Skylon. SKYLON.

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 enginerocket 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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15 thoughts on “Skylon. SKYLON.

  1. Smurf says:

    Interesting. I was a little confused about how it would work but you’ve explained it well.

    I also had a cheap knockoff of that transformer.

  2. D. says:

    You should try Kerbal Space Program :)

  3. tangent says:

    You say several times that you can’t see how Skylon can work because NASA has been developing the comparatively-simple scramjet concept for decades and has only just got it working. That is a mistake, you cannot compare Skylon with a scramjet. Skylon developers are not trying to solve any problems relating to a scramjet. They are building an air-breathing jet engine that transforms into a rocket engine.

    Britain knows how to build a jet engine, we’ve been building them for decades. The Skylon developers also know how to build a rocket engine. Ariane 5 is powered by a rocket engine based on one that was pioneered in the UK. The secret that enables Skylon to reach mach 5, the bit with the magic pixie dust, is the pre-cooler. And yet you say that Skylon is nowhere near even finishing the proof-of-concept stage. That is not correct, a full-scale pre-cooler has been built and tested and it works very well. There are now no significant design barriers to Skylon’s completion.

    You may continue to be sceptical but please don’t compare Skylon’s design issues with scramjet, nor anything to do with a scramjet. It is a red herring.

    • hentzau says:

      I mention scramjets because I like these posts to have some sort of educational quality to them and it’s useful to compare the different types of engine and what they can and can’t do.

      And the pre-cooler, man. I do not claim to have any detailed knowledge of Skylon, but I do know about laboratories and the law of thermodynamics. The internet tells me they’ve built a working pre-cooler *in a laboratory*. Perhaps the internet is out of date, and the project has moved past that point, but if there is one thing I learned during my research it is that things tend to work *very* differently in laboratories than they do in their intended real-world environment. Sorry to bring up the scramjets again, but that X-43A NASA built? That is, to me, a practical proof of concept: doesn’t do anything directly useful but demonstrates that scramjet technology can work as designed in the environment that it’s designed for. It may be possible to use the pre-cooler to remove that much heat from the air in a laboratory, but as far as I’m concerned that’s not the same thing as a practical demonstration that Skylon can work as designed.

      (As a cruder example, we’ve known about the principles governing railguns and even had working laboratory examples to investigate hypervelocity impact physics for nearly half a century, yet I’m not seeing the latest and greatest naval vessels mounting railgun tech any time soon.)

      So yes, I’ll continue to be skeptical. If in twenty years time we’re all flying into space on board Skylon spacecraft you can, if you remember this conversation, come back here and say “I told you so!” Until then — or at least, until I see a working example of the Sabre engine flying — I’m going to maintain my position that Skylon cannot work.

    • jezty says:

      …just to complete your point about the Rolls Royce rockets, there is someone pretty important at Reaction Engines with regard to those original “Ariane rockets”.

      This is from wikipedia about Alan Bond, the founder, Managing Director and Chief Technical Analyst at Reaction Engines:

      “He worked on liquid rocket engines, principally the RZ2 (liquid oxygen / kerosene) and the RZ20 (liquid oxygen / liquid hydrogen) at Rolls Royce under the tutelage of Val Cleaver, and he was also involved with flight trials of the Blue Streak at Woomera.

      He then worked for about 20 years at UK Atomic Energy Authority’s Culham Laboratory on nuclear fusion, on the JET and RFX nuclear research projects. He was engaged in studies for the application of fusion to interplanetary space travel. He is the leading author of the report on the Project Daedalus interstellar, fusion powered starship, published by the British Interplanetary Society.

      In the 1980s, he was one of the creators of the HOTOL spaceplane project, along with Dr. Bob Parkinson of British Aerospace. Alan Bond brought a Liquid Air Cooled Engine design he had invented to the HOTOL project, and this became the Rolls Royce RB545 rocket engine.

      In 1989, he formed Reaction Engines Ltd[1] with…..”

  4. jezty says:

    Erm you rightly stated that Skylon has a bit of a ramjet. I like the way you burst out laughing and put it on record before you have any clue how this design works. Professional.

