Powered flight tests of Virgin Galactic's SpaceShipTwo (SS2) suborbital spaceplane are expected to resume by early June following the vehicle's first rocket-propelled flight over Mojave, Calif., on April 29.

Although only 16 sec. of SS2's 13-min. flight were powered by its hybrid RM2 rocket motor, the event was hailed by Virgin Galactic founder Richard Branson as the “single most important flight test to date.” Coming 2.5 years after the first glide flight of SS2, the powered test also marks the start of the final envelope-expansion phase and the “very realistic goal of full spaceflight by the year's end,” he adds.

Whether that is achievable will depend on the success of the forthcoming flights that will end with a maximum apogee demonstration flight to 361,000 ft. At that point, SS2 developer Scaled Composites will turn the vehicle over to Virgin Galactic, which plans to begin commercial suborbital services from Spaceport America, N.M., in 2014. Tests will follow a “methodical build-up” to the maximum apogee flight says Scaled President Kevin Mickey, gradually expanding the supersonic aerodynamic flight envelope, launch weight and structural loads.

Scaled is optimistic tests will remain on track partly because of experience gained during flight tests of the SpaceShipOne (SS1) vehicle during the build-up to winning the Ansari X-Prize almost a decade ago. “We learned a tremendous amount on SS1, which will minimize any test surprises,” says Mickey. The 17-flight SS1 test program included just six rocket-powered flights in 2003-04, and uncovered several issues with directional stability, flight control and avionics systems.

“With SS1, we leapt to the ultimate goal fairly quickly. The goal of this program is different to SS1. It is our job to help Virgin and The Spaceship Co. mature this into a commercial operation, and we'll take steps to ensure the vehicle's safety and robustness,” he adds. The Spaceship Co. (TSC) is Scaled Composites' wholly owned manufacturing sister business .

Although Mickey acknowledges that “with any testing, you don't know what you don't know,” he says the overall level of maturity of the SS2 design and systems makes the upcoming test effort more predictable. For example, maximum dynamic pressure (Max q) testing, which is the next major target, was almost completed on the rocket-powered April 29 flight. “That will be an important data point,” Mickey says.

The powered test flight began when SS2, mounted below the wing of the WhiteKnightTwo carrier aircraft, took off from Mojave at 7.03 a.m. local time. Launch occurred 45 min. later from an altitude of almost 47,000 ft. Following release from the carrier aircraft, Scaled test pilots Mark Stucky and Mike Alsbury “lit the rocket almost immediately,” says Mickey. The rocket boosted the vehicle to Mach 1.22 and an apogee of 56,200 ft. following rocket burnout after 16 sec.

“The pilots described it as being a gentler ignition than they expected. It was a smoother, gentler acceleration,” Mickey says. Transonic transition was nominal “which was the milestone of this flight,” he adds. The vehicle glided back to Mojave where it landed safely just after 8 a.m.

“It's a good start, but we still have to get through Max q and supersonic reentry and various other milestones,” says Virgin Galactic President/CEO George Whitesides. However, in terms of the overall flight-test program, Whitesides says the success of the initial powered flight is a pivotal achievement. “If you look at the overall risk embedded in the program, this flight reduced a large portion of that risk. With any supersonic vehicle, this is probably one of the most important test milestones to get through, and the vehicle's performance through Mach 1 looked good in terms of flutter. In terms of a single flight, this was probably the greatest burden of risk in the flight-test program.”

The start of powered tests is also expected to boost business for Virgin Galactic, the world's first registered spaceline. Passenger bookings have passed 570, according to Whitesides. “We are looking at up to 600 pretty soon. Most people are probably eager to have it tested out,” he says. In addition, business is anticipated from scientific experiments, including flight tests of developmental space-access technology payloads for NASA's Flight Opportunities Program.

Work on additional RM2 rocket motors meanwhile continues at Sierra Nevada Corp., which Whitesides confirms will continue to be the baseline choice for propulsion. “We've had a bunch of contracts looking at tweaks to the hybrid rocket, but our baseline is what we're doing now. We will keep working with different folks to keep improving (the propulsion system) but the qualification program went really well.”