Rotating Detonation Engines Promise 25% Efficiency Boost Over Rockets

Scott Manley

Supersonic shockwaves in rotating detonation engines enable constant-volume combustion that theoretically yields 25% more work than conventional cycles, though fluid instability and material limits threaten real-world gains.

AI-assisted summary of What Is A Rotating Detonation Engine - And Why Are They Better Than Regular Engines — original source. Check the original for full context.

Source published: 2020-05-11

Key ideas

Rotating detonation engines (RDEs) are predicted to be up to 25% more efficient than standard rocket engines, dwarfing the roughly 10% gain typically achieved by shifting from open-cycle to complex closed-cycle systems. (0:04)
Unlike conventional jet and rocket engines that rely on subsonic deflagration, detonation engines operate via supersonic shock waves that trigger near-instantaneous pressure and temperature spikes. (1:16)
Thermodynamically, detonation approximates the constant-volume Humphrey cycle rather than the constant-pressure Brayton cycle, enclosing significantly more area on a pressure-volume diagram for greater work extraction. (6:12)
By generating shock pressures exceeding 100 atmospheres, air-breathing RDEs could bypass heavy turbocompressor stages and support hypersonic flight up to Mach 5. (3:02)

Detailed notes

## Background Published on YouTube by Scott Manley on May 11, 2020, this video explores the emergence of detonation engine technology and claims of its superiority over conventional rocket and jet propulsion. According to the description, the video examines the core definitions of detonation engines, highlights the differences between pulsed and rotating architectures, and unpacks why thermodynamic principles predict these engines will outperform traditional designs.

## TL;DR Scott Manley examines rotating detonation engines (RDEs), which promise an efficiency gain of up to 25% over conventional rocket and jet engines by replacing subsonic deflagration with supersonic shock waves. After contrasting intermittent pulse detonation engines (PDEs) with the continuous annular shock waves of RDEs, he explains how detonation can achieve over 100 atmospheres of compression without heavy rotating compressor stages. Manley then details the underlying thermodynamics, using pressure-volume diagrams to contrast the constant-pressure Brayton cycle of traditional engines against the constant-volume Humphrey cycle of detonation engines. While the theoretical work output is substantially higher, he concludes that significant material and fluid-dynamic hurdles must be resolved before RDEs can realize their mathematical promise in production-ready hardware.

## Atmosphere & style The presentation carries Scott Manley's signature blend of approachable humor, technical rigor, and hands-on demonstrations, transitioning smoothly from a slow-motion kitchen experiment with burning alcohol to thermodynamic state diagrams. His delivery is optimistic yet analytically grounded, balancing genuine excitement over a potential 25% efficiency leap with pragmatic skepticism regarding extreme temperatures and wave stability challenges. Watching the video feels like an engaging, fast-paced masterclass in aerospace engineering that demystifies abstract propulsion physics through intuitive visual analogies and clear, practical context.

## Bulletin - [00:04](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=4s) Rotating detonation engines (RDEs) are predicted to be up to 25% more efficient than standard rocket engines, dwarfing the roughly 10% gain typically achieved by shifting from open-cycle to complex closed-cycle systems. - [01:16](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=76s) Unlike conventional jet and rocket engines that rely on subsonic deflagration, detonation engines operate via supersonic shock waves that trigger near-instantaneous pressure and temperature spikes. - [06:12](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=372s) Thermodynamically, detonation approximates the constant-volume Humphrey cycle rather than the constant-pressure Brayton cycle, enclosing significantly more area on a pressure-volume diagram for greater work extraction. - [03:02](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=182s) By generating shock pressures exceeding 100 atmospheres, air-breathing RDEs could bypass heavy turbocompressor stages and support hypersonic flight up to Mach 5.

## Key takeaways - [00:04](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=4s) A small-scale RDE prototype generating under 100 kg of thrust represents tangible experimental progress toward validating decades of theoretical propulsion research. - [00:04](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=4s) Detonation engine technology holds potential applications beyond rocketry and aviation, including marine turbine propulsion for the US Navy. - [01:16](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=76s) Combustion mode is defined by reaction front velocity relative to the local speed of sound: deflagration is subsonic, whereas detonation is supersonic. - [01:16](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=76s) In a detonation wave, unburned fuel cannot move out of the path of the supersonic front, causing near-instantaneous pressure and temperature rises with no time for thermal expansion during the burn. - [03:02](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=182s) Early pulse detonation engines (PDEs) demonstrated flight viability in 2008 using long tubes firing sequentially at 80 Hz, but intermittent purge-and-fill cycles limited steady thrust output. - [03:02](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=182s) Historical pulsejets like the V-1 flying bomb or hobbyist valveless tubes operate via subsonic deflagration rather than true supersonic detonation. - [03:02](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=182s) RDEs replace intermittent tube firing with continuous azimuthal propagation, guiding one or more detonation shock waves around an annular channel fed continuously with fresh propellant. - [03:02](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=182s) Air-breathing RDEs generate over 100 atmospheres of pressure across the shock front, potentially eliminating heavy mechanical turbocompressors in hypersonic vehicles up to Mach 5. - [03:02](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=182s) Maintaining stable operation in an RDE is complex because multiple circulating shock waves can overtake each other, extinguish, or let unburned propellant escape. - [06:12](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=372s) Standard jet and rocket engines operate on the Brayton cycle with constant-pressure (isobaric) heat addition. - [06:12](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=372s) Detonation engines execute the Humphrey cycle, where isochoric (constant-volume) combustion spikes pressure vertically on a P-V diagram to enclose greater net work output. - [09:48](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=588s) Much like real Stirling engines fail to meet ideal cycle efficiency due to continuous mechanical linkage motion, RDEs face real-world material and fluid dynamic losses that erode theoretical gains. - [09:48](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=588s) Extreme temperature and pressure spikes inside detonation chambers place severe demands on materials, requiring engineering solutions before laboratory gains translate into operational hardware.

## Detailed walkthrough ### [00:04](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=4s) The Promise of Rotating Detonation Engines

Scott Manley introduces recent news surrounding a small-scale technology demonstration of a rotating detonation engine (RDE). Although this specific prototype is compact and generates less than 100 kilograms of thrust, it serves as a tangible validation of a propulsion concept that researchers across the globe have theorized for decades. The intense interest stems from thermodynamic expectations: a rotating detonation engine is predicted to be up to 25% more efficient than standard rocket engines. Manley quips about the phrase “garden variety,” clarifying with a laugh that he naturally means the rocket garden at Kennedy Space Center, since he does not have any rockets planted in his own backyard.

Putting that performance jump into perspective, Manley highlights that rocket engineers go through enormous mechanical complexity just to squeeze out marginal gains: “When you consider that switching from a open cycle engine to a much more complicated closed cycle engine gets you a 10% improvement, a 25% improvement would be a massive.” Furthermore, the utility of detonation engines extends far beyond spaceflight and jet aircraft; organizations like the US Navy are actively investigating detonation technology to drive turbines aboard naval ships.

