进来学习一下。
wiki里找到一段关于这两类引擎的区别，不知是否说在了点子上：
The motion impulse of the engine is equal to the fluid mass multiplied by the speed at which the engine emits this mass:
I = m c
where m is the fluid mass per second and c is the exhaust speed. In other words, a vehicle gets the same thrust if it outputs a lot of exhaust very slowly, or a little exhaust very quickly. (In practice parts of the exhaust may be faster than others, but it's the average momentum that matters, and thus the important quantity is called the effective exhaust speed - c here.)
However, when a vehicle moves with certain velocity v, the fluid moves towards it, creating an opposing ram drag at the intake:
m v
Most types of jet engine have an intake, which provides the bulk of the fluid exiting the exhaust. Conventional rocket motors, however, do not have an intake, the oxidizer and fuel both being carried within the vehicle. Therefore, rocket motors do not have ram drag; the gross thrust of the nozzle is the net thrust of the engine. Consequently, the thrust characteristics of a rocket motor are different from that of an air breathing jet engine, and thrust is independent of speed.
The jet engine with an intake is only useful if the velocity of the gas from the engine, c, is greater than the vehicle velocity, v, as the net engine thrust is the same as if the gas were emitted with the velocity c-v. So the thrust is actually equal to
S = m (c-v)
This equation implies that as v approaches c, a greater mass of fluid must go through the engine to continue to accelerate, but all engines have a designed limit on this, and also that the vehicle can't accelerate past its exhaust velocity as it would have zero thrust.
(http://en.wikipedia.org/wiki/Jet_engine)
[ 本帖最后由 deepgreen 于 2008-10-24 01:08 编辑 ] |
显身卡
