Unlike conventional rockets, winged vehicles face the added challenge of encountering a potentially lethal form of unsteady oscillation, or flutter, as they pass through the atmosphere on the way to and from the airless voids of suborbital space.

However, following two supersonic flights through the heart of the potential flutter zone, Scaled Composites' test pilots, evaluating Virgin Galactic's SpaceShipTwo (SS2), report the vehicle is rugged and stable, without any of the transonic issues that could have bedeviled the design. While the subsonic build-up test flights unearthed some unexpected results before the start of powered flights in April, aerodynamic modifications have proved effective during the vehicle's early supersonic flights under rocket power.

The findings have built confidence in achieving the next major milestone for the program, a supersonic reentry using SS2's unique tail-plane feathering braking system. The test, if successful, will open the way for longer-duration rocket burns, which will culminate with a maximum apogee demonstration flight to around 360,000 ft. On completion of this milestone, Scaled Composites will turn the vehicle over to Virgin Galactic, which plans to begin commercial suborbital services from the Spaceport America site, located in the Jornada del Muerto desert basin in New Mexico, in 2014. The two-crew SS2 is designed to carry six passengers or a science payload.

Describing the most recent powered flights at the Society of Experimental Test Pilots Symposium in Anaheim, Calif., Scaled Composites test pilots Mark Stucky, Michael Alsbury and Clint Nichols say the focus is now on expanding the high-speed, high-altitude envelope following the second powered flight on Sept. 5. That test, which reached Mach 1.43 and an apogee of 69,000 ft., included the first use of the feathering reentry system on a powered flight, and saw the vehicle's Sierra Nevada Corp.'s second solid rocket motor, RM2, fired for 20 sec.—4 sec. more than the first powered flight on April 29.

The second flight “met all objectives” and built on the first powered flight, which “took us into the middle of the flight regime, where flutter was predicted,” says Stucky. However, no flutter was encountered and “the only real surprise” was an uncommanded pitch-up of around 10 deg. during acceleration. Subsequent analysis indicated that the development of a “pitch bubble” was responsible for the event. This was solved by placing four vortex generators on the upper surface of the wing, close to the leading-edge root area. The small devices were built and installed in short order, thanks to the Mojave, Calif.-based company's 3-D printer.

Scaled also gave new details about the tail-stall event on the 16th glide flight in September 2011. That episode led to a redesign of the inboard strakes and contributed to a nine-month hiatus in test flights. The test card called for releasing SS2 from the WhiteKnightTwo carrier aircraft, and immediately entering a rapid descent, to check for flutter in the transonic region. The crew planned to rap the stick to check for damping as the nose passed 30 deg. down, and they expected the nose to reach -25 deg. before tracking upward and capturing Mach 0.8 and an initial dive angle of 18 deg.

Instead, Stucky expounds, “Upon release, the downward pitch rate was quick. And unlike the previous flight and nominal simulation, it wasn't slowing. I began trimming aft and applied maximum upward elevon, without any noticeable effect.” The spacecraft then became inverted and, despite the crew dumping water ballast, it immediately entered a left spin, he says. “I instinctively applied full opposite stick and rudder without any noticeable effect. After four turns, we spontaneously decided to activate the feathering device. It was designed for carefree reentry from space, not as a spin recovery system, but it functions well in that role.” SS2 quickly rolled upright and recovered, he adds.

Post-flight analysis indicated the tail had stalled negatively, was unable to counter the downward-pitch rate and that the steep dive at that particular speed and altitude was outside the SS2 design envelope. The enlarged strake effectively “doubles the negative alpha [angle-of-attack] range,” says Stucky.