Space tourism front-runner Virgin Galactic hopes to launch customers toward the edge of space later this year. To get them there, in a winged craft called SpaceShipTwo, the company will light up hybrid-fuel rockets.
Hybrid-fuel engines marry two classic designs: liquid-fuel (like the space shuttle’s main engines, which combine liquid hydrogen and liquid oxygen) and solid-fuel (like the shuttle’s boosters, which use solid aluminum and ammonium perchlorate).
Solid-fuel engines are powerful, but they burn until the fuel is gone—whether you like it or not. Pilots can throttle liquid-fuel engines, but they’re very complex machines. Hybrid-fuel engines are the in-betweens. They burn solid fuel with a liquid oxidizer, which pilots can adjust, but they are far simpler than liquid-fuel engines.