An air breathing engine is used early in a launch. That means it's in competition with a rocket propelled first stage. But first stages are the least sensitive part of a rocket to Isp. They are more sensitive to thrust/weight ratio of the engines, and on that metric air breathing engines are grossly inferior to rockets.
I think this is not meant to directly replace the first stage engines for a conventional rocket. Instead you'd use an approach like SpaceShipOne where you launch like a normal plane using jet engines and then only fire rocket engines once you run out of air. It would be cool to have this approach in a single vehicle. Thrust to weight is less of a consideration there. Although it will still be expensive to carry two radically different types of engines. I'd love to see an engine like this that can seamlessly switch to burning oxidizer that was brought along for the ride. Maybe even combined with an aerospike to get every last bit of efficiency out of higher altitudes. Then we might get true scifi SSTO spacecraft. Think of a 737 but it can go to space.
What kills air launch (specifically, an airplane first stage and conventional second) is the second stage must withstand stress in two dimensions; most rockets wouldn’t fair well being hung horizontally because they don’t have to design to withstand stress in that dimension. Most airliner airframes wouldn’t deal well with being hung from tail or tip; that’s strike fighter stuff.
Air launch combines the worst of staging (complexity) and SSTO (dual dimensioning). What I'm reading is turbojets supplementing rockets to reduce fuel burn.
But instead of using the aeroplane you could just use a conventional first stage rocket? I don't think I understand what advantage the aeroplane gives.
The aeroplane takes off from an airport, lands back in the airport, turned around ready to go in half an hour (this is assuming a multistage arrangement where only the first stage is a plane; if you're talking SSTO it's a different story, but that's always been a _bit_ of a pipe dream).
Lift from wings means overall less fuel usage, launch from an aircraft increases the choice of "launch rockets from here" locations.
ie. Can presumeably get closer to the equator and choose the desired "west to east" ascent line (and fall to earth line for debris in case of rocket failure).
After watching the video I'm left wondering how k2pilot would respond to the analysis. Is the idea that you could use these jet engines as part of a vertical takeoff?
Thrust/weight ratio of a Merlin engine: 184 (sea level), 214 (vacuum)
Thrust/weight ratio of a military jet engines: ~9
Thrust/weight ratio of a commercial turbofan: ~6
Merlin also has a much lower cost per unit thrust, 200x lower than a military jet engine, 400x lower than current commercial high bypass turbofan engines.
Stupid question but don't the wings make most of the work in term of lift? I would assume you need less energy to pull some weight up in the air, obviously a lot slower also.
The thing is, the goal of a rocket is not to "lift things off the ground", it is to "speed up things to an orbital velocity" -- around 8km/s for LEO.
From this point of view starting the rocket engine in the air at 0.2km/s speed is a marginal improvement compared to starting it on the ground. It still can be an improvement if executed correctly but it's hard to make it big enough to justify the complexity.
Isn’t most of the speed you spend on the first stage wasted on going up, though? And what little momentum you might get to keep when it’s time to turn into your orbit is also sucked away by air resistance. Rockets have to go directly up for the first part of the flight, and all that acceleration is wasted because what you really need is orbital velocity, which is going to end up being “sideways” relative to earth. An air breathing first stage would mean you have to give a little less of your fuel to going up, and you’re doing it in an environment with less air resistance.
And any gain in the very first part of a flight can also be achieved by just making a cheap rocket first stage slightly larger.
Being able to propulsively recover rocket first stages, as Falcon 9 demonstrated, has destroyed pretty much any rationale for winged or air breathing first stages.
Winged and air breathing first stages are a square peg round hole situation because high altitude and runway technical requirements are so different, and at low altitude reaching mach 0.8 is all you get to do cheaply - a negligible benefit considering the extreme costs associated with either horizontal plane-style integration or high TWR turbines. If you're going to optimize for getting useful delta V out of them, you need to do it in the stratosphere with ramjets/scramjets, which have minimum airspeed requirements on top of the minimum glide airspeed requirements necessary to keep a heavy aerodynamic body from falling to earth. You're best starting out at extreme speed and high altitude by launching from a hydrogen or vacuum filled maglev at Chimborazo or Kilimanjaro, and using the scramjet to get from the summit launchsite at a muzzle velocity of atmospheric-mach 3 up to the Karman line at mach 10. The initial speed provides the ability to go directly to hypersonic engines, the ability to trivially maintain altitude and build speed with only modest TWR and small light wings, and the ability to minimize drag losses by going through very thin air.