- Demonstration engine thrust: Under 100 kg - Projected efficiency improvement: Up to 25% over conventional rocket cycles - Open-cycle to closed-cycle comparison: Switching to closed-cycle typically yields only ~10% improvement - Alternate applications: Naval ship turbine power

---

[01:16](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=76s) Deflagration vs. Detonation

The critical distinction in these engines comes down to the scientific definition of combustion. In everyday language, people associate “detonation” loosely with explosions, but not all explosions are detonations. Scientifically, combustion is categorized by the speed of its reaction front relative to the speed of sound in the burning medium. A subsonic combustion front is termed a *deflagration*, whereas a supersonic, shock-based combustion front is a *detonation*. Conventional jet engines and rocket engines rely on deflagration, whereas detonation engines harness supersonic shock waves.

```mermaid flowchart TD accTitle: Combustion Modes Comparison accDescr: Diagram contrasting subsonic deflagration with supersonic detonation combustion processes.

subgraph Deflagration["Subsonic: Deflagration (Jet/Rocket Engines)"] D1["Subsonic Flame Front"] --> D2["Propellants Undergo Thermal Expansion"] D2 --> D3["Constant Pressure Combustion (Brayton)"] end

subgraph Detonation["Supersonic: Detonation (Detonation Engines)"] R1["Flame Accelerates Past Local Speed of Sound"] --> R2["Shock Wave Forms (Near-Instantaneous P & T Rise)"] R2 --> R3["Constant Volume Combustion (Humphrey)"] end ```

Manley illustrates this principle with a kitchen demonstration where he evaporated alcohol inside a transparent bottle and ignited it. Filmed in slow motion, the flame front can be seen propagating down into the vessel. Under the right air-fuel mixture and geometry, a combustion front can accelerate until it outpaces the local speed of sound. At that threshold, unburned gas cannot move out of the path of the oncoming reaction: “And at this point the unburned mixture doesn't get pushed out of the way of the incoming flame front and the flame front becomes a shock wave where the pressure and temperature rises near instantaneously.” Because the reaction occurs so rapidly that propellants have no time to undergo thermal expansion, detonation behaves as a constant-volume process, delivering an enormous pressure spike compared to the constant-pressure burn inside traditional engines.

---

[03:02](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=182s) Pulse Detonation Engines vs. Rotating Detonation Engines

To harness detonation for propulsion, engineers first developed the Pulse Detonation Engine (PDE), which saw an airborne flight test back in 2008. The test aircraft utilized long, straight tubes resembling drain pipes extending past the tail of the plane. The operational sequence required a cycle: injecting a purge of clean air, filling the tube with an air-fuel charge, and firing a spark plug at the base. The flame initially propagated as a deflagration before accelerating down the length of the pipe into a fully developed detonation. The test aircraft flew with four tubes firing at 20 pulses per second each, producing an overall acoustic frequency of roughly 80 Hz. Manley notes that non-detonation pulse engines also exist—such as the mechanical flap-valve engine on the WWII German V-1 flying bomb or the valveless U-shaped pulsejets built by YouTuber Colin Furze—but those rely on subsonic deflagration rather than supersonic detonations.

```mermaid flowchart LR accTitle: Pulse Detonation vs Rotating Detonation Engine Cycles accDescr: Comparison of intermittent purge-and-fire PDE tubes versus continuous annular detonation waves in an RDE.

subgraph PDE["Pulse Detonation Engine (Intermittent)"] P1["1. Air Purge"] --> P2["2. Fuel/Air Fill"] P2 --> P3["3. Spark Ignition"] P3 --> P4["4. DDT in Long Tube"] P4 --> P5["Exhaust & Repeat (80 Hz aggregate)"] end

subgraph RDE["Rotating Detonation Engine (Continuous)"] R1["Continuous Annular Propellant Injection"] --> R2["Detonation Wave Travels Azimuthally"] R2 --> R3["Continuous High-Pressure Exhaust"] R3 --> R2 end ```

While PDEs demonstrated the concept, the intermittent purge-and-pulse sequence prevents them from producing steady, uninterrupted thrust. The Rotating Detonation Engine overcomes this limitation: “So the rotating detonation engine takes this concept and tries to make the detonation a continuous process.” Instead of moving down an extended pipe, the detonation shock wave is guided continuously around a circular, annular channel. Fresh propellant enters ahead of the traveling wave, detonates upon contact, and vents out the open nozzle end, allowing the wave to circle indefinitely.

Physical implementation, however, has proven extraordinarily difficult despite extensive computational fluid dynamics (CFD) modeling. The annular chamber often supports multiple shock waves chasing each other around the circumference, all of which must sustain identical propagation speeds. If chamber conditions fluctuate, one wave can overtake another, the wave can blow itself out entirely, or unburned fuel can escape out the back and combust inefficiently or explosively. If mastered, an air-breathing RDE could eliminate massive mechanical turbocompressors—because a hundred-plus-atmosphere detonation shock easily dwarfs the few atmospheres of mechanical compression provided by standard rotating blades—enabling hypersonic propulsion up to approximately Mach 5.

- 2008 PDE flight test: 4 tubes operating at 20 Hz each (80 Hz combined) - PDE design constraint: Extreme tube length required for deflagration-to-detonation transition (DDT) - Key RDE advantage: Replaces intermittent cycles with continuous wave propagation in an annular channel - Compression capability: Shock-wave pressure reaches 100+ atmospheres without mechanical compressor stages - Hypersonic speed regime: Air-breathing RDEs are projected to operate efficiently up to roughly Mach 5

---

[06:12](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=372s) The Thermodynamics of Heat Engines and Cycles

To explain why propulsion scientists hold such high hopes for an unproven technology, Manley transitions into the classical thermodynamics of heat engines. At a fundamental level, any device that extracts heat from a high-temperature source, dumps residual heat into a cold sink, and diverts a fraction of that thermal flow into mechanical work is a heat engine. Whether an engine utilizes pistons, cylinders, or turbine blades—or is as vast as Earth's weather systems circulating atmospheric gases via solar heating—it adheres to these foundational thermodynamic rules.

“So you can describe the operation of every heat engine by tracking how its pressure, volume, temperature, and entropy change over time.” Manley uses an idealized Stirling engine cycle on a pressure-volume (P-V) diagram to illustrate how these thermodynamic states operate in closed cycles:

On these diagrams, the net mechanical work generated per cycle corresponds directly to the area enclosed by the cycle lines. Rockets and conventional jet engines operate via the Brayton cycle, where combustion occurs isobarically (at constant pressure, expanding in volume across states 3 to 4 as fresh air or propellants circulate through). In contrast, detonation engines execute the Humphrey cycle, where combustion occurs isochorically (at constant volume), fundamentally altering the shape of the curve and boosting theoretical thermal efficiency.

[09:48](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=588s) The Detonation Thermodynamic Cycle and Engineering Realities

Looking at the thermodynamic cycle on a pressure-volume diagram, the detonation process drives the curve upward vertically rather than horizontally, fundamentally reshaping the enclosed cycle. Because work output corresponds directly to the area enclosed within that thermodynamic loop, this geometry provides significantly more usable area. Scott Manley highlights the theoretical promise of this behavior, noting: “So therefore it's anticipated that a detonation cycle should get more power out of the same heat input.”

