Calculating Thrust from Rocket Engine Ejection Velocity

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Discussion Overview

The discussion revolves around calculating the thrust produced by a rocket engine based on the ejection velocity of burned gases and their weight flow rate. The scope includes mathematical reasoning and technical explanation related to rocket propulsion principles.

Discussion Character

  • Technical explanation, Mathematical reasoning, Debate/contested

Main Points Raised

  • One participant proposes using the equation F = M/32.2 * Ejection Velocity to calculate thrust, suggesting a thrust of 527 lbs.
  • Another participant challenges this approach, stating that thrust should not depend on gravitational acceleration, as it is a function of the rocket's environment.
  • A different participant presents the equation Thrust = dm/dt * Vel, indicating agreement with the thrust calculation and clarifying the units of thrust.
  • One participant expresses confusion regarding the interpretation of "the weight of gas ejected is one pound per second," suggesting it could be understood as a mass flow rate of 1 lbm/s, which may affect the calculation.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the correct interpretation of the weight of gas ejected and its implications for the thrust calculation. Multiple competing views remain regarding the appropriate approach to the problem.

Contextual Notes

There are unresolved issues regarding the assumptions made about the units of weight and mass, as well as the dependence of thrust calculations on the gravitational context.

recon223
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The burned gases of a rocket engine are ejected from the rocket nozzle at a speed of 17,000 ft/sec. If the weight of gas ejected is one pound per second, find the thrust developed by the rocket.


I don't really have an equation to use here but my guess is F = M\32.2 * Ejection Velocity


F in lbs. = [ 1 lb / 32.2] * 17,000 = 527 lbs of thrust


Anyone care to verify that this is correctly calculated or have any ideas on a different calcualtion?
 
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Analysis of units does not support your approach. Also 32.2 ft/s2 is the terrestrial acceleration due to gravity. The rocket could be anywhere-- interstellar space or the moon. Thrust is a function of the rocket, not gravity.
 
Last edited:
Thrust=dm/dt*Vel

Your work looks correct to me. 1 lbf=32.4 Lbm*ft/Sec2
The units of thrust is lbf
 
This part got me-- "the weight of gas ejected is one pound per second". I read this as a dm/dt of 1lbf/s. If this is read as 1lbm/s then things to fall into place.
 

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