# Airframe dynamics of a model rocket roll control system?

1. Sep 1, 2007

### leright

Hello,

I am hoping to build a model rocket with a roll control system for my senior project. Since the idea of building or obtaining a wind tunnel seems infeasible I will need to theoretically calculate how this rocket will behave with various canard fin angles and at various air speeds. I assume first I will need to measure the moment of inertia of the rocket airframe. Can someone give me an idea of how to do this? I assume somehow I would mount the rocket to some type of rod (which itself doesn't significantly affect the moment of inertia) and measure the angular acceleration as a function of torque, right? So I would need some type of gyro or accelerometer and a torque transdicer of some kind, right? Can someone help me out here?

Also, I would need to be able to determine how much torque is applied to the rocket airframe at different airspeeds and different fin angles. Can someone help me with this?

2. Sep 1, 2007

### Cyrus

You might want to see if you can find the MOI by using a simple pendulum. The period is related to the MOI about the axis of rotation. Just a simple first though. I tried it once but you need very very low friction. Id probably not try it though.

3. Sep 2, 2007

### leright

I don't quite follow you. Sure, with a bob swinging on a string, the period of the pendulum will depend on the MOI (which is a function of the weight of the bob and the length of the string.) However, I am trying to measure the moment of inertia of the rocket about its roll axis.

4. Sep 2, 2007

### AlephZero

The inertia for the pendulum is I + mr^2 where I is the MOI of the bob about its CG, m is the mass and r is the length of the string. If the pendulum has a small bob and a long string I is negligible compared with mr^2 but that isn't always the case.

To use this method to find I, you want to make r small. Putting the pivot of the pendulum directly on the outside of the case would be one way. Then I would be the same order of magnitude as mr^2.

Another way would be mount the rocket horizontally on a rotating spindle, and use a pulley and a weight on a string to accelerate it with a known torque. With a small weight, the accleration will be small, so you can measure the time for the string to unwind a known length and calculate the acceleration. Do it with a few different size weights, and you should be able to allow for any friction in the pivot.