Comment by psunavy03
1 day ago
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?
1 day ago
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?
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.
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.
> SSTO is just marginally possible, if it is possible
Looking at the specs it would appear the first stage of a Falcon 9 plus a nosecone could get itself to orbit with no cargo. Barely.
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.
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.
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There has been some progress on scramjet propulsion.
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.
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.
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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?
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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.
There's been progress on scramjets for cruise missions. For acceleration missions, like launchers, scramjets make no sense at all.
That really depends on how fast you can cruise. High speed scramjets above mach 15 will make space missions possible. The craft will be at the sufficient height and just enough speed, so that a rocket engine won't have to add too much deltav. Scramjets are still in their infancy. There are already developments on for variable-geometry multi-mode ramjets for this purpose.
PS: I have seen early-stage (but successfully tested) scramjets being developed for this purpose.
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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.
Because of course SSTO has benefits over TSTO, simplicity of operation being one example.
The argument for SSTOs was that staging was too scary. But experience since then shows this argument was bogus. Staging can be made highly reliable.
Even a small amount of delta V provided by a first stage makes the job of the "almost SSTO" second stage much easier. And a low delta V first stage can be rugged, with high high safety factors, and is easy to recover at the launch site.
Put another way: if you have an SSTO, its payload increases dramatically if you stack it on a very low performance recoverable first stage.
I don't see any way SSTOs are going to be preferable to TSTOs, especially if the SSTO has to use hydrogen to get off the ground.
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