Hybrid rocket
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A hybrid rocket propulsion system typically comprises a solid fuel and a liquid or gas oxidizer. These systems are superior to solid propulsion systems in the respects of safety, throttling, restartability, and environmental cleanliness. However, hybrid systems are slightly more complex than solids, and consequently they are heavier and more expensive.
Common oxidizers include gaseous or liquid oxygen and nitrous oxide.
The Reaction Research Society (RRS), although known primarily for their work with liquid rocket propulsion, has a long history of research and development with hybrid rocket propulsion.
Several universities have recently experimented with hybrid rockets. BYU, the University of Utah and Utah State University launched a student-designed rocket called Unity IV in 1995 which burned the solid fuel Hydroxy-terminated polybutadiene (HTPB) with an oxidizer of gaseous oxygen, and in 2003 launched a larger version which burned HTPB with nitrous oxide.
Portland State University also launched several hybrid rockets in the early 2000's.
SpaceShipOne, the first private manned spacecraft, is powered by a hybrid rocket burning HTPB with nitrous oxide. The hybrid rocket engine was manufactured by SpaceDev. SpaceDev partially based its motors on experimental data collected from the testing of AMROC's (American Rocket Company) motors at NASA's Stennis Space Center's E1 test stand. Motors ranging from as small as 1000 lbf (4.5 kN) to as large as 250,000 lbf (1.1 MN) thrust were successfully tested. SpaceDev purchased AMROCs assets after the company was shut down due to lack of funding.