However, transitioning from pure theory to functional machinery introduces unavoidable practical compromises. Real-world systems suffer from efficiency losses and physical constraints that degrade performance away from the idealized thermodynamic cycle. For instance, the theoretical advantage of operating at substantially elevated pressures and temperatures can easily be restricted by the thermal and mechanical limitations of available materials. Manley draws an analogy to the Stirling engine, where the ideal cycle assumes pistons move instantly to a position, remain stationary, and then move back; in practice, mechanical linkages force the pistons to cycle continuously in a smooth sine-wave motion, preventing operational Stirling engines from ever matching their theoretical maximum efficiency.

Detonation engines currently face this exact bridge between mathematics and implementation. While researchers have achieved initial experimental milestones to prove they are more than mere concepts on paper, substantial engineering hurdles remain. The technology must still mature into robust, practical hardware capable of proving that the thermodynamic gains calculated on paper can survive the harsh realities of real-world combustion. As Scott Manley summarizes: “So right now detonation engines have taken a few small steps to being more than a theoretical device but beyond that they do need to be developed into real working devices and actually demonstrate they're able to achieve the levels of power and efficiency that the mathematics suggests.”

- **Thermodynamic advantage**: Constant-volume (detonative) heat addition plots vertically on the pressure-volume diagram, expanding the internal area of the cycle to extract greater work from an equivalent amount of heat. - **Material constraints**: The extreme peaks in temperature and pressure generated by detonation waves test the physical endurance of engine materials, often requiring operational derating below theoretical optimums. - **The Stirling engine analogy**: Real engines deviate from their theoretical cycles—just as sinusoidal piston movement degrades Stirling efficiency below textbook ideals, fluid dynamics and real combustion effects will erode ideal detonation engine curves. - **Current status**: Rotating detonation engines have moved beyond purely abstract concepts into early laboratory and flight demonstrators, but have yet to consistently demonstrate their full theoretical efficiency in production-ready hardware.

## Notable quotes > [09:48](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=588s) it's anticipated that a detonation cycle should get more power out of the same heat input. — Scott Manley

> [09:48](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=588s) detonation engines have taken a few small steps to being more than a theoretical device but beyond that they do need to be developed into real working devices and actually demonstrate they're able to achieve the levels of power and efficiency that the mathematics suggests. — Scott Manley

## Mentioned Scott Manley, Rotating Detonation Engine (RDE), Pulse Detonation Engine (PDE), US Navy, Kennedy Space Center, V-1 flying bomb, Colin Furze, Stirling engine, Brayton cycle, Humphrey cycle

## Video description There's been a few stories recently about detonation engines and how they're supposed to be superior to traditional jet or rocket engines. So, what is a detonation engine, what's the difference between a pulsed and rotating engine and why does thermodynamics predict these are better than traditional engine designs.

[Source](https://www.youtube.com/watch?v=rG_Eh0J_4_s)

中文简报

旋转爆震发动机有望比传统火箭提升25%的效率旋转爆震发动机中的超声速激波可实现定容燃烧，理论上比常规循环多产生25%的功，但流体不稳定性与材料极限可能制约实际收益。 Scott Manley探讨了旋转爆震发动机（RDE）。该发动机通过用超声速激波取代亚声速爆燃，有望比传统火箭和喷气发动机实现高达25%的效率提升。在对比了间歇式脉冲爆震发动机（PDE）与RDE的连续环形激波后，他解释了爆震如何在不需要笨重旋转压气机级的情况下实现超过100个大气压的压缩。随后，Manley详细阐述了其背后的热力学原理，利用压力-体积图（P-V图）对比了传统发动机的定压布雷顿循环与爆震发动机的定容汉弗莱循环。尽管理论做功输出大幅提高，但他总结道，在RDE于可投产硬件中兑现其数学承诺之前，必须先攻克严峻的材料和流移动力学障碍。旋转爆震发动机（RDE）的效率预计比标准火箭发动机高出多达25%，远超从开式循环转为复杂的闭式循环通常获得的约10%增益。 (0:04) 与依赖亚声速爆燃的传统喷气和火箭发动机不同，爆震发动机依靠超声速激波工作，能瞬间引发极高的压力与温度尖峰。 (1:16) 在热力学上，爆震更接近定容汉弗莱循环而非定压布雷顿循环，在压力-体积图上围出明显更大的面积，从而能够提取出更多的功。 (6:12) 通过产生超过100个大气压的激波压力，吸气式RDE有望省去笨重的涡轮压气机级，并支持高达5马赫的高超声速飞行。 (3:02)

Reading summary

Rotating detonation engines (RDEs) offer the potential for a theoretical efficiency improvement of up to 25% over conventional rocket cycles by replacing subsonic deflagration with continuous supersonic detonation shock waves. Thermodynamics of Detonation vs. Deflagration (1:16) Conventional jet and rocket engines rely on deflagration, a subsonic combustion process characterized by constant-pressure heat addition via the Brayton cycle. In contrast, detonation engines utilize a reaction front that accelerates beyond the local speed of sound into a supersonic shock wave. Because this reaction occurs faster than unburned propellants can thermally expand, combustion behaves as a near-instantaneous, constant-volume process under the Humphrey cycle. On a thermodynamic pressure-volume diagram, this detonative heat addition drives the cycle upward vertically rather than horizontally, yielding significantly more enclosed area for an equivalent heat input and generating pressure spikes exceeding 100 atmospheres. Evolution From Pulsed to Continuous Annular Cycles (3:02) Early efforts to apply detonative combustion produced the Pulse Detonation Engine (PDE), flight-tested in 2008 using four drainpipe-like tubes firing at 20 pulses per second each to achieve an 80 Hz acoustic frequency. PDEs required an intermittent sequence of air purging, fuel charging, and spark ignition, requiring long physical tubes to allow the deflagration-to-detonation transition (DDT) to mature. The rotating detonation engine solves this intermittent operation by sustaining continuous detonation inside an annular ring. In an RDE, fuel and oxidizer enter continuously ahead of a shock wave that circles the channel azimuthally, exhausting high-pressure gas without interruption while potentially eliminating the need for heavy mechanical turbocompressors in air-breathing hypersonic flight up to Mach 5. Practical Constraints and Engineering Hurdles (9:48) Despite theoretical projections, translating RDE mathematics into functional machinery presents severe fluid dynamic and material challenges. Sustaining steady operation requires multiple coexisting shock waves in the annular chamber to maintain identical propagation speeds; minor fluctuations can cause waves to collide, blow out entirely, or allow unburned propellant to escape uncombusted. Furthermore, extreme peaks in pressure and temperature push structural materials beyond their physical limits, often forcing systems to run below their optimal thermodynamic state. Much like mechanical Stirling engines lose efficiency because continuous sinusoidal piston movement cannot replicate idealized step-changes, practical detonation engines inevitably see real-world combustion inefficiencies erode their theoretical gains. While sub-100 kg thrust test articles have proven that rotating detonation engines are viable beyond theory, the technology remains in early experimental validation, with research organizations like the US Navy exploring applications while engineers work to prove that theoretical thermodynamic efficiency gains can survive in production-grade hardware.

