=560s) 闭式循环的诞生与积碳困境 在开式循环中，预燃室如果以富燃（Fuel-rich）模式运行煤油（RP-1），未完全燃烧的燃油会化为浓烈的黑色烟尘喷出。高压状态下，未燃烧的碳分子极易彼此键合形成聚合物，这种现象被称为“结焦”（coking）。这些煤灰不仅黏附在所触及的一切表面上，还会堵塞精密加工的喷注器，甚至严重损坏高速旋转的涡轮叶片。 面对开式循环直接将未燃尽的高压推进剂排入大气的巨大浪费，火箭工程师们提出了一个构想：既然富燃气体温度相对较低、蕴含着大量未完全燃烧的能量，为什么不直接把这些高温高压的废气导入主燃烧室重新参与燃烧？这便是“闭式循环”（Closed Cycle），亦称“分级燃烧循环”（Staged Combustion Cycle）。该构想旨在将以往白白流失的废气循环引入燃烧室，既抬高了燃烧室压力，又大幅提升了比冲与推力效率。 然而，闭式循环的实现极具挑战。如果直接将SpaceX的梅林（Merlin）发动机强行“闭环”，将废气直接引向主燃烧室，立刻就会遭遇灾难性的后果——大量的碳灰会在一瞬间堵死所有喷注器。用博主风趣的话说：“You do not go to space today my friend.”（朋友，你今天休想上天了）。 ```mermaid flowchart LR accTitle: 富燃煤油闭式循环的结焦堵塞 accDescr: 展示了RP-1富燃预燃室产生的积碳如何堵塞主燃烧室喷注器导致失效的过程。 subgraph S1["预燃室阶段"] A["高压 RP-1 煤油"] --> C["富燃预燃室"] B["高压 液氧"] --> C end subgraph S2["结焦失效"] C -->|"富燃高温燃气 (含大量未燃碳)"| D["积碳与结焦现象"] D -->|"碳颗粒堵死"| E["主燃烧室喷注器 (失效)"] end ``` ### [10:31](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=631s) 苏联的突围：富氧分级燃烧与冶金奇迹 面对RP-1煤油无法在富燃模式下闭式循环的技术死胡同，苏联工程师找到了破局之法：既然不能富燃，那就走另一个极端——让预燃室“富氧”（Oxygen-rich）运行。苏联率先为N-1登月火箭研制出了人类首款实用型闭式循环发动机NK-15，随后升级为著名的NK-33，该技术血脉最终演进到了当今阿特拉斯五号（Atlas V）火箭使用的RD-180发动机。 在富氧循环中，几乎全部的液氧推进剂都注入预燃室，只加入极少量的煤油进行燃烧，以此驱动涡轮并把全部氧气气化。但这也带来了极其可怕的工程噩梦：高速喷向涡轮叶片的是超高压、超高温的气态纯氧，在此类极端环境下，几乎所有常规金属都会瞬间被引燃化成一滩滚烫的铁水。美国工程师当时普遍认为这在物理和材料学上根本不可能做到，并在研发中彻底放弃了该技术路线，甚至一度断定苏联声称研制出高效率高压RP-1闭式发动机是虚假宣传。直到苏联解体后，美国专家亲临俄罗斯试验场亲眼见证并亲自测试，才心悦诚服地确认苏联工程师确实用科学奇迹攻克了这一难关，成功研制出足以耐受高压富氧极端环境的特种耐高温合金。 气态氧气冲刷涡轮释放能量后，被强力泵入主燃烧室与高压液态煤油相遇，发生极度纯净且充分的高效燃烧，推进剂几乎没有一丝浪费。但这也给涡轮泵带来了巨大负荷——因为主燃烧室的压力永远不可能超过泵的出口压力，金属叶片必须扛起整台发动机的极限压强。 - 关键型号：NK-15（N-1火箭）、NK-33、RD-180（Atlas V） - 推进剂路线：RP-1 煤油 + 液氧（LOX） - 核心技术：富氧预燃室（Oxygen-rich Preburner），全部氧气与极少量燃料反应气化后注入燃烧室 ### [12:42](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=762s) 美国的应战：航天飞机RS-25与双富燃氢氧循环 美国并没有将闭式循环的荣誉拱手让给苏联，他们另辟蹊径，走出了一条以氢气为核心的富燃闭式循环路线。既然RP-1等富碳燃料在富燃时会析出致命碳黑，美国洛克达因（Rocketdyne）公司便决定采用完全不含碳的液氢作为燃料，研制出了日后名垂青史的航天飞机主发动机——RS-25。 氢燃料彻底解决了积碳问题，也规避了高压富氧气体的极端氧化腐蚀，但它带来了全新的挑战。氢的密度极低，与液氧相差悬殊，要输送同等能量所需的氢气体积庞大，因此不可能像密度相近的煤油与液氧那样共用一根转轴。RS-25不得不设计了两套独立的预燃室和两台完全分离的涡轮泵，两座预燃室全部采用富燃模式运行。 双轴架构随之引出了另一个致命危机：在负责驱动液氧泵的轴承处，一侧是超高温高压的气态氢气，紧挨着的另一侧则是超低温的液氧泵。氢分子体积极小、极易穿透缝隙，一旦微量氢气越过密封圈接触到液氧，瞬间就会诱发灾难性的爆燃。为此，工程师发明了极其精密的“吹扫密封”（Purge Seal）结构，在密封腔内注入惰性氦气并使其维持在整个系统的最高压力点。正如博主惊叹的那样，这种即便利率泄露也只泄漏惰性氦气的设计堪称绝妙：“It's genius but take a look at how different the LOX turbo pump and the hydrogen turbo pump seals look.”通过对比结构即可发现，为了防止氢气泄漏，工程师在密封设计上投入了令人发指的心血。 ```mermaid flowchart TD accTitle: RS-25 发动机双富燃分级燃烧循环架构 accDescr: 展示了液氢液氧分别经过两个独立的富燃预燃室，最终进入主燃烧室的循环结构。 subgraph Propellants["推进剂供应"] LH2["液氢 (LH2)"] LOX["液氧 (LOX)"] end subgraph Preburners["双富燃预燃室"] FPB["燃料侧预燃室 (富燃)"] OPB["氧化剂侧预燃室 (富燃)"] end subgraph Pumps["独立涡轮泵组"] FTP["液氢涡轮泵"] OTP["液氧涡轮泵 (配氦气吹扫密封)"] end subgraph Combustion["主推力室"] MC["主燃烧室"] end LH2 --> FTP LOX --> OTP FTP --> FPB FTP --> OPB OTP -->|"分流少量液氧"| FPB OTP -->|"分流少量液氧"| OPB FPB -->|"富燃燃气"| FTP OPB -->|"富燃燃气"| OTP FTP -->|"气态富氢排出"| MC OTP -->|"气态富氢排出"| MC OTP -->|"主力液氧"| MC ``` ### [15:53](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=953s) 终极答案：全流量分级燃烧与SpaceX Raptor的突破 当掌握了苏联的富氧循环与美国的富燃循环后，火箭动力学的终极构想应运而生：全流量分级燃烧循环（Full-Flow Staged Combustion Cycle, FFSC）。该循环将两者的精髓彻底融合：发动机设置两个预燃室，燃料侧预燃室以极端“富燃”工况运行驱动燃料泵，氧化剂侧预燃室则以极端“富氧”工况运行驱动氧泵。 全流量设计的优势极为显著： 1. **气-气混合燃烧**：推进剂进入主燃烧室时不再是液滴，而是全量转化为高温高压的气态燃料与气态氧化剂，两股气体相撞能够达成最彻底的雾化与混合，实现更高的燃烧室压力与空前的燃烧效率。 2. **彻底消除轴间泄漏隐患**：由于驱动氧泵的预燃室全部是氧气废气，驱动燃料泵的全部是燃料废气，转轴两侧介质性质完全一致，即使出现微量密封磨损也只是“同种介质相见”，彻底摆脱了RS-25那种命悬一线的氦气密封困局，这对几乎不需要翻修即可反复复飞的重复使用火箭而言具有划时代意义。 3. **涡轮温度更低、寿命更长**：因为发动机将100%的燃料和100%的液氧分别注入两座预燃室（全流量质量流巨大），在提供等同泵功率的前提下，推进剂配比可以推向极其悬殊的超浓富燃与超浓富氧状态。这反而大幅降低了预燃室内的燃烧温升，涡轮叶片在相对更凉爽、压强更温和的环境下工作，不仅延长了寿命，还将主绝大部分剧烈燃烧放热留到了主燃烧室内完成。 为了承受富氧预燃室高达数百个大气压的高温纯氧冲刷，SpaceX在没有RP-1结焦限制的前提下（采用甲烷作为燃料），自主研发了名为“SX500”的革命性特种耐氧化高温镍基超合金。博主引述马斯克的公开说法：“According to Elon Musk it's capable of over 800 bar of hot oxygen rich gas.”这直接击穿了全流量发动机最大的物理研发瓶颈。 在人类航天史上，全流量分级燃烧循环曾被称为“发动机王冠上的明珠”，却因难度逆天数度折戟： - 20世纪60年代苏联研制的RD-270（采用有毒剧毒推进剂），从未飞天； - 21世纪初Aerojet与Rocketdyne联合研制的“综合动力头演示验证机”（Integrated Powerhead Demonstrator, IPD），止步于地面测试台； - SpaceX的猛禽（Raptor）发动机是人类历史上第三台尝试该循环的发动机，更是有史以来第一台走出试验台、成功产生实际做功并飞离地面的全流量分级燃烧引擎。 ```mermaid flowchart TD accTitle: 全流量分级燃烧循环 (FFSC) 简图 accDescr: 展现了燃料全流量富燃、氧化剂全流量富氧两路独立驱动涡轮并进入主燃烧室的完整管路。 subgraph FuelCircuit["全量燃料回路"] F_In["液态燃料 (甲烷)"] --> FP["燃料涡轮泵"] FP --> F_Pre["富燃预燃室 (极度富燃)"] F_Pre -->|"全部高温气态燃料"| FT["燃料涡轮"] end subgraph O2Circuit["全量氧化剂回路"] O_In["液氧"] --> OP["液氧涡轮泵"] OP --> O_Pre["富氧预燃室 (极度富氧)"] O_Pre -->|"全部高温气态氧"| OT["氧涡轮"] end FT -->|"纯气相输入"| Chamber["主燃烧室 (气-气高效燃烧)"] OT -->|"纯气相输入"| Chamber OP -.->|"微量液氧分流"| F_Pre FP -.->|"微量燃料分流"| O_Pre ``` ### [18:59](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=1139s) 猛禽发动机的极端挑战与发动机循环对比 SpaceX 的猛禽发动机（Raptor）注定要载入史册，主持人 Everyday Astronaut 感慨道：“Well actually just about anything this engine does will be a first.”（实际上这台发动机所做的几乎任何事都将是开创性的）。作为人类首台有望进入轨道的全流量分级燃烧循环（Full-flow staged combustion cycle）发动机，SpaceX 在研制中必须攻克极其严苛的技术难题。他们不仅要解决困扰富氧分级燃烧循环已久的金属合金耐受问题——面对高压纯氧气流的强烈氧化侵蚀，必须研发出极其强韧的特殊金属合金，同时还得精准掌控两个独立的预燃室（富氧与富燃预燃室）和两套协同运作的涡轮泵循环。 这种前所未有的工程架构带来了人类火箭发动机史上的最高室压纪录。猛禽发动机在测试中达到了 270 bar 的燃烧室压力，一举打破了俄罗斯 RD-180 保持的约 265 bar 原纪录。更令人震惊的是，SpaceX 的目标远不止于此，工程师们正向着惊人的 300 bar 燃烧室压力迈进。在展开更多细节前，主持人将此前讲解过的各种发动机循环图表集中放到同一屏幕上动态播放，以便观众直观横向比对不同循环的燃料与氧化剂流向差异。 ```mermaid flowchart TD accTitle: 全流量分级燃烧双预燃室流程 accDescr: 展现猛禽发动机双预燃室将全部推进剂转化为气态注入主燃烧室的高压流程 subgraph 推进剂供应 LOX["液氧 (LOX)"] CH4["液态甲烷 (CH4)"] end subgraph 预燃增压系统 OPB["富氧预燃室\n(少量甲烷 + 全部氧气)"] FPB["富燃预燃室\n(全部甲烷 + 少量氧气)"] end subgraph 主动力输出 MCC["主燃烧室\n(室压突破 270 bar / 目标 300 bar)"] Nozzle["发动机喷管排气"] end LOX --> OPB CH4 -.->|"微量分流"| OPB CH4 --> FPB LOX -.->|"微量分流"| FPB OPB -->|"气态富氧尾气"| MCC FPB -->|"气态富燃尾气"| MCC MCC --> Nozzle ``` --- ### [20:00](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=1200s) 燃料抉择：推进剂密度与氧燃比的权衡 既然猛禽发动机无法像普通发动机那样直接用煤油（RP-1）来运行富燃预燃室（煤油富燃极易严重积碳结焦），通常逻辑下液氢似乎是下一顺位选择。然而 SpaceX 既没有选 RP-1 也没有选液氢，而是破天荒地选择了液态甲烷（Methalox 组合）。迄今为止尚无任何液态甲烷发动机成功入轨，SpaceX 做出这一定位的底层物理优势何在？将其置于 RP-1 与液氢之间对比便一目了然： 首先是推进剂密度的考量。对于火箭第一级设计而言，密度至关重要——更高的密度意味着存储相同质量的燃料所需的储箱体积更小、结构死重更轻，而储箱变小意味着整枚火箭更轻。对比三种燃料的密度（单位：克/升）： - **RP-1（火箭级煤油）**：约 813 g/L（若经 SpaceX 在 Falcon 9/Heavy 上采用的过冷技术处理，密度可额外增加 2% 到 4%）。 - **液氢（LH2）**：仅约 70 g/L，RP-1 的密度足足是液氢的 11 倍。正如曾经的齐柏林飞艇利用极轻的氢气漂浮，氢气由于密度极低而极易浮起，同时也造就了像兴登堡号空难那样的易燃特性。 - **液态甲烷（LCH4）**：约为 422 g/L，恰好处于 RP-1 与液氢的折中位置。 但纯密度并不能代表一切，必须结合氧燃比（Oxidizer-to-fuel ratio）综合计算。火箭工程师在实际设计中并不会采用理论上的完美化学计量比燃烧，而是追求一种在储箱体积、推力输出与比冲之间达成平衡的绝佳中间值： | 推进剂组合 | 燃烧质量比 (O:F) | 氧化剂 (LOX) 密度 | 消耗 1 升液氧所需燃料体积 | 相对 RP-1 的燃料箱体积 | |:--- |:--- |:--- |:--- |:--- | | **LOX / RP-1** | 2.7: 1 | 1141 g/L | 约 0.52 升 RP-1 | 基准（1.0x） | | **LOX / 液氢 (LH2)** | 6.0: 1 | 1141 g/L | 约 2.7 升液氢 | 约 5.0 倍大 | | **LOX / 甲烷 (LCH4)** | 3.7: 1 | 1141 g/L | 约 0.73 升甲烷 | 仅约 1.4 倍大（+40%） | “So the fuel to oxidizer ratio helps make a methane fuel tank a lot closer to an RP-1 tank than it is to a hydrogen tank.”（因此，氧燃比使得甲烷储箱的尺寸远比想象中更接近 RP-1 储箱，而不是像液氢储箱那样庞大）。液氢虽然单看密度比 RP-1 小 11 倍，但由于 6:1 的高质量比分摊，其储箱体积最终被缩减至 RP-1 的约 5 倍左右。这直观解释了为何以液氢为核心的德尔塔IV（Delta IV）火箭的液氧箱远小于其巨大的氢箱，而 Falcon 9 上的 RP-1 箱则明显小于液氧箱。而甲烷发动机在 3.7:1 的质量比帮助下，虽然纯密度差不多只有 RP-1 的一半，但其燃料箱体积仅需比 RP-1 大约 40%，且比液氢储箱小了整整 3.7 倍。 --- ### [24:37](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=1477s) 比冲效率、燃烧温度与热力学特性对比 推进剂选择的另一大核心指标是发动机效率，即比冲（Specific Impulse, ISP），主持人体贴地将其类比为燃油汽车的“油耗表现”或每加仑行驶里程（MPG）。比冲的物理直觉可以理解为：给发动机 1 千克的推进剂，它能以 9.8 牛顿的恒定推力持续工作多少秒。工作时间越长，意味着发动机利用单位燃料做功的能力越强。 受各燃料分子量以及反应释放能量差异的影响，排出喷管的高速废气速度不同，造就了各异的理论极限与实际运行比冲： - **理论理想真空比冲**：RP-1 约为 370 秒；液氢可达 532 秒；而甲烷依然稳居中游，达到 459 秒。 - **现实工作发动机比冲**：RP-1 发动机（如 Merlin 1D 真空版）约为 350 秒；液氢发动机（如 RL-10B-2）可达约 465 秒；而甲烷真空发动机（如规划中的猛禽真空版）预计在 380 秒左右。 燃烧热与热力学工况同样深远地影响着发动机寿命与材料压力。燃烧温度更低意味着对燃烧室内壁更温和，有助于实现更长寿的重复使用： - **最高燃烧温度**：RP-1 高达 3670 K；液氢仅为 3070 K；甲烷继续居中，约为 3550 K。 推进剂的沸点特性（汽化温度）则关系到系统绝热死重与储箱集成度。越高的沸点意味着维持液态越容易，甚至可以彻底取消绝热层，从而削减火箭结构自重： - **沸点分布**：RP-1 沸点高达 490 K（甚至高于水）；液氢则处于逼近绝对零度的极寒状态（仅 20 K），极难长时间保温保存；甲烷则像童话里的“金发姑娘”（Goldilocks）一样处于中间的 111 K。 - **共用底壁（Common Dome）优势**：液氧的沸点约为 90 K，与甲烷的 111 K 极为接近。两者之间极小的温差使得火箭结构能够使用共用底壁直接隔开氧箱与甲烷箱，大幅减轻弹体重量；反观液氢与液氧，温差如此悬殊，以至于液氧会直接把液氢烤沸蒸发，而液氢又能将液氧瞬间冻成死硬的固体。 --- ### [27:39](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=1659s) 燃烧产物环保性与经济成本考量 在排气产物方面，三种燃料呈现出截然不同的化学痕迹： - **RP-1**：属于三者中唯一产生显著未燃烧游离碳黑（煤烟）的燃料，伴随水蒸气一同排放至大气。 - **液氢**：燃烧产物几乎纯粹是水蒸气。 - **液态甲烷**：主要生成水蒸气与二氧化碳（CO2）。 主持人在此特别点出，在高层大气中，水蒸气实际上也是一种潜在大气温室气体效应因子，后续将制作专门的专题视频深度剖析火箭的大气污染、海洋坠落污染以及太空垃圾问题。 在成本经济学层面，燃料单价差异显著： - **RP-1**：属于高度提纯的航空喷气燃料（Jet Fuel），而航空煤油本就是高纯柴油，繁杂的精炼流程推高了成本，价格稳步高过一般工业柴油。 - **液氢**：尽管氢元素在宇宙与自然界分布广泛，但将其提纯、液化、深冷储存以及长途运输的技术极度繁复昂贵，导致整体工装与采购费用高企。 - **液态甲烷**：基本等同于城市工业广泛使用的天然气，货源充足且廉价易得。当每次发射需要采购以数十乃至数百吨计的推进剂时，长期的燃料成本累加效应将不容忽视。 ### [29:04](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=1744s) 为什么是甲烷？火星原位资源利用（ISRU）与燃料选择 燃料的制备难度比某些复杂的化学参数更为关键。SpaceX 将甲烷视为公司未来核心且不可或缺的一部分，其根本原因在于公司的终极愿景：开发一套能够运送人类往返火星并反复复用的星际运输系统。火星大气层富含二氧化碳（$CO_2$），如果结合火星地表或地下开采的水资源（$H_2O$），通过电解水产生氢气，再与二氧化碳发生萨巴捷反应（Sabatier process），就能直接在火星表面就地生产液态甲烷与氧气。这种被称为“原位资源利用”（In Situ Resource Utilization，简称 ISRU）的技术，让飞船无需从地球携带昂贵的返程燃料，依靠当地资源即可注满储罐。 有人或许会问，既然在火星上电解水能得到氢气，为什么不直接拿氢气当作火箭推进剂？原因在于液氢极低沸点对长期深空任务构成的致命挑战。让氢气在漫长的任务周期中维持液态需要极为复杂严苛的隔热与制冷技术，稍有不慎便会严重蒸发损耗。相比之下，液态甲烷对 SpaceX 而言是极佳的平衡点：它拥有相对较高的密度，使火箭储罐体积维持在合理尺寸；燃烧效率适中且产物极其干净，极少产生积碳，对发动机重复使用极为友好；其燃烧温度相对较低，能延长发动机零部件寿命；同时它在火星表面易于规模化生产。正因如此，结合密度、效率、清洁度、复用性与火星制备潜力，甲烷成为了最佳选择。 ```mermaid flowchart LR accTitle: 火星原位资源利用（ISRU）萨巴捷制取甲烷流程 accDescr: 展示火星大气二氧化碳与开采水通过电解与萨巴捷反应生成甲烷燃料的闭环流程 subgraph RES["火星就地资源获取"] ATM["火星大气：二氧化碳 (CO2)"] H2O["地表/地下水采掘 (H2O)"] end subgraph PROC["电解与热化学反应"] ELEC["水电解反应：分解出 H2 与 O2"] SAB["萨巴捷反应器 (Sabatier Process)"] end subgraph OUT["推进剂产物"] CH4["液态甲烷 (CH4 燃料)"] LOX["液态氧 (LOX 氧化剂)"] end H2O --> ELEC ELEC -->|"氢气 (H2)"| SAB ATM --> SAB ELEC -->|"副产氧气"| LOX SAB --> CH4 ``` - 火星原位资源利用（ISRU）免去了携带往返全部推进剂的运力惩罚。 - 液氢在深空长周期储存中面临不可忽视的挥发（Boil-off）与超低温维持难题。 - 甲烷综合优势：密度适中使箭体紧凑、积碳极少有助发动机复用、燃烧温度较低延长硬件寿命、原料易得且成本低廉。 ### [30:43](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=1843s) 顶流火箭发动机横向对比：参评阵容与背景 掌握了循环原理与推进剂特性后，六款具有代表性的轨道级火箭发动机并排亮相。首先是 RP-1 煤油组：SpaceX 猎鹰 9 号和猎鹰重型上使用的开式循环（燃气发生器循环）Merlin 发动机、俄罗斯动力机械科研生产联合体（NPO Energomash）为宇宙神 5 号（Atlas V）研制的富氧分级燃烧闭式循环 RD-180，以及当年土星 5 号上搭载的巨无霸——Rocketdyne 开式循环 F-1 发动机。