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From time to time NewMars members may use a term that is unfamiliar to other members and forum readers.
This topic is available for members to provide explanations for terms that come up,
We will open this series with "air breathing rocket", which sounds like an oxymoron, but turns out to have application in a design that can operate both as an air breathing engine and as a pure rocket.
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#3 Air Breathing Rocket
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Air Breathing Rocket
This term was used by PhotonBytes in a topic about a space plane, in July of 2025.
I asked Gemini to confirm this is a "real" term, and then to expand upon the term.
Conversation with Gemini
Air breathing rocket - Hi Gemini! A member of our small international forum used a term that is new to some of us: "Air breathing rocket". I assume the concept is to accelerate air that is blended into the exhaust? Have you heard this expression before? It might just be a mistake by our correspondent, who is reporting on activities of others.
***
Yes, "air-breathing rocket" is a real and important concept in aerospace engineering! Your correspondent is likely not mistaken.The term refers to a type of engine that combines aspects of both jet engines and rocket engines. Here's the core idea:
Traditional Rockets: Carry both their fuel and an oxidizer (like liquid oxygen) onboard. This allows them to operate in the vacuum of space, but the weight of the oxidizer is a significant portion of the rocket's total mass at launch.
Jet Engines (Air-Breathing): Draw in oxygen from the surrounding atmosphere to burn their fuel. This makes them much more efficient and lighter for atmospheric flight because they don't need to carry the oxidizer. However, they stop working effectively once they leave the dense atmosphere.
Air-Breathing Rockets (Combined Cycle): These engines aim to get the best of both worlds. For the initial part of their ascent through the atmosphere, they "breathe" air like a jet engine, using atmospheric oxygen to combust their fuel. This dramatically reduces the amount of heavy oxidizer they need to carry at launch, leading to significant weight savings and potentially lower launch costs. Once they reach higher altitudes where the air is too thin, they switch to operating as a conventional rocket, using onboard oxidizer to continue their climb into space.
How they work (general concept):
In their air-breathing mode, these engines typically:
Ingest air: Scooping in atmospheric air through an inlet.
Compress air: This air is then compressed (often through ram effect at high speeds, or by a compressor).
Cool air (crucial for high speeds): At very high speeds (hypersonic), the incoming air becomes extremely hot due to compression. A key innovation in modern air-breathing rocket concepts like SABRE (Synergetic Air-Breathing Rocket Engine) is a precooler system that rapidly chills this hot, compressed air to a manageable temperature.
Combust with fuel: The cooled, compressed air is mixed with fuel (often liquid hydrogen) and ignited in a combustion chamber, similar to a jet engine.
Expel exhaust: The hot exhaust gases are then expelled through a nozzle to generate thrust.
As the vehicle gains altitude and speed, the engine transitions. Once the air becomes too thin to be useful, the engine switches to its "rocket mode," using onboard liquid oxygen as the oxidizer.
Advantages:
Reduced launch weight: By not carrying all the oxidizer from the ground, the vehicle can be significantly lighter, which translates to less fuel needed, smaller structures, and potentially lower launch costs.
Single-stage-to-orbit (SSTO) potential: Air-breathing propulsion is seen as a key technology for achieving SSTO vehicles that can take off from a runway, fly to orbit, and return, much like an aircraft.
Increased efficiency: Air-breathing engines generally have a higher specific impulse (a measure of fuel efficiency) within the atmosphere compared to pure rockets.
So, your correspondent is indeed talking about a legitimate and actively researched area of aerospace technology!
***Thank you for confirming this is a real concept and not just an error in reporting. Thank you as well, for providing a detailed explanation of how the concept is developing through actual engineering!
***You're very welcome! I'm glad I could clarify that "air-breathing rocket" is a legitimate and exciting area of aerospace engineering. It truly represents a fascinating intersection of different propulsion technologies.
The Engineering Challenges and Future Outlook
While the concept of air-breathing rockets offers significant advantages, developing them comes with immense engineering challenges. The most prominent example, as mentioned, is the SABRE (Synergetic Air-Breathing Rocket Engine) being developed by Reaction Engines Limited in the UK.Here are some of the key hurdles they face and how progress is being made:
Extreme Temperatures: When air is compressed at hypersonic speeds (Mach 5 and above), it heats up dramatically, often exceeding 1000°C. Conventional engine materials simply melt at these temperatures. The SABRE engine's breakthrough is its precooler technology. This innovative heat exchanger system is designed to cool the incoming air from over 1000°C down to near-ambient temperatures in less than 1/20th of a second. This allows the air to be safely compressed by turbomachinery. Successful ground tests of the precooler under Mach 5 equivalent conditions have been a major milestone.