    Most of the “thrust” of your criticism is that no one has demonstrated scramjets reliably yet. Nobody has demonstrated time travel yet, but since it has bloody nothing to do with Skylon it is completely irrelevant and makes your argument read like that of a 14 year old.

    It doesn’t have a scramjet. You said so yourself. You got somewhere by saying it is a kind of rocket with the atmospheric air cooled to be used. It is in fact a combined cycle turbojet/rocket (a well known and in fact very old design) that doesn’t know it has superheated air coming toward it as the air is cooled by over 1000 degrees at peak speeds. The tricky bit is cooling the air. If you can do that the rest is accepted as very much possible. So that’s the turbojet bit.

    At the best limit of materials and cooling ability around mach 5, the air inlet is closed, it switches to pure rocket mode and it burns on-board oxygen. ESA have said that if the cooling works, there is no reason why this shouldn’t work. Reaction Engines are about to complete an ESA funded study and announce whether the cooling works or not in the next few weeks.

    Oh yes. They did add some ramjets too, but they aren’t a fundamental component. They are just there to get a bit more thrust out of the exhaust gasses rather than just waste them.

    • hentzau says:

      I get the impression that you and the above commenter may be involved in the Skylon research somehow. If that’s the case, and/or if I’ve offended you by being so dismissive, then I apologise. I use a heavily acerbic and caustic tone in my posts for comic effect, which has now backfired more than once when somebody responsible for something I wrote about came and complained in the comments. That’s fine; this blog is a personal project and so I’m not particularly bothered about maintaining a professional tone, but I still don’t like knowing I’ve upset people.

      See my above reply for a more reasoned critique of why I think Skylon has some issues; we agree that cooling the air is the tricky part and when I see the precooler fly as part of the Sabre engine I’ll accept Skylon can work. Till then, I don’t believe it can be done outside of a laboratory. It’s difficult to predict technological progress, and there’s nothing wrong with the Skylon *concept*, physically, it just requires you to do something that I think is somewhat beyond modern technology. In fifty or a hundred years, maybe, but not now.

      • jezty says:

        I’m not involved in the research, but I do follow it. I feel a bit harsh now having read your apology… Your post seemed so far from related to Skylon, yet a total citicism of the concept, I felt compelled to comment though.

        Well. We’ll know in the next couple of weeks. As things stand, if they have proven the cooler during these tests, “someone” has 250 million for them to build and test the engine which I believe is a 3 year project to completion (3 years of the 10 year project for the whole vehicle), so a complete answer to your scepticism is only 3 years away, not 50 – or 100.

    • Smurf says:

      “ESA have said that if the cooling works, there is no reason why this shouldn’t work”

      That’s a pretty big ‘if’ though. You and Hentzau pretty much agree on this point, don’t you? I’m not sure what your issue is.

      • jezty says:

        Do you selectively read? As a contrast with the tone of his post:

        “Reaction Engines are about to complete an ESA funded study and announce whether the cooling works or not in the next few weeks.”

        Reaction Engines is made up of former British Aerospace and Rolls Royce experts in their field. ESA are not funding this full scaler cooler test because they think Reaction Engines are making a scramjet or anything else that isn’t true.

        Reaction engines was foudned by former British Aerospace and Rolls Royce experts in their fields. I think ESA are thinking it is a smaller “if” than you are attempting to claim – and so am I. The cooler really IS the magic ingredient that makes the difference that they have been working on for 30 years.

      • Smurf says:

        Of course they’re going to think it’s possible, and it may well be. One of the big joys of science and engineering is taking on projects that are insanely difficult and trying to find a solution. I’m not saying it can’t be done. I’m saying it’s going to be very, very difficult to do.

        And no, I don’t selectively read, thanks for asking.

    • aosher says:

      Good grief, is there any subject so obscure that it can’t produce angry internet men?

      • jezty says:

        Angry? I thought I was being very reasonable under the circumstances :)

        btw I don’t think it’s an obscure subject. Soon you probably won’t either.

  5. [...] quite popular in “hard” sci-fi novels. Remember the atmospheric ramjets I talked about in the Skylon post? This is like that, except instead of using the Earth’s atmosphere as reaction mass the Bussard [...]

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