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[QUOTE]Originally posted by Guardian 2000: [QB] [QUOTE]Originally posted by Timo: [qb]I think you have got the wrong starting point. Thrust reversers *work*. I've seen thrust being reversed, and a jet taxi caboose first under the power of such reversed thrust. Sure, the process is wasteful, but mainly because of all the turbulence and thermodynamic losses that wouldn't be present if we dealt with ideal gas and laminar flow. This would not work if a thrust reverser merely killed forward thrust.[/qb][/QUOTE]False analogy. First, atmospheres introduce new variables into the situation, since you can get some rebound off of the surrounding air (or even the ground in a Harrier or hovercraft). Second, you're talking about a turbofan gas turbine engine, not a direct reaction engine or a rocket . . . same basic principles, to be sure, but significantly different execution than in, say, a space shuttle main engine. Ninety percent of the thrust of a turbofan engine comes not from the combustion of the fuel and air, but from the massive fan at the front of the engine nacelle which is driven by the gas turbine part of the engine. Thrust reversers on an aircraft are predominately non-propulsive, though (and I am correcting an earlier statement after further research) can provide some effect, thanks in large part to the atmospheric effects. [QUOTE][qb]So there *must* be something wrong with your analysis. What could it be? Let's try trace again what happens to a gas particle in a rocket engine. First, it obtains kinetic energy from an exothermic reaction (chemical, nuclear) or is given the energy by some other means (EM fields, laser or solar heating). It has a nonzero momentum p now, too. Now, the particle moves out through the open nozzle. It didn't deliver momentum to the ship by any visible mechanism. Did it violate conservation of momentum? Not from its own POV. It was its own system. The wider system of ship and particle conserved momentum as well, because the particle didn't gain its p by stealing it from the ship - it got the p from the chemical/other reaction, which wasn't necessarily coupled to the ship. So statistically only half of the particles matter at this stage - namely those that impact the front wall of the rocket burn chamber and thus deliver a momentum to it that has at least some component pointing toward the bow of the ship. When they deliver this momentum, they also lose some of their velocity.[/qb][/QUOTE]And above is the problem with your notion, wherein you presume low efficiency for the engine. Modern chemical rocket engines, such as the shuttle main engines, use a multistage combustion approach, leading to excellent combustion efficiency (99%) and excellent thrust efficiency. The impulse engines of a Galaxy Class starship, according to the TNG:TM, produce thrust by channeling the exhaust products from the spherical fusion reactor in an aftward direction, then through an accelerator, and finally more thrust is obtained via the spacetime driver coils . . . all of this is translated to the spaceframe by whatever technobabble means. We are not given the efficiency of this engine configuration, but I'd expect it to be a helluva lot better than 50%. The remainder of your argument is based on the low engine efficiency presumption and therefore will only hold true in that case. G2k [/QB][/QUOTE]
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