ROCKETRY
PROJECT
ASSIGNMENTS
January 19, 2002
Some
Design Considerations in Model Rocketry
OABB Model Rocketry Project
Concepts from “Handbook of Model Rocketry” by G. Harry Stine
There are eight axes of motion that affect the flight of a model rocket. It is your job as a designer to control this motion so your rocket is stable and efficient. These axes are:
1
Thrust – moves the rocket forward
2
Drag – opposes thrust and slows the rocket
3&4 Yaw
– swings the rocket nose left and right
5&6 Pitch
– swings the rocket nose up and down (similar
to yaw in simple models)
7&8 Roll
– spins the rocket about an imaginary line through the nosecone and out the
back
Most of these are very complex, but we can simplify to the things we can easily affect. You control thrust through selection of your engine. You control drag through basic shape, material, and finish of the rocket and its components – the nosecone, body tube, fins, launch lug, and anything else that makes up your rocket. Yaw and pitch are more difficult and deal with the basic design and where the Center of Gravity (CG) and the Center of Pressure (CP) are relative to one and other. And you control roll through the basic design and careful placement of fins. Gravity is another force that acts on your rocket, but you cannot control. In general, thrust is good and drag is bad. And uncontrolled yaw, pitch, and roll can be dangerous.
One important factor of a rocket is stability:
POSITIVE STABILITY: is where in a model rocket the Center of Gravity is ahead of the Center of Pressure. It has large fins set far back on the body tube. It will fly straight when launched and will weathercock into the wind at launch.
NEUTRAL STABILITY: where the CP and CG lie at the same location on the model. This might be caused by a lightweight nose or by fins that are too small, or both. There is no stabilizing and restoring forces present in the model during flight. It’s free to wander anywhere in the sky, and some of its wanderings may be wild and certainly unpredictable. It may become stable or unstable at any moment because of the burnoff of the propellant, and then it might keep right on going in the direction it happens to be pointed at that instant.
NEGATIVE STABILITY: where the CG lies behind the CP. In this case, the aerodynamic forces on the fins try to make the model fly tail-first, which it doesn’t want to do under power. Once the nose swings in pitch or yaw after leaving the launch rod, a force exists to keep it swinging. The unstable model usually pinwheels end over end and winds up going nowhere except to flop to the ground.
It might sound simple – just put large fins far back on your rocket and it will be stable – and it might be. But that will also add a lot of unnecessary drag. A really long rocket may help position the CG and CP properly, but it will also add weight. And the CG changes as the rocket’s engine burns, and the CP changes as the rocket’s position changes relative to its flight path. And speaking of weight – it seems as if the lighter the rocket, the higher it will fly. But a rocket that is too light for its size will act as if you were trying to throw a feather high into the air. Sometimes adding weight to a rocket will make it fly higher! As you design your rocket, you will find that there are a lot of trade-offs - to gain one thing, you must give up another..
A student at M.I.T., Gordon K. Mandell studied the stability of rockets and came up with a few simple rules:
- A rocket should be at least 10 times longer than it’s diameter (the distance across the body tube).
- The center of gravity should be between 1 and 2 times the diameter ahead of the center of pressure. The diameter is also called the caliber.
- Align the fins to get a zero roll-rate, because roll can contribute to instability by causing added pitch
- If you need large fins, increase the dimension OUT from the body (called span) because this increases the force needed to stabilize the rocket without moving the CP further away from the CG.
For our first rocket design, we will use ½ A or ¼ A engines. Select components from Estes. Play with the RockSim program and simulate launches of your design. Try changing the design – body tube type and length, nose cone shape and length, total weight, fin size and shape – play with everything and then simulate the launch, looking for the maximum altitude flight with the small engines.
Next meeting (February 16, 2002) we’ll build the rockets you have designed and shoot them off. There will be a prize for the highest, stable flight. Before you will be allowed to shoot off your rocket, you must prove to your leaders that the rocket is stable on the RockSim program.