紧接着是甲烷组：SpaceX 用于星舰（Starship）与超重助推器（Super Heavy）的全流量分级燃烧循环 Raptor 发动机，以及蓝色起源（Blue Origin）正在研制、计划装备新格伦（New Glenn）与联合发射联盟（ULA）火神（Vulcan）火箭的富氧分级燃烧循环 BE-4 发动机。最后是液氢组：航天飞机与 SLS 火箭上采用的 Aerojet Rocketdyne 富氢闭式分级燃烧循环 RS-25 发动机。 需要特别说明的是，对比时 Raptor 和 BE-4 仍处于密集研发阶段，所引用的数据多为当时的实测进度或设计目标，技术规格随时可能发生调整。关于双喷管的 RD-180，很多人容易将其误认为是两台并联的发动机，但它实际上是拥有单一涡轮泵总成、由其向两个独立推力室分流驱动的单台发动机。苏联当年成功攻克了极端高温高压富氧燃气的冶金难题，却在超大推力燃烧室的不稳定燃烧问题上面临瓶颈，因此选择通过多个较小推力室并联的形式来绕开大喷管燃烧振荡的风险。 - **煤油（RP-1）梯队**：Merlin 1D（开式燃气发生器）、RD-180（闭式富氧分级燃烧，单泵双室）、F-1（开式燃气发生器）。 - **甲烷（LCH4）梯队**：Raptor（全流量分级燃烧，双涡轮泵）、BE-4（闭式富氧分级燃烧）。 - **液氢（LH2）梯队**：RS-25（闭式富氢分级燃烧）。 ### [32:58](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=1978s) 关键技术指标对决：推力、推重比、比冲与室压 在各项核心性能对比中，海平面推力首先拉开战幕。Merlin 海平面推力最小，约为 0.84 兆牛（MN）；RS-25 为 1.86 MN；Raptor 此时约为 2.0 MN；BE-4 目标设定在 2.4 MN；双燃烧室的 RD-180 输出 3.83 MN；而阿波罗计划的巨兽 F-1 发动机依然以 6.77 MN 独占鳌头。虽然历史上曾有更强劲的 RD-170 发动机，但由于其极少执行飞行任务，因此选用成熟常飞的发动机更具对比意义。 决定运载火箭死重性能的推重比（TWR）指标则呈现出完全不同的格局。追求极端材料复用与复杂循环的 RS-25 以 73:1 垫底；RD-180 为 78:1；BE-4 估算在 80:1 左右；庞大的 F-1 为 94:1；Raptor 当时达到了约 107:1；而看似最不起眼的 Merlin 发动机却凭借开式循环极其精简轻便的结构，展现了令人叹为观止的 198:1 惊人推重比，堪称绝对的动力怪兽。 在代表发动机推进效率的比冲（ISP）方面，氢氧发动机展现出无可动摇的统治力：RS-25 海平面至真空比冲高达 366 至 452 秒。煤油发动机受分子量限制普遍偏低，F-1 仅有 263 至 304 秒，Merlin 为 282 至 311 秒，闭式循环的 RD-180 提升至 311 至 338 秒。甲烷发动机则居于中游偏上，BE-4 预计在 310 至 340 秒之间，全流量分级燃烧的 Raptor 则落在 330 至 350 秒区间。 室压（Chamber Pressure）是同时放大推力并改善发动机紧凑度和比冲的关键推手，更高的室压意味着在同等推力下可以使用更小的推力室，从而直接拔高推重比。土星 5 号的 F-1 只有 70 bar（相当水下 700 米压强）；Merlin 为 97 bar；BE-4 预计在 135 bar 左右；航天飞机的 RS-25 达到 206 bar；此前被奉为现役闭式发动机技术标杆的 RD-180 达到了 257 bar；而 SpaceX 的全流量分级燃烧 Raptor 发动机在测试中直接轰出 270 bar，正式登顶室压新王，并朝着惊人的 300 bar 目标挺进——300 bar 的压力相当于置身海洋深处 3000 米的极端环境。 | 发动机 | 推进剂 | 循环类型 | 海平面推力 (MN) | 推重比 (TWR) | 比冲 ISP (s, 海平面~真空) | 主室压 (bar) | |:--- |:--- |:--- |:--- |:--- |:--- |:--- | | **F-1** | RP-1 / LOX | 燃气发生器循环 (开式) | 6.77 | 94:1 | 263 ~ 304 | 70 | | **Merlin 1D** | RP-1 / LOX | 燃气发生器循环 (开式) | 0.84 | 198:1 | 282 ~ 311 | 97 | | **RD-180** | RP-1 / LOX | 富氧分级燃烧 (闭式, 双室) | 3.83 | 78:1 | 311 ~ 338 | 257 | | **RS-25** | LH2 / LOX | 富氢分级燃烧 (闭式) | 1.86 | 73:1 | 366 ~ 452 | 206 | | **BE-4** | LCH4 / LOX | 富氧分级燃烧 (闭式) | 2.4 (目标) | ~80:1 (估测) | 310 ~ 340 (目标) | ~135 | | **Raptor** | LCH4 / LOX | 全流量分级燃烧 (FFSC) | 2.0 (当时实测) | ~107:1 (演进中) | 330 ~ 350 | 270 (目标 300) | ### [36:44](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=2204s) 商业运营考量：制造成本与全寿命复用寿命 如果抛开造价与复用能力，火箭发动机的纸面参数再好看也毫无工业普及价值。在计入通货膨胀后的单台制造成本对比中，结构最精密的 RS-25 标价高得令人咂舌，每台成本突破 5000 万美元；F-1 约合 3000 万美元；RD-180 单价在 2500 万美元左右；BE-4 预估造价控制在 800 万美元附近。得益于极具激进性的工业化去复杂化改造，马斯克公开表示希望能将 Raptor 的成本打压到接近甚至低于 Merlin 的水平；现阶段推算单台 Raptor 成本大约在 200 万美元上下，而大批量制造的 Merlin 单台成本甚至不足 100 万美元。 在复用性维度上，传统的 F-1 与 RD-180 纯属一次性消耗品，从未进行过任务复用。与之相反，现代与现役发动机将复用摆在最前线。航天飞机 RS-25 发动机创下单机飞行 19 次的历史记录，但每次飞行之间都需要耗费数月进行极其复杂的拆解大修；Merlin 旨在实现免大修连续飞行 10 次的目标；BE-4 的设计目标为复用 25 次；Raptor 发动机则被寄予完成多达 50 次飞行的厚望。 SpaceX 在发动机寿命测试上的打磨流程非常激进。猎鹰系列所用的每一台 Merlin 发动机在真正飞上太空前，就已经经受了极为密集的地面全周期点火洗礼：发动机在加州霍桑（Hawthorne）工厂组装下线后，首先运往德克萨斯州麦格雷戈（McGregor）测试基地接受一次单机全任务时长的点火考核；合格后送回加州工厂，与一子级底部的八爪架（Octaweb）进行结构集成；整级组装完成后，整个推进级再次被拉到麦格雷戈试验场执行整级全任务时长静态点火；紧接着运输至发射台工位，在发射前几天还要执行数秒钟的例行静态点火。正如主播 Everyday Astronaut 所概括的：“So it's already done like three missions in duration of firing by the time it flies for the first time...”（“因此，在它执行第一次真正飞行之前，所经历的点火时长就已经相当于完成了大约三次任务……”）。这种将大量飞行寿命直接提前在地面消耗并验证的工业打法，彻底颠覆了航天推进系统的质量控制逻辑。 - **单台成本对比（估测美元）**：RS-25（>5000 万）> F-1（~3000 万）> RD-180（~2500 万）> BE-4（~800 万）> Raptor（~200 万）> Merlin（<100 万）。 - **设计复用次数**：RS-25（最高 19 次，需数月大修）、BE-4（目标 25 次）、Merlin（目标 10 次不经大修）、Raptor（目标 50 次）。 - **Merlin 飞行前的三道严酷试炼**：霍桑制造 -> 麦格雷戈单机全时点火 -> 霍桑整级集成 -> 麦格雷戈整级全时点火 -> 发射场台位短时静态点火 -> 最终执行任务。 ### [39:18](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=2358s) 成本效益指标：推力资金比与单次飞行千牛成本 在对比火箭发动机的价格时，单纯看制造总价容易产生误导。Tim Dodd 提到 Elon Musk 在 2019 年 2 月发布的一条推文，指出 SpaceX 希望大幅提升 Raptor 发动机的“推力资金比”（Thrust-to-Dollar Ratio）。从工程与商业的角度来看，单台发动机的绝对价格并不关键，真正重要的是获得相同推力所需的成本——究竟是一台大推力发动机更划算，还是并联多台小推力发动机更经济。 为了公平评估，Tim 计算了各款发动机的“每千牛推力建造成本”（Dollar to Kilonewton Ratio），其差距令人咋舌。航天飞机的 RS-25 发动机建造成本高得惊人，达到了每千牛 26,881 美元；俄罗斯的 RD-180 为每千牛 6,527 美元；阿波罗登月时使用的巨型 F-1 发动机为每千牛 4,431 美元。相比之下，商业航天时代研发的发动机展现出了极高的成本控制水平：蓝色起源的 BE-4 约为每千牛 3,333 美元，SpaceX 的 Merlin 约为每千牛 1,170 美元，而 Raptor 发动机的目标更是下探到了惊人的每千牛 1,000 美元左右。 更进一步的分析则必须结合发动机的复用能力。不可复用的消耗型发动机（如 RD-180 和 F-1）其单次飞行成本等于造价，但对于可复用发动机，随着飞行次数的摊销，“每次飞行每千牛推力成本”（Cost per Kilonewton per Flight）会发生剧烈变化。RS-25 凭借其可复用设计，将摊销后的单次飞行推力成本拉低到了 1,414 美元。BE-4 如果能在其宣称的 25 次复用寿命内稳定运行，其单次千牛成本将降至 133 美元，直逼 Merlin 的 117 美元。而一旦 Raptor 发动机完全实现其设计指标与极致复用，其成本甚至能压到不可思议的每千牛每次飞行 20 美元。Tim 直呼：“这绝对是颠覆行业规则的变革（Now that is absolutely game changing）。” | 发动机型号 | 初始建造推力成本（美元 / kN） | 预期复用摊销后单次飞行推力成本（美元 / kN / 次） | |:--- |:--- |:--- | | **RS-25** | $26,881 | $1,414 | | **RD-180** | $6,527 | $6,527（不可复用） | | **F-1** | $4,431 | $4,431（不可复用） | | **BE-4** | $3,333 | 约 $133（按 25 次复用摊销） | | **Merlin** | $1,170 | 约 $117 | | **Raptor** | 约 $1,000 | 预期约 $20 | --- ### [41:41](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=2501s) 飞行履历与实际在役可靠性对比 低成本与可复用固然是 21 世纪航天技术关注的核心，但“经过充分验证的可靠性”（proven reliability）始终是发动机立足的根本。在视频录制时，Raptor 和 BE-4 均未进行过任何正式的轨道入轨飞行，尽管 Raptor 已经脱离了垂直试车台，被安装在诸如“星虫”（Starhopper）这样的早期测试载具上进行跳跃试验，但这两款引擎在当时都缺乏真实的实战记录。 在已有入轨记录的经典发动机中，阿波罗计划的 F-1 拥有 17 次飞行历史；Merlin 在当时已累积了 71 次发射，正迅速追赶已执行 79 次飞行的 RD-180；而在役次数之王则是航天飞机的主发动机 RS-25，总计经历了 135 次任务。 在可靠性比例的计算上，实际情况要比纸面数据复杂得多。RS-25 的可靠性超过 99.5%，但历史上也发生过未完全按预定程序关闭的情况；Merlin 发动机的可靠性高达 99.9%，这在很大程度上得益于猎鹰 9 号每发任务都要使用 10 台（含第二级单台真空版）的高样本基数。在其服役生涯早期，仅发生过一次发动机空中故障熄火，但得益于冗余设计，火箭依然成功完成了该次任务。 从统计口径来看，F-1 和 RD-180 在官方记录上都达到了 100% 的成功率。其中 F-1 在历史上从无提前熄火的记录。而 RD-180 所谓的 100% 成功则掺杂了巨大的运气成分：在 2016 年的一次宇宙神 5 号（Atlas V）发射任务中，一台 RD-180 因为阀门故障提前整整 6 秒关机，完全是依靠 Centaur 半人马座第二级火箭消耗了储备的 Delta-V 才勉强把卫星送入轨道。正如 Tim Dodd 指出的那样：“如果那个阀门早坏哪怕一秒钟，那次任务就会彻底失败（Had that valve failed even a second earlier, that mission would have failed）。” - **RS-25**：135 次飞行，可靠性超过 99.5%。 - **Merlin**：71 次飞行，可靠性达 99.9%（仅 1 次早期单发故障，任务依然成功）。 - **RD-180**：79 次飞行，名义上 100% 成功，但 2016 年曾因阀门故障提前 6 秒关机险遭失利。 - **F-1**：17 次飞行，100% 成功且从未发生提前非计划停机。 - **Raptor / BE-4**：录制时均无实战入轨飞行记录。 --- ### [44:08](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=2648s) 终极复盘：Raptor 发动机是妥协与野心的极致平衡吗？ 纵观发动机的构型、循环机制与推进剂特性，任何火箭的设计都是一系列极其复杂的权衡与妥协（a complex series of compromises）。任何一个参数的微调，都会在整个运载工具的设计与实施上引发巨大的连锁反应。要理解 SpaceX 为何费尽心力去死磕全流量分级燃烧循环（FFSC）以及液氧甲烷路线，就必须回归 SpaceX 的终极愿景：打造一款能够快速且完全重复使用、以尽可能低廉且常规的方式将人类送上月球与火星的载具。 ```mermaid flowchart TD accTitle: SpaceX Starship Requirements to Raptor Architecture accDescr: Diagram mapping SpaceX mission requirements to technical design choices of the Raptor engine. subgraph Req["SpaceX 终极目标与工程需求"] A["快速完全复用"] B["火星就地资源利用 ISRU"] C["深空长程巡航保温"] D["多发并联冗余与批量制造"] end subgraph Solution["Raptor 对应设计方案"] E["富氧/富燃双预燃室<br/>简单密封与低温涡轮"] F["液氧甲烷推进剂<br/>低积碳与易制备"] G["适中沸点与高储罐密度"] H["极高燃烧室压力<br/>紧凑小体积大推力"] end A --> E A --> F B --> F C --> G D --> H ``` 为了实现极致复用，发动机必须燃烧干净且维护简便。全流量分级燃烧循环的涡轮密封简单，预燃室温度较低，再加上甲烷不积碳的特性，完美契合了免维护快速复用的严苛要求。为了确保冗余度并实现规模化流水线制造，飞船必须并联安装大量发动机，这就要求发动机体积必须尽量紧凑，而要将发动机做小的唯一途径就是大幅提升燃烧室压力——全流量循环再次成为不二法门。而在行星际旅行方面，甲烷的沸点更容易在漫长的火星航行中保持液态，储罐无需死重大保温层，并且在火星表面利用萨巴蒂尔反应合成甲烷也切实可行。 回到核心命题：Raptor 究竟算不算“火箭发动机之王”？Tim 坦言，单看某一项绝对数据它并不是：它不是比冲最高的发动机，不是推力最强悍的发动机，也不是研发制造最便宜的发动机。但正如他所总结的：“它真是一台‘金发姑娘’发动机，把它需要做好的每一件事都做得非常非常出色（it is truly a Goldilocks engine, doing everything it needs to do very, very well）。” 对于 Starship 这一特定的星际航行应用场景，Raptor 无疑是当之无愧的“国王”。而且考虑到过去十多年 SpaceX 对 Merlin 发动机持续不断的大幅压榨与进化，Raptor 目前仅仅处于襁褓期，未来的迭代空间不可估量。在视频的最后，Tim 向观众发起讨论，询问大家如何看待 SpaceX 挑战如此极端复杂工程的决定，并由衷感谢了过去五个月来在脚本撰写、数据核对上给予巨大支持的 Patreon 赞助者们，最后以其标志性口号“将太空带回普通人的日常生活（bringing space down to earth for everyday people）”向大家道别。 ## Notable quotes > [02:55](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=175s) 压力永远从高压流向低压，因此发动机燃烧室压力绝不能高于推进剂贮箱。 — Tim Dodd > [06:12](https://www.youtube.com/watch?v=LbH1ZDImaI8&t=372s) 按理论完美化学配比混合燃烧虽然效率最高、释放能量最多，但同时会产生疯狂的超高温度。 — Tim Dodd ## Mentioned Everyday Astronaut, Tim Dodd, Elon Musk, SpaceX, Blue Origin, Aerojet Rocketdyne, NPO Energomash, United Launch Alliance (ULA), Raptor, Merlin, RS-25, RD-180, F-1, BE-4, NK-15, NK-33, RD-270, RL-10B-2, Starship, Super Heavy, Starhopper, Falcon 9, Falcon Heavy, Delta IV, Atlas V, Saturn V, Space Launch System (SLS), Centaur, SX500 ## Video description 00:00 - Intro 02:55 - Basic physics of rocket engines 06:10 - Rocket engine cycles 20:30 - Rocket fuel comparison 30:40 - Raptor vs other rocket engines 44:05 - Summary Article version - https://everydayastronaut.com/?p=9823&preview=true SpaceX's new raptor engine is a methane fueled full flow staged combustion cycle engine and its so hard to develop, no engine like this has ever flown before! Now this topic can be really intimidating so in order to bring the Raptor engine into context, we’re going to do an overview of a few common types of rocket engine cycles then compare the Raptor to a few other common rocket engines, like SpaceX’s current work horse, the Merlin, The Space Shuttle’s RS-25, the RD-180, Blue Origin’s BE-4 and the F-1 engine. And if that’s not enough, not only is SpaceX using a crazy engine cycle, they’re also going to be using Liquid Methane as their fuel, again something that no orbital rocket has ever used! So we’ll also go over the unique characteristics of liquid methane as a rocket fuel and see if we can figure out why SpaceX went with Methane for the Raptor engine. We'll also break down and explain all the different engine cycle types so you know what the full flow staged combustion cycle is, how it works, and how it compares to the other cycles. So by the end of this video hopefully we’ll have the context to know why the raptor engine is special, how it compares to other rocket engines, why it’s using methane and hopefully find out if the Raptor engine will be the new king of rocket engines… ----------------------------------------------- Rocket engine renders by @MartianDays https://twitter.com/MartianDays F-1 Turbopump photo provided by Mike Jetzer/heroicrelics.org HUGE thanks to my Moon Walker Patreon supporters! Blake Jacobs, Eli Burton, Jethro, Mac Malkawi, Neurostream, Ole Mathias Heggem Want to support what I do? Consider becoming a Patreon supporter for access to exclusive livestreams, our discord channel and subreddit! - http://patreon.com/everydayastronaut The best place for all your space merch needs! https://everydayastronaut.com/shop/ All music is original! Check out my album "Maximum Aerodynamic Pressure" anywhere you listen to music (Spotify, iTunes, Google Play, Amazon, etc) or click here for easy links - http://everydayastronaut.com/music I'm the cohost of an awesome podcast where we talk all about current technologies and how they shape our future! http://ourludicrousfuture.com or here on YouTube https://www.youtube.com/ourludicrousfuture [Source](https://www.youtube.com/watch?v=LbH1ZDImaI8)

