Hypersonic SABRE engine: what's the straight dope?

Factual Questions
1

CNN has an article discussing how BAE Systems is investing 20 million pounds in a company that is working on a hypersonic engine known by the acronym SABRE, basically a rocket engine that breathes air when possible.

link

According to the article, this will “revolutionize transport in the next two decades”. The claimed breakthrough is a precooler which cools 1000 degree C air coming from the intake to -150 C air in 1/100 sec.

What’s the straight dope on this? Will we be cruising to Europe in 30 minutes in the future? Is it like nuclear fusion in that it’s coming Real Soon Now? How would the economics for something like this work? I have heard that the real economic engine of air travel is freight which subsidizes passenger travel and that this wasn’t an option for the Concorde coupled with the fact that the Concorde couldn’t fly overland because of sonic booms. Also, would passengers be in free fall for part of the trip? If so, I am going to hold out for the intercontinental hyperloop train.

Thanks,
Rob

2

It looks genuine, but the article doesn’t really make clear what the advantage is for this extremely complex engine design. That it can go runway to runway and still hit hypersonic speeds, without dual engine systems or such, would appear to be the point.

There are so many other issues with super/hypersonic passenger systems that a reliable and cost-effective engine is only an interesting milestone - if and when.

3

From what I can gather, the real break through in this design is its self cooling element, allowing the jet engine to perform at much higher speeds. I would imagine a lot more military uses coming before any civilian ones.

4

We aren’t going to have a working spaceplane in two decades–it’s just too difficult a problem, with too little funding, and not enough points in its favor.

That said, the SABRE engine isn’t snake oil. It’s just very early in its development.

The SABRE is a hybrid engine. It can act as a normal rocket, burning liquid hydrogen and oxygen. It offers no advantages over normal rocket engines in this regard.

However, when in the atmosphere (and within a certain speed envelope) it can get the oxygen from air instead. It accomplishes this through a sophisticated heat exchanger, which uses the hydrogen fuel to cool the incoming air enough that it can be used as the oxidizer.

In principle, the design solves a longstanding problem that spaceplanes have–they tend to need several different engines, depending on the regime. One might need a turbojet for low-speed atmospheric flight, a ramjet for high-speed atmospheric flight, and a rocket for space flight. The SABRE can operate in all three regimes and so you only need one engine instead of three.

They’ve demonstrated some aspects of the heat exchanger and haven’t yet hit any showstoppers AFAIK, but they’re still a long way off from demoing a full engine, let alone a full spaceplane. So there’s a lot of development yet to go.

It’s not clear to me that spaceplanes are really the right way to go, anyway. They bring a lot of mass and complexity that pure rockets don’t have. They use less fuel than a rocket, but the fuel isn’t the expensive part. The rocket itself is expensive and currently discarded, but SpaceX and others are making progress towards reusable rockets, and at much higher mass efficiency than a spaceplane with all its attendant bits (wings, landing gear, etc.).

I wish them the best but I honestly think it’s a dead end. Reusable rockets will tide us over until we can develop cheap ground-launch systems (electromagnetic accelerators, etc.).

5

Meanwhile, BAE Systems can come up with plenty of very interesting and profitable uses for an engine that can accelerate from a subsonic cruise to supersonic speeds (before coming to a very abrupt stop at its destination…)

6

eta - this was to Dr Strangelove …You’re probably right but I think Concorde and I think VTOL Harrier … if BAE has done due diligence like they said …

7

$31M is pocket change to BAE. I’m sure they see it as a high-risk but potentially high-payoff investment. And like lazybratsche said, it’s possible that they have use-cases for it that aren’t spaceplanes. There are already cruise missiles that use sophisticated propulsion methods that aren’t appropriate for more general use (though a hydrogen-powered missile sounds a bit unwieldy).

8

Apart from the heat exchange - which tbf is impressive - I also read they solved another problem (with ice forming) with cool geometry. I guess they’re working though the issues sequentially (because of limited funding) and BAE will get them to at least a ground system.

9

Not sure about the “much” higher thing. The main advantage would seem to be the turnaround time-don’t have to go grab a crane and a ship or transport aircraft to bring the rocket back to the pad, just gas back up and off you go again.

10

We don’t have an example of a true spaceplane so it’s hard to make comparisons, but consider the Rockwell X-30 (“National Aero-Space Plane”) project. It had a propellant fraction of 0.74. And that’s with very fancy (and still undeveloped) composite fuel tanks, among other things.

Compare to the first stage of the Falcon 9 1.1 (with resuability kit), which has a propellant mass fraction of 0.94. That’s an enormous difference in mass efficiency.

The SABRE engine will have even more obstacles to turnaround time compared to a SpaceX-style reusable rocket. The engines are more complex, implying more complicated inspection. They also run on a more difficult fuel (liquid hydrogen as compared to kerosene or methane [the likely fuel of choice a couple decades from now]). And all the thermal protection stuff, which as we know can result in loss of vehicle with even fairly minor damage.

For whatever difficulties the Falcon 9 has in reusability, they’ll be compounded many-fold in a spaceplane. Gas-n-go is a very long ways off.

