This Hacker News discussion revolves around the viability of advanced rocket engine designs and the fundamental challenges of achieving Single-Stage-To-Orbit (SSTO) spaceflight.
Aerospike Engines: Potential and Challenges
A significant portion of the conversation focuses on aerospike engines, particularly their perceived potential for future rocket designs and the obstacles they face. Some users see promise in the technology, especially with advancements in other areas like rotating detonation engines.
- "I have some hope that rotating detonation engines will make aerospikes viable." - hinkley
- "My guess is aerospikes are making a comeback though because of interest in hypersonic weapons system. I could also see them being useful for the second stage of something like Starship which mostly operates at high altitudes but has to land at low altitudes." - PaulHoule
However, the practicalities of aerospike design, particularly thermal management and complexity, are frequently raised as significant hurdles.
- "The idea with the constantly moving flame front is that it spreads the heat out. The limitation with aerospikes is getting enough coolant through the spike. Bells are simpler to cool, which as I understand more than makes up for them needing more cooling." - hinkley
- "Doesn't seem like a front rotating around the spike would gain that much "spreading out" over a continuous front. At the end of the day, its a spike that narrows to a very small point." - ambicapter
- "Wrt. aerospike engine - sounds nice, yet hardware wise it is heavier than the classic engine, and just look at that large number of pieces - just all those small mini-engines - it is made of and compare to Raptor 3." - trhway
- "The thermal management system, which really have to be solved before worrying about that optimization." - PaulHoule
The discussion also touches on whether prominent aerospace companies like SpaceX are overlooking aerospikes, considering their innovative track record.
- "Similarly, I assume there are valid reasons SpaceX has chosen not to use aerospike Raptors, especially given they haven’t been able to make it work, that's a strong data point as to the state of the art." - psunavy03
The Elusive Goal of Single-Stage-To-Orbit (SSTO)
The concept of Single-Stage-To-Orbit (SSTO) vehicles is a central theme, with users expressing skepticism about its practical realization and historical challenges. The "white whale" analogy is used to describe the persistent pursuit of SSTO.
- "The abstract brings up SSTOs, but has there been anything in recent invention that will make them anything other than the white whale people have been chasing since forever?" - psunavy03
- "SSTO is just marginally possible, if it is possible you need exotic materials and engines and you're never going to get a good payload fraction and adding wings, horizontal takeoff, horizontal landing and such just makes it worse." - PaulHoule
The primary reasons cited for SSTO's difficulty include the tyranny of the rocket equation, the need for extremely lightweight structures, and the challenges of carrying all necessary propellant.
- "The tyranny of the rocket equation ensures that while a SSTO carrying all of it's oxygen is possible, it's never going to be able to carry enough mass to be useful." - bryanlarsen
- "SSTOs are, like everything else going to orbit, delimited by weight. If you are going to make the fuel tanks internal to the vehicle and not something that falls off and sheds their weight mid-flight, you have to get vehicle weight to the absolute minimum." - ordinaryradical
- "It’s a hard job, you need plastics that can handle orbital temperature cycling (+300 to -300 F every 30 mins), atomic oxygen (nasty corrosion), UV with no atmospheric protection, FST for crew exposure…" - ordinaryradical
- "Why make an SSTO when you can make a TSTO? First stage recovery is a solved problem and will always greatly relax the engineering problems over making a SSTO." - pfdietz
SpaceX's Approach to Innovation and Reusability
SpaceX's engineering philosophy and its success with reusable rockets, particularly the Falcon 9, are discussed as a counterpoint to bold, potentially unproven technologies. Users debate whether SpaceX is truly radical or simply executes conservative, incremental improvements.
- "I'd argue that the brilliance of SpaceX is the opposite. They stick to technology and markets that are proven and use technically conservative approaches. Falcon 9 is about relentless improvement in small ways, not bold new ideas..." - PaulHoule
- "Landing and re-using their Falcon first stages was pretty radical though." - dcminter
- "Rumor has it they were struggling with the payload fraction w/ the first generation of Starship and they switched to a second generation that struggles with blowing up. A big advantage of the two-stage architecture is that you can develop the two stages independently." - PaulHoule
The development of Starship and its two-stage architecture is seen as a pragmatic approach, allowing for independent development and iterative improvement, much like the Falcon 9's evolution.
- "SpaceX is very close to demonstrating an architecture that ameliorates almost all of the drawbacks of two stage to orbit architectures." - bryanlarsen
- "If somebody develops an engine that makes air breathing most of the way to orbit feasible, this has a chance of competing a Starship style architecture." - bryanlarsen
Air-Breathing Engines and Their Limitations
The potential of air-breathing engines, particularly for reducing the reliance on onboard oxidizer tanks, is explored. However, the discussion highlights significant efficiency losses and scaling problems associated with these concepts.
- "In my very uninformed opinion the only way we'll get useful SSTO is if we can get a meaningful amount of oxygen from the atmosphere rather than carrying it up in heavy tanks." - bryanlarsen
- "The problem is that oxygen makes up only about 20% of the air. So you have need to accelerate all of this N2 that gives you nothing in energy and the result is a much lower Isp..." - mandevil
- "For scramjet, assuming we got a decent one, napkin is about the same. The best, my favorite, is air-augmented - scram-compress the air and channel it on the outside of the hot bell nozzles of the already working rocket engines - unfortunately the scaling mentioned above comes into play for meaningfully sized rockets though it has worked great for small ones." - trhway
- "For acceleration missions, like launchers, scramjets make no sense at all." - pfdietz
The general consensus is that while air-breathing propulsion might offer benefits for certain applications, the drawbacks in terms of specific impulse and the need to move large amounts of non-reactive air make them generally unsuitable for large-scale rocketry compared to carrying all propellant.
Scrutiny of Visuals and Credibility
A side but notable theme is the critique of the visuals presented in the linked paper, with multiple users pointing out what appear to be AI-generated or inaccurate illustrations, casting doubt on the paper's overall credibility.
- "Is that an AI generated image of the Venture Star? It's missing portside wings.." - gatkinso
- "The name "Venturestar" is properly rendered in that image but "NASA" and "Lockheed Martin" are thoroughly mangled the way I'd expect text to be mangled in an AI image." - PaulHoule
- "Yes, if you look close, the paper is replete with error-filled generative reproductions of existing illustrations in the citations; including Fig. 6 (MC Escher struts), Fig. 7 (sprouting greeble tubes), and Fig. 8 (actuators replaced by tubes connected to mystery manifolds). Even Fig. 2 shows the spike geometry magically changing… Casts serious doubt on the credibility of the rest of the work." - Ginger-Pickles