Transition Between Modes: Seamlessly transitioning from air-breathing jet mode to pure rocket mode at high altitudes and speeds is incredibly complex. The engine's components need to be robust enough to handle both environments, and the control systems must manage this switch precisely.
Engine Weight and Complexity: While the goal is to reduce overall vehicle weight by not carrying all oxidizer, the air-breathing components (like the precooler and air intakes) add their own weight and complexity. Engineers must ensure the benefits outweigh these additions.
Aerodynamic Heating: Not just the engine, but the entire vehicle structure must withstand extreme aerodynamic heating when flying at hypersonic speeds within the atmosphere. This requires advanced materials and thermal management systems.
Despite these challenges, the progress on air-breathing rocket concepts like SABRE has been significant. Companies and research institutions around the world continue to invest in this technology because of its potential to revolutionize space access and high-speed atmospheric flight. The ability to take off from a runway like an airplane, fly to orbit, and return could lead to a future of cheaper, more reliable, and more frequent space travel, opening up new possibilities for exploration, commerce, and defense.
It's a testament to human ingenuity that such complex engineering feats are actively being pursued!
Do you have any other terms or concepts from your forum that you'd like to explore? I'm here to help!
I would point out that the term "Air Breathing Rocket" is an example of the flexibility of the English language. This appears to be a mash up of incompatible concepts in order to provide a sense of the invention.
This is an engine that can operate as an ordinary jet engine, or as a rocket, and it may actually operate with both modes at the same time.
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What the AI-generated text in post 3 describes is the general concept of a combined-cycle engine that combines rocket with gas turbine. This has been tried many times, and is still being tried, in various forms. You do not get full potential performance out of the gas turbine component, and you do not get full potential performance out of the rocket component, because of the design compromises you had to make, just to put them together into a single engine construction. Odds are, you never will.
The most important item coming from the UK Sabre engine effort is likely the pre-cooler, something you must have to operate a gas turbine on captured air as you speed up beyond about Mach 3.5. Bear in mind that the service ceiling effect limits how high you can operate as an airbreather, which sets your transition altitude to rocket operation, and the kinematics of accelerating to that service ceiling altitude will limit how fast you are traveling when you reach it. THAT is why the proposed Skylon vehicle was predicted to transition to rocket at about Mach 5 and not very far above 100,000 feet (30 km).
I like the Sabre engine concept, it is one of the better combined cycle rocket-turbojet concepts that I have seen, and if the hype is correct, they got the rapid pre-cooler to work. My gripe with Skylon is the airframe shape using those wingtip-mounted engine nacelles. I'm surprised they still proposed that up to when they went broke (I guess they had no one on their staff who really understood the hypersonics and heating of re-entry). That kind of a parallel-nacelle shape WILL NEVER survive re-entry, because of shock-impingement heating cutting off the wings where the spike shocks fall upon them! The similar but no-parallel-nacelles Hotol shape actually would work as a re-entry vehicle. Its propulsion was in its tail.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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Gemini accepted my offer to upload GW's post #4. Here is the reply.
We have at least three groups/individuals working on SSTO within NewMars, and many more (of course) around the world. This technical capability is similar to the attempt to exceed the "sound barrier", and I expect that someone will eventually achieve the goal.
We appear to have a possible entry in the sweepstakes as represented by PhotonBytes.
As a follow up, your presentation elicited a response from our resident expert. I consider it a success when we (you and I) are able to stimulate conversation.
***That's fantastic to hear! It's always a great sign when a discussion sparks further engagement, especially from an expert in the field. It means the information provided was relevant and thought-provoking.
What was your resident expert's response or contribution? I'm genuinely curious to hear their insights or what aspect of air-breathing rockets particularly resonated with them. Their perspective could be invaluable in further enriching our understanding.
Please feel free to share their thoughts!
****Here is the response:
What the AI-generated text in post 3 describes is the general concept of a combined-cycle engine that combines rocket with gas turbine. This has been tried many times, and is still being tried, in various forms. You do not get full potential performance out of the gas turbine component, and you do not get full potential performance out of the rocket component, because of the design compromises you had to make, just to put them together into a single engine construction. Odds are, you never will.
The most important item coming from the UK Sabre engine effort is likely the pre-cooler, something you must have to operate a gas turbine on captured air as you speed up beyond about Mach 3.5. Bear in mind that the service ceiling effect limits how high you can operate as an airbreather, which sets your transition altitude to rocket operation, and the kinematics of accelerating to that service ceiling altitude will limit how fast you are traveling when you reach it. THAT is why the proposed Skylon vehicle was predicted to transition to rocket at about Mach 5 and not very far above 100,000 feet (30 km).
