Virgin Galactic confirms that following a series of successful hot-fire rocket tests, it has reverted to an improved form of the original rubber-based fuel for powering the company's suborbital SpaceShipTwo (SS2), the second version of which is nearing completion in Mojave, California. Virgin Galactic-which plans to resume flights of SS2 in 2016-used a recently developed and more energetic nylon-based fuel for last October's ill-fated fourth and final powered flight (PF4) of the prototype SS2, during which the vehicle broke up following the copilot's earlier-than-planned unlocking of the feathering tail mechanism. Company officials recently hinted they were considering a return to the rubber-based hydroxyl-terminated polybutadiene (HTPB) fuel used in initial test flights. The switch was confirmed by CEO George Whitesides at the International Symposium for Personal and Commercial Spaceflight in Las Cruces. Perfecting the propulsion system has been the most challenging aspect of the suborbital spaceplane project. Virgin Galactic based its original power plan on a scale-up of the basic HTPB-fueled hybrid motor used in the original X-Prize-winning SpaceShipOne, but in 2014 announced it was opting to use a polyamide-based grain to achieve a step change in thrust and propulsion consistency. Meanwhile, parallel development of the HTPB fuel continued through 2014, and by the time PF4 occurred, so much improvement had been made that Virgin decided it would revert to the original fuel.
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