How Does Movement Affect Heat and Thrust in Rocket Engines?

[Stewart Snider]

Consider a rocket engine held stationary that produces 1000 kg of thrust.
Now, allow that engine to move on a track at, say, 100m/s, controlling its velocity with a brake, keeping the fuel burn rate the same. The brake will get hot.

But since the fuel burn rate remains the same, the rocket's heat output remains the same as well, right? All we've done is allowed the thing to move. But with movement, we're generating heat in the brake in addition to the rocket exhaust. It would seem we're getting the heat from the brake for free. This violates the law of conservation of energy, so I know there must be a flaw here.

Can anyone tell me what I'm missing?

 

[Dale]

Can anyone tell me what I'm missing?

The KE of the exhaust

 

[Stewart Snider]

Do you mean the kinetic energy of the exhaust decreases as the engine moves? If so, I don't understand how any part of the engine knows it's moving.

 

[Ibix]

It doesn't need to know that it's moving. The exhaust exits with the same velocity with respect to the rocket but a lower velocity with respect to the ground because the rocket is moving.

 

[Dale]

Do you mean the kinetic energy of the exhaust decreases as the engine moves?

Yes.

If so, I don't understand how any part of the engine knows it's moving.

I don't understand what you are asking here. Can you ask your question without anthropomorphising?

 

[CWatters]

Do you mean the kinetic energy of the exhaust decreases as the engine moves? If so, I don't understand how any part of the engine knows it's moving.

Ask yourself...

Does the rocket engine eject the exhaust gas at a fixed speed relative to the ground or the engine?