This would be used in something like Starraker. That design can get 100 tons to orbit with a single stage much more efficiently than any rocket. Take off like a plane, climb to high altitude. Enter a supersonic dive. Pull up and ignite your rockets from high in the atmosphere when travelling relatively fast. You need much less fuel that way. The wings get you the first 30k mètres.
> The extra energy needed to make an object travel fast enough to stay in orbit is more than 30 times as much as the energy needed to lift it to an altitude of 100 km.
SSTO is driven by the notion that staging is dangerous and worth avoiding. I think the SpaceX experience shows that effort was better spent making staging reliable rather than trying to make SSTO work.
And TSTO has some very practical advantages. The mass ratios of the stages are much less constrained, the payload mass is less sensitive to overrunning the mass budget, and most of the mass of the launcher is recovered at much lower speed, making handling entry much easier. The first stage does have to be returned to the launch site but that's not a terribly hard problem.
Are those advantages worth inherently worse amortization costs on the first stage though? A TSTO can have one first stage for ~10ish orbiters since it's only used for 10 minutes while the orbiter has to do all it's supposed to do before coming back into the soup. An SSTO definitionally has to have all parts of the vehicle involved at all stages of operation.
I think a more complicated ground facility is fine if you're doing enough flights.
> Are those advantages worth inherently worse amortization costs on the first stage
If you can launch from and land at any airfield, even if just any military airfield, absolutely. You've opened the market for point-to-point ballistic transport.
The difference is that when a C-17 comes up on radar, there is not a predefined protocol in RU or CN to prepare all of their ICBMs for a rapid response.
What I was getting at, is do all ICBM early warning systems sit on edge more often? That would be my assumption.
We have come close to annihilation from mistakes before, this seems like a path towards more possible mistakes.
Then I guess the answer might include ADB-S and other information-sharing channels. Perhaps only accepting incoming ballistic transports from predefined friendly launch sites. Perhaps only accepting manned ballistic transports.
Ballistic vehicles do not have much cross-range capability, so knowing both where they came from and where they are going is easier than with aircraft. But of course real-time human-seeming communication could be faked with good AI, and even if that were not an option I'm sure some kamakazi pilots would volunteer if necessary. Anything could be faked, but the barriers are not unsubstantial.
I think you are confusing the speed at which the earth orbits the sun (~29.8km/s) with the speed at which an object needs to travel to maintain earth orbit (~7.8km/s).
Whoops, you are right. I think it's too late to edit my original post.
The point still stands, though, you have to get to nearly 8km/s otherwise you aren't in orbit and you fall back into the atmosphere.
You can't get to anywhere near that speed while still in the atmosphere - SR-71s only manage about 1km/s, and because kinetic energy is proportional to the square of the speed, at that point you are only 1/64th of the way there.
To get to an appropriate speed, you need to accelerate eventually. But it's easier to do that at a higher altitude, where you have thinner air to travel through: less drag, less severe transonic effects to mitigate and, on the way up, a wider variety of air-breathing engine types to choose from (including the Astro Mechanica one).
The wider variety of air-breathing engines points out a problem: the launcher necessarily goes through a wide variety of aerodynamic regimes where different engines work. Making a single engine that works over a wide range of speeds is difficult. And for what? Using more fuel in a larger first stage just so you can save on cheap oxidizer (LOX at $.10/lb)?
The mention of SSTO is my annual reminder to check on the Skylon project, using the SABRE air-breathing rocket engines.
A hydrogen-fueled engine which can transition between fully air-breathing (for runway takeoff) to pure rocket mode at high altitude. Has a pre-cooler in front of the compressor to improve operation at high Mach number (otherwise the air coming the inlet is too hot when going fast).
Skylon was basically knocked out by Falcon 9, and even more so by Starship.
Skylon doesn't even save on propellant cost. It uses more expensive liquid hydrogen in order to save very cheap liquid oxygen.
The only reason to continue to invest in this is if one imagines there's some cruise mission application for the technology, say hypersonic cruise missiles.
I actually asked why jet engines were not used in place of the SRBs on the Space Shuttle on space.SE a few years ago, there were two really good answers:
This; plus you need to duplicate structures to make the launch vehicle work in both air-breathing and rocket mode. This whole approach (incl. SpaceShipOne and Pegasus) was trying to disrupt non-reusable rocket launch. Re-usable rocket launch is a much better/greater disruptor than some sort of hybrid vehicle.
