I was at an amateur astronomy congress last weekend. One of the talks was by an rocket builder who later demonstrated the principle behind hybrid rockets. The rocket motor consisted of a combustion chamber made of a perspex tube. This made it possible to see inside the combustion chamber during the demonstration. The solid component of the fuel was also perspex and the gaseous component of the fuel was nitrous oxide. The demonstration rocket did not have a nozzle since this was just a demonstration of the basic principles of hybrid rocket fuels. The point of the demonstration was to show that hybrid rockets can be cheap, controllable and safe.
Hybrid rocket demonstration