Hybrid rockets that burn inert, non-toxic fuel with a liquid oxidizer may find new applications in the years ahead, following a hybrid sounding rocket experiment late last year that moved the technology beyond the realm of hobbyists and failed commercial ventures.

A 57-ft.-long single-stage hybrid sounding rocket built by Lockheed Martin's Michoud Operations in New Orleans lifted a set of aerodynamic reentry experiments more than 44 mi. to the fringes of space, where they were allowed to fall back to Earth at hypersonic speeds. The rocket, which used heated helium to force oxygen into a carefully shaped casting of fuel that was essentially synthetic rubber, generated 60,000 lb. of thrust for the Dec. 18 flight.

"AS FAR AS WE CAN TELL there has never been any [hybrid] sounding rocket anywhere near this size that's been flown before," said Randy Tassin, vice president for program management and technical operations at the Michoud facility. "In our minds, anyway, it's a historic event."

NASA tested a much larger hybrid rocket motor on the ground at Stennis Space Center, Miss., last year, and is studying hybrid technology as a possible element in the new Next-Generation Launch Technology (NGLT) program that spun out of the old Space Launch Initiative. Unlike the big solid-fuel boosters that boost the space shuttle and large expendable rockets to orbit today, hybrid-fuel rockets can be throttled, shut down and restarted in flight. And the inert fuel--hydroxyl-terminated polybutadiene (HTPB) in the sounding rocket flight--is much safer to handle than standard solid propellant based on ammonium perchlorate and other highly volatile chemicals. Lockheed Martin, which used its own funds in a Space Act agreement with NASA to develop the prosaically named Hybrid Sounding Rocket, is studying several other possible uses for hybrid-fueled rocketry.

"It could be a good low-cost, high-speed test capability that would help universities and a lot of other R&D programs get off test experience at a real low cost," Tassin said. "It obviously, in a big picture, could eventually be a space launch booster, replacing a solid or a liquid system. We have looked at it for crew-escape propulsion systems, and it has a potential there [as well as] for targets and sounding rockets, and [possibly] upper stages. So really, if you look at it, it has a wide open field in terms of where you could use it."