精炼阅读摘要

科普学者Scott Manley指出，旋转爆震发动机（RDE）利用超音速激波实现等容燃烧，其理论热效率预计比传统火箭发动机高出最多25%，但该技术从热力学概念走向实际工程应用仍面临严峻的流体动力学与材料耐受性挑战。爆燃与爆震的物理机制差异 (1:16) 传统火箭与喷气发动机依赖亚音速燃烧模式，即爆燃（Deflagration）。在此过程中，未燃气体有充裕时间因受热而膨胀，燃烧实质上属于定压过程（遵循Brayton循环）。爆震（Detonation）则截然不同，当反应前沿在特定燃料配比和结构中加速并突破当地声速时，未燃混合气无法及时被推开，燃烧前沿随即转变为超音速激波。由于反应速度极快，推进剂在燃烧发生前来不及发生热膨胀，使得燃烧过程表现为等容燃烧（遵循Humphrey循环），在微秒级时间内产生极高的温度与数百个大气压的压力尖峰。从脉冲爆震到连续旋转爆震的演进 (3:02) 为了将爆震现象应用于动力系统，工程师早期开发了脉冲爆震发动机（PDE）。2008年进行的PDE飞行测试使用了延伸至机尾后方的长管，通过间歇性执行空气吹扫、充入燃气、火花塞点火以及爆燃转爆震（DDT）循环产生动力；测试机搭载的4根管道各自以20赫兹频率工作，合并声学频率约80赫兹。然而，PDE的间断脉冲与长直管结构无法提供平稳的连续推力。相比之下，旋转爆震发动机（RDE）通过环形燃烧室使超音速激波沿着圆周方向持续旋转传播，新鲜推进剂在波前注入并立即被引爆，排气经喷管排出，从而实现了连续爆震和不间断的推力输出。热力学循环优势与潜在应用前景 (6:12) 根据热力学循环的压容图（P-V图），机械功输出直接由封闭曲线所围成的面积决定。传统发动机的等压燃烧线呈水平展开，而爆震发动机所对应的Humphrey循环在热量加入时呈现垂直上升的等容特征，显著增加了封闭图形的面积，理论上可在消耗等量热量的情况下榨取更多机械功。传统开式循环火箭发动机转向结构极其复杂的闭式循环仅能换取约10%的性能提升，而RDE有望实现最多25%的效率跃升。此外，空气吸入式RDE形成的超音速激波能产生上百个大气压的压缩比，有望省去沉重的机械涡轮压气机，支持高达5马赫的高超音速飞行；美国海军等机构亦在研究将其用于驱动水面舰艇的涡轮发电机。流体控制与工程实现的现实瓶颈 (9:48) 尽管计算流体动力学（CFD）模型得到广泛应用，但物理制造RDE极度困难。环形燃烧室内部常存在多个同向追逐的激波，这些激波必须维持完全一致的传播速度；一旦燃烧室环境产生波动，激波可能相互吞并、直接熄灭，或者导致未燃推进剂被过早喷出而引发燃烧效率下降乃至外部爆炸。类比斯特林发动机在工程中因活塞正弦运动而无法达到理想循环，RDE在现实中同样会因流体损失和材料耐温耐压极限而被迫下调运行参数，无法直接达到理想的数学上限。目前的旋转爆震发动机已通过小型原型机验证了物理可行性（例如推力低于100千克的测试样机），但该领域仍处于从小规模实验向实用化硬件跨越的起步阶段，其宣称的热力学增益仍有待全尺寸工程硬件在严苛工况下的进一步验证。