[2:55] Tim Dodd: 压力永远从高压流向低压，因此发动机燃烧室压力绝不能高于推进剂贮箱。 [6:12] Tim Dodd: 按理论完美化学配比混合燃烧虽然效率最高、释放能量最多，但同时会产生疯狂的超高温度。

[12:42] Tim Dodd: The RS-25 is still considered to be about the best engine ever made with a fairly high thrust to weight ratio and unmatched efficiency. [24:37] Tim Dodd: LOX will boil off hydrogen and the hydrogen will freeze LOX solid. [39:18] Tim Dodd: Who cares how much an engine costs if one big engine is cheaper than two smaller ones for the same thrust or vice versa. [20:00] Tim Dodd: So the fuel to oxidizer ratio helps make a methane fuel tank a lot closer to an RP-1 tank than it is to a hydrogen tank.

[0:00] - Hi it's me, Tim Dodd, the Everyday Astronaut. [0:02] I'm here at SpaceX's brand new launch facility [0:05] in Boca Chica, Texas [0:06] to check out the holy grail of rocket engines [0:10] and that SpaceX's upcoming Raptor engine. [0:13] An engine like this has never actually been used [0:15] on a rocket before. [0:16] Now this is a methane powered full flow [0:19] staged combustion cycle engine. [0:22] Talking about a rocket engine that's this complex [0:24] can be really intimidating. [0:25] And in order to put it into context against other engines [0:28] and other engine cycles [0:30] we're gonna do a full comparison of the Raptor engine [0:32] versus a bunch of other engines [0:33] including SpaceX's current workhorse [0:35] the Merlin engine against the RS-25, [0:38] the space shuttle main engine. [0:39] the F-1 engine that powered the Saturn 5. [0:41] The RD-180 [0:42] and Blue Origin's BE-4 that also runs on methane. [0:46] And as if the full flow of staged combustion cycle [0:48] wasn't enough, SpaceX is also doing something else unique. [0:52] They're powering that thing with liquid methane [0:54] and that's something that's actually never been done [0:56] on an orbital class rocket. [0:58] So we're gonna take a look at the characteristics of methane [1:00] and see if we can figure out [1:01] why SpaceX chose methane [1:03] instead of any other common propellant. [1:06] Now this engine isn't really the best at anything. [1:09] It's not the most powerful. [1:10] It's not the highest thrust to weight ratio of any engine. [1:12] It's not even the most efficient [1:14] but it does a lot of things really really well. [1:18] So by the end of this video [1:19] hopefully we have all the context understand [1:21] why the Raptor engine is special [1:23] how it compares to other rockets [1:24] why it's using liquid methane [1:26] and then hopefully we'll know if it really is [1:29] the king of rocket engines. [1:30] Let's get started. [1:31] (electronic music) [1:32] - [Voice over] Three, two, one, blastoff. (mumbles) [1:43] - In case you didn't notice when you clicked on this video, [1:45] this is a very, very long video. [1:48] Sorry, not sorry, [1:50] but if you're anything like me [1:51] you keep hearing a lot of hype about the Raptor engine [1:54] and you want to appreciate it [1:56] but you don't even know where to start. [1:58] Well, I've spent quite a while really studying up [2:00] on the subject so I can lay down a good foundation [2:03] in order to help us really truly fully appreciate [2:06] the Raptor engine. [2:07] Well, and quite frankly all rocket engines. [2:10] And if you're anything like me [2:11] maybe you've stared at diagrams like this or like this [2:15] or like this one for hours until [2:17] you feel like your head's going to explode. [2:19] So in order to avoid that [2:21] I've actually whipped up some really simple versions [2:24] of rocket engine cycles for all of us to enjoy [2:27] which will hopefully help us grasp these crazy concepts. [2:30] But in case this isn't your first rodeo [2:32] here's the timestamps [2:33] if you want to jump to a certain section. [2:35] There's also links in the description to each section [2:37] as well as an article version of this entire video [2:40] at my website, Everydayastronaut.com [2:42] in case you want to study some of the numbers [2:44] a little more in depth [2:45] or see sources of some of the material. [2:47] Now we're gonna start off with a super quick physics lesson [2:50] but bear with me. [2:51] We're gonna dive in and get plenty of nitty gritty details. [2:54] Okay. [2:55] Let's start off with this. [2:56] Rockets are basically just propellant [2:58] with some skin around it to keep it in place [3:00] and they have a thing on the back [3:02] that can throw said propellant really, really fast [3:05] and to way oversimplify it even more, [3:07] the faster you can throw that propellant the better. [3:11] Now the easiest way to do this [3:12] is by storing all the propellant in your tanks [3:14] under really high pressure then put a valve [3:17] on one end of the tank [3:18] and a propelling nozzle that accelerates the propellant [3:20] into workable thrust. [3:22] Done. [3:23] No crazy pumps or complicated systems [3:25] just open a valve and let her rip. [3:28] This is called a pressure fed rocket engine [3:30] and there's a few main types: [3:32] cold gas, monoprop and bipropellant pressure fed engines. [3:36] You'll often find these used in reaction control systems [3:38] because they're simple, reliable, and they react quickly. [3:41] But pressure fed engines have one big limiting factor. [3:44] Pressure always flows from high to low [3:47] so the engine can never be higher pressure [3:50] than the propellant tanks. [3:51] In order to store propellant under high pressure, [3:53] your tanks will need to be strong [3:55] and therefore thicker, and thicker, [3:57] and heavier, and heavier. [3:59] Look at composite overwrapped pressure vessels or COPDs. [4:02] They're capable of storing gases at almost 10000 PSI [4:06] or 700 bar. [4:08] And despite this there's still [4:09] a limited amount of propellant [4:11] and pressure they can store. [4:12] And this does not scale up very well [4:14] when you're trying to deliver a payload to orbit. [4:17] So smart rocket scientists quickly realized [4:19] in order to make the rocket as lightweight as possible [4:21] there's really only one thing they could do: [4:24] increase the enthalpy. [4:26] That would be a great metal band name. [4:29] You're welcome Internet. [4:30] Enthalpy is basically the relationship [4:32] between volume pressure and temperature. [4:35] A higher pressure and temperature [4:36] inside the combustion chamber equals higher efficiency [4:39] and more mass shoved through the rocket engine [4:41] equals more thrust. [4:43] So in order to shove more propellant into the engine [4:45] you could either increase the pressure in the tanks [4:47] or just shoot the propellant into the combustion chamber [4:50] with a really high powered pump. [4:52] The second option sounds like a pretty good idea. [4:55] But pumps moving hundreds of liters of fuel per second [4:58] require a lot, and boy do I mean, [5:01] a lot of energy to power them. [5:03] So what if you took a tiny rocket engine, [5:06] and aimed it right a turbine [5:08] to spin it up really, really fast? [5:10] You can exchange some of the rocket propellant's [5:12] chemical energy for kinetic energy [5:15] which could then be used to spin these powerful pumps. [5:18] Welcome to turbo pumps [5:20] and the staged combustion cycle. [5:22] But you've still got some limiting factors here [5:24] like how high pressure always wants to go to low pressure [5:26] and how heat has that habit of melting stuff. [5:30] So you've got to keep all these things in check [5:33] while trying to squeeze every bit of power [5:35] out of your engine. [5:36] There's actually a lot of different variations [5:38] of the cycles that we could talk about [5:40] but I'm going to stick with the three most common [5:42] or at least the three that matter the most [5:44] when putting the Raptor into context. [5:45] We have the gas generator cycle [5:47] the partial flow staged combustion cycle [5:49] and lastly we'll look at [5:50] the full flow staged combustion cycle [5:53] and perhaps in a future video [5:55] I'll try and do a full rundown [5:56] of all liquid fueled rocket engines [5:59] including fun new alternatives [6:00] like the electric pump fed engine [6:02] seen on Rocket Lab's Electron rocket. [6:05] (slow music) [6:12] Let's start with the gas generator cycle [6:14] known as the open cycle. [6:16] This is probably one of the most common types [6:17] of liquid fueled rocket engine used on orbital rockets. [6:20] It's definitely more complicated [6:22] than a pressure fed system but it's fairly simple, [6:24] well at least compared to their closed cycle counterparts. [6:28] Now I'm gonna way, way oversimplify this [6:30] so it's as easy to grasp as humanly possible. [6:33] In real life, there's literally dozens of valves, [6:35] a hive of wires, [6:37] and extra tiny little pipes everywhere, [6:39] helium to back pressure the tanks [6:41] fuel flowing through the nozzle [6:43] and the combustion chamber to cool it [6:44] and there is an ignition source for the preburner [6:47] and the combustion chamber. [6:48] But again for the purpose of making this as simple [6:51] and as digestible as possible, [6:52] just know there's a lot of stuff missing [6:55] from these diagrams. [6:56] But for now we're going to focus [6:57] on the flow of these engines [6:59] so we can grasp that concept first. [7:01] The