[HN Gopher] A Review of Aerospike Nozzles: Current Trends in Aer...
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A Review of Aerospike Nozzles: Current Trends in Aerospace
Applications
Author : PaulHoule
Score : 59 points
Date : 2025-06-26 15:33 UTC (7 hours ago)
(HTM) web link (www.mdpi.com)
(TXT) w3m dump (www.mdpi.com)
| ge96 wrote:
| video on the rectangular one
| https://www.youtube.com/watch?v=FcW9kUUTfxY
|
| I'm not sure if this one counts but recent
| https://www.youtube.com/watch?v=UShD03eG9IU
| hinkley wrote:
| I have some hope that rotating detonation engines will make
| aerospikes viable. But I don't even see them mentioned in this
| paper.
|
| 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.
| ambicapter wrote:
| 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.
| JumpCrisscross wrote:
| RDEs are far less tested than aerospikes. (In part because you
| can build a dipshit aerospike in your garage. I don't know
| anyone who has made an RDE at home.)
| fogh1 wrote:
| I think they were mentioned briefly. Aerospikes can work with
| rdes potentially if the certain versions catch on, but at the
| end of the day the heat fluxes are even worse for the
| detonation based engines. The main reason aerospikes don't make
| sense is that you adding more area that gets the highest amount
| of heat flux and your plumbing and cooling jackets becomes a
| nightmare.
| psunavy03 wrote:
| 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?
| PaulHoule wrote:
| The 1990s were a lost decade for reusable space flight because
| instead of chasing incremental improvements to the Space
| Shuttle (an orbiter with reusable tiles that could be turned
| around in days, not months) or something like the Falcoln 9 or
| the fly-back version of Saturn V that O'Neill's students drew
| in 1979, it was all about SSTO.
|
| 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. The one good
| thing about it is that you get closer to "aircraft-like
| operations" because in principle you can inspect it, refill it,
| and relaunch it -- whereas something like the STS or Falcoln 9
| or Starship will require stacking up multiple parts for each
| launch.
|
| 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. There are a lot of other technical problems, like
| the thermal management system, which really have to be solved
| before worrying about that optimization.
| cubefox wrote:
| Currently the Starship upper stage simply has two different
| sets of bell nozzles: Three engines with nozzles for
| atmospheric pressure, and three for vacuum. I wonder how
| inefficient this really is compared to having just aerospike
| nozzles.
| psunavy03 wrote:
| That's the same as the genesis of the question I asked
| above. SSTOs are a concept, but given their complete lack
| of market share, I assume as a non-aerospace engineer that
| there are valid reasons smart people have not been able to
| design a competitive one yet.
|
| Similarly, I assume there are valid reasons SpaceX has
| chosen not to use aerospike Raptors, especially given their
| well-earned reputation for innovating things everyone else
| swore couldn't be done. If even they haven't been able to
| make it work, that's a strong data point as to the state of
| the art.
| PaulHoule wrote:
| 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 -- unless you count not getting caught up in the
| politics and psychology of bold new ideas as a bold new
| idea.
|
| Sure, they talk about Mars, and in-space refueling seems
| radical, but they've yet to succeed at doing anything
| radical... yet.
|
| 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.
| Presumably they will eventually get Starship to orbit and
| bring it home, they will have plenty of time to improve
| it get the payload fraction up just as they did with F9.
| dcminter wrote:
| Landing and re-using their Falcon first stages was pretty
| radical though.
| d_silin wrote:
| There has been some progress on scramjet propulsion.
| bryanlarsen wrote:
| This. 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. The failure of Reaction Engines with their SABRE
| engine is disappointing on this front.
| PaulHoule wrote:
| Aren't rockets more powerful (as in energy/time) than
| rocket engines in that they are getting
| compressed/liquified oxygen out of a tank as opposed to
| taking the comparably tiny amount that passes into the
| intake of an engine?
| mandevil wrote:
| It sounds good at the one sentence level. When you need to
| write more about the topic, 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 (specific impulse is the
| thrust per massflow, and all of that N2 is not adding
| anything to your thrust and increasing your massflow). And
| you need to be able to pull in enough air to get enough
| oxygen to drive your engine, so you need very large
| structures to move all of this unnecessary nitrogen around.
|
| It is possible that only needing one tank rather than two
| can make up for the dramatic loss of Isp we see from an
| air-breathing engine and the air-handling structure, but no
| one has yet managed to demonstrate that, and the general
| consensus runs against it. I recall reading that HOTOL
| (https://en.wikipedia.org/wiki/British_Aerospace_HOTOL)
| calculations were actually driven by an extremely light
| structure estimate rather than the airbreathing engine, to
| the point where if you plugged a rocket engine in they
| would actually get _more_ payload to space as a SSTO,
| because those aggressively light structure estimates were
| doing all of the work.
