I was asked in the comments if today’s SpaceShipOne flight was propelled by N2O NO2 noted that the fuel is nitrous oxide and rubber propellant.
At How Stuff Works, there’s a page —How Space Ship One works— that explains the engine and its technology. SpaceShipOne burns rubber.
In solid rocket fuel, the oxidizer is embedded into the fuel; in a liquid system the two components are stored separately on the craft and combined during ignition. The problem with this latter system is that traditional fuels and oxidizers are expensive and dangerous to store. To cut down on both cost and risk, SpaceShipOne is propelled by a mixture of hydroxy-terminated polybutadiene (tire rubber) and nitrous oxide (laughing gas). The rubber acts as the fuel and the laughing gas as the oxidizer.