Upside Down Rocket: Thrust & Conversion to Newtons

[smarthimandrew]

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?

 

[bigfooted]

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

 

[russ_watters]

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.

 

[CWatters]

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

 

[pmacias]

I can confirm that the reading on the scale's display would indeed be the thrust of the rocket. However, there are other factors at play that may affect the reading, such as air resistance and the weight of the rocket itself.

To convert the thrust value to Newtons, you would need to use the formula F=ma, where F is the force (thrust), m is the mass of the rocket, and a is the acceleration. This formula is derived from Newton's Second Law of Motion, which states that the force applied to an object is equal to its mass multiplied by its acceleration.

Once you have the mass and acceleration values, you can simply multiply them together to get the thrust in Newtons. It is important to note that the mass should be in kilograms and the acceleration should be in meters per second squared (m/s^2) to ensure the correct unit conversion.

In summary, securing an upside down model rocket to a kg scale would provide a reading of the rocket's thrust, and this value can be converted to Newtons using the formula F=ma. Other factors may affect the reading, but this conversion will provide an accurate measurement of the rocket's thrust force.