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Comment by nickhalfasleep | Hacker News Reader

They talk over and over about their engine, when there are plenty other hard problems they do not have solutions for. For instance light enough cryogenic fuel tanks don't exist. This was part of the reason the Venture Star / X-33 never met their weight budget.

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Came here to say the same thing. And note that you're "burning" 400MW of energy in the heat exchanger (transferring it to the helium) at some point you have to re-cool that helium back to liquid) if you outgas the helium you are a net loss because helium has a limited supply. Doing it with hydrogen doesn't get nearly the thermal mass. So it is a really really cool engine, but it remains to be seen how practical it can be.

The second thing, consider the experience from the SR-71. To stay light and still be able to withstand the frictional heating of the air at mach 5 requires some pretty exotic materials. And matching thermal properties for materials at mach 5 vs sub mach. Covering the whole thing in thermal tiles adds weight which comes out of the payload.

To me, adding on the possibility that it could be some sort of Mach 5 super airliner, suggests that the practicalities of it as an SSTO platform are perhaps not nearly so compelling as once thought. Fortunately they were not talking about the multi-gigawatt ground lasers to boil hydrogen. That idea, while nominally physically possible, is impractical to imagine the rest of the world being "ok" with.

http://www.reactionengines.co.uk/sabre_howworks.html

Seems that they are using the heat absorbed from the air, via the helium to eventually heat the hydrogen going into the engine.

http://www.reactionengines.co.uk/images/sabre/sabre_cycle_10...

Any idea why they can't use hydrogen and oxygen directly? Or are they just using a smaller quantity of helium as a 'working refrigerant'?

Liquid Oxygen is (surprise!) a powerful oxidisation agent whilst Hydrogen will cause early failure of the metal parts via hydrogen embrittlement. Liquid helium is inert and much less likely to cause problems I'd guess.

This seeems to use gaseous helium but other than that it seems to be close to the reason. The last thing you want is for any of those heat exchangers to break, as you are effectively mixing the components of rocket fuel.

The main reason though seems to be the temperatures involved. They are exposing hot air to near liquid helium (thanks to being cooled the the liquid hydrogen), thus cooling the air before it reaches the compressor etc that will make it suitable for the rocket engine proper.

At the same time they seem to be heating the hydrogen up. Easier to ignite a gas, or something close to it, than liquid. Just look at how your typical car engine works (never mind a fuel air explosive).

the hypothetical aurora spy plane also hypothetically uses exactly this solution.

I thought the liquid helium is only for an engine testbed. Production Skylon engines will use liquid hydrogen and oxygen, effectively using their criogenic fuel as a heat sink.

No, they use helium still, due to hydrogen embrittlement of the exchanger, but the helium goes to another exchanger with the hydrogen.

The plan is to dump the waste heat into the cryogenic fuel I believe.

To me, adding on the possibility that it could be some sort of Mach 5 super airliner, suggests that the practicalities of it as an SSTO platform are perhaps not nearly so compelling as once thought

Ding!

Yeah, that was my reaction. If SpaceX get the reusable first stage tech running on Falcon 9, then they shave $40M off the cost of each launch (the first stage fuel and oxidant costs $300,000; the stage itself, including nine motors, costs around $40M). Even if each flight costs a couple of mill to refurbish and can only fly 10 times, they'll still have cut the $40M price to around $6-7M.

Meanwhile, here's Skylon, a decade late off the starting blocks. With the Sabre engine running on liquid hydrogen, which is a beast to handle, with valuable He as a consumable.

Frankly, the airliner option looks silly to me. The only real target market for a Mach 5 in-air vehicle is the military.

Not much of a target at that, otherwise they'd still be flying the SR-71, which doesn't need as many... unproven technologies.

'tis a shame that whatever Starlite (http://en.wikipedia.org/wiki/Starlite) is is lost knowledge.

How does this all work in space?

They can't maintain orbit at Mach 5, it's just not fast enough. IIRC you need Mach 25 for LEO.

There are now LH2 composite tasks at a decent tech readiness level

http://www.nasa.gov/press/2014/august/nasa-completes-success...

That is a tank serviceable for a few hours which keeps itself cool on the pad with boil-off. An economic aircraft would have to stand thousands of hours with airframe stresses.

I hadn't heard about the fuel tank problem before. Where can I learn more?

Do you think new materials like graphene will dramatically change spaceflight when they're mass produced?

s/when/if/

Graphene may yet be a pipe dream. If we manage to manufacture it in large and perfectly uniform quantities, it's a wonder-material. If we don't - it could make for slightly better carbon fibre composites.

In short, cryogenic liquids, especially Hydrogen, is incredibly destructive to any container. It's pretty much the smallest molecule around and will wiggle into and make anything brittle over time.

http://www.nasaspaceflight.com/2006/01/x-33venturestar-what-...

Aluminium isn't susceptible.

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