I've always been skeptical of these liquid-air cycles, but this scheme strikes me as especially problematic in that an ejector ramjet would be mechanically simpler and have the same top speed. For the curious, an ejector-ramjet (or ram-rocket or air-augmented rocket or any of a number of other names) is just a ramjet with a small rocket motor behind the center body of the diffuser. The exhaust plume entrains the inlet air, boosting the combustion chamber pressure by cooling the rocket exhaust plume. (It also acts as a fairly, err, robust flameholder.) If you're wed to LH2 as a coolant, just bleed it in along the walls of the rocket combustion chamber, and it will mix with the main mass flow and combust. Et voila! The cooling system is the fuel injection system for the ramjet mode.
Now you just spend all the compressor and heat-exchanger machinery weight that you saved by not going with the Skylon engine by having dedicated thrusters elsewhere on the vehicle for the pure rocket mode.
As someone somewhere else said, Skylon strikes me as a characteristically British solution to the problem, just due to it's sheer over-reliance on plumbing.
>Every space launch for the past 60 years has involved blasting off vertically and jettisoning separate stages once the fuel they carry is exhausted.
This isn't quite correct, right? There has been at least one LEO launch systems which utilized a horizontal take-off. The Pedasus rocket from Orbital:
> The Pegasus rocket is a winged space launch vehicle capable of carrying small, unmanned payloads (443 kilograms (980 lb)) into low Earth orbit. It is air-launched, as part of an expendable launch system developed by Orbital Sciences Corporation (Orbital). The Pegasus is carried aloft below a carrier aircraft and launched at approximately 40,000 ft (12,000 m). It flies as a rocket-powered aircraft before leaving the atmosphere.
The trick is using a conventional jet engine (as opposed to a (sc)ramjet), but cooled with the cryogenic rocket fuel. In effect, this is using the fuel as a heat sink, then expelling the fuel in the jet or rocket exhaust.
It's not a conventional jet engine - it has an intake turbine, but it's powered by the same helium cycle used by the precooler, and not by combustion byproducts.
The compressed air (or liquid oxygen when in rocket mode) and hydrogen is then combusted in a way similar to a conventional rocket engine. REL calls it an "air-breathing rocket engine", but it shares more in common with a ramjet than a conventional turbojet.
Well, this is their secret sauce, which they claim will make the SABRE engine work where other scramjets have failed.
The tests they're currently running are of the pre-coolers, which have a proprietary design to prevent icing. If it works, they will have overcome the biggest hurdle to building a hybrid scramjet/rocket engine.
When going hypersonic, lack of heat is of least concern ;-)
Icing occurs around plane parts where air at ambient temperature is further cooled, for example by pressure drop. As it is the case in throttle body of piston engine.
Here incomming air is highly compressed -- and thus heated up -- due to impacting plane parts at hypersonic velocities.
> Here incomming air is highly compressed -- and thus heated up -- due to impacting plane parts at hypersonic velocities.
The problem is that at high mach numbers, the incoming air is too hot to successfully combust. They have to chill it (using the cryogenic fuel), and chilling it would usually result in icing.
The whole point of these tests is to prove that their precooler system resists icing.
"Beyond Mach 5.5, the air would still be unusably hot despite the cooling, so the air inlet closes and the engine relies solely on on-board liquid oxygen and hydrogen fuel as in a normal rocket." that's straight from the wiki page.
I don't think the Skylon will ever be practical, but it's easy to imagine a multi-reusable-stage launch system combining air-breathing jet engines and wings to take you to 60,000 ft, a second one with Sabre-derived designs and tiny wings to add a couple Mach and getting to the upper stratosphere and a third one that can get to orbit.
What I never understand when reading about Skylon, is why the project try to do single stage to orbit directly? A reusable first stage would in itself be valuable and neither cost so much to develop, nor be risky by walking the edge of weight gain etc. ssto could be version 2.
(I'm not that knowledgeable on the subject, flame me if your day has been bad, just add information. :-) )
Edit: Symmetry, SpaceX is itself trying for reusable stages. As far as I can tell, it should be the sweet spot?
Single Stage to Orbit (or any type of reusable launch system) invariably adds complexity and weight, which in turn eats up any gains by being able to reuse stages. It just isn't worth it to add complexity unless you go whole hog and make the entire thing SSO.
Why are they complaining so much? If it works DARPA, SpaceX, VirginGalactic, ESA and NASA will pay more than you can ever spend. When they didn't get the attention of these guys so far, there is probably a mayor catch.
Edit:
What i mean is this part:
> To succeed, it needs to capture a healthy slice of the market for satellite launches. That's the biggest potential earner. But to do that, it must prove viable.
The story from the garage and share of satellite launches, that doesn't sound like a plan. That sounds like someone who want's to be like NASA, there was nothing in the article about reducing costs and LEO or anything business related.
> He intervenes. "Not 'could'," he says. "It's not 'Skylon could make space travel easier'. It's 'Skylon will make space travel easier'."
Skylon, makes space travel easier. Now on Kickstarter.
Because their financial projections are bunk and are ridiculously understated. Every piece of technology on the vehicle is custom made. Even if you accept all their projections you still have to assume that they develop it in time and that it works as intended. Any weight overruns will literally make the vehicle unusable.
If they have a revolutionary technology and they can really show that these billions will provide the most competitive way of Space travel. There are a dozen companies and well funded government agencies who have the money and could provide most of the infrastructure they need.
Why is nobody interested?
If there are big reservations, why do they start with satellite launches? Why not start by making more powerful engines than Rolls Royce, and sell these to all the candidates who are interested.
I have a concept for a better Operating System, i need a big chunk of OS sales. I need Billions to have something comparable to Microsoft. Btw it's a British operating system, the government should give me the money.
They are starting with engines. The rest of their ideas are simply concepts at this stage. It's a small company.
The problem is that plenty of people have tried and failed to produce a successful air-breathing rocket engine. Nobody wants to invest any money in Reaction Engines until they can prove that the core concept that the company is built on works. And proving this costs a certain amount of cash. Some of this is being financed by the ESA, in very much the same way that NASA is financing new commercial spaceflight efforts.
In the next year or two we're either going to see them succeed and garner lots of investment, or fail and fade into obscurity.
Now you just spend all the compressor and heat-exchanger machinery weight that you saved by not going with the Skylon engine by having dedicated thrusters elsewhere on the vehicle for the pure rocket mode.
As someone somewhere else said, Skylon strikes me as a characteristically British solution to the problem, just due to it's sheer over-reliance on plumbing.