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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.
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 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).
https://en.wikipedia.org/wiki/Skylon_(spacecraft)
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:
https://space.stackexchange.com/questions/6256/why-were-jet-...
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.
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.
Put wings on it and go up sideways. It solves the thrust/weight issue the same way a hill climber does: instead of going directly up they zig zag.
You need the lateral velocity anyway, for orbit.
Depends on your launch profile, of course.
You could try to stay air-breathing as long as possible while building some of your orbital velocity.
I'm not saying there's One True Mission Profile here. Depending on your needs and constraints, it might be a good plan in some cases.
See also: https://xkcd.com/1244/ (sometimes)
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.
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.
1. https://twitter.com/1stPrinciplesFM/status/17629458095729254...
I'm sceptical but optimistic.
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.
Space is better for marketing and gaining traction. I strongly suspect they’re quietly talking to missile manufacturers.
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?" )
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.
Great points. So not only is the tech iffy, but the application is misguided.
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.
Andrew Cote gives the missing details here:
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.
Press X to doubt. I'll believe it when I see it.
"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.
Hope I'm wrong though!
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.
What happened to the Reaction Engines / Skylon / SABRE?
Still being worked on. It is a potentially higher-performing system than this one, but also dramatically more complex and expensive.
This hybrid engine is interesting in how fundamentally simple and easy to build it is.
> The key insight is to use electric motors to drive a compressor
Tech Ingredients made a "hybrid jet engine" based on this concept in 2018: https://www.youtube.com/watch?v=_ax0pI4Jp18&t=775s
Can anyone speculate as to what kind of advancements in precision machining might enable this sort of new jet engine design?
I feel, as a layperson when it comes to aerospace, that so much of the innovation is bounded by advancements (or lack thereof) in machining.
As Nitter is broken and Twitter has ... numerous issues, is there a non-Twitter source for this?
https://nitter.poast.org/k2pilot/status/1763007610993991722
Seems to be the last one online
x is perfectly easy to use or to customize and allows instant sharing of short bits of information in whatever format.
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.
Someone made the positive version of "Unsafe at any speed"
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.
Unfortunately they got shut down by NIMBYs. https://spacenews.com/noise-complaints-help-bring-down-launc...
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)
Getting into orbit is more about horizontal speed than altitude.
Sounds about as practical as an electric supercharger. I would love to be wrong; I guess time will tell.
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
This seems to not have consideration for hypersonic air flow. If this is just a lifting engine it can only provide a small fraction of delta v.
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...
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.
> Rotating Detonation Engine of some kind?
No, if only by the sound.
By the way, what happened to SABRE [1]. Slowly plugging along, or a victim of Brexit?
[1] https://en.wikipedia.org/wiki/SABRE_(rocket_engine)#History
They look otherworldly too.
Recent NASA test: https://m.youtube.com/watch?v=UShD03eG9IU
> no one wants a kilometer of tailpipe hanging off the ass end of their vehicle
speak for yourself
"The key insight is to use electric motors to drive a compressor."
Is this for the air intake? There must be a speed limit to this surely, and then you have to close intake and switch to rocket mode or something?
And then there's the weight of the batteries and motors - I guess you could re-purpose those as oxidizer pump or whatever maybe.
And is that an aerospike? A nice idea, but problematic for thermals.
I mean I assume they have something working, but I'm skeptical of the concept.
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.
Typical discussion: https://aviation.stackexchange.com/questions/90862/are-there...