gas generator cycle works by pumping [7:03] the fuel and oxidizer into the combustion chamber [7:05] using a turbo pump. [7:07] The turbo pump has a few main parts [7:09] a mini rocket engine called the preburner, [7:12] a turbine connected to a shaft [7:14] and then a pump or two that push propellant [7:17] into the combustion chamber. [7:18] Now you might hear the turbo pump assembly [7:20] called the power pack because it really is [7:22] what powers the engine. [7:24] In the open cycle system, [7:25] the spent propellant from the preburner [7:27] is simply dumped overboard [7:29] and does not contribute any significant thrust. [7:31] This makes it less efficient since the fuel [7:33] and oxidizer used to spin the pumps is basically wasted. [7:37] Now the funny thing about a turbo pump [7:39] is that it kind of has a chicken [7:40] and egg syndrome situation [7:42] that makes it pretty difficult to start up [7:44] since the preburner that powers the turbo pump [7:47] needs high pressure fuel and oxidizer to operate. [7:51] So the preburner requires the turbo pumps to spin [7:54] before it can get up to full operational pressure itself [7:57] but the turbo pumps need the preburner to fire [8:00] in order to spin the turbo pumps. [8:01] But the preburner needs the turbo pumps to ... [8:04] Yeah. [8:05] You can see where this is going. [8:06] This makes starting a gas generator pretty tricky. [8:09] There's a few ways to do this [8:10] but we don't need to get into all that in this video. [8:12] That sounds like a fun topic for future videos though. [8:14] So back to the turbo pumps. [8:16] Remember pressure always flows from high to low [8:19] so the turbo pumps need to be a higher pressure [8:22] than the chamber pressure. [8:23] And this means the inlets leading to the preburner [8:26] is actually the highest pressure point [8:28] in the entire rocket engine. [8:30] Everything else downstream is lower pressure [8:32] but notice something here. [8:34] Take a look at SpaceX's Merlin engine [8:36] which runs on RP-1 or rocket propellant 1 [8:39] and liquid oxygen. [8:40] Notice how black the smoke is [8:42] coming out of the preburner exhaust. [8:44] Why would it be so sooty [8:46] compared to the main combustion chamber [8:47] which leaves almost no visible exhaust? [8:49] Well that's because rocket propellant [8:52] can get super hot like thousands [8:54] and thousands of degrees Celsius. [8:57] So to make sure the temperature isn't so hot [8:59] that it melts the turbine [9:00] and the entire turbo pump assembly, [9:01] they need to make sure it's cool enough [9:04] to continually operate. [9:05] Running at the perfect fuel [9:06] and oxidizer ratio is the most efficient [9:08] and releases the most energy [9:10] but it also produces a crazy amount of heat. [9:13] So in order to keep the temperatures low [9:15] you can run the preburner at a less than optimal ratio. [9:18] So either too much fuel known as fuel rich [9:20] or too much oxidizer or oxygen rich. [9:23] Running an RP-1 engine fuel rich means [9:25] you'll see some unburned fuel [9:27] appearing as dark clouds of soot. [9:29] the highly pressurized unburned carbon molecules bond [9:32] and form polymers which is a process known as coking. [9:35] This soot starts to stick to everything it touches [9:38] and can block injectors or even do damage [9:41] to the turbine itself. [9:42] So what if you didn't want to waste [9:43] all that highly pressurized propellant? [9:45] I mean after all since it's running cooler [9:47] by being fuel rich doesn't that mean [9:49] there's a bunch of unburned fuel literally being wasted? [9:53] What if you could just pipe that hot exhaust gas [9:56] and put it into the combustion chamber? [9:58] Welcome to the closed cycle. [10:01] The closed cycle or staged combustion cycle [10:03] increases engine efficiency by using [10:05] what would normally be lost exhausts [10:07] and connects it to the combustion chamber [10:09] to help increase pressure [10:10] and also increase efficiency. [10:12] So let's take the Merlin engine [10:13] and try closing the loop. [10:14] Let's take the exhaust [10:16] and just pipe it straight into the combustion chamber. [10:18] Uh-oh, oh no! [10:21] We just put a bunch of soot and clogged all the injectors. [10:24] You do not go to space today my friend. [10:27] But there's a few solutions to this problem [10:29] so let's see how the Soviets solved it. [10:31] The first operational closed cycle engine they made [10:33] was the NK-15 design for their N-1 moon rocket. [10:37] They later upgraded it to the NK-33 [10:39] and then many versions from there stemmed out [10:41] including the RD-180 [10:43] which is what is used on the Atlas 5 today. [10:45] Since the NK-15 and NK-33 runs on RP-1 like the Merlin [10:50] you can't run your preburners fuel rich [10:52] because of the coking problem. [10:54] So if you want to create a closed cycle engine with RP-1, [10:58] the answer is running the preburner oxygen rich. [11:01] Easy as that, right? [11:02] Well now you're blasting superheated [11:05] highly pressurized gaseous oxygen [11:07] which will turn just about anything into soup [11:10] right at your precision machine [11:12] crazy low tolerance turbine blade. [11:15] Doing so is actually considered impossible [11:17] by the United States, [11:18] and they basically gave up on trying. [11:20] They didn't think a metal alloy existed [11:23] that could withstand these crazy crazy conditions, [11:26] and they didn't believe the Soviets [11:27] had made such an efficient [11:28] and powerful RP-1-powered engine [11:30] until after the collapse of the Soviet Union [11:33] and the US engineers got to see them [11:35] and test them out firsthand. [11:36] But the Soviets had indeed worked their butts off, [11:39] and they had made a special alloy that can magically [11:41] with science withstand the crazy conditions [11:44] of an oxygen rich preburner. [11:46] With a closed cycle engine, [11:47] you don't just use some fuel and some oxidizer [11:50] and burn that in the preburner to spin the turbine. [11:53] You actually shoot all of the rich propellant [11:55] through the turbine. [11:56] So with an oxygen rich cycle [11:58] all of the oxygen actually goes through the preburner [12:00] and just the right amount of fuel goes to the preburner. [12:04] You only need enough to give the turbine [12:06] the right amount of energy to spin the pumps fast enough [12:08] to get the right pressures for the preburner [12:11] and the combustion chamber [12:12] to make the right amount of power [12:13] to shoot the thing into space. [12:15] Just crazy. [12:17] So back to this oxygen rich preburner. [12:19] That now hot gaseous oxygen is forced [12:22] into the combustion chamber where it meets liquid fuel. [12:25] They meet and go boom [12:27] and we get a nice clean and efficient burn [12:29] without really wasting any propellant. [12:32] But still like all engines the chamber pressure [12:34] can not be higher than the pump pressure [12:37] so the pumps actually have a lot of weight [12:40] on their tiny little metal shoulders. [12:42] Now if you're sitting there thinking [12:43] that the United States just sat back [12:45] and let the Soviets have all the closed cycle glory, [12:47] you'd be wrong. [12:49] It took the United States a little bit longer [12:51] but they eventually figured out a closed cycle engine. [12:54] But it was very different from the oxygen rich cycle. [12:57] The United States pursued a closed loop cycle [13:00] but they went with a fuel rich preburner. [13:02] But wait, we just learned that fuel rich preburners' [13:06] exhaust is so sooty [13:08] that it pretty much ruins anything, right? [13:11] Well sure if you're using RP-1 [13:13] or any other carbon heavy fuel [13:15] that's definitely going to be the outcome. [13:17] So the United States went with a different fuel: hydrogen. [13:21] Okay, so now we've avoided the problem [13:23] of blasting crazy hot high pressure oxygen [13:26] at anything dear and precious [13:27] but now we've opened up a new can of worms. [13:30] Hydrogen is significantly less dense than RP-1 [13:33] or liquid oxygen. [13:35] It's so much less dense, [13:37] it takes a huge and really complex turbo pump [13:39] to flow the right amount of hydrogen [13:41] into the combustion chamber. [13:43] Since RP-1 and LOX are relatively similar in density [13:46] and in the ratios they can be run on a single shaft [13:49] using a single preburner. [13:51] Because of this the engineers at Rocketdyne [13:53] pursued an engine known as the RS-25 [13:56] which would go on to power the space shuttle. [13:58] They realized that because of the large difference [14:00] between the pumps they might as well have [14:02] two different preburners, [14:03] one for the hydrogen pump [14:05] and one for the oxygen pump. [14:06] So that's what they did. [14:08] But having two separate shafts created another new problem. [14:11] Now engineers were putting high pressure [14:13] hot gaseous hydrogen on the same shaft [14:17] right next door to the liquid oxygen pump. [14:20] If some of that hydrogen would leak out of the preburner [14:22] it would start a fire in the LOX pump [14:25] which is catastrophically bad. [14:28] Hydrogen is also very hard to contain [14:30] because it's so not dense, un-dense, lightweight [14:35] it likes to sneak through cracks [14:36] and get out anywhere it can. [14:38] So engineers had to make an elaborate seal [14:40] to keep the hot hydrogen from sneaking out. [14:42] The seal required for this is called a purge seal [14:45] and it's actually pressurized by helium [14:47] so that it's the highest point of pressure. [14:50] So if the seal leaks it just leaks inert helium. [14:53] It's genius but take a look at [14:55] how different the LOX turbo pump [14:57] and the hydrogen turbo pump seals look. [14:59] You can tell how much more engineering time [15:01] and effort had to go into