I like that in scifi, people treat space like the ocean asks

I guess you’re probably getting sick of the “go to this link and tell me why this sucks” questions but…

Go to this link and tell me why this sucks.  Or why it’ll work.  Whatever.

I know you’ve already gone over why reusable spacecraft are terrible but does adding the fact that this is a single stage make it even worse?  Or better?

Yeah, so I was looking at that and thinking “Oh, a two-stage scramjet/rocket hybrid, that’s not a completely terrible idea,” and then I got to the following sentence:

The proposed engine for the vehicle is not a scramjet, but a jet engine running combined cycles of a precooled jet engine, rocket engine and ramjet.^[2]

And literally burst out laughing. This is a little ambitious of them to say the least, especially considering we haven’t even perfected basic scramjet engines yet.

Okay, you’ve got your airbreathing jet engine, which is designed to operate in an atmosphere at speeds of up to Mach 3-4 (Mach X is simply X times the speed of sound in the local medium, which in this case happens to be air at or near sea level pressure). Jets use turbines to suck in air through the front of the engine and then expel it out the back, providing thrust according toNewton’s third law. Jet engines are comparatively efficient in terms of fuel because they use the surrounding medium – air – as their reaction material, obviating the need to carry lots and lots of rocket fuel along with the jet plane.

Then you’ve got your ramjets and your scramjets (supersonic combustion ramjet). The idea here is that instead of using a turbine to physically pull air into the engine, you simply use the very high speed of the craft to ram compressed air into it as a natural consequence of its supersonic motion. This is helpful because it means ramjets are theoretically rather simple devices compared to jet engines, but the tricky part of their design is that in order to efficiently combust the air as it moves through the engine it has to be moving at subsonic velocities, hence the need for the air to be slowed down by a compressor before it can be heated and chucked out the back of the engine.

Scramjets are ramjets, except more so. Here we’re not bothering to slow the air down as it moves through the engine, instead combusting it at supersonic velocities. This is really goddamn hard because the air is going to be spending sod-all time actually inside the scramjet engine, giving you a window of mere milliseconds to burn your fuel and accelerate it efficiently. Even worse, air moving that fast which is compressed that much is actually pretty hot! Building an engine that can deal with air moving through it at those temperatures is one of the core obstacles facing the designers of scramjet aircraft.

So while scramjets and ramjets are conceptually simple designs, it’s only relatively recently that any headway has been made with their practical implementation thanks to these hefty engineering challenges. Further, because of the way they function there’s a couple of restrictions on their use. Ramjets cannot be used to accelerate a stationary or slow-moving craft because they rely on the fast motion of the craft through the atmosphere in order to force air into the engine. And since scramjets require the air to enter the engine at supersonic velocities, they can only function after the craft itself has broken the sound barrier.

The good news is that because the air going into the engine is already moving very fast ramjets and scramjets are pretty nippy. Ramjets are capable of speeds up to Mach 6, while scramjets are thought to be able to do anywhere between Machs 12 and 24 – the fastest scramjet aircraft built so far has managed Mach 10.

Why is this potentially useful for spaceflight purposes? As it happens, Mach 24 is about orbital velocity. I doubt scramjets would be capable of anywhere near that kind of velocity, but it wouldn’t matter too much since you’d still need rocket engines to operate in a vacuum. If you’re sticking rocket engines onto the thing anyway, you may as well use them to kick it up to orbital velocity after the scramjet engine has done most of the hard work for you. A practical implementation of the scramjet concept would be a couple of rocket boosters stuck to a space plane with a jet/scramjet engine. The jet gets the plane off the ground, it ascends to 30,000 feet or so, the scramjet boosts it up to Mach 10-12 and then the rocket boosters kick in and take it into an orbital trajectory. When you want to de-orbit you make a burn with the rocket engines and then dump them on the way down while the plane component lands at an airfield somewhere. By using air-breathing engines that don’t have to carry around their own reaction mass with them for part of the flight you drastically reduce the amount of propellant the spacecraft has to lug up into orbit, which in turn reduces the launch costs.

Skylon, though, is a one-stage to orbit spacecraft design. The way it’s supposed to work is that instead of one stage being a jet/scramjet hybrid aircraft and the second stage being a rocket engine, it will instead have just one engine design that’s supposed to do everything – get the craft off the ground, accelerate it up to supersonic velocities within the atmosphere, and then take it into space. This is an awful lot to ask of a single engine, especially since NASA and company have been developing the comparatively-simple scramjet concept for decades and they’ve only just gotten to the point where it works, and as far as I’m concerned it’s just asking for trouble.

Skylon’s engine is basically a rocket engine that runs off of liquid oxygen. When it’s in the atmosphere, it uses a magic precooler to cool oxygen within the air it’s moving through down to temperatures of -120 to -140 ^oC, which liquefies it and allows it to be used as fuel for the engine. When it’s in vacuum, it switches to its own internal supplies of liquid oxygen. Sounds simple, right?

Except it has one tiny, tiny problem it has to overcome in order for this engine concept to work, and it’s the same one the scramjet designers spent thirty years dealing with: air moving at supersonic velocities is very hot. Very hot. It is also moving very fast. It is very fast and hot air. The Skylon engine, therefore, has to somehow remove about 200-250-odd degrees of heat from this air in the short period of time that it’s actually inside the engine in order to turn it into the liquid oxygen fuel the engine needs to run. I think this is a rather tall order, and am entirely unsurprised to learn that Skylon is nowhere near even finishing the proof-of-concept stage. I don’t think it ever will, because I don’t see how it can physically work using today’s technology.  Scramjets push the boundaries of what modern aerospace engineering is capable of.  In my opinion, the Skylon engine design exceeds it completely.

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