SpaceShipTwo is powered by a hybrid rocket motor — one that combines elements of solid rockets and liquid rocket engines by using a solid fuel source and a liquid oxidizer. Hybrid rocket motors are not a new idea; they have been used in many other applications, including on SpaceShipOne. Although each type of rocket engine has important advantages for different applications (for example, LauncherOne uses liquid rocket engines), hybrids are particularly well suited for SpaceShipTwo because they can be shut down safely and quickly at any point during the flight, unlike solid motors, but do not require the cryogenic propellant tanks or complex plumbing needed by liquid rocket engines.