11

The major advantage of such an engine is getting much of the launch done using external air - eliminating the need to carry heavier liquid oxygen to burn with the hydrogen for a portion of the flight. As mentioned above, the most optimistic designs before this had a spaceplane that was three-quarters propellant by weight.

There’s also the issue of the “tuning” of the rocket engine, which I haven’t seen discussed for Sabre. The shape of a traditional rocket engine “bell” shape is tuned for a specific speed range (through the atmosphere?), and it is less efficient outside that speed. IIRC the scramjet that NASA periodically tests has an external-burning wedge shape to allow it to be more efficient over a wider range of speeds.

12

I can’t comment on the economics of space planes and hypersonic transports but the engineering is serious and promising. The heat exchanger Reaction Engines are developing is, as I understand it, a real break-through and - after years of absolutely no support - they have enough evidence to convince the UK government to cough up money at a time of deep financial cuts and get BAE to put hard commercial cash in. I am sure Dr. Strangelove has it right, it is a “high-risk but potentially high-payoff investment.”

Interesting the OP asks about the comparison with fusion. Alan Bond, the founder and Chief Engineer of Reaction Engines whose concept it was, worked on nuclear fusion for some years when there was almost no support or funding for SABRE. He continued work on the engine in the background and the Reaction Engines test facility is on the same site as the JET (Joint European Torus) and MAST (Mega Amp Spherical Tokamak) fusion experiments at Culham in Oxfordshire.

13

In the space industry, the history of aiming for few stages and reusable components has not been good. The shuttle being a prominent example. It’s often cheaper to just take simple, dumb, propellant-inefficient designs. Think 18-wheeler rather than F1 even if the F1 is sexier.

As a plane, it’s no good either. Flying is already very fast for our common routes. Look at how little demand there was for the Concorde. It’s silly to work on civilian flight using scramjet or SABRE when civilian flight using turbojet failed as a business model and civilian flight using ramjet never happened. There are much lower hanging fruits for reducing air travel time than exotic tech. Here too, think 18-wheeler or bus rather than F1.
As a component in Kerbal Space Program, it’s pretty fun.
A lower tech version of it, using solid fuel + ambient oxygen then transitioning to solid fuel and oxidizer, may be useful for missiles. Improvements on that may allow delivery of small satellites for cheaper.

14

I agree with the general theme of the replies so far, it is serious engineering but very much a long-term bet with uncertain commercial applications.

But what the hell, it is cool. And the money investied by the government is incidental and even if a “space plane” doesn’t pop out at the other end I’d imagine decent spin-off benefits for both the UK govenrment and BAE.

15

That’s pretty much it. To expand a little, though, it can go from using atmospheric air to an internal oxygen fuel source.

Concorde was doomed by American aircraft companies who got it blocked from flying on the profitable routes.

(I used to work at BAE Systems long ago.)

16

SABRE has been well-reported in the trade press for years now. It appears they’re really onto something.

If they can make the heat exchanger work long-term this will revolutionize missiles. You’ll be able to build high supersonic cruise missiles with multi-thousand km ranges. Right now missiles are either slow and long range e.g. cruise missiles or fast and short range. This engine promises the best of both worlds: fast and far.

If that effort proves successful then eventually the tech *might *be applicable to either space launch vehicles or very high speed aircraft. But that’s very much a generation 2 or 3 or 4 derivation, and assumes all the bumps in that long development road are overcome. Scale, reliability, and longevity are tough beasts to master.
BAE’s recent investment is definitely a big vote of confidence. And as others said above, on BAE’s scale this is still a pocket change investment. They will hit some milestones soon where they have to decide if the promise is bright enough to dump the next couple hundred million into it. We shall see.

17

The problem with concorde was - who wants to spend $7000 for a flight from the USA to Europe and save 3 or 4 hours when you can get to Europe for $500 but it takes those extra 3 or 4 hours? (and it’s recommended that you be there 2 hours or more before departure, and you could wait an hour after for customs and baggage… what are you really saving?) Plus it was a fuel hog that came into service just as fuel prices were skyrocketing. If it could have made the 14-hour USA-Australia flights in 4 or 5 hours - that would probably have been a real attraction.

Not being able to fly trans-continental, and not being able to fly trans-Pacific - the Concorde was a good idea but not quite ready for prime time. Fuel prices and deregulated airline prices did it in.

One of the shuttle’s biggest flaw (one of them!) was hanging off the side of the boosters. If it was on top, nobody would care about falling ice, and possibly an exploding fuel tank would be easier to escape… but then the rocket engines would be on the disposable first stage… etc. etc. Another major flaw, was that it was designed to be a truck, rather than a minivan for passengers. The size IIRC was dictated by cargo bay needed for those giant secret Keyhole satellites that could read license plates from orbit. Better to have a different rocket configuration for heavy lifting. Of course, then there’s details like the need to inspect the whole structure (especially tiles) after each flight, and the complex high maintenance design plus cost constraints that meant as soon as a shuttle landed, many parts were stripped to put into the one waiting to launch.