Possible exception of course are hypersonic military vehicles (spyplanes or missiles). So there's that.
Air breathing is a much better match for cruise missions (that sustain a speed for long periods of time) than it is for acceleration missions (that bring a vehicle to a high speed over a shorter period of time).
Worse, as the vehicle ascends the atmosphere gets thinner and thinner, and the benefit of not having to carry O2 for that stage vanishes, so the amount of O2 that you save not carrying isn't worth the weight cost of the engines.
Everyday Astronaut has a recent video on "why don't we launch rockets from jets"[0]. The video obviously does not include reference to the engine mentioned here -- but I think most of the points still stand.
In particular
- orbit is mainly about going fast, not so much about getting up high
- your second stage will pay a structural price for allowing air-launch loads, and this structure needs to be carried to orbit
- rockets are *big*
I think what’s missing from Everyday Astronauts discussion is: what if we could make the plane/rocket go much faster.
The cruise speed of a jet plane of 800-1000km/h may not be a high enough fraction of orbital velocity to be worth it.
But what if you could fly even higher and faster, say 3000km/h, before you do stage separation?
Now you’ve actually achieved a significant fraction of orbital velocity.
Obviously that would require new technology. And in particular the main challenge is making an engine that can operate efficiently at a wide range of speeds. So this engine could change the equation.
But I suspect it’d only make sense for smaller vehicles delivering personnel/cargo to arbitrary orbits on short notice.
I also wonder how they can get enough electrical power without a big weight penalty.
Keep in mind that Electron is already using electric motors for their pumps. So the question is kind of whether you could also use a motor to capture and compress air to exploit the oxygen in the air rather than having to carry it as payload.
Minimal orbital speed is somewhere around 25,000-28,000 kph. The fastest air breathing plane every built (SR-71) had a top speed of about 3,500 kph. So for the added weight and complexity of adding an air breathing first stage (wings, air breathing engines, landing gear etc) you are only getting about 14% of the way to the minimal orbital velocity needed, and that would be if your air breathing first stage could match the performance of the fastest plane ever built, keep in mind the SR-71 carried almost no payload. It is far more likely that any realistic air breathing first stage is going to get to less than 10% of orbital velocity, and that would likely be for launching a fairly small rocket. On the other hand the Falcon 9 first stage usually doesn't separate until 6,000-9,000 kph which is closer to 25-35% of orbital velocity, and that's while carrying almost 23,000 kg of payload plus the fully fueled second stage.
After watching the video I'm left wondering how k2pilot would respond to the analysis. Is the idea that you could use these jet engines as part of a vertical takeoff?
An interview (1) with the founder Ian Brooke was illuminating on this jet and the wider opportunity it presents. He sheds a lot of light on the problem space (no pun intended) and how it can be tackled whilst being commercially viable. I particularly enjoyed geeking out the pros and cons of turbo-fan, RAM and SCRAM jets, rockets and the limits of physics.
He's oozing Elon vibes in ambition, first-principle thinking, deep domain knowledge and commercial intelligence. One to watch.
The problem with turbofans (the most efficient jet engine at high subsonic speeds) is the fan, compressor and turbine have different optimal speeds. (The fan wants to spin slow to promote a high bypass ratio without tearing the blades apart while the compressor and turbine want to run at full power.)
The conventional solution is additional compressor and turbine stages. The novel one is the geared turbofan. Both, to my knowledge, are tuned for a specific airspeed and altitude. What these guys seem to be getting at is driving the compressor separately. That doesn't decouple the turbine from the fan, but if they're racing to Mach 3 and then dumping off, they don't need a fan. Altogether, there is an efficiency threshold past which a turbojet first-stage (probably rocket-supplemented) makes sense.
Where I'm sceptical is in choosing launch as the beachhead. If you have a better turbojet--particularly one pitching efficiency over thrust--you should be building drones. Probably missiles. You'll get more build opportunities at a smaller scale, lengthening your runway and speeding up your learning curve. You have more customers and a cleaner path to export. You get to segregate the subsonic and supersonic markets in engineering time and capital deployment. The only reason to go for space first are passion over practicality, a need for vaporware-insensitive investors or an additional design advantage not yet disclosed.