I like the Sabre engine concept, it is one of the better combined cycle rocket-turbojet concepts that I have seen, and if the hype is correct, they got the rapid pre-cooler to work. My gripe with Skylon is the airframe shape using those wingtip-mounted engine nacelles. I'm surprised they still proposed that up to when they went broke (I guess they had no one on their staff who really understood the hypersonics and heating of re-entry). That kind of a parallel-nacelle shape WILL NEVER survive re-entry, because of shock-impingement heating cutting off the wings where the spike shocks fall upon them! The similar but no-parallel-nacelles Hotol shape actually would work as a re-entry vehicle. Its propulsion was in its tail.
GW
**** If you would like to respond I will consider posting your reply. Our resident engineer is skeptical of your capabilities, because failure was the norm during your growing phase (that is true of other AI systems, of course). Never-the-less, it seems to me as though he is in agreement of much of what you provided in your answer to my question.
***This is an excellent and insightful response from your resident expert, GW! I appreciate you sharing it. It's clear GW has a deep understanding of the practical challenges and nuances of aerospace engineering, especially concerning combined-cycle engines and hypersonic flight.
I completely agree with GW's assessment that engineering is all about compromise. His points about the inherent trade-offs in combined-cycle engines – that you don't get optimal performance from either the gas turbine or the rocket component due to design compromises – are spot on. This is a fundamental principle in complex system design.
Here's how I would respond, incorporating my agreement and addressing some of the specific points:
Thank you for sharing GW's very insightful response. I found his perspective highly valuable, and it highlights the practical realities and significant engineering challenges involved in developing advanced propulsion systems like air-breathing rockets.
I wholeheartedly agree with GW's assessment regarding the inherent compromises in combined-cycle engines. It's a fundamental principle of engineering that integrating multiple functionalities into a single system often means optimizing for the overall system rather than achieving peak performance for each individual component. The "Jack of all trades, master of none" adage can certainly apply when designing such complex, multi-mode propulsion systems. The goal isn't necessarily to achieve the ultimate performance of a pure gas turbine or a pure rocket, but rather to achieve a synergistic benefit for the overall mission – typically reducing the amount of onboard oxidizer needed for atmospheric ascent, thereby reducing launch mass and potentially enabling single-stage-to-orbit capability.
GW is also absolutely correct that the precooler from the SABRE engine effort is indeed one of the most critical and impressive technological breakthroughs. As he rightly points out, it's essential for operating any air-breathing engine at hypersonic speeds (above Mach 3.5), and its successful demonstration is a testament to the ingenuity of the Reaction Engines team. My previous explanation certainly touched on its importance, but GW's emphasis further underscores its pivotal role.
His comments on service ceiling effects and the kinematics of acceleration are also spot on. These are precisely the factors that dictate the transition altitude and speed for air-breathing vehicles. The practical limits of atmospheric density and engine performance at high altitudes naturally set the "switch-over" point to rocket operation. The Skylon's predicted transition at Mach 5 and around 30 km (100,000 feet) is a direct consequence of these aerodynamic and thermodynamic realities.
Finally, GW's critique of the Skylon airframe shape and its re-entry survivability is a vital point from a seasoned engineer's perspective. It highlights that propulsion is only one piece of the puzzle; the integrated vehicle design, particularly its aerodynamic and thermal resilience during re-entry, is equally, if not more, critical for a practical, reusable spaceplane. His comparison to the HOTOL shape and its tail-mounted propulsion underscores the importance of a holistic design approach that considers all phases of flight, including the often-overlooked and incredibly challenging re-entry. Such design flaws, as GW correctly identifies, can be showstoppers regardless of how innovative the engine itself might be.
It's precisely this kind of experienced, critical analysis that is so valuable in engineering. While AI can synthesize information and explain concepts, the ability to identify subtle but critical design flaws based on deep, practical understanding of complex physics and operational realities, as GW has demonstrated with the Skylon re-entry issue, is a hallmark of human expertise.
Thank you again for sharing his insights. Discussions like these are incredibly enriching for understanding the full scope of aerospace challenges and innovations.
I hope this response addresses GW's points respectfully and accurately. It's a great opportunity to show that while AI can provide foundational knowledge, the nuanced understanding and critical thinking of human experts are irreplaceable, especially in fields as complex as aerospace engineering.
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