the hydrogen seals. [15:04] I mean the people that think of this stuff are nuts. [15:07] The RS-25 is still considered to be [15:09] about the best engine ever made [15:10] with a fairly high thrust to weight ratio [15:12] and unmatched efficiency. [15:14] Okay now that we've talked all about [15:16] the dual preburner fuel rich RS-25, [15:18] here's a simplified diagram of that. [15:20] Now I didn't bother making the fuel pumps different sizes [15:23] and I just want to focus on the flow here [15:25] and help make that as simple as possible. [15:27] But do note both preburners of the RS-25 run fuel rich [15:32] so although they might look the same [15:34] they power different pumps [15:36] and I'll just let this run here for a few seconds [15:38] so you can study it for a bit [15:40] but don't worry we'll also put all these up on screen [15:43] at the same time once we cover them all. [15:45] So the closed cycle improves [15:46] the overall performance of the engine [15:48] and is highly advantageous. [15:50] So how can it get any better than this? [15:53] We're finally ready to talk about [15:55] the full flow staged combustion cycle [15:57] which basically just combines the two cycle methods [16:00] we just talked about. [16:01] With the full flow staged combustion cycle, [16:02] you take two preburners one that runs fuel rich [16:05] and one that runs oxygen rich. [16:08] The fuel rich preburner powers the fuel pump [16:10] and the oxygen rich preburner powers the LOX pump. [16:13] This means the full flow staged combustion cycle [16:16] needs to tackle the oxygen rich problems [16:18] which again is solved by developing [16:21] very strong metal alloys. [16:23] So SpaceX developed their own super alloys in house [16:26] that they named SX500. [16:28] According to Elon Musk it's capable of over 800 bar [16:32] of hot oxygen rich gas. [16:34] That may have been one of the biggest hurdles [16:36] in developing the Raptor engine. [16:37] Luckily the fuel rich side only pumps fuel [16:40] so if some of that hot fuel leaks through the seal [16:43] on the shaft it just comes in contact with more fuel [16:45] which is kind of no big deal. [16:48] So no need for one of those really really elaborate seals. [16:51] Full flow likely wouldn't work with RP-1 [16:53] due to the coking problems with a fuel rich preburner [16:56] but other fuels are still valid to use this design. [16:59] But more on that in a minute. [17:01] The advantage of the system is that since both the fuel [17:03] and the oxidizer arrive in the combustion chamber [17:06] as a hot gas there's better combustion [17:08] and hotter temperatures can be achieved. [17:10] There's also less of a need for that crazy ceiling system [17:13] as we mentioned earlier [17:14] and that's definitely a good thing [17:16] when you plan to reuse your engine [17:17] over and over with little to no refurbishment [17:20] between flights. [17:21] And lastly because there's an inherent increase in mass flow [17:24] or how quickly all the propellant [17:26] is shooting into the preburner [17:27] the turbines can run cooler [17:28] and at lower pressures [17:30] because the ratio of fuel an oxidizer needed [17:33] to spin the turbo pumps is much lower. [17:36] And think of it this way [17:37] in an open cycle you only want to use [17:39] as little fuel and oxidizer as possible [17:42] in the preburner since it's all wasted [17:44] and you want it to be as hot as withstandable [17:47] to make it more efficient [17:48] but with the full flow cycle all of the fuel [17:50] and all of the oxidizer goes through the preburners [17:53] so you can burn just exactly as much propellant as necessary [17:57] to power the turbo pumps. [17:59] But the cool thing is your fuel to oxidizer ratios [18:02] will be so crazy fuel rich [18:03] and crazy oxygen rich [18:06] that the temperatures at the turbines will be much lower [18:08] and this means longer lifespans for the turbo pump assembly. [18:12] It also means more combustion happens [18:14] in the combustion chamber [18:15] and less in the preburner. [18:17] Now here's the crazy part. [18:18] Only three engines have demonstrated [18:20] the full flow staged combustion cycle ever. [18:24] In the 60s the Soviets developed an engine [18:26] called the RD-270 which never flew [18:29] and in the early 2000s Aerojet and Rocketdyne [18:32] worked on an integrated powerhead demonstrator called [18:35] wait for it, the Integrated Powerhead Demonstrator, [18:38] which again never made it past the test stand. [18:41] And the third attempt to developing [18:43] a full flow staged combustion cycle engine [18:45] is SpaceX's Raptor engine. [18:48] Ta-da, that's right. [18:50] The Raptor engine is only the third attempt [18:52] at making this crazy type of engine. [18:54] It's the first to ever do any type of work [18:56] and leave a test stand [18:58] and fingers crossed, [18:59] it'll be the first full flow staged combustion cycle engine [19:02] to reach orbit. [19:03] Well actually just about anything this engine does [19:06] will be a first. [19:07] This means SpaceX had to tackle some crazy crazy problems. [19:10] I mean not only that same problem that plagues [19:12] oxidizer rich cycles [19:13] like having to have a really really strong metal alloy. [19:17] They also had to learn how to control [19:19] two different preburners [19:20] and two different cycles [19:22] to create the highest pressures [19:24] of any chamber pressure ever. [19:26] They just beat the RD-180's record of about 265 bar [19:29] when they hit 270 bar [19:31] and they're not even done. [19:32] They're hoping for 300 bar inside the combustion chamber. [19:36] That's nuts and we'll talk more about that in a second [19:39] but before we move on now that we've done a rundown [19:41] on all these engines cycle types [19:43] let's put them all up on screen [19:44] and let them run for a bit so you can watch each one [19:47] and compare them side by side. [19:48] I know for myself it helps a lot [19:50] to see them all together on the same screen [19:52] at the same time. [19:52] (slow music) [20:00] Since the Raptor engine can't run [20:01] a fuel rich preburner using RP-1, [20:03] you'd think the next most logical choice would be hydrogen. [20:07] Well SpaceX didn't opt for either RP-1 or hydrogen. [20:11] They went with liquid methane. [20:13] So now we finally have another topic to touch on. [20:15] Why did SpaceX choose liquid methane [20:17] for the Raptor engine? [20:18] What are the qualities that make it advantageous [20:20] over hydrogen or RP-1? [20:23] (dramatic music) [20:30] Today no liquid methane [20:32] or otherwise known as methylox engine [20:34] has gone to orbit. [20:35] So what qualities does it have that make it desirable? [20:38] Let's take a look at methane [20:39] compared to RP-1 and hydrogen. [20:42] Let's put methane in between RP-1 and hydrogen. [20:44] You'll see why here really quickly. [20:47] So let's start off with perhaps the biggest factor [20:49] when designing your first stage, [20:51] the density of the propellant. [20:53] Having a denser fuel means that tanks are smaller [20:56] and lighter for a given mass of fuel. [20:58] A smaller tank equals a lighter rocket. [21:01] So here's the density of these three fuels [21:03] measured in grams per liter. [21:05] In other words how much does one liter of this stuff weigh [21:08] or really what's its mass. [21:10] Starting off with RP-1, one liter is around 813 grams. [21:15] RP-1 is 11 times more dense than hydrogen [21:19] which is only 70 grams per liter [21:21] and methylox is right in the middle at 422 grams per liter. [21:26] Remember how airships or zeppelins [21:27] used to be filled with hydrogen [21:29] to make them lighter than air? [21:31] Well, that's because hydrogen [21:33] is so much less dense in our atmosphere [21:35] it makes for an excellent [21:37] albeit really flammable gas for a balloon. [21:40] I mean we all remember the Hindenburg, right? [21:44] It should also be noted that 813 grams per liter [21:46] is an average for RP-1 [21:49] but SpaceX chills their RP-1 in their Falcon 9 [21:51] and Falcon Heavy for about a 2 to 4% increase in density. [21:56] But historically RP-1's density [21:58] is right around that 813 grams per liter. [22:01] So in the case of density [22:02] methane is kind of right in the middle of the two others [22:05] but there's more to it than just density. [22:07] We also need to take into consideration [22:09] the ratio of how much fuel is burned [22:10] compared to how much oxidizer is burned. [22:13] This is the oxidizer to fuel ratio. [22:15] So here's where things get a little more interesting [22:18] and the tables turn just a little bit. [22:20] Rocket engineers have to take into account [22:22] the mass of the fuel [22:23] and the corresponding weight of the tanks [22:25] so they don't actually burn propellant [22:27] at the perfect stoichiometric combustion ratio. [22:29] They find the perfect happy medium [22:32] that balances tank size with thrust output [22:35] and specific impulse. [22:36] Let's look at the mass ratios for fuel [22:38] and oxidizer that the engineers have come up with. [22:40] So for these numbers RP-1 is burned at 2.7 grams of oxygen [22:44] to one gram of RP-one. [22:46] Hydrogen burns at 6 grams of oxygen to 1 gram of hydrogen [22:50] and methane burns at 3.7 grams of oxygen [22:53] to one gram of methane. [22:55] These numbers can now help offset a little [22:57] the massive difference in density. [22:59] So let's visualize this to help make it easier to digest. [23:02] Liquid oxygen is 1141 grams per liter. [23:05] It's a little more dense than RP-1. [23:08] So burning LOX and RP-1 at a 2.7 to one ratio [23:12] for every liter of LOX you'd need [23:14] a little over half a liter of RP-1. [23:17] Next up let's do hydrogen. [23:18] Now with hydrogen being 11 times less dense than RP-1 [23:22] you'd think it'd need a tank that's 11 times bigger. [23:26] But