A type of rocket developed in the 1990s that is safer, cleaner, and cheaper than conventional rockets that use solid and liquid propellants. NASA successfully fired a prototype rocket motor in August 1999 and completed its fourth in a series of large-scale hybrid motor tests in 2002 at the Stennis Space Center. The hybrid rocket has solid fuel converted into a rubbery form. An oxidizer is continuously injected into its core, producing hot exhaust gases that vaporize a small layer of fuel, causing a reaction to produce more gases. The rocket can be turned off after ignition using a simple switch. NASA's hybrid rocket motor has 112 500 kg of thrust. NASA's Ames Research Center collaborated with Stanford University to develop a non-toxic, easily handled fuel.