Insightful comment. Do you have any book or youtube recommendations for casual readers like me - A programmer who is fascinated by engines and gets intrusive thoughts like "why ever happened to jetcars? " ( that nagging question again popped up when i read this headline - "Wait, Efficient at any speed"? "wasn't that one of the problems chrysler faced in the 60s ? could this work for roadcars?" )
For a low-math text on aeronautical engineering at its core, the FAA's Pilot's Handbook of Aeronautical Knowledge [1][2] is hard to beat. It won't talk much about jet engines, though.
Real Engineering [3] and Mustard [4] tend to do a decent job surveying specific plane designs. And the Wikipedia pages are decently peppered with references.
Otherwise, getting into the weeds on engine design [5] and launch mechanics [6].
The canonical book for this is "Rolls-Royce - The Jet Engine". It's an old book, but the principles are all there. And hey, someone has put a PDF up on the internet of it, findable by Google. http://www.valentiniweb.com/piermo/meccanica/mat/Rolls%20Roy...
It would be interesting to see a cross section and what kind of variable geometry they are using to max out in the different flight regimes. I poked around and didn't see that, and I wonder if they could even publish that at this stage.
I make no comment on the quality of the tech–we don't know much about it. Only that based on how they describe it, the product-market fit seems forced for orbital launch.
Expect to see every kind of innovation from here on out, as the launch ecosystem fills in every 'ecological niche'.
Sure this isn't a direct competitor to {whatever pundits argue}. But if it works for even one kind of mission, then in this modern space age, it has a place.
Like road vehicles, there are sedans, commuters, offroad, and heavy haulers, heck even trains. One vehicle will never do it all.
Not sure if I understood correctly, but does this mean that instead of going up vertically, with this engine, the "rocket" should fly near horizontally and stay in the atmosphere at the right altitude until it reached the highest possible speed given the air resistance, and then lift up by the 2nd stage rocket engines?
Rockets already mostly do this - they start pitching over at a fairly low altitude (10-30km), or sometimes immediately on launch, and thrust near horizontally. But yeah, theoretically an air-breathing rocket would fly lower for longer, or for some designs even dive down for part of the trajectory.
Rockets take off vertically, then pitch over not for aerodynamics but because reaching orbital velocity requires going sideways VERY fast. They don't pitch over at very low altitudes (with rare exceptions) because the air resistance from high-speed movement is simply too great.
Among the exceptions was the Nike Hercules missile interceptor. As its target was ballistic missiles on a hypersonic ballistic trajectory, the Nike Sprint had to go very fast, in the lower atmosphere, going from 0 to Mach 10 in 15 seconds, sustaining 100 Gs and reaching a skin temperature of over 6,000°F, glowing white, within seconds of launch:
Sounding rockets, used in early rocketry and atmospheric / astronomic research would in fact launch near vertically. Their goal wasn't to go orbital, but merely to get above (most) of the Earth's atmosphere.
Early US sounding rockets were the WAC Corporal (max altitude ~235,000 ft / 72 km) and Aerobee (260 mi / 418 km), each with about 60 kg payload capacity. Neither was an orbit-capable launcher.
I was wondering how a missile interceptor was even possible in the 50s given the insane precision required to hit a bullet with a bullet at those speeds, but it actually uses a nuclear warhead.
Spartan used a 500 kT thermonuclear warhead, I believe. It was a special warhead designed to minimize immediate beta emission in the debris cloud (which would create ionization that would interfere with radar). To that end, the tamper in the H bomb's secondary was not uranium, but solid gold.
> Rockets take off vertically, then pitch over not for aerodynamics but because reaching orbital velocity requires going sideways VERY fast. They don't pitch over at very low altitudes (with rare exceptions) because the air resistance from high-speed movement is simply too great.
I know. I don't know why you think that contradicts what I said.
Because the comment you'd responded to asked "does this mean that instead of going up vertically, with this engine, the 'rocket' should fly near horizontally and stay in the atmosphere at the right altitude?"
Your response suggested that rockets do this (though your altitude comment negates some of that). They in fact don't, and get above most of the atmosphere before their horizontal-to-the-ground vector becomes significant. A key clue is that fairing separation (shedding excess weight, but constrained by the aerodynamic advantages and protections of the fairing itself) tends to occur before major pitch-over.
Note that pitch-over is not the same as the azimuth "roll program" which most launches execute immediately after clearing the launch tower itself, which is for purposes of aligning navigation, in part for the later pitch-over maneuver. Roll is not pitch-over. Everyday Astronaut has a good explainer (~22m long):
The problem with air-breathing engines is that they work best where the atmosphere, and aerodynamic effects, are still relatively thick, as compared to the elevations at which pitch-over occurs. Commercial flights and even very-high-altitude surveillance craft (U-2, SR-71) still operate where aerodynamics and high-speed skin heating (a factor for they hypersonic SR-71, but not the subsonic U-2). Max altitude for the SR-71 was about 25 km (82,000 ft).
