Re: Skylon SSTO

Pat Flannery wrote:
I get a feeling that those Sabre engines are very blue-sky when it gets right down to it, and would be extremely difficult if not impossible to get to work if you ever tried to build them.

I think they're easier than you would expect, but they're a very new type of engine; and they're closed cycle which is never entirely fun to R&D. To put this into perspective the SR-71's engines were thought to be capable of Mach 4.5, but this engine is only aiming for another Mach but the precooler takes out most of the nasties from travelling at high speed- the engine sees almost identical conditions from Mach 0 all the way up to Mach 5.5. In other words, it works the same on the bench as it does in the sky...

Another problem is the vehicle's shape; whereas the long and skinny aspect works fine from a low-drag point of view, it also means the front LH2 tank has a lot of surface area for its capacity.

I don't think that this is a problem, the tanks are vaguely similar dimensions to the Space Shuttle Main Tank, but with thicker insulation.

Anyway in terms of drag the important thing isn't the cross-sectional area, it's the ballistic coefficient, which is largely independent of diameter, but is critically dependent on length. That's why the Skylon tank/vehicle is pretty long (~80m). The minimum diameter is then determined by bending moments to avoid buckling.

Pat
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