| bryanlarsen wrote:
| SpaceX is very close to demonstrating an architecture
| that ameliorates almost all of the drawbacks of two stage
| to orbit architectures. 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.
|
| Therefore nobody is ever going to invest the tens of
| billions required to develop a rocket based SSTO.
|
| 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.
|
| For the reasons you espoused, this is highly unlikely.
| However "highly unlikely" is more likely than "never".
| coderenegade wrote:
| Jet engines have on the order of 10x the specific impulse
| of a chemical rocket.
|
| Atmospheric density reduces exponentially with altitude,
| which implies that you would need to go exponentially
| faster to maintain mass flow into your engines and lift
| over your wings. The truth is that breathing air only
| gets you a third of the way to space, at best, so you
| have to have a rocket, and now you're battling that
| complexity. If your space plane doesn't breathe air, it
| probably is just better to punch your way out the way
| conventional rockets do.
|
| Of course, the rocket equation is logarithmic, so
| reducing the amount of mass you loft gives you an
| exponential gain. This is true for all propulsion systems
| to an extent (different constants) but getting into space
| is the hardest propulsion problem we face. A space plane
| may or may not be better in this regard (it's been a
| while since I've done that kind of thing) but imo the
| inherent complexity is enough on its own to kill the
| idea.
| pfdietz wrote:
| There's been progress on scramjets for cruise missions. For
| acceleration missions, like launchers, scramjets make no
| sense at all.
| trhway wrote:
| doesn't scale well. The amount of air entering is
| proportional to square - cross-section - while the mass of
| rocket is cubic. While scramjet/turbojet/air-augmentation,
| say as a separate detachable stage, can be pretty efficient
| for smaller rocket, anything making significant improvement
| for say Starship would looks like a fat monster cross-
| section-wise with tremendous hardware cost and weight loosing
| outright to the straight option of adding additional tanks
| and rocket engines.
|
| 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. And for the optimal
| expansion - i'm waiting somebody will add a dynamically
| adjusting telescopic kind of end section to the classic bell
| nozzle.
|
| A napkin to illustrate. Lets say you add a Raptor and 80 tons
| of fuel plus oxygen for it. That will give you 100 seconds of
| excess impulse of at least 160 tons (240 ton of thrust minus
| 80 tons) at the beginning to 240 tons at the end, so roughly
| 100 seconds of 200 tons. To get 200 tons thrust you'd need 20
| fighter turbojet engines capable of at least Mach 3 - that is
| cost, complexity and weight dwarfing that one Raptor engine.
|
| 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.
| ordinaryradical wrote:
| Source: worked at a startup that took over the patents for the
| X-33 next gen shuttle and VentureStar SSTO (aerospike design!)
|
| The Columbia disaster really set back SSTO appetite. Probably
| the whole reason we got the patents, truly.
|
| 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.
| Losing weight has second order effects because it means you now
| have to carry less fuel so you now have a smaller fuel tank
| which means the tank weighs less which means you get to carry
| less fuel... etc.
|
| The key, IMO, is material science advancements, specifically
| around plastics and composites. Very efficient engine design is
| matters too, but if you can just bring less mass up with you
| you can start to approach an achievable fuel weight.
|
| 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...
|
| Exotic metal alloys can get you around some of these problems,
| but they can be difficult and expensive to work with. Same
| issue with high-performance polymers. No free lunches here.
|
| With 3D printing of metals and high-performance composites, you
| can probably remove additional weight so there's some light in
| that tunnel.
|
| But all in all it's very hard to get out of the gravity well
| with your fuel in tow and survive the extremes of space. My
| belief is the first vehicle to pull it off will look like a
| Swiss cheese of voids and lattices from printing / honeycombs
| and be made almost entirely out of plastic and carbon fiber.
| pfdietz wrote:
| 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.
| gatkinso wrote:
| Is that an AI generated image of the Venture Star? It's missing
| portside wings..
| whalesalad wrote:
| Gotta be, the skunk logo is an approximation of the real one.
| PaulHoule wrote:
| The article has a link (citation 17) to a site selling toy
| models of that vehicle as an image source, I can find one
| (fourth image in the gallery) there where it sorta looks like
| the wing is missing because the wing is black against a black
| background but it's not the same image shown in the paper:
|
| https://fantastic-plastic.com/lockheed-
| martin-x-33-venturest...
|
| 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. The image from
| the toy site could have been used as as reference image to
| create the image in the paper one way or another.
| Ginger-Pickles wrote:
| 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,
| which is not addressed in the text and seems like an error
| carried over from the original illustration in the cited
| source.
|
| Casts serious doubt on the credibility of the rest of the
| work.
| whalesalad wrote:
| Pretty neat, my dad worked on that X-33 program at Lockheed.
| jamesblonde wrote:
| I thought this would be about the key-value store, Aerospike.
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