Liquid-fulled rocket engine efficency.

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Discussion Overview

The discussion revolves around the efficiency of liquid-fueled rocket engines, particularly focusing on how altitude affects thrust and engine performance. Participants explore concepts related to thrust generation, aerodynamic drag, and nozzle design in varying atmospheric conditions.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants suggest that the increase in thrust efficiency at higher altitudes is related to the decreasing atmospheric pressure, which may reduce the need for ullage space in the tanks.
  • Others argue that aerodynamic drag decreases with altitude, which could influence the overall efficiency of the rocket.
  • One participant notes that rocket nozzles have specific pressure ranges for optimal performance, indicating that bell-shaped nozzles are more effective at lower altitudes while conical nozzles perform better in thinner air.
  • Another point raised is that pressure thrust contributes to increased thrust as altitude rises, with both thrust and specific impulse (Isp) improving due to lower atmospheric pressure.
  • A participant illustrates this concept by comparing a pressurized container opened on the ground versus in space, suggesting that the latter would generate thrust due to the absence of atmospheric pressure.

Areas of Agreement / Disagreement

Participants express various viewpoints on the factors influencing rocket engine efficiency at altitude, with no consensus reached on the primary reasons for increased thrust efficiency. Multiple competing explanations remain under discussion.

Contextual Notes

Some claims depend on specific definitions of engine efficiency and may not account for all variables affecting rocket performance at different altitudes. The discussion does not resolve the complexities of nozzle design and its impact on thrust generation.

flatmaster
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"Acceleration increased during S-IC flight for two reasons: decreasing propellant mass; and increasing thrust as F-1 engine efficiency improved in the thinner air at altitude."

This is from Wikipedia, so who knows. Why would a thiner atmosphere increase the thrust efficency of the engine? Something to do with the ullage space in the tanks not needing to be filled with atmosphere?
 
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I don't know whether or not this would be considered 'engine efficiency', but obviously aerodynamic drag decreases with altitude.
Also, rocket nozzles have specific air-pressure ranges at which they work best. Bell-shapes work better lower, while conical ones are better in thinner air.
 
It's mostly due to the pressure thrust. As altitude increases and atmospheric pressure decreases, there is a contribution to the thrust due to static pressure at the exit of the nozzle. Both thrust and Isp increase with altitude.
 
FredGarvin said:
It's mostly due to the pressure thrust. As altitude increases and atmospheric pressure decreases, there is a contribution to the thrust due to static pressure at the exit of the nozzle. Both thrust and Isp increase with altitude.

Ah, this makes sense. You can imagine a 1atm pressurized container. Opening this on the ground does nothing. Opening this in space would create thrust. Thanks!
 

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