Hybrid rockets
My former thesis advisor has been involved with a project to develop hybrid rockets using wax as fuel. It's basically a steel casing with a rocket nozzle on the end and filled with wax. There's an opening down the axis of the rocket for injecting oxygen. Cleverness in the design comes in the pattern used for the opening in the wax and the injector used for the liquid oxygen. The beauty of the concept is its relative cheapness and extreme safety.
Gary