Detailed notes — zh

## Background 该视频由知名航天科普创作者 Scott Manley 于 2020 年 5 月 11 日发布在 YouTube 平台。视频简介指出，近期有关爆震发动机及其优于传统喷气或火箭发动机的报道引发关注，本期内容旨在解析爆震发动机的定义、脉冲爆震与旋转爆震的区别，以及热力学为何预测其性能将超越传统发动机设计。 ## TL;DR 视频深入探讨了新兴的推进技术——旋转爆震发动机（RDE）及其背后的物理机制。Scott Manley 首先阐释了亚音速爆燃与超音速爆震的根本差异，并对比了早期的脉冲爆震发动机（PDE）与连续工作的环形旋转爆震发动机。通过热力学中布雷顿循环与汉弗莱循环在 P-V 图上的做功面积对比，视频揭示了定容爆震燃烧实现最高 25% 效率提升的理论根源。最后，视频客观指出了该技术在材料耐受极限、流体损失及激波控制等工程现实中所面临的巨大落差与落地挑战。 ## Atmosphere & style 整期视频洋溢着 Scott Manley 标志性的理智严谨与幽默风趣，他擅长用在厨房点燃酒精瓶的小实验或调侃肯尼迪航天中心的“火箭花园”来化解晦涩难懂的物理概念。讲述节奏明快且极富逻辑性，借助生动的热力学图解和计算模拟，将复杂的工程难题拆解得抽丝剥茧、通俗易懂。观看体验既让人对颠覆性航天技术的理论前景感到振奋，又在客观审视工程落差的冷静分析中保持理性的科学求实态度。 ## Bulletin - [00:04](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=4s) 旋转爆震发动机（RDE）理论上可比传统火箭发动机提升高达 25% 的效率，远超从开式循环转向闭式循环带来的约 10% 增益。 - [06:12](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=372s) 爆震燃烧属于超音速定容燃烧过程（汉弗莱循环），相较于传统喷气与火箭发动机采用的亚音速定压燃烧（布雷顿循环），能在 P-V 图上围出更大的做功包络面积。 - [03:02](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=182s) 吸气式 RDE 内部瞬间激波压力可达 100 个大气压以上，有望省去庞大沉重的压气机叶片，并支持高达 5 马赫的高超音速飞行。 - [10:02](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=602s) 尽管近期已完成小推力（低于 100 千克）测试验证，但材料耐温耐压极限以及多波同步控制等工程挑战仍阻碍其完全兑现理论性能。 ## Key takeaways - [00:04](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=4s) 旋转爆震发动机（RDE）近期完成原型演示，虽然推力不足 100 千克，但标志着该技术正式从纯理论走向实物验证。 - [00:04](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=4s) 热力学预测爆震发动机效率最高可提升 25%，对于推进领域而言这是远超传统闭式循环升级（约 10%）的巨大技术飞跃。 - [00:04](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=4s) 爆震发动机的应用场景不仅限于火箭和喷气飞机，美国海军亦在探索将其用于驱动舰船上的燃气轮机。 - [00:04](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=4s) 爆震与常规爆燃的本质区别在于燃烧波阵面的速度：爆燃低于音速，而爆震超过音速并形成极高压力的激波阵面。 - [03:02](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=182s) 早期的脉冲爆震发动机（PDE）依赖长管实现爆燃向爆震转变（DDT），但其不连续的脉冲输出限制了工程实用性。 - [03:02](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=182s) RDE 通过将超音速爆震波限制在环形燃烧室内连续周向旋转，解决了脉冲爆震发动机推力不连续的缺陷。 - [03:02](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=182s) RDE 的主要工程难题在于维持燃烧室严苛工况，确保多道爆震波以完全相同的速度稳定追逐，避免工况失谐或熄火。 - [06:12](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=372s) 传统喷气与火箭发动机遵循定压燃烧的布雷顿循环，在 P-V 图上吸热阶段表现为水平线，工质可自由膨胀。 - [06:12](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=372s) 爆震发动机遵循定容燃烧的汉弗莱循环，由于燃烧瞬间完成无时间膨胀，P-V 图吸热段垂直陡增，包围出显著更大的有效做功面积。 - [10:02](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=602s) 现实中爆震波产生的瞬时极端温度与冲击压力对结构材料构成了巨大挑战，实际系统难以完全达到理想热力学极限。 - [10:02](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=602s) 如同斯特林发动机活塞正弦运动偏离理想突变一样，流体阻力与非理想膨胀等物理损失也会削弱爆震发动机在现实中的效率提升。 ## Detailed walkthrough ### [00:04](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=4s) 旋转爆震发动机的技术前景与爆震物理学最近，关于一种被称为“旋转爆震发动机”（Rotating Detonation Engine，简称 RDE）的新型高效火箭发动机技术的演示引发了广泛关注。虽然目前测试的模型设计体积极小，产生的推力不足 100 千克，但这标志着该技术走向了实用化验证。全球各地的研究人员已经围绕这种理论构想探索了数十年，热力学预测它的效率最高可比传统的普通火箭发动机提升 25%。Scott Manley 幽默地打趣道，他所说的“普通品种”（garden variety）其实是指肯尼迪航天中心的“火箭花园”（Rocket Garden），因为他自家的后花园里可没有火箭。对于火箭推进领域而言，25% 的效率提升将是一场巨大的技术飞跃。相比之下，传统的火箭发动机即使从开式循环（如燃气发生器循环）转向极其复杂且昂贵的闭式循环（如分级燃烧循环），通常也只能换来区区 10% 的效率增益。正如 Scott Manley 所指出的：“When you consider that switching from a open cycle engine to a much more complicated closed cycle engine gets you a 10 improvement, a 25 improvement would be a massive.”（考虑到从开式循环发动机转向更复杂的闭式循环发动机只能带来 10% 的提升，25% 的提升将是巨大的。）此外，爆震发动机不仅局限于火箭或喷气飞机，美国海军甚至希望利用这种技术来驱动其舰船上的燃气轮机。理解这种发动机的核心在于区分“爆燃”（Deflagration）与“爆震/爆轰”（Detonation）。虽然人们常把爆震等同于爆炸，但在现代科学定义中，爆震是指燃烧波阵面（火焰前锋）的移动速度超过了该燃烧介质本身的声速。Scott Manley 拿自己在厨房做的趣味实验举例：在一个瓶子里蒸发少许酒精并将其点燃，慢动作回放显示火焰前锋缓缓沿着瓶身向下蔓延。火焰的速度取决于混合比等诸多参数，如果条件恰到好处，火焰前锋会持续加速并突破音速。一旦超音速，未燃烧的混合气体根本来不及被推开，火焰前锋就会转变为一道激波，导致其内部的压力和温度几乎在瞬间剧烈飙升。常规喷气发动机和普通火箭利用的是亚音速燃烧，即爆燃过程；而爆震发动机依赖超音速激波推进。爆震过程最显著的特征在于，激波内部的压力攀升幅度极其惊人。这是因为燃烧在瞬间完成，推进剂甚至来不及发生热膨胀。因此在科学定义中，爆震燃烧属于“定容燃烧过程”（Constant Volume Process），而常规喷气发动机中的传统燃烧则属于“定压燃烧过程”（Constant Pressure Process）。 - **推力规模**：演示装置体积很小，产生推力小于 100 kg。 - **理论效率提升**：相较于传统常规火箭发动机，预计效率提升高达 25%。 - **对比基准**：传统发动机由开式循环升级至闭式循环仅带来约 10% 的提升。 - **潜在应用范围**：火箭、航空喷气发动机以及美国海军舰船的涡轮动力系统。 --- ### [03:02](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=182s) 从脉冲爆震到旋转爆震：架构演进与工程难点探索爆震动力的最早形式是“脉冲爆震发动机”（Pulse Detonation Engine，简称 PDE），该技术在 2008 年就曾完成过飞行演示。PDE 的结构相对简单：它是一根长管，前端配备进气阀门。工作时，首先用空气吹除净化管道，然后喷入燃料与空气混合气并使其充盈管腔，接着利用火花塞在管底点火。火焰最初以亚音速的爆燃形式燃烧，在沿着长管传播的过程中逐渐加速转变为超音速爆震，这种“爆燃向爆震转变”（Deflagration to Detonation Transition，DDT）的距离需求正是发动机管道必须做得很长、外观就像伸出飞机尾部的大排水管的原因。2008 年的测试机装有 4 台发动机，每台每秒脉冲 20 次，整体产生 80 Hz 的脉冲频率。Scott Manley 补充道，像二战德国 V-1 飞弹（依靠机械簧片阀门）或 YouTube 创作者 Colin Furze 制造的 U 型脉冲喷气发动机，其燃烧均为亚音速，因此都不是真正的爆震发动机。虽然 PDE 理论效率优于常规喷气发动机，但其周而复始的“排气净化-喷注-点火-爆震”循环导致推力输出是不连续的。为了克服这一缺陷，“旋转爆震发动机”（RDE）应运而生。RDE 的构想是将爆震限制在环形燃烧室内，使其成为连续过程：爆震波不再沿着长管单向喷出，而是在环状燃烧室内部沿周向高速旋转，推进剂混合物从侧向连续注入，被迎面而来的爆震波引爆并向后喷出产生推力。只要维持特定的热力学条件，爆震波就能在环形腔内永远旋转下去。 ```mermaid flowchart LR accTitle: RDE 环形爆震工作机制 accDescr: 展示推进剂注入、环形超音速爆震波传播以及超音速燃气排出的循环流程 subgraph RDE_Cycle["环形燃烧室"] A["连续喷入新鲜推进剂"] --> B["超音速爆震波沿环形腔周向旋转"] B --> C["瞬间定容燃烧/激波高压"] C --> D["向后喷出膨胀工质产生推力"] D -.