Aviation Week reports that Virgin Galactic is developing a liquid-propellant engine that will ultimately replace the hybrid rocket motor used in SpaceShip Two.

This report is being treated as a revelation, but it’s not really surprising. Doug Shane of Scaled Composites spoke of such a possibility at the Air Force Research Laboratory’s Responsive Access to Space Technology Exchange conference back in 2006. Shane, who later replaced Burt Rutan as president of Scaled, said that changing out hybrid motors after each flight was an acceptable way of getting the suborbital spaceflight business started but they would want to switch to a “different kind of engine” at some point.

This is, however, the first official confirmation that a parallel liquid-propellent engine development program for SpaceShip Two is actually under way.

A liquid-propellent engine might allow Virgin Galactic to reduce its seat prices and be more competitive with XCOR Aerospace.

The development of the SpaceShip Two hybrid-rocket motor has been slow and plagued with problems, leading some observers to suggest that Scaled and Virgin were oversold on the benefits of hybrid-rocket motors.

Proponents of hybrid motors say they are simpler and faster to develop. It’s true that hybrid motors are conceptually simpler than liquid-propellent engines, based on engine block diagrams and part counts. A space vehicle is more than just the engine, however, and a space transportation system is more than just the vehicle. While a hybrid motor may reduce the engine parts count, the need to replace the hybrid core after every flight adds operational complexity in the maintenance segment, which reduces flight rate and runs up cost.

Hybrid motors are faster to develop to the point of initial test firing. Once engine testing begins, however, hybrids are at a disadvantage. Liquid-propellent engines are much faster to refuel and recycle between test firings. So, completing the number of test firings required for flight readiness may take less time with a liquid-propellent engine.

These are tradeoffs which Scaled and Virgin have certainly examined closely. It’s not surprising that their views on engine design have evolved based on data acquired during the engine-development program.

The prospect of a future, liquid-fueled SpaceShip Two variant will provide fodder for new speculation about Virgin Galactic’s near-term plans. How soon will SpaceShip 2.1 appear? (Presumably sometime around 2015, since the vehicle will use the same engine as the LauncherOne upper stage, which is scheduled to fly in that time frame.) Will Virgin constrain production of SpaceShip Two airframes until the new engine appears? Will it be possible to retrofit early airframes with the new engine, or will the liquid-propellent engine require new-build airframes?

Another question is what this implies for Sierra Nevada, which has been developing hybrid motors for SpaceShip Two as well as its own DreamChaser. The change won’t affect Sierra Nevada’s near-term business, but the loss of Virgin Galactic as a long-term customer can’t possibly be good news.

Written by Astro1 on July 24th, 2012 , Scaled Composites, Virgin Galactic

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