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Towards a combined-cycle SSTO.
On Jan 26, 5:18*am, Robert Clark wrote:
Looking at the numbers I'm now convinced you can make a single stage to orbit vehicle with a combined ramjet/rocket engine, and without having to use scramjets. AFAIK the only advantage to a jet mode is not having to carry oxidizer for that part of the ascent within atmosphere dense enough *at a specific velocity* to obtain sufficient oxygen to take the place of the oxidizer. "Sufficient" tails off with height of course, so you have to tailor velocity profile to density (modulo oxygen concentration per altitude). Rather than favoring a particular design a priori, it would seem that the first step is determining the maximum altitude at which any kind of air-breather will work *better than a rocket* at that altitude. That's your potential final engine configuration before going to pure rocket. Anybody done that? Then, determine the altitude and velocity domains in which a specific type of jet is most efficient. Then, determine the feasibility of combining the winners so they can transition from one mode to the next *without awkward loss of thrust* during transition. IOW, work backwards from a high-altitude efficiency benchmark to determine what sort of engine you use to launch with. I'll mention recalling reading that at sea level, piston engines are more efficient than any kind of jet. I'd love to see a SSTO with props... Heard of the Pulse Detonation Engine? http://en.wikipedia.org/wiki/Pulse_detonation_engine Still under research but very promising. The idea is to combine the turbo-ramjet/rocket into a single engine. This is what Skylon wants to do with their Sabre engine. But the Sabre will use hypersonic airbreathing propulsion up to Mach 5.5 before the rockets take over. This will require complicated air-cooling methods using heat exchangers with flowing liquid hydrogen for the Skylon. ISTM that as long as the air-cooling tech is passive and doesn't introduce enough drag to offset the no-oxidizer-aboard advantage, and the hydrogen cooling tech isn't as heavy as the oxidizer would have been, then fine. Otherwise, no point. But the above suggested analytical path may indicate it isn't worthwhile to include either turbojet or ramjet mode. Mark L. Fergerson |
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Towards a combined-cycle SSTO.
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