Airframe dynamics of a model rocket roll control system?

In summary, the individual is trying to build a model rocket with a roll control system and needs help calculating the moment of inertia. They also need help determining how much torque is applied to the rocket airframe at different airspeeds and different fin angles.
  • #1
leright
1,318
19
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?
 
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  • #2
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
cyrusabdollahi said:
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.

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
leright said:
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.

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.
 

1. What is the purpose of a roll control system in a model rocket?

A roll control system is used to stabilize the flight of a model rocket by controlling its rotation or roll. This helps to keep the rocket flying in a straight path and prevents it from spinning out of control.

2. How does a roll control system work?

A roll control system typically consists of fins or vanes attached to the rocket's body near the tail. These fins can pivot or move to adjust the rocket's direction and keep it stable during flight. The movement of the fins is controlled by a servo motor or other mechanism.

3. What factors affect the effectiveness of a roll control system?

The effectiveness of a roll control system can be affected by various factors such as the size and shape of the fins, the placement of the fins on the rocket, the weight distribution of the rocket, and the speed and direction of the wind during flight.

4. Are there different types of roll control systems?

Yes, there are different types of roll control systems depending on the design and purpose of the model rocket. Some common types include canard fins, delta fins, and cruciform fins. Each type has its own advantages and limitations.

5. How can the performance of a roll control system be improved?

The performance of a roll control system can be improved by carefully selecting the size and placement of the fins, balancing the weight of the rocket, and conducting test flights to fine-tune the system. Additionally, using a gyroscopic stabilization system can also improve the effectiveness of the roll control system.

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