->|"持续维持循环"| B end ``` 然而，要让 RDE 稳定运转具有极高的工程壁垒。虽然大量的计算流体力学（CFD）模型展示了爆震波的传播方式与调控机制，但物理实物验证一直进展艰难。最大的难题在于维持环形燃烧室内部严苛的环境，确保爆震波能稳定维持。实际上，工程师往往需要让多道爆震波在环形腔内相互追逐运转，且它们的速度必须绝对一致。如果工况失谐，某道波就可能追上另一道波；推进剂甚至可能在未燃烧的情况下直接被排出室外在外部引燃，从而大幅降低效率，甚至引发灾难性爆炸；亦或是爆震波自身直接熄灭。如果攻克这些难关，RDE 的回报将极其丰厚。吸气式 RDE 可以大幅淘汰传统喷气发动机前端庞大沉重的压气机叶片——毕竟压气机压气带来的几个大气压，相比于爆震激波内部瞬间产生的 100 个大气压以上的超高压来说显得微不足道。业内认为，吸气式爆震发动机有望在高达 5 马赫（Mach 5）的高超音速飞行中提供有效推力，而这是传统喷气发动机无法企及的领域。 - **PDE 飞行测试历史**：2008 年完成试飞，采用长管结构以实现爆燃转爆震（DDT）。 - **PDE 运行频率**：试验机单管频率 20 Hz，4 管合计约 80 Hz。 - **非爆震脉冲对比**：V-1 飞弹及常见脉冲喷气管并非爆震，因为其燃烧速度低于声速。 - **RDE 压力优势**：爆震激波内部瞬间压力可达 100+ 个大气压，远超常规压气机的增压能力。 - **高超音速潜力**：吸气式 RDE 有望支持高达 Mach 5 的动力推进。 --- ### [06:12](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=372s) 热力学循环对比：布雷顿循环与汉弗莱循环为了解释为什么科研人员对这种尚未完全成熟的技术抱有如此巨大的期望，必须深入到热机热力学（Thermodynamics of Heat Engines）的基本原理。热机是一种理想化的能量转换装置，它从高温热源吸收热量，将其排入低温冷源，并在此过程中抽取一部分热量转化为有用的机械功。以带活塞的封闭气缸为例：加热内部气体使活塞膨胀向外做功；随后将其置于冷源降温，气体收缩，活塞复位；整个过程中系统将部分转移的热量转化成了机械功，其做功量与吸收热量的比值即为发动机的热效率。从广义上讲，世界上所有的发动机本质上都是热机，无论是带活塞、气缸的内燃机，还是拥有旋转涡轮叶片的航空发动机，只要能通过热流转换提取出机械功，都遵循这一法则——甚至地球的大气天气系统也是一种热机，太阳的热量驱动着气流循环。在热力学中，任何热机的运行状态都可以通过压力（P）、体积（V）、温度（T）和熵（S）的变化来完整描述。在经典的压容图（P-V 图）中，循环路径所围成的封闭几何面积直接对应着发动机在一个循环中所转换输出的有用净功，包络面积越大，效率和做功能力通常就越高。常规的火箭发动机和喷气发动机遵循的是“布雷顿循环”（Brayton cycle）。在布雷顿循环中，燃烧加热过程发生在恒定压力下（即定压燃烧），工作流体在燃烧室内受热并自由膨胀。在 P-V 图上，这一步（例如从状态 3 到状态 4）表现为一条水平线：压强保持不变，而体积随着热量的加入急剧膨胀；排气后工质被新鲜进入的空气替代。与之形成根本性对比的是，爆震发动机遵循的是“汉弗莱循环”（Humphrey cycle）。在汉弗莱循环中，热量是通过超音速爆震波在瞬间注入的，燃烧过程以“定容”（Constant Volume）方式进行。由于工质根本没有时间发生体积膨胀，在 P-V 图上，热量加入阶段不再是布雷顿循环那样的水平线，而是垂直陡峭向上的过程，从而在理论上圈出了远比布雷顿循环更为庞大的做功面积，奠定了其极高理论效率的基石。 | 循环类型 | 典型应用发动机 | 吸热/燃烧特征 | P-V 图加热段表现 | |:--- |:--- |:--- |:--- | | **布雷顿循环 (Brayton Cycle)** | 常规喷气发动机、传统火箭发动机 | 定压加热（Constant Pressure），工质自由热膨胀 | 水平线（压力恒定，体积增大） | | **汉弗莱循环 (Humphrey Cycle)** | 脉冲爆震发动机、旋转爆震发动机 (RDE) | 定容加热（Constant Volume），激波瞬间超压 | 垂直线/急剧上升（体积不变，压力陡增） | - **热机本质**：任何将高低温热源流动转化为机械功的系统皆为热机。 - **P-V 图几何意义**：循环曲线内部包络的闭合面积代表该循环转化为有用功的能量大小。 - **布雷顿与汉弗莱的分野**：布雷顿循环吸热时压力恒定（定压），汉弗莱循环吸热时体积瞬间锁定（定容），这正是爆震发动机实现更高热力学效率的理论根源。 ### [09:48](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=588s) 爆震循环的热力学优势与理论效率从热力学循环的压容图（P-V 图）来看，传统的等压燃烧（如布雷顿循环）在燃烧过程中压力基本保持不变，而爆震燃烧则表现为近乎垂直上升的定容燃烧过程。这种急剧的压力骤升从根本上改变了热力学循环曲线包络的几何形状，从而显著扩大了循环内部所包围的有效面积。在热力学中，P-V 图闭合曲线内部的面积直接对应于发动机向外输出的功，因此理论计算明确表明，爆震循环在消耗完全相同热量输入的前提下，能够释放出更多的有效动力。正如 Scott Manley 所指出的：“it goes vertically and this changes the shape of your cycle and actually gives you more area to work inside so therefore it's anticipated that a detonation cycle should get more power out of the same heat input.” ```mermaid flowchart TD accTitle: 爆震循环与传统等压循环做功对比 accDescr: 展示等容燃烧相比等压燃烧如何通过扩大P-V图循环面积来提升热效率 subgraph Comparison["循环模式对比"] A["燃料与氧化剂注入"] --> B{"燃烧方式"} B -->|"传统燃烧 (缓燃 / Deflagration)"| C["等压燃烧过程 (Brayton 循环)<br>压力基本恒定"] B -->|"爆震燃烧 (Detonation)"| D["等容燃烧过程 (Humphrey 循环)<br>压力垂直骤升"] end subgraph Outcome["能量输出与效率"] C --> E["P-V 图包络面积较小<br>相对较低的热效率"] D --> F["P-V 图包络面积显著增大<br>同等热输入下输出更多功"] end ``` ### [10:02](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=602s) 工程现实的制约与实际落地的技术鸿沟尽管热力学理论展现出极其诱人的前景，但现实中的工程制造与物理法则不可避免地会引入各种效率损失，使实际循环偏离理想状态。首先是极端环境对材料的苛刻考验：由于爆震波产生的瞬时峰值温度与冲击压力极高，受限于现有的结构材料耐温耐压极限，工程师在现实中往往难以完全利用这些理论极限工况。为了阐明这种理想与现实的落差，Scott Manley 举了斯特林发动机（Stirling engine）作为生动的类比：在理想的斯特林热力循环中，活塞理应向上移动后瞬间完全静止，随后向下移动再瞬间保持静止；但在机械运转的现实世界里，活塞的实际往复运动呈现的是平滑的正弦波轨迹，这种运动方式的妥协导致实体斯特林发动机的效率始终无法达到理论计算的高度。目前，旋转爆震发动机虽然已经完成了初步的概念验证与地面试车，迈出了摆脱纯理论模型的关键几步，但要将其转化为主流可用的推进系统，依然面临漫长的工程研发道路。正如 Scott Manley 总结道：“so right now detonation engines have taken a few small steps to being more than a theoretical device but beyond that they do need to be developed into real working devices and actually demonstrate they're able to achieve the levels of power and efficiency that the mathematics suggests.” - **材料耐受极限**：爆震波伴随的超高温和剧烈冲击压强，对燃烧室与喷注结构构成了极为苛刻的材料挑战。 - **机械与流体损失**：实际流体阻力、传热损失以及激波衰减等非理想效应，会侵蚀掉部分理论预期的效率增益。 - **验证现状**：旋转爆震发动机正处于从实验室原理验证向实用化火箭与喷气动力系统跨越的攻关阶段，亟待通过全尺寸、长时间的运行测试来兑现数学公式所承诺的高效率。 ## Notable quotes > [10:02](https://www.youtube.com/watch?v=rG_Eh0J_4_s&t=602s) so right now detonation engines have taken a few small steps to being more than a theoretical device but beyond that they do need to be developed into real working devices and actually demonstrate they're able to achieve the levels of power and efficiency that the mathematics suggests. — Scott Manley ## Mentioned Scott Manley, 旋转爆震发动机 (RDE), 脉冲爆震发动机 (PDE), 肯尼迪航天中心 (Kennedy Space Center), 美国海军 (US Navy), Colin Furze, V-1 飞弹, 布雷顿循环 (Brayton cycle), 汉弗莱循环 (Humphrey cycle), 斯特林发动机 (Stirling engine) ## Video description There's been a few stories recently about detonation engines and how they're supposed to be superior to traditional jet or rocket engines. So, what is a detonation engine, what's the difference between a pulsed and rotating engine and why does thermodynamics predict these are better than traditional engine designs. [Source](https://www.youtube.com/watch?v=rG_Eh0J_4_s)