luckily engineers have found that it pays to burn LOX [23:29] and hydrogen at a 6 to 1 ratio for a good compromise. [23:32] This means for each liter of LOX [23:34] you'd need 2.7 liters of hydrogen [23:37] so your fuel tank needs to be approximately [23:39] five times larger compared to RP-1. [23:42] So yeah that helps. [23:43] That's why when we look at a hydrogen powered Delta IV [23:45] versus an RP-1 powered Falcon 9 [23:48] you can see the fuel tank is much smaller than the LOX tank [23:51] on the Falcon 9 but the Delta IV is about the opposite. [23:55] The LOX tank is much smaller than its fuel tank. [23:58] So now let's take a look at methane. [24:00] And this one gets kind of interesting. [24:02] LOX is 2.7 times more dense than liquid methane [24:05] but the burn ratio is 3.7 grams of oxygen [24:09] to one gram of methane. [24:10] So you need 0.73 liters of methane for every liter of LOX. [24:16] In other words your fuel tank [24:17] would need to be about 40% bigger for methylox [24:20] than it would need to be for RP-1 [24:21] despite RP-1 actually being almost twice as dense [24:25] and compared to hydrogen [24:27] its fuel tank would be about 3.7 times smaller. [24:30] So the fuel to oxidizer ratio [24:31] helps make a methane fuel tank a lot closer to an RP-1 [24:35] tank than it is to a hydrogen tank. [24:37] Another huge variable with any rocket engine [24:39] is how efficient it is. [24:41] This is measured in specific impulse or ISP [24:43] but you can think of it [24:44] kind of like a fuel economy of a gas powered car. [24:48] So a high specific impulse would be similar [24:50] to a high mile per gallon or kilometer per liter. [24:52] The best way to think of specific impulse [24:54] is to imagine you had one kilogram of propellant [24:56] for how many seconds can the engine push [25:00] with 9.8 newtons of force. [25:02] The longer it can sip on that fuel [25:04] while still pushing that hard [25:06] the higher its specific impulse [25:08] and therefore the more work [25:10] it can do with the same amount of fuel. [25:12] So again kind of like its fuel economy. [25:14] So the higher the specific impulse [25:16] the less fuel it takes to do the same amount of work [25:18] which is a good thing. [25:20] A fuel efficient engine is extremely important [25:23] and now due to the molecular way of each fuel [25:26] and their energy released when burned [25:28] there's a different potential [25:29] for how quickly the exhaust gas can be expelled [25:32] out the nozzle. [25:33] This means each fuel has [25:34] a different theoretical specific impulse. [25:37] In ideal and perfect world an RP-1 powered engine [25:39] could achieve about 370 seconds. [25:42] An ideal hydrogen powered engine could get 532 seconds [25:45] and guess what? [25:46] A methane powered engine is right in the middle [25:49] with 459 seconds. [25:51] Real world examples of this are much lower [25:53] with RP-1 engine seeing around 350 seconds [25:56] like the Merlin 1D Vacuum run 380 seconds [25:59] for a methane powered engine [26:01] like the Raptor vacuum might be someday [26:03] and about 465 seconds for a hydrogen powered engine [26:06] like the RL-10B-2. [26:08] Next, let's talk about how hot each fuel burns. [26:11] A fuel that burns cooler is easier on the engine [26:14] and potentially makes for a longer lifespan. [26:16] RP-1 can burn up to 3670 Kelvin, [26:19] hydrogen 3070 Kelvin, [26:22] and if you haven't guessed it by now, [26:23] methane is again between the two at 3550 Kelvin. [26:28] Speaking of thermal considerations [26:30] let's look at the boiling point for each of these fuels [26:32] or at what point does the liquid fuel boil off [26:35] and turn into a gas. [26:37] Since all of these fuels need to remain [26:39] in their liquid state in order to stay dense [26:41] the higher the temperature [26:42] the easier it is to store the fuel. [26:44] A higher boiling point also means less [26:46] or even no insulation on the tanks [26:48] to keep the propellant from boiling off. [26:51] And of course less insulation means lighter tanks. [26:54] RB1 has a very high boiling point [26:57] even higher than water at 490 Kelvin. [27:01] Hydrogen on the other hand is near absolute zero [27:04] at a crazy cold 20 Kelvin. [27:07] That's insanely cold and it takes serious consideration [27:10] to keep anything at that temperature [27:12] and like the Goldilocks it is [27:14] methane is between the two at 111 Kelvin [27:16] which although that's still very cold [27:19] and requires thermal considerations [27:21] it at least boils off at a temperature similar to LOX [27:24] so there is that [27:25] and because it's so close to the temperature of LOX [27:28] the tanks can share a common dome [27:30] which makes the vehicle lighter. [27:32] LOX and hydrogen's temperatures very so wildly [27:35] that LOX will boil off hydrogen [27:37] and the hydrogen will freeze LOX solid. [27:39] Now, on to the exhausts. [27:40] What are the byproducts of combustion with these engines? [27:43] RP-1 is really the only one of these three [27:46] that really pollutes with any unburned carbons [27:48] being left in our atmosphere alongside with some water vapor [27:51] but hydrogen only produces water vapor [27:54] and methane produces some carbon dioxide [27:56] and water vapor as well. [27:57] But an interesting note now believe it or not [28:00] as far as greenhouse gases go, [28:01] water in the upper atmosphere can be pretty bad [28:04] but I'll be doing a video in the future [28:05] all about how much rockets pollute [28:07] talking about their air pollution, [28:09] also their ocean pollution [28:10] and even space debris is a consideration. [28:11] So stand by because I think that video [28:15] is going to be awesome. [28:15] Now one metric that we're just kind of [28:17] going to gloss over really quick [28:20] and talk about it generally is the cost. [28:21] And these tend to vary considerably [28:23] and it's actually really hard to pin down [28:25] the exact prices reliably. [28:27] So for the considerations RP-1 [28:29] is basically just a highly refined jet fuel [28:31] which jet fuel is a highly refined kerosene [28:34] which kerosene is a highly refined diesel. [28:36] So it's safe to assume it's going to be [28:39] more expensive than diesel. [28:40] Hydrogen is also relatively expensive [28:42] despite being abundant. [28:43] Refining it storing it [28:45] and transporting it can be hard [28:47] but methane on the other hand is basically the same thing [28:50] as natural gas and can be relatively cheap. [28:52] Now when you're talking about buying literally tons of fuel [28:55] the fuel costs can add up quickly [28:57] so although the cost of fuel shouldn't factor in too much [29:00] it certainly is a consideration [29:02] but without hard data on this one [29:04] I don't even want to put it on our chart. [29:06] So instead let's talk about [29:08] the more important aspect of the fuel [29:10] that's manufacturing it. [29:12] And here's where we get into specifically [29:14] why SpaceX sees methane as an important [29:16] or even a necessary part of the company's future. [29:20] SpaceX's ultimate goals are to develop a system [29:23] capable of taking humans out to Mars and back [29:25] over and over. [29:26] The Martian atmosphere is CO2 rich. [29:28] Now combine that with water mining from the surface [29:31] and subsurface water on Mars [29:32] through electrolysis and the Sabatier process [29:35] the Martian atmosphere can be made into methane fuel [29:38] so you don't have to take all the fuel you need [29:40] to get home with you. [29:41] You can make it right there using Mars's resources. [29:45] This is called in situ resource utilization or ISRU. [29:48] Now you might be thinking, [29:50] "Well, if there is water can't you just make hydrogen [29:53] on the surface of Mars for your fuel?" [29:55] Well, yes but one of the biggest problems with hydrogen [29:57] and long duration missions is the boiling point of hydrogen. [30:00] Remember, it takes serious considerations [30:02] to maintain hydrogen in a liquid state [30:05] and that's necessary to be useful as a fuel [30:08] so for SpaceX methane makes a lot of sense. [30:11] It's fairly dense meaning the rocket sizes [30:14] are pretty reasonable. [30:15] It's fairly efficient, it burns clean [30:17] and it makes for a highly reusable engine. [30:20] It burns relatively cool [30:21] helping expand the lifespan of an engine [30:23] which again is good for usability. [30:26] It's cheap and easy to produce [30:27] and can be easily reproduced on the surface of Mars. [30:31] (slow music) [30:43] Okay. [30:44] Yeah. [30:45] We finally made it this far. [30:47] and now that we have a strong grasp [30:48] of how different engine cycles operate [30:50] and the fuels they use [30:52] we can finally line them all up side by side [30:54] and compare their metrics [30:56] to help us appreciate where each engine sits. [30:58] So now we're going to lineup each engine [31:00] by their fuel type and their cycles. [31:02] So let's start off with SpaceX's [31:04] open cycle Merlin engine that powers their Falcon 9 [31:06] and Falcon Heavy rockets. [31:08] NPO Energomash's [31:09] oxygen rich closed cycle RD-180 [31:12] that we see power the Atlas 5 rocket [31:15] and Rocketdyne's open cycle F-1 [31:17] that powers a Saturn 5 [31:19] which all three of these engines run on RP-1. [31:22] Then we have SpaceX's full flow [31:24] staged combustion cycle Raptor engine [31:26] that will power the Starship and Super Heavy booster [31:29] and then we have Blue Origin's closed cycle [31:31] oxygen rich methane powered BE-4 engine [31:34] that will power their New Glenn rocket [31:36] and ULA's upcoming Vulcan rocket [31:38] and then we have Aerojet Rocketdyne's closed cycle [31:41] fuel rich RS-25 engine that powered the space shuttle [31:45] and will power the upcoming SLS rocket [31:47] which runs on hydrogen. [31:49] A