USING THE ENGINES IN ROCKETS
It is assumed that the experimenter has had previous experience with model rockets and so no attempt will be made to provide details of their construction. The major difference between a home made rocket engine and a commercial one when used in a model rocket is the outside dimensions. Commercial cardboard tubes can often be obtained from model rocket suppliers that are close enough to the outside diameter of the engine to work sufficiently well. A friction fit is used to hold the engine in place. Masking tape is wrapped around the nozzle end of the engine so that a tight fit is obtained and a five to ten pound force must be used to push the engine into place. If the engine is much smaller than the engine-holder tube tape should be added also to the top of the engine casing so it will not be loose but still slide readily into the tube.
An easier method of obtaining a good fit with commercial tubes is to build up the outside of the rocket engine casing when rolling them so that they will readily fit the available tubes. Never reduce the wall thickness to obtain a match fit. The casings also should have smooth outside surfaces free from wrinkles and should have perfectly round cross sections to fit well in the engine holder tube.
An engine holder tube can be made to any size in the same manner the engine casing was made. The tube, however, would only need to be a few layers thick to be rigid enough. Any cylindrical item such as dowels, plastic pipe, metal rods, etc. can be used as mandrels.
When preparing the rocket for flight, it might also be a good idea to add a little extra flame proof wadding to protect the recovery system.
The most important aspect of building a rocket and preparing it for flight is to check the balance. Remember that the center of gravity must be ahead of the center of pressure for stable flight. The home made engine is somewhat heavier for the same total impulse (power) as an equivalent standard commercially manufactured engine and so rockets may tend to be tail heavy. Either additional weight can be added to the nose or additional fin area can be added to the tail to make the rocket stable. Never use a rocket intended for commercial engines with your home made engine without first checking the stability. Consult the references at the end of the book for texts on model rocketry. There are many that will give detailed instructions on building model rockets.
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