Source quotes

[10:02] Scott Manley: so right now detonation engines have taken a few small steps to being more than a theoretical device but beyond that they do need to be developed into real working devices and actually demonstrate they're able to achieve the levels of power and efficiency that the mathematics suggests.

Source quotes

[9:48] Scott Manley: it's anticipated that a detonation cycle should get more power out of the same heat input. [9:48] Scott Manley: detonation engines have taken a few small steps to being more than a theoretical device but beyond that they do need to be developed into real working devices and actually demonstrate they're able to achieve the levels of power and efficiency that the mathematics suggests.

Original transcript

[0:04] hello it's scott manley here [0:06] recently there's been news about a [0:07] demonstration of the technology behind a [0:10] more efficient rocket engine known as [0:12] a rotating detonation engine while their [0:15] design is small [0:16] and generates less than 100 kilograms of [0:18] thrust it's a practical demonstration of [0:20] a device which is being pursued by [0:22] researchers all over the world [0:24] a device that has been theorized for [0:26] decades and is predicted to be up to 25 [0:29] more efficient than the ordinary garden [0:31] variety rocket engine [0:33] and of course when i say garden variety [0:35] i mean the rocket garden at kennedy [0:37] space center because [0:38] i don't have any rockets in my garden [0:40] anyway [0:41] being able to get an improvement of [0:42] efficiency of a rocket engine would be [0:44] great [0:45] when you consider that switching from a [0:47] open cycle engine to a much more [0:49] complicated [0:50] closed cycle engine gets you a 10 [0:52] improvement [0:53] a 25 improvement would be a massive [0:57] and detonation engines aren't specific [0:59] to rocket engines or jet [1:01] engines the us navy would like to use [1:03] those to power the turbines on some of [1:05] their ships [1:06] so what is a rotating detonation engine [1:09] and how [1:10] does it work and how can it squeeze more [1:12] performance out of the same mass of [1:14] propellant [1:16] the important word here is detonation [1:18] which refers to a very specific [1:20] combustion process [1:21] now when we hear the word detonation we [1:23] associate it with explosions but not all [1:26] explosions are detonations [1:28] in modern scientific terminology a [1:30] detonation is a combustion process where [1:32] the flame front moves [1:34] faster than the speed of sound in the [1:36] material that's burning [1:38] so here's a fun combustion experiment i [1:40] did in my kitchen [1:41] i evaporated some alcohol in this bottle [1:44] and then lit the mixture on fire [1:45] and in slow motion you can see the flame [1:48] front traveling down [1:49] into the bottle so the speed of that [1:52] flame [1:52] is dependent upon many factors and it [1:54] can be made to burn [1:55] faster by getting the mixture ratio [1:57] correct and if things get just right the [2:00] flame front can accelerate and start to [2:02] move faster than the speed of sound [2:04] in the gas and at this point the [2:06] unburned mixture [2:08] doesn't get pushed out of the way of the [2:09] incoming flame front and the flame front [2:11] becomes a shock [2:12] wave where the pressure and temperature [2:15] rises near instantaneously [2:17] this is a detonation wave so combustion [2:20] processes with subsonic flame fronts get [2:22] referred to as [2:23] deflagration processes and supersonic [2:26] shock wave shock-based [2:28] combustion is detonation regular jet [2:31] engines use deflagration [2:33] and detonation engines use a detonation [2:36] the stand out characteristic of the [2:38] detonation process [2:40] is that the pressure rise in the shock [2:41] wave is much much higher [2:44] because there's no time for the burning [2:45] propellants to undergo any thermal [2:47] expansion [2:48] in scientific terms the combustion is a [2:50] constant volume process [2:52] whereas the combustion in a jet engine [2:54] is a constant pressure process [2:57] so how can the detonation process be [2:59] exploited to make a better engine [3:02] the simplest example of a detonation [3:04] engine is the pulse [3:05] detonation engine and one of these was [3:07] demonstrated in flight [3:08] in 2008 this is a simple engine that was [3:11] a long [3:12] tube with some inlet valves at one end [3:14] first there would be a [3:16] purge of air and then a fuel air mixture [3:20] would then be injected and as that [3:21] expanded to fill the tube [3:23] you would then hit it with a spark plug [3:25] which would start the combustion at the [3:26] base of the tube [3:28] and initially it would burn as a [3:29] deflagration but then as it progressed [3:31] down it became a detonation [3:33] and if you did this just right you could [3:36] make several pulses per second and get [3:38] thrust out of it [3:40] so this transition was why the engine [3:42] had to be so long [3:43] it actually extended far past the rear [3:46] of the plane they looked like big long [3:47] drain pipes [3:49] so the engines in that test vehicle [3:51] would pulse 20 times per second [3:53] and there were four off them so they had [3:54] like an 80 hertz pulsing [3:57] and at this point i should also mention [3:58] that there are other pulse jets [4:00] the the v1 missile was powered by a [4:03] pulsed engine but it used mechanical [4:06] slats and it wasn't a detonation [4:07] help there's even the u-shaped pulse [4:10] jets built by [4:11] uh colin furs who's also on youtube but [4:13] these are not [4:14] detonation engines because the [4:16] combustion isn't supersonic [4:18] so in theory pulse detonation engines [4:21] can exceed the efficiency of [4:22] regular jet engines but the purge and [4:24] pulse cycle means that it's not [4:26] generating thrust all the time [4:29] so the rotating detonation engine takes [4:31] this concept and tries to make the [4:33] detonation a continuous process [4:36] instead of the detonation wave traveling [4:38] down a long tube [4:39] it's confined to travel around a [4:41] circular cavity [4:43] the propellant mixture is fed in along [4:45] the tube [4:46] and it detonates and is expelled at the [4:48] other therefore the detonation wave [4:50] travels around this cavity in the circle [4:53] as [4:53] long as the conditions sustain it that's [4:56] the theory [4:57] but of course making it work is the [4:59] trick and there's been lots of [5:00] theoretical [5:01] cfd models showing how waves might work [5:03] how they could be tuned and controlled [5:05] but the actual physical demonstrations [5:07] have been elusive [5:09] the hard part is maintaining the [5:11] conditions inside your annular [5:12] combustion chamber