XCOR chief engineer Dan DeLong and his team had been considering the LOX-loading problem for years. They saw rapid refueling as a key to lowering the cost of spaceship operations, the goal of most of the space entrepreneurs, and they had already considered many possible solutions. They will be helped in this case by the fact that X-Racers making pit stops will have tanks pre-chilled by their first loads of LOX-
tanks holding cryogenic liquids like LOX take longer to fill than other tanks because the first liquid hitting the tank walls boils like water on a hot skillet-but the rest of XCOR's plan to reduce load times calls for using a new, proprietary device to fill the vehicle's tank.
Last October, XCOR engineers ran a test on a spare EZ-Rocket tank on the tarmac just outside the company's hangar at Mojave Airport. The tank filled with 250 pounds of liquid oxygen in 50 seconds. As far as Greason knows, that's never been done before.
The X-Racer's increased fuel requirements presented another challenge-delivering fuel under pressure. Since the Velocity aircraft isn't designed for pressurized tanks, the X-Racer's rocket would have to be pump-fed. Pump-fed rockets typically use high-speed turbopumps to force the kerosene in at velocities similar to pressurized tanks, but, at a million dollars a pop, a turbopump was out of the question. Instead XCOR tested a new reciprocating piston pump. Though less complex and less expensive than a turbopump, it delivers fuel in spurts, like a bicycle pump squirting air into a tire. DeLong and his team designed an accumulator that would build up pressure in a separate chamber and deliver it to the engine in a steady flow. DeLong and his engineers recently set up the piston pump on a bench on the XCOR shop floor and ran water through it to simulate rocket fuel.
The system performed very well, and with the two biggest engineering dilemmas resolved, Greason felt confident enough to sign off on building the RRL's planes. Diamandis and Whitelaw were off to the races-almost.
Next comes the question of how to make the races competitive for the pilots and engaging for viewers. What no one wants is airplanes screeching around together for 15 minutes and then all pitting simultaneously.
The answer is a staggered start-the pilots will take off in pairs a few minutes apart. They'll be competing against the clock for the best overall time but racing to maneuver around one another to keep their times low. With up to 10 X-Racers planned for the early RRL races, several of them will have their engines lit at any given time, taking off, climbing, and roaring through high-G turns at up to 230 mph. Each plane's 1,800-pound-thrust rocket engine will blow through its fuel in four minutes of total burn time, requiring at least four landings for pit stops during each hour-long race, with up to 15 minutes of flight time between stops. The pilots will stretch their flight time by gliding when possible and reserving rocket boosts-each lasting between 5 and 30 seconds-for crucial moments like passing other racers.
Managing their planes' energy, Searfoss says, will be a big part of the strategy. But during those four minutes of boost time, the plane's high thrust-to-weight ratio will give it acceleration like that of an F15, which should make things exciting for the pilots.