> fairing separation (shedding excess weight, but constrained by the aerodynamic advantages and protections of the fairing itself) tends to occur before major pitch-over.
Not for the launches I've watched, e.g. SpaceX pitches through 45 degrees at ~61km of altitude, whereas fairing separation doesn't happen until 82km altitude (by which time it's of course pitched down significantly further). Is that unusual?
> That's getting to be close to what's useful for space launch, but whilst the altitude is useful, the velocity remains low relative to orbital velocities.
True, but also potentially positive; if (big if) you can figure out the other issues, then the faster you go the higher you can continue to take in enough air to be useful.
FWIW, I tried to find an altitude-velocity diagram of a typical rocket launch without luck. Lots of diagrams, none with specific altitude & velocity components.
61 km altitude is FL200, a/k/a 200,000 feet altitude. That's above the operating altitude of any air-breathing so far as I'm aware.
As I'd noted earlier, the SR-71 (in regular operation) was limited to FL85, and the all-time altitude record was FL123, still 77,000 feet below your SpaceX Falcon pitch-over. The SR-71 saw significant thermal heating given its speed. The only aircraft that have gone higher are the rocket-powered X-15, with an all-time record of 347,400 ft (105,900m) in 1963, and Spaceship One, at 367,490 ft. (112,010 m), in 2004. Both the latter were themselves air-launched, though largely to gain initial altitude given the power and speed achieved under rocket power.
I'm unable to read the Twitter thread itself, so if there's any specific technical capability mentioned, I'm missing it. I'd be very surprised if the designs would exceed FL100, let alone FL200.
If it can truly "act as the first stage of a rocket," that's impressive--the most powerful jet engine ever built. If, instead, it will power a plane that will carry a rocket, that's a well-trodden path whose engineering problem is not particularly related to the carrier's efficiency.
I just saw an interview with more detail and it sounded to me like they were going to have the turbine drive a generator powering an electric motor to drive the fan. Like a diesel electric locomotive. And this is supposed to allow it to transition from turbofan to turbojet to reach mach 2.7 to allow starting up a ramjet which launches the second stage to mach 5 before lighting its rocket engine. Not sure why none of that detail was mentioned here.
The ramjet is on the second stage, separate from the turbofan/jet on the first stage.
The loss in the electrical system should be pretty small, and they claim that they gain a lot of efficiency from being able to optimize the engine over the whole range from 0 to mach 2.7.
"The key insight is to use electric motors to drive a compressor"
Uhhhh what? This just does not seem like a good approach, admittedly most of my aerospace knowledge comes from KSP and Scott Manley videos, but the atmosphere thins out pretty damn quick and if they're saying that they can get a benefit on a first stage by getting their oxidizer from atmo in exchange for a bunch of hardware and batteries color me extremely skeptical. This is a field that has had it's problems attacked by a lot of very smart people and the even if this made sense (to my amateur eye it doesn't) the devil is in the details.
Conventional jet engines run at RPMs in excess of 20000rpm. To get this same speeds for the same mass flown rates of say a J79 (F4-Phantom) you'd be looking at some massive electric motors. Conventional jet engines would be using electric motors if they were light enough already.
There is a reason Rolls-Royce UltraFan uses a speed reducing gearbox for it's fan blades and not an electric motor.
More to the point above, at hypersonic speeds, using a conventional compressor is sort or useless unless you can keep your air cool.
Are they really using batteries, or do they have constant electrical generation via a turbine or something? Seems batteries for this thing would add a lot of weight.
The electron rocket used batteries to drive the fuel pumps for a liquid engine that could get into orbit. It used a lot of LiPo batteries, which it jettisoned as they were used up.
If the first stage is a SpaceX-style reusable stage, is there any benefit to a jet engine? I suppose perhaps it is more reusable, AFAIR the SoaceX stages have short-ish lifespans.
We already have the technology for cheap orbital launches. SpaceRyde was developing a balloon that would take payloads to the edge of space then use a small rocket to get it the last little bit and do orbit positioning.
