Aircraft "thrust" vs. power?

"Does that mean a jet engine has unlimited power? Surely there must be a limit, based at least on the maximum rate of fuel delivery, and perhaps before that based on a maximum rpms like a car. But maybe the plane reaches its top speed before its maximum power?"

Aircraft drag is a function of v^power, where power is greater than 1.

"What about rockets in space? Do they also produce constant thrust?"

Yes, and the rocket's net mass is constantly decreasing, since propellant is evacuated.

"Does it have to do with the exhaust gas? The rocket "at rest" would emit exhaust gas at nonzero speed, with thrust equal to that applied to the rocket. Therefore some power is spent there."

The key is, the exhaust products leave at essentially a fixed speed with respect to the spacecraft. At some point, for someone at rest, the exhaust products will still be moving in the same direction as the rocket proper... if slower.

http://en.wikipedia.org/wiki/Spacecraft_propulsion

To look at thrust, you must understand the basic principles behind it. Molecules/atoms are ejected out one of the jet at high speeds (and high temperatures). From this, applying conservation of momentum, the jet would move in the opposite direction.

Jets do not have unlimited power, and are restricted by the airflow into and out of the engine, and some other factors like fuel delivery. It maybe easier to understand that a planes top speed would be defined by the maximum output of the engine, and not the other way around.

As for rockets in space, there is no air drag. Therefore, a rocket already going 8000kph would not need additional input from the thrusters to maintain that velocity. On Earth we are limited by air resistance, so everything that begins to move will slow down unless additional energy is put in. The only reason thrust would be needed in space is to accelerate or decelerate.

For example, a jet in the earth's atmosphere would require the thrusters to be on at all times in order to maintain a constant velocity. That is because air resistance is acting against the thrust. So at constant velocity, you would have F(air drag) = F(thrust)

In space, there is no air drag. Therefore you would only consider the interaction between the ejected air molecules and the rocket. (also remember that rockets must have fuel tanks and propellant tanks [full of gas] to move since air doesn't exist in the vacuum)

I know what you mean and I can't find good references quickly.

But aircraft thrust drops off with speed, and of course they cannot produce unlimited power

Rockets are governed by the rocket equation; the maximum speed eepends on the specific impulse of the fuel

there is a top end limit to the power jet engines can produce before over heating or burning out. a lot of it is up to other things as well- weather, maintenance, fuel quality, pilot experience.

rockets in space- so there is nearly no friction in space, but they use maneuvering thrusters (mainly) once the rocket/payload leave earth's gravity well. they also use entirely different types of fuel- solid state for launch, and nitrogen (usually) for the thrusters. don't forget inertia in space- it will be a huge factor- because the rocket/payload won't (technically) stop moving until docked or landing.

no, jet engines do not have unlimited power.

the absolute limit is equal to the ability to burn fuel.

heat of combustion of the fuel, in J/kg, times fuel flow rate, kg/sec = J/sec = Power.

the actual efficiency and power is affected by many different factors;.