Model Rocketry

Parts of a Model Rocket

Rocket Engines Most are single use engines. Classified by the amount of power they produce.

How Does a Rocket Engine Work? Newton’s Third Law: For every action, there is an equal and opposite reaction. Engine throws mass (hot gases) out back end, resulting in a force in the opposite direction.

Force = mass x acceleration (Newton’s Second Law) Throw a baseball while on roller blades: what happens?

Power rating is total thrust developed times the time in seconds the engine burns.

Engine Designations

From the letter and first number, you can get burn time: – Power/Thrust=time – For C6-5, 10Ns/6N=1.67s The longer the engine burns, the higher the rocket flies. The higher the thrust, the more rugged you need to build.

Rocket Recovery Systems Streamer recovery Parachute recovery Tumble or featherweight recovery Glide recovery Horizontal-spin recovery Helicopter recovery Drag recovery

Rocket Stability CP = Center of Pressure CG = Center of Gravity

Center of Pressure Location where all the aerodynamic forces acting on the rocket are in balance. Difficult to determine without a wind tunnel. Can be estimated with a cardboard cutout of rocket shape.

Center of Gravity Location where the rocket will balance, or rotate if unrestrained. Simple to locate: balance the rocket on the edge of a ruler.

Swing Test Do a swing test on new designs to demonstrate stability before the first launch. Install motor (no ignitor), recovery device, and wadding. Tie a 6-10 foot string on rocket at CG and tape in position. Swing in a circle at shoulder height. Should fly straight. Correct stability before flying. CG should be body tube diameters ahead of CP.

Achieving Stability

Move CG Forward Decrease mass at rear of rocket Add mass to nose of rocket Lengthen rocket body Recess engine inside body tube

Move CP Rearward Move fins rearward Eliminate any forward fins Sweep fins to rear Increase fin area Increase number of fins

Tracking Model Rockets