so that the [5:14] detonation wave continues to move [5:17] frequently they're working with multiple [5:18] detonation waves chasing each other [5:20] and they all need to move at exactly the [5:22] same speed if you get the conditions [5:25] wrong then one [5:26] wave might start to catch up on the [5:28] other it's possible that you can have [5:31] unburned propellant expel out the back [5:33] and burn there losing your efficiency or [5:35] even worse [5:36] exploding or the detonation wave could [5:38] simply just blow itself [5:40] out but the advantages could be huge [5:44] an air fed detonation engine could [5:46] eliminate many of the moving parts [5:48] it wouldn't need the compressor that's [5:50] typical at the front of a jet engine [5:52] and after all a few atmospheres of [5:54] compression is kind of wimpy compared to [5:56] the hundred plus atmospheres you can get [5:58] inside a detonating shock wave [6:01] it's believed that air breathing [6:03] detonation engines could provide usable [6:05] thrusts for hypersonic propulsions up to [6:07] about mach 5 [6:08] which isn't possible with conventional [6:10] jets [6:12] so now many of you probably want to know [6:14] why scientists have so much expectations [6:16] of the performance of a device that's [6:18] not [6:19] really been built what is the [6:22] theoretical underpinning for [6:23] all this hope and for that unfortunately [6:26] i'm going to have to give you a little [6:27] bit of a lesson in thermodynamics of [6:29] heat engines now if you've done physics [6:31] you know all this but [6:32] there's other people here that might [6:34] want to know so a heat engine [6:36] is an idealized device that takes heat [6:38] from a high temperature source and moves [6:41] to a low temperature sink while [6:43] siphoning off a portion of that to do [6:45] useful work and a simple example is [6:48] where you take a [6:48] cylinder that's closed with a moving [6:50] piston and you heat the gas inside it [6:53] that will cause the piston to move and [6:55] it will do work [6:57] then you can attach it to a cold [6:59] reservoir which will cool the gas and [7:00] the cylinder will retract [7:02] and it'll also do work now in that [7:04] process you've moved some heat from the [7:06] hot reservoir to the cold reservoir but [7:08] you've also got some mechanical work out [7:10] of it [7:11] and you can actually figure out the [7:12] efficiency of the engine by the [7:13] mechanical workout to the amount of heat [7:15] transferred [7:16] so as it turns out that on some level [7:19] every engine [7:20] is a heat engine and it doesn't matter [7:22] if it has pistons or [7:24] cylinders or turbine blades if you can [7:26] get mechanical workout from heat flow [7:28] it's a heat engine even the weather on [7:31] the planet earth is a kind of heat [7:33] engine as the sun's heat causes gas to [7:35] circulate and move around [7:37] so you can describe the operation of [7:40] every heat engine [7:41] by tracking how its pressure volume [7:43] temperature and entropy change over time [7:46] so the simple gas in a cylinder i [7:48] described [7:49] is kind of like an idealized sterling [7:51] engine [7:52] and i have a diagram here which shows [7:54] how the pressure and the volume [7:56] changes over time on this so if we just [7:59] start at position one that's where the [8:00] cylinder is hot [8:02] and it's full of compressed gas so as we [8:05] go from one to two [8:06] the gas is now expanding the pressure is [8:09] dropping [8:09] and the volume is increasing and it's [8:11] doing mechanical work [8:13] at that point you put it in touch with [8:14] the cold reservoir the temperature drops [8:17] down from two to three so that moves [8:18] vertically the volume remains the same [8:21] and now because the temperature has [8:23] dropped the pressure has dropped [8:25] so the cylinder wants to contract and [8:27] does so along these lines with volume [8:29] decreasing [8:30] and pressure increasing back to the [8:32] ambient we end up at [8:33] four where we allow the gas to warm [8:36] again from the hot reservoir so it rises [8:38] back [8:38] up to one so that is the cycle of the [8:40] engine and you can draw other diagrams [8:42] that show how [8:43] its temperature changes with respect to [8:46] the entropy [8:47] the really important thing about these [8:48] diagrams is that you can actually figure [8:51] out how much [8:52] energy is transferred or converted to [8:54] work by looking at the area enclosed by [8:57] these lines [8:58] so you can figure out the efficiency of [8:59] the engine by looking at the volume [9:01] inside there [9:02] so rockets and jet engines they use the [9:06] brayton cycle which [9:07] looks like this and in this case the [9:09] heat is transferred or applied [9:12] at constant pressure with the working [9:14] fluid expanding [9:15] inside the combustion chamber so on this [9:17] diagram that steps three to four where [9:19] the pressure remains the same but the [9:21] volume increases because the temperature [9:22] is being added [9:24] so note that in this diagram by the way [9:27] that it's not a closed gas inside a [9:29] cylinder [9:30] the at 0.8 you've got the gas being [9:33] exhausted [9:34] and then being replaced by fresh inlet [9:36] air [9:37] now detonation engines use the humphrey [9:40] cycle [9:41] which adds the heat at constant volume [9:43] which means that instead [9:45] of steps three to four going [9:46] horizontally [9:48] it goes vertically and this changes the [9:50] shape [9:51] of your cycle and actually gives you [9:53] more area [9:54] to work inside so therefore it's [9:56] anticipated that a detonation cycle [9:59] should get more power out of the same [10:01] heat input [10:02] so look that's the theory and of course [10:04] in practice there are efficiency losses [10:07] and [10:07] engineering considerations that push [10:09] your cycle away from ideal [10:11] for example you might not be able to [10:13] exploit higher temperatures and [10:15] pressures because of the materials that [10:16] you're working with [10:18] or say in the pistons in the sterling [10:20] engine those are supposed to move up [10:22] and stay still and then down and stay [10:24] still but in practice they move kind of [10:26] in a sine wave pattern [10:28] so this real working sterling engines [10:31] don't get the same efficiency as [10:33] idealized engines so right now [10:36] detonation engines have taken a few [10:38] small steps to being more than a [10:40] theoretical device but beyond that they [10:43] do need to be developed [10:44] into real working devices and actually [10:47] demonstrate [10:48] they're able to achieve the levels of [10:50] power and efficiency [10:52] that the mathematics suggests [10:59] 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