I've been trying to figure this out for days. I'm told that atmospheric pressure imposes a limit on maximum possible exhaust velocity in the earth's atmosphere, and that under STP conditions that limit is approximately 15,000 feet per second. But that doesn't make any sense. Suppose you had an engine running at that exhaust velocity, and you halved the diameter of the throat of the combustion chamber through which the exhaust was flowing. That would reduce the area of the throat to a quarter of its previous size, and in accordance with Bernoulli's principle, the exhaust would have to accelerate to four times its previous velocity by the time it escaped the combustion chamber. At that point your engine would be producing four times its previous amount of thrust. Or, if you throttled back and reduced the fuel flow to a quarter of normal, it would produce the rated amount of thrust but burn for four times as long, dramatically improving its efficiency. Is there any reason, apart from the structural strength of the engine and materials, why this wouldn't work? Is there any reason why redesigning the chamber, throat and nozzle to to actively compress and accelerate the exhaust flow could not be done? If anyone can explain this to me in simple terms without equations, I'd be extremely grateful. Thanks very much.