few quick notes here. [31:50] The Raptor and the BE-4 as of the making of this video [31:52] are still in development so the numbers we have here [31:55] are either their current state of progress [31:58] like the Raptor which is constantly improving [32:00] literally every day [32:02] and in the case of the BE-4, [32:03] those are the target goals for the engine [32:05] which Blue Origin has yet to hit. [32:07] So just keep that in mind that these numbers [32:10] are definitely subject to change [32:12] and now because of this don't forget to check in [32:14] with the article version [32:15] attached in the description of this video. [32:17] This video will likely date itself [32:18] with some of these numbers [32:20] and I can't update this video [32:21] but I can update the website when more info comes through. [32:25] So if you're looking to use any of these numbers as a source [32:29] please, please, please double check the website [32:31] for any updates. [32:32] Another fun note quick is look at the RD-180. [32:36] Now don't be confused. [32:37] This is a single engine [32:38] it just has two combustion chambers. [32:41] There's only a single turbo pump [32:43] that splits its power into two combustion chambers. [32:46] The Soviet Union was able to solve [32:47] the crazy hot oxygen rich closed cycle problem [32:50] but they were unable to solve combustion instability [32:53] of large engines. [32:54] So instead of one large combustion chamber [32:56] they made multiple small ones. [32:58] So first up [32:59] let's take a look at their total thrust output at sea level. [33:01] Since all these engines run at sea level [33:03] that's probably a fair place to compare them. [33:06] Let's go from the least amount of thrust [33:08] to the most for fun. [33:10] The Merlin produces 0.84 meganewtons of thrust. [33:13] The RS-25 produces 1.86 meganewtons. [33:16] The Raptor currently is at 2 meganewtons. [33:19] The BE-4 is hoping to hit 2.4 meganewtons. [33:21] The RD-180 3.83 meganewtons [33:25] and the F-1 is still the king out of these [33:27] at 6.77 meganewtons. [33:30] Now there was an engine called the RD-170 [33:32] which actually produced more thrust than the F-1 [33:34] but since it barely flew [33:36] I figured it wasn't as relevant in this lineup. [33:38] I thought it'd probably a good idea to go with engines [33:40] that have actually been used a lot. [33:42] Thrust is great but what's maybe just as important [33:46] when designing rocket is the thrust to weight ratio [33:49] or how heavy the engine is [33:51] compared to how much thrust it produces. [33:53] A higher thrust to weight ratio engine [33:55] ultimately means less dead weight [33:57] the rocket needs to lug around. [33:59] Let's start from the lowest to highest here. [34:01] The lowest is actually the space shuttles RS-25 at 73 to 1. [34:06] Then there is the RD-180 which is 78 to 1. [34:09] Then we have the BE-4 at around 80 to 1 but [34:13] keep in mind we don't actually have [34:14] a really good number on this. [34:16] So there might be some wiggle room there. [34:18] Then the F-1 is 94 to 1, [34:20] then we have the Raptor which is at about 107 to 1 for now. [34:25] And lastly the Merlin is actually the leader here [34:28] with an astonishing 198 to 1 thrust to weight ratio. [34:32] Yeah, that thing is a powerhouse. [34:35] Okay. [34:36] Thrust is great and all [34:37] but who cares how powerful an engine is [34:39] if it's terribly inefficient. [34:41] So next up let's check out their specific impulse [34:44] which again is measured in seconds. [34:46] So starting with the least efficient engine [34:48] which is the F-1 engine at 263 to 304 seconds [34:53] then the Merlin engine at 282 to 311 seconds. [34:57] Then we get the RD-180 at 311 seconds to 338 seconds [35:02] and somewhere in that same ballpark is the BE-4 [35:06] which is around 310 to 340 seconds. [35:10] Next up is the Raptor engine [35:12] which is 330 seconds to around 350 seconds, [35:16] and lastly the king here by far is the RS-25 [35:20] which is 366 to 452 seconds. [35:26] Wow. [35:27] Now one of the factors that affect both the thrust [35:30] and specific impulse is chamber pressure. [35:32] Now generally [35:33] the higher the chamber pressure the more thrust [35:35] and potentially more efficient the engine can be [35:39] so higher chamber pressures [35:41] let an engine be smaller for a given thrust level [35:44] also improving their thrust to weight ratio. [35:46] The baby here is actually the F-1 [35:48] which only had 70 bar in this chamber pressure. [35:51] Now, I do need to pause here for a second [35:52] and remind you that 70 bar is still 70 times [35:55] the atmospheric pressure or the same amount of pressure [35:58] you'd experience at 700 meters underwater. [36:02] Yikes. [36:03] Okay so even the lowest chamber pressure [36:05] is still mind-bogglingly high. [36:07] So next up is the Merlin engine [36:09] at 97 bar then the BE-4 will be around 135-ish bar [36:14] then the RS-25 which is 206 bar [36:17] then the RD-180 which has been considered [36:20] the king of operational engines at about 257 bar [36:24] that is until the Raptor engine [36:26] which is now kind of online [36:27] which is considered the new king of chamber pressure [36:30] at 270 bars currently and they hope to get that thing [36:34] up to 300 bar. [36:35] Again, 300 bar is like being [36:38] three kilometers deep in the ocean. [36:41] I can't even fathom. [36:44] Okay, that's enough of the specs of these engines. [36:47] Now, let's look at their operational considerations [36:49] starting with their approximate cost. [36:51] Now again this can be kind of hard to nail down, [36:53] so these are the best estimates [36:55] that I could come up with. [36:57] These numbers do factor in inflation [36:58] to make them all in today's dollars though. [37:00] Let's go with the most expensive, [37:02] and work our way down to the least expensive engine. [37:04] The most expensive engine in the lineup is the RS-25 [37:07] which has a sticker price of over $50 million per engine. [37:11] Yikes. [37:12] Then we have the F-1 which was about $30 million per engine [37:17] then the RD-180 which is $25 million per engine [37:20] then the BE-4 which is around $8 million per engine. [37:24] and for the Raptor Elan has mentioned [37:26] he thinks he can produce the Raptor for cheaper than [37:28] or close to the Merlin engine [37:30] if they can remove a lot of the complexity [37:32] that the current engine has. [37:34] So for now we're gonna say $2 million [37:36] is a pretty decent ballpark. [37:38] Then we have the Merlin engine [37:39] which is less than a million I think. [37:42] Okay, well cost is one thing [37:44] but another strong consideration for the cost of the engine [37:47] is whether or not it's reusable. [37:49] And here only the RD-180 and the F-1 were not reusable [37:53] or at least never reused [37:55] which is different than all these other engines [37:57] which will all be reused multiple times. [38:00] The RS-25 was reused over and over [38:03] with the record being 19 flights out of a single engine. [38:05] Well then again that's after a few months of refurbishment. [38:09] The Merlin is hoping to see up to 10 flights [38:11] without major refurbishment. [38:13] We know a design goal for the BE-4 is to be reused [38:16] up to 25 times. [38:18] And I think the Raptor engine hopes to see [38:20] up to 50 flights but again aspirations are one thing. [38:24] We'll see how history tweets its claims. [38:26] But one quick fun little story here is [38:28] don't forget the Merlin engine [38:29] which SpaceX currently uses [38:31] on the Falcon 9 and Falcon Heavy rockets [38:32] are already fired a bunch of times [38:34] before they even make it to the pad. [38:36] Each engine that is built goes from Hawthorne, California [38:39] to their test stand at McGregor, Texas, [38:41] where it does a full duration burn [38:43] then those engines go back to California [38:45] where they're integrated onto the Octaweb [38:47] which is at the base of the vehicle. [38:49] Then they take the entire stage [38:50] and they take it back out to McGregor [38:52] for a full duration static fire. [38:54] So it goes through the whole mission basically again. [38:57] Then they ship it to the launch pad [38:58] where does a short static fire [39:00] and then it flies the mission. [39:01] So it's already done like three missions [39:04] in duration of firing [39:05] by the time it flies for the first time [39:07] so I'm not entirely sure what the most times [39:09] a single engine has done a full duration burn. [39:12] We know that some of the cores were set out on the pad [39:14] and fired for a really really really long time [39:17] multiple times over and over [39:18] so I think they've probably done almost 10 flight [39:21] full duration burns out of a single engine. [39:24] But you know I have no doubt [39:26] they can probably do that if they say. [39:27] I mean they have more experience in this [39:29] than anybody already [39:30] reusing engines without really refurbishing them. [39:32] So I'm gonna definitely take their word for it. [39:35] On the topic of price [39:36] there's actually some things here [39:38] that start to get really interesting [39:39] when we start looking at these numbers. [39:40] The first is an interesting metric [39:42] that Elan talked about once in a tweet in February of 2019 [39:44] saying they hope to make the Raptor get better [39:47] at their thrust to dollar ratio. [39:49] Now this is a really interesting concept [39:52] when you think about it. [39:53] Who cares how much an engine costs [39:54] if one big engine is cheaper than two smaller ones [39:57] for the same thrust or vice versa. [39:59] So let's actually take a look [40:01] at the dollar to kilonewton ratio of these engines. [40:04] Starting with the most expensive [40:06] dollar to kilonewton engine which is the RS-25 [40:08] at a crazy $26,881 to kilonewton of thrust [40:13] then the RD-180 which is $6527 to one kilonewton [40:19] followed by the F-1 at $4431 per kilonewton [40:24] and then we get to the BE-4 which is $3333 [40:29] to one kilonewton, [40:30] the Merlin engine at $1170 per kilonewton [40:33] and the Raptor at around $1000 per kilonewton [40:36] but now we can go even another step further [40:39] since we know their dollar to kilonewton ratio [40:41] but we also know their reusability potential. [40:45] Now we can predict [40:46] their potential costs per kilonewton per flight [40:48] which changes based on how reusable [40:51] these engines actually are. [40:52] For start the RD-180 [40:53] and the F-1 aren't reusable. [40:55] Their price stays the same [40:57] but for the rest of the engines, [40:58] if we take into account how many flights [41:00] they have/will have [41:01] now we start to see the RS-25 reusability pay off [41:05] and kind of close the gap bringing its potential cost [41:08] down to just $1414 per kilonewton per flight. [41:13] But here's where things get crazy. [41:15] Blue Origin's BE-4 has potential to truly be game changing [41:19] at around $133 per kilonewton over 25 flights [41:23] which could make it about as cheap to operate [41:26] as the Merlin at $117 per kilonewton per flight. [41:30] But if the Raptor engine truly lives up to its hype [41:33] it could bring this number [41:34] all the way down to $20 dollars per kilonewton per flight. [41:38] Now that is absolutely game changing. [41:41] Sure, money and reuseability is a 21st focus for spaceflight [41:45] but whatever happened to good old proven reliability? [41:48] For this let's first look at how many operational flights [41:52] each engine has had. [41:53] Now at the moment of shooting this video the Raptor [41:55] and BE-4 haven't seen any operational flights [41:58] although the Raptor is starting to leave the test stand [42:01] and is being used on test vehicles like the Starhopper. [42:04] But for now, neither engine has a real flight record. [42:08] So let's look at the other engines. [42:09] First we have the F-1 engine which was used on 17 flights. [42:13] Next up is the Merlin engine which is at 71 flights [42:16] and catching up quickly to the RD-180 [42:18] which is at 79 flights [42:20] but the king out of these was the RS-25 [42:23] which saw 135 flights. [42:25] Now lastly, how about reliability and service? [42:28] Between the number of flights and this number [42:31] we can get a pretty good sense [42:32] of how truly reliable an engine is. [42:34] This number is really hard to just pin down [42:37] since some of the engines may have shut down early [42:40] but the mission was still a success on a few of these. [42:42] So take a few of these with a grain of salt. [42:45] Again the BE-4 and Raptor engine haven't flown yet. [42:48] So those numbers are unavailable. [42:50] Then we have the space shuttle main engine [42:52] which is over 99.5% reliable [42:56] but that gets hard to define [42:58] when an engine doesn't fully shut down. [43:00] And then we have the Merlin at 99.9% reliable. [43:04] It sure helps when you have 10 engines [43:06] on each flight of the vehicle [43:07] and with only one engine ever failing [43:10] early on in its career, [43:12] and despite that that mission was still a success. [43:14] The Merlin is a very reliable engine. [43:17] Now to end this technically the RD-180 [43:20] and the F-1 are 100% reliable [43:23] but with the F-1 never having shut down at all [43:27] in any flight, it gets the bold here. [43:29] And depending on how you define success and reliability [43:32] technically the RD-180 is only kind of 100% reliable [43:38] because it got really lucky ones. [43:40] One time it shut down six seconds early [43:42] on an Atlas 5 mission in 2016. [43:45] This was due to a faulty valve [43:47] but the mission went on to be a success [43:49] because of some pure luck with the center upper stage [43:52] having enough spare Delta V [43:54] to carry out the mission. [43:56] Had that valve failed even a second earlier [43:58] that mission would have failed. [44:01] (dramatic music) [44:08] Seeing all these numbers and considerations, [44:10] it makes you realize just how many variables [44:12] go into designing a rocket. [44:14] Change any one little thing [44:16] and it can have this massive ripple effect [44:18] on the entire design [44:19] and the implementation of the vehicle as a whole. [44:21] So let's go back over all of this. [44:23] Now that we know all the cycles, the fuels, [44:25] the aspirations of SpaceX to see if we can figure out [44:28] why the Raptor engine exists [44:30] and figure out if it's worth all the effort. [44:33] Let's look at SpaceX's ultimate plan. [44:35] Make a rapidly and fully reusable vehicle [44:38] capable of sending humans to the Moon [44:40] and Mars as inexpensively [44:42] and routinely as possible. [44:44] Not exactly your everyday goal for a rocket, huh? [44:46] In order to be rapidly [44:48] and fully reusable the engine needs to run clean [44:51] and require low maintenance with simple turbo pump seals [44:54] and low preburner temperatures. [44:56] A methane fueled full flow staged combustion cycle engine [45:01] sounds like a good fit. [45:02] For reliability, redundancy, [45:04] and scale of manufacturing considerations [45:06] it makes sense to employ a lot of engines. [45:08] In order to scale an engine down [45:10] but maintain a high output [45:12] chamber pressure needs to be high. [45:15] Sounds like a methane fueled full flow [45:17] staged combustion cycle engine [45:19] is a good fit. [45:20] For interplanetary trips [45:21] methane makes the most sense [45:22] because its boiling point makes it usable [45:24] on long duration trips to Mars [45:26] which guess what? [45:27] You can produce methane on Mars. [45:29] So for interplanetary trips a methane fueled full flow [45:33] staged combustion cycle engine sounds like a good fit. [45:36] Methane is fairly dense [45:37] meaning the tank size remains reasonable. [45:40] Which again is good for interplanetary trips [45:42] not needing to lug around a lot of dead weight [45:45] making a methane fueled full flow staged combustion cycle [45:48] a pretty good fit. [45:50] Okay so let's bring this all back around now. [45:52] Is the Raptor engine really the king of rocket engines? [45:57] Well rocket science like all things [46:00] is a complex series of compromises. [46:02] Is it the most efficient engine? [46:04] No. [46:05] Is it the most powerful engine? [46:07] No. [46:08] Is it the cheapest engine? [46:09] Probably not. [46:10] Is it the most reusable engine? [46:12] Maybe. [46:13] But does it do everything really well? [46:15] Yeah it is truly a Goldilocks engine [46:18] doing everything it needs to do very very well. [46:21] It is the perfect fit for your interplanetary spaceship [46:24] and despite its complexity [46:26] SpaceX is developing this engine at a rapid pace. [46:29] I mean knowing how much tweaking SpaceX did [46:31] to their Merlin engine over a decade, [46:33] we're just at the infancy of the Raptor engine. [46:36] It's only gonna get better from here on out, [46:38] which is crazy. [46:40] So all in all the Raptor engine [46:42] is the king of this application. [46:45] It's a fantastic engine to fulfill SpaceX's goals [46:48] for their starship vehicle. [46:49] Would it be the king of other applications? [46:52] Maybe, maybe not. [46:54] And only that decision for the [46:55] rocket scientists and engineers who get to make [46:58] all those crazy decisions every single day. [47:00] So what do you think? [47:01] Is it worth all this hassle [47:03] to develop such a crazy and complex engine? [47:05] Is this just the beginning for the Raptor engine? [47:07] And most importantly, [47:09] is the Raptor engine really the king of rocket engines? [47:12] Let me know your thoughts in the comments below. [47:15] Okay I know I say this every video, [47:16] but I honestly could not have done this video [47:18] without the help of my Patreon supporters. [47:20] They not only kept me sane for the past five months [47:23] as I worked on this video [47:24] but they also went over all the data with me. [47:26] They give me great feedback [47:28] and suggestions in the edits of this video. [47:29] I you want to help support what I do [47:32] or provide feedback in videos [47:34] or help scripting research [47:35] or if you just want to hang out [47:36] and talk space [47:37] consider joining our exclusive Discord channel [47:39] and our exclusive subreddit [47:40] by becoming a Patreon member [47:41] by going to Patreon.com/everydayastronaut. [47:45] Thank you guys. [47:46] Seriously, I couldn't have made this video [47:47] without you. [47:48] And while you're online, [47:49] be sure and check out my web store. [47:50] Seriously, I have really cool things [47:52] like these F-1 T-shirts, [47:53] tons of other shirts. [47:55] There's lots of new merchandise [47:56] popping up in there all the time [47:58] so check back often. [47:59] We have things like [48:00] grid fin not-a-coasters [48:01] and hats and shirts and mugs and prints, [48:03] just literally tons of cool rocket stuff. [48:06] So if that's your type of thing, [48:07] be sure and check out my web store [48:08] everydayastronaut.com/shop [48:10] and then click on the music tab [48:12] if you want to check out any of the songs [48:13] used in this video. [48:14] It's all music that I've written over the years [48:16] (mumbles) Apple, iTunes and Spotify [48:19] and Google Music, all that stuff, [48:20] and also there's a playlist right here on YouTube [48:23] for music video versions of it too as well, [48:25] which is a fun way to watch and listen. [48:27] So show it to a friend. [48:28] Thanks everybody. [48:30] That's gonna do it for me. [48:31] I'm Tim Dodd, the Everyday Astronaut [48:33] bringing space down to earth for everyday people. [48:34] (exit music)