Quote:
Originally Posted by darcane
Nonsense?
Jet engines are rated in Thrust (which is really just a Force) rather than Power. With a constant thrust, power increases proportionally to speed. If you eliminate aerodynamic drag by being in space, then Acceleration is constant if Thrust is constant.
This is why he specified a rocket engine (which is a type of jet engine).
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Hi darcane,
Jets, and of course rocket engines are specified at least 2 levels of thrust. Sea level at 0 speed; and maximum. If the engine is for the military one or more contract requirements will also be published. The maximum thrust occurs at exactly one altitude and speed. Above or below either and the thrust will be less. Sometimes there will also be a chart of thrust vs speed at constant fuel burn (that's at the maximum power level) and constant altitude. That chart generally shows a slight increase in thrust below some point, because the engine might make more power as it runs leaner. Then the thrust declines just faster than the speed increases. True for most rockets that specify thrust also - it is the maximum available, often the altitude and speed are also given.
For rocketry the
specific impulse is often specified. In the wikipedia article you will find this interesting statement:
Quote:
For rockets and rocket-like engines such as ion-drives a higher I_{sp} implies lower energy efficiency: the power needed to run the engine is simply:
dm/dt * v_e^2/2/
where ve is the actual jet velocity.
whereas from momentum considerations the thrust generated is:
dm/dt * v_e
Dividing the power by the thrust to obtain the specific power requirements we get:
v_e/2
Hence the power needed is proportional to the exhaust velocity, with higher velocities needing higher power for the same thrust, causing less energy efficiency per unit thrust.
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-mort