Upside Down Rocket: Thrust & Conversion to Newtons

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Discussion Overview

The discussion centers around the measurement of thrust from a model rocket when it is secured upside down to a scale. Participants explore the implications of this setup, including the conversion of thrust measurements to Newtons and the factors that may influence the readings on the scale.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Conceptual clarification

Main Points Raised

  • One participant questions whether the scale would read the thrust of the rocket or if other factors would influence the reading.
  • Another suggests using a free body diagram to analyze the forces acting on the rocket, noting that both thrust and gravity act downwards.
  • A different participant mentions that model rocket engine thrust values are published and can be referenced, and explains how a scale measures force and converts it to mass based on gravitational acceleration.
  • One participant describes a method to measure thrust by resetting the scale to zero with the rocket in place and then lighting the rocket, indicating that the scale would then read the thrust in kilograms, which can be converted to Newtons by multiplying by 9.8.
  • This participant also introduces the concept of impulse related to model rocket motors, providing examples of how different thrust values can be classified under the same motor class based on their impulse ratings.

Areas of Agreement / Disagreement

Participants express various viewpoints regarding the measurement of thrust and the conversion process, with no clear consensus reached on the implications of the setup or the best method for measurement.

Contextual Notes

Some assumptions about the environment (e.g., measuring on Earth) and the definitions of thrust and impulse are present but not fully explored or resolved in the discussion.

Who May Find This Useful

Individuals interested in model rocketry, thrust measurement techniques, and the physics of forces may find this discussion relevant.

smarthimandrew
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If I were to take a model rocket and secure it upside down to a kg scale, what would I be reading on the scale's display? I would assume it would be the thrust of the rocket but are there other factors at play here? If it is the thrust of the rocket, how would I convert the said value to Newtons?
 
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You can draw a free body diagram to analyze the forces on the rocket.

The trust vector is pointing down, as well as gravity: F + m*g.

EDIT: assuming you measure this on Earth and not in space
 
Model rocket engine thrust is published - you can just look it up. And a typical scale measures force and converts to mass by assuming you are on the surface of the Earth and applying f=ma. So it is easy enough to convert back.
 
Put rocket on scale pointing downwards.
The display will read the weight of the rocket in Kg.
Press the reset button on the scales so they read zero.
Light the rocket and the scales will read the thrust in Kg.
To convert to Newtons multiply by 9.8.

Edit:

I'm not an expert but model rocket motors are classified by the impulse they produce. The impulse is the force * duration.

So for example a Class B that is rated 5.00 N·s might produce..

5N for 1 Second
2.5N for 2 Seconds
1N for 5 Seconds
etc

They would all be Class B I believe.

I've yet to find a model shop that will sell me a 6Z class motor...

http://en.wikipedia.org/wiki/Model_rocket_motor_classification
 
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