Powered vehicles can be a great deal of fun, and they are certainly exciting to watch. One thing that we always need to keep in mind is safety.
With a powered vehicle there will always be some form of stored energy that has the potential to be released suddenly. When operating/starting these vehicles, safety glasses are a good idea, and maybe even hearing protection.
Rocket-powered vehicles can potentially be the most dangerous, so extra care should be taken when designing and launching these type of vehicles.
The derby is held in an area where the back of the property is about 70 meters from the start position. If a rocket-powered vehicle is able to stay on the track, it will eventually run out of track, where the grass-covered bumpy ground will stop it fairly quickly; no problems there. If however, as is often the case, the vehicle becomes airborne, it can fly for a considerable distance. We have a rocket trajectory and distance calculator available and would be happy to help you figure the maximum range that your rocket-powered vehicle has in the worst-case condition. An example is shown below.
A derby car with a mass of 85 grams. (fairly standard pinewood derby car with no added weight)
An A3-4T Estes rocket engine. (common small rocket engine)
Vehicle launches upward at an angle of 45 degrees. (worst case, as this angle gives the greatest airborne distance)
That's 57 meters too much.
Increase the mass of the car to 130 grams.
By adding a small amount of mass to the car, we can keep it from becoming a danger to the neighbors. The link below will take you to a chart showing the specifications of the Estes model rocket engines available. Unless you are designing a very heavy vehicle, the small engines should provide plenty of thrust. In 2013, an A3-4T engine propelled a fairly standard car down the length of the track in less than a second. That was plenty of thrust.