How Does Movement Affect Heat and Thrust in Rocket Engines?

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A stationary rocket engine producing 1000 kg of thrust generates heat from fuel combustion, but when it moves at 100 m/s, additional heat is produced in the brake due to friction, raising questions about energy conservation. The heat output from the fuel remains constant, but the kinetic energy of the exhaust changes relative to the ground, not the engine. The discussion highlights that the exhaust velocity is fixed relative to the rocket, leading to a lower velocity relative to the ground as the rocket moves. This distinction clarifies that the engine does not need to "know" it's moving; rather, the exhaust's kinetic energy is affected by the rocket's motion. The conversation ultimately seeks to reconcile the apparent contradiction in heat generation and energy conservation principles.
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?
 
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Stewart Snider said:
Can anyone tell me what I'm missing?
The KE of the exhaust
 
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.
 
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.
 
Stewart Snider said:
Do you mean the kinetic energy of the exhaust decreases as the engine moves?
Yes.

Stewart Snider said:
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?
 
Stewart Snider said:
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?
 

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