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.
You can't get to very high chamber pressures with those, and then maybe aerospike was a way to work around the limitations.
Then XLR-129 and SSME came along with staged combustion cycle and you can just have higher pressure engines. They can both run at sea level and still have a decent efficiency in vacuum.
The linear aerospike for X-33 was kind of a neat tech demo and fit in with the whole shape of the vehicle and composites, non-tubular tanks and large base area. Maybe too many new things at once in retrospect.
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.
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.
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.
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.
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.
SpaceX took a lot of ideas which had been individually proven before, and then put in the work to perfect them and integrate them in a production ready spacecraft. That is important work and good engineering, but not radical. An aerospike had literally never been flown to orbit at that time (I think still not), so it would have been a way worse fit for the SpaceX method of developing the Falcon 9.
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.
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".
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 looked into that kind of thing, so no opinion either way) but imo the inherent complexity is enough on its own to kill the idea.
The general idea is that you can get much better results in terms of deltav if you can find at least part of the reaction mass from elsewhere without carrying it onboard. Even inert nitrogen is useful as a reaction mass. Another way to get a good result is to use separate sources of reaction mass and energy. Then use that energy to accelerate the reaction mass as much as possible, so that you get a decent deltav by the time you exhaust the reaction mass. This is what ion and plasma thrusters do.
However, the requirement of the high thrust disappears once you finish the vertical climb. There's no danger of falling back to ground once you reach orbit. What you need at this stage instead, is to add velocity (deltav) to the craft to change its orbit/trajectory. This can be done even at very low thrust, because you have all the time you need. The limiting factor now is that you have only a finite amount of propellant onboard. You want to add as much deltav as possible before it runs out. A high thrust doesn't help because the engine will simply consume the propellant faster and exhaust it before you get the required deltav. This is where specific impulse comes into play. The maximum deltav you can get is proportional to the specific impulse of the engine (see rocket equation for details). As you can imagine, high specific impulse is critical for space missions requiring high deltav, like the New Horizons spacecraft that imaged Pluto or the Parker solar probe (interestingly, getting to the sun is harder than escaping the solar system). Rockets/jets with low thrust and high specific impulse are called sustainers.
The general trend seen is that specific impulse drops off as thrust increases. For example, the space shuttle solid booster has Tmax = 15 MN, Isp = 268s, and space shuttle orbiter cryogenic engine RS25 has Tmax = 2.28 MN, Isp = 452s. Meanwhile, the NEXT xenon ion thruster used in the DART mission has Tmax = 236 mN and Isp = 4200s. Note that the thrust has changed from Mega newtons to milli newtons. You would hardly recognize it if the ion engine thrusted against your body.
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.
PS: I have seen early-stage (but successfully tested) scramjets being developed for this purpose.
We have to ask: what exactly is a scramjet vehicle delivering? It's enabling the use of air instead of liquid oxygen. But how valuable is this? LOX is the second cheapest industrial liquid after water. The fuel part of a rocket propellant combination typically dominates the propellant cost. If a scramjet launcher uses more fuel (especially hydrogen) than a rocket vehicle would, it will end up increasing propellant cost per unit payload to orbit. It will also likely increase propellant volume per unit payload to orbit, especially if LH2 is used (LH2 being just 5% of the density of LOX).
All scramjet launchers need a rocket to reach stable orbit (since a scramjet cannot produce thrust at apogee to circularize above the atmosphere. So one can ask, what the tradeoff between the delta-V this rocket provides and that of the scramjets? From what I've heard, all such trade studies end up optimizing to 100% rocket and 0% scramjet.
A scramjet stage will be very light compared to an equivalent rocket stage, since it carries only the energy source (fuel) and not the full reaction mass. If this scramjet stage is able to impart a velocity close to the orbital velocity by the time it reaches the upper atmosphere, the subsequent rocket stage will have much less work to do to get it into orbit. And that translates to much less propellants (including oxidizer) and much less mass in the upper stage. It's not necessary to collect oxygen from the atmosphere to see an advantage.
Obviously, the raising of the perigee at apogee is going to need this rocket engine again. There are no launcher concepts that depend purely on scramjets.
Its not the cost, its the mass you're trying to reduce. So far, the engineering challenges have made it unfeasible, but its not a surprise that people look at the hundred tons of LOX on a rocket and imagine exchanging it for payload (or aircraft style re-usability).
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.
I'm not sure if this one counts but recent https://www.youtube.com/watch?v=UShD03eG9IU
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.
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.