The major advantage, I assume, is that a less complex rolling-takeoff aircraft-like spacecraft could remove the need for a lot of the current infrastructure - No need to mate to large fuel tanks, no need for giant crawler to take it out to a launch pad with miles of isolation from nearby structures, no need for 100 people monitoring every piece of the flight. Launch from any airport with a decent runway (and cryogenic facilities, I assume). Yeah, that’ll be the day.

18

I’ll go further than that and say that it is probably that we will never develop a working single-stage-to-orbit (SSTO) spaceplane with any conventional chemical propulsion technology, period. The spaceplane concept is a cartoon solution to the real problem of making space access routine which is based on the idea that if you have something that looks like an airliner it will be able to operate like an airliner. The problem with that line of thinking is that it fails to consider that the requirements for an orbital spacecraft are very different than those for an airliner (even a supersonic one) and trying to make a vehicle perform both roles means it will likely perform poorly at either of them; it’s like asking someone to make a fuel efficient main battle tank; you’re going to get something with still mediocre fuel economy but that is a terrible tank on the battlefield.

Part of the problem is the amount of mass that has to be carried on the way up; not just the wing structure but the thermal protection systems that would be required to protect the large amount of frontal heated surface. This is all parasitic mass that in an SSTO takes away from payload capability on a 1:1 basis; every kilogram of wing surface you bring up is a kilogram of payload you leave behind. The counterargument is that this can be offset by the lift generated by the wing during ascent and (for a multi-regime ramjet, oxidizer taken from the atmosphere) but you also have to account for drag due to the additional frontal and wetted surface as well as the fact that you’re only going to get significant lift for a small portion of that ascent after which the weight is just non-lift-generating dead mass. On the way down you have to have some means to protect the structure from thermal loading, either with heat-insulating tiles and blankets, or with some kind of active cooling system that requires a complex flow systems to keep a metallic or ceramic coating cool enough that it doesn’t ablate. The only real advantage is the ability to glide into an approach near the end of ascent. It is the only time that such a vehicle would behave like an airliner. That may seem to be a savings (not having to carry propellants for terminal descent) but any savings is massively overwhelmed but the amount of structure you would need to carry.

The notion that space launch vehicles will be “reuseable” in the sense that aircraft are today can only be made in ignorance of the fact that the environments and loads that space launch vehicles are exposed to are beyond that seen even by high performance aircraft by orders of magnitude. Over the short life of a launch vehicle it experiences the equivalent in terms of vibration that an aircraft may see in years of operation, and to absolute levels that are much higher. Rocket engines operate at temperatures and pressures that are dramatically higher than high bypass turjets. The amount of propellants that flow through a propellant feed system on a rocket in just a few minutes could fly an aircraft around the world dozens of times, and are cold enough to shatter many normal materials practically on contact. I’m dubious that even more modest and plausible attempts at reusability, such as SpaceX’s plans to return their first ascent stage back to a pad and refuel, will end up being fiscally viable or will result in being able to turn around a stage and reuse without substantial refurbishment. The kind of robustness required for this kind of operation with high reliability would demand prohibitively large mechanical margins or the ability to detect incipient flaws and weaknesses with minimal inspection, and adding this additional mass virtually demands a more propellant mass effiicient propulsion system than a chemical combustion powered rocket will every theoretically provide.

NASA did a study on reusabilities in the 'Seventies (and Orbital Sciences redid the study in the late 'Nineties) which concluded that you would need to get 50 to 60 reuses out of a vehicle with only very limited refurbishment in order to be more advantageous than a single use vehicle. That seems counterintuitive because to do this with an airliner would be ridiculous, but the difference is that an airliner can survive hundreds of flights before engines have to be replaced and thousands before the airframe needs repair; the majority of the cost in operating a plane is in the capital cost and fuel. With a rocket, the majority of the cost isn’t structure and propellants; it is the labor to integrate and test it and all of the other associated operational costs. “Throwing away” the rocket after a single flight isn’t nearly the lost investment that it would be an aircraft because the degree of effort to ensure the integrity of a used stage dwarfs the material and fabrication costs.

This isn’t to say reusability in an SSTO can’t ever be accomplished–in fact, there are a number of plausible concepts which would require only modest improvements in materials and propulsion–but the concept of an airliner-as-space-launch-vehicle just isn’t realistic once you start looking at what would be required in terms of performance and robustness, and incremental improvements in the architecture of conventional multi-stage rockets aren’t much better.

As for the SABRE engine, if it can be made workable it may be suited to hypersonic vehicles, possibly even operating in a suborbital mode (although the use of hydrogen as a fuel is enormously problematic for any commercial applications) but even with this innovation of the cooling system there are a large number of problems that would still have to be addressed in a practical vehicle. A rocket or suborbital plane is far more than just its propulsion system.

Stranger

19

Wikipedia (primary sources available at the article):

You might be right about the vibrations and such-I don’t know if using the same engine to achieve orbit (in pure rocket mode) will minimize such wear and tear, or not.

20

What are those concepts and what makes them plausible?