How Does Movement Affect Heat and Thrust in Rocket Engines?

  • Context: Undergrad 
  • Thread starter Thread starter Stewart Snider
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Discussion Overview

The discussion revolves around the effects of movement on heat generation and thrust in rocket engines, particularly focusing on a scenario where a rocket engine is stationary versus one that is moving while maintaining the same fuel burn rate. Participants explore the implications of kinetic energy and heat generation in relation to the conservation of energy.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that a stationary rocket engine producing 1000 kg of thrust generates a consistent amount of heat, but questions how movement affects this heat generation when the engine is moving at 100 m/s.
  • Another participant introduces the idea that the kinetic energy of the exhaust may decrease as the engine moves, prompting further inquiry into the relationship between the engine's movement and the exhaust's velocity.
  • A participant clarifies that the exhaust exits at the same velocity relative to the rocket, regardless of the rocket's movement relative to the ground.
  • There is a challenge to the anthropomorphizing of the engine, with a request for clearer questions regarding how the engine perceives its movement.

Areas of Agreement / Disagreement

Participants express differing views on how movement affects the kinetic energy of the exhaust and the implications for heat generation, indicating that the discussion remains unresolved with multiple competing perspectives.

Contextual Notes

There are unresolved questions regarding the relationship between the exhaust velocity and the rocket's movement, as well as the implications for energy conservation. The discussion does not clarify how these factors interact under the conditions described.

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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