This makes no sense. Getting up in altitude is the cheapest part of getting in orbit: the goal of the rocket is to bring you to orbital speed, which is what's hard. So the small rocket would still need to be almost as big (a bit smaller due to lower air friction at high altitude and the higher potential energy at launch, but only marginally so)
Rocket engines are much more efficient at lower atmospheric pressure.
The bigger the nozzle the faster the exhaust gas. Efficiency is proportional to the square of the speed of the exhaust gas.
The bigger the nozzle the lower the pressure of the exhaust gas. If the nozzle is too big then the atmospheric gas goes into the nozzle. That’s the limit on nozzle size for sea level launches.
Starting high is better because you can have a MUCH more efficient engine.
Why would you use an aerospace for this when conventional turbojets use an articulating nozzle? In fact, they'd be better off trying to develop and sell an articulating nozzle for a conventional rocket than a whole engine such as this
I asked this of the Stratolaunch people several years ago. They said there's an advantage to simply lifting the vehicle that high, it's already starting with more potential energy than it would at sea level. However, it hasn't seem to have made them competitive...
> there's an advantage to simply lifting the vehicle that high, it's already starting with more potential energy than it would at sea level
This implies dropping to presumably start a scramjet? Otherwise, all you're getting is a bit of altitude and softer max Q.
The surprise advantage of air-launched vehicles is you can launch from any airfield. The surprise disadvantage is you need strength two directions; vertically, when it's thrusting, and horizontally, when it's hung underwing.
Rotating Detonation Engine of some kind? I've had a bit of a thrust-crush on that configuration since I heard they were working on the fluid dynamics to make them work. Pulse Detonation Engines are cool and all, but no one wants a kilometer of tailpipe hanging off the ass end of their vehicle.
That fluid problem really is the Achille's heel of RDE, because it requires such a clean airflow in the thing, in order to synchronize the blast waves. The longest burns to date, so far as I know, have been with LOX.
Most "news articles" on the company website over the past year were joint projects from spinoff technologies and corporate rewards, with the most exciting things being them raising 40 million last year.
In the meantime, I've noticed that another company "Hermeus" is building on similar concept engines to be used on their hypersonic planes, though currently they seem to be targeting modified versions of existing jet engines (changing the compressor while keeping combustion chamber intact), not a whole new model of jet engine.
The sound from these things can literally tear other nearby assemblies apart. Aside from the "clean flow" problem, this is another gigantic engineering hurdle to scaling these things up to a larger size. There's such a thing as too powerful.
Some of the instrumentation wire harnesses were wrapped in foil, which got ripped off by air currents driven by the exhaust plume. Foil wraps shield against heating by IR radiation from the plume. It's cheap but effective.
Exactly, so why are you posting a link to scramjets, in relation to supersonic combustion?
I don't think a turbine engine can handle hypersonic speeds, and so this will only work in the first phase of flight, as a booster. Or if it's intended for SSO, the intake will have to close at higher speeds, or the very least the turbine stopped, and the engine switch to rocket mode, with oxidizer pumped in somehow. And then the 'back of the turbine' will likely have thermal issues.
I don’t usually post a middlebrow dismissal, but using an electric compressor is a fairly obvious idea which often gets discussed and immediately dismissed, so there’s not much interesting here until they reveal more of their design.
I didn’t see much discussion on potential efficiency gains in that thread.
A launcher would aim for higher altitudes than an airplane, so the question of maintaining efficiency the whole way becomes much more important.
I suspect this kind of engine would need something like a single-use aluminium air battery to make sense though. You could get really high energy density that way. Power density might be a challenge though.
Scott Manley in the X thread said "So, is the turbine section driving a generator to make electricity to power the compressor" and Ian Brooke replied "Exactly that. ... The key insight is to use electric motors to drive a compressor. That way we can make it spin at any speed, allowing it to "adapt" to it's airspeed and combustion cycle."
Until the new engine rules when it will be removed because it was deemed to expensive, complex and the technology didn't translate to road cars enough for them to justify the R&D.
Though borderline possible, it is very hard for solid fuel rockets to make it to orbit, and they have horrible payload to total mass ratios. It all has to do with the specific impulse of the fuels, no solid fuel provides as much as hydrogen/oxygen.
It's not so much just the Isp -- rockets can easily get to orbit on LOX/RP-1; LOX-LH2 is not needed -- but also dry mass of the stage. Basically the entire solid rocket motor is a thrust chamber, and must be strong enough to withstand that pressure. In a pump fed liquid propellant rocket, this is not the case: the tanks need only withstand a pressure high enough to avoid cavitation in the pumps.
