A while back I joined the local pyrotechnic organization. The meetings have been quite interesting. The speaker at one meeting operates a company that makes custom rocket motors, mostly for the military. His talk was on the design of composite rocket fuel. The talk started out with stories of growing up doing experiments in his parents kitchen. Some of which would probably get you put in jail these days. The stories were followed by a description and photo tour of his company. Then things got rather technical. The talk was probably better suited for a group of amateur rocket builders rather than fireworks enthusiasts, but it sure was interesting. There was a lot of information packed into his talk. Here are some of the points about composite rocket fuel.
- Composite fuel is used in everything from the space shuttle boosters to rocket-propelled grenades.
- The oxidizer of choice is ammonium perchlorate because it is inexpensive and all the combustion products are gasses.
- The fuel and oxidizer are finely ground solids with a liquid resin binder.
- There should be as little binder as possible yet still have the mixture pourable.
- Pour ability is controlled by the particle sized distribution of the solids.
- Iron oxide is added to the mixture as a catalyst to increase the burn rate.
- Carbon black is added to make the fuel opaque to infrared radiation from the burning fuel which can ignite the fuel where it should not be burning.
- Aluminum is sometimes added to the mixture. Its combustion product is a solid but burning aluminum raises the temperature so much, increasing the volume of the gasses, that it is worth adding.
- It is important to test the fuel mixture to determine the burn rate at various pressures. A fuel that increases burn rate with increasing pressure too much can cause the rocket engine to explode.
- Fireworks rockets that whistle use the pressure sensitivity to make the whistle. The fuel is formulated to be especially sensitive to pressure increases. A pressure wave from the burning fuel front is reflected off the nozzle. When this pressure wave reaches the burning fuel front it increased the burn rate which causes another pressure wave. I always thought it was a special nozzle.
- Nozzles are most often made of phenolic impregnated with fused silica.
After the talk we all went outside to see a rocket motor firing. The motor is used to extract the parachute used to parachute an entire small airplane. If the pilot gets into trouble a lever can be pulled which will fire the rocket motor to extract the parachute. This motor produces 500 pounds of thrust for one second. The motor is electrically fired so they had to jury rig some jumper cables and an extension cord to a car battery to fire the motor. Boy was it loud!