Scaled Composites hopes to achieve one additional glide test of Virgin Galactic’s SpaceShipTwo (SS2) by year-end and says that even if bad weather prevents the attempt, the program is already ahead of schedule following a trouble-free initial unpowered flight on Oct. 10.
“Testing has been going quite a bit better than we’d originally hoped, and we’ve been able to make glide flights ahead of what we’d anticipated in terms of flight-to-flight turn-around time,” says Pete Siebold, Scaled director of flight operations. The Virgin Galactic program therefore remains on target to becoming the world’s first commercial space line, with routine suborbital operations from Spaceport America, N.M., as early as 2012.
For the glide tests, the 60-ft.-long, 42-ft.-wingspan SS2 is released from the WhiteKnightTwo (WK2) mothership at an altitude of 45,000 ft. This is about 5,000 ft. lower than the planned release altitude for rocket-powered flights because the WK2 is currently flying with landing gear down and locked pending a modification. This design change, made following a partial gear collapse on a training flight in August, is “really close” to being implemented, says Siebold. “We have tests planned, and we’re working to get it done as soon as it’s ready,” he adds. Even with gear down, Siebold says WK2’s performance “continues to amaze us, and speaks volumes about what it will be able to do in the near future.”
The gear restriction results in reduced flight times of 5-10%, meaning that more testing has to be packed into each glide. “One of the challenges in testing a glider with a restricted lift/drag ratio [around 7:1 for SS2], is that by definition the flights can’t be very long,” Siebold says. Although the theoretical maximum is as much as 15 min., the first flight is so far the longest duration of around 13 min.
Since the initial drop in October, Scaled has progressively increased the flight envelope of SS2 over three glide flights. The latest, on Nov. 17, increased top speed to 246 KEAS (knots equivalent airspeed) and loads to 3.5g. Lasting approximately 11.5 min., the last flight also further explored the flutter envelope, stall characteristics, and SS2’s stability and control. As part of the buildup to powered tests with the rocket motor, and with weights representative of passenger loads, the vehicle has also been fitted with a water ballast tank to expand the aft center of gravity (cg).
“We’ve been flying at a nominal cg, but on the last flight we flew with jettisonable ballast to allow us to explore any aft-cg issues,” says Siebold. “We launched at a further aft-cg location and demonstrated the ballast-dump capability before landing at a forward cg.” Stored in the trailing-cone location where the Sierra Nevada RM2 hybrid rocket will be located for powered flights, the ballast container is believed to store at least 1,200 lb. of water, or sufficient to simulate the weight of six passengers.
Although additional RM2 ground tests are planned, Siebold says parts of the propulsion system will be incorporated gradually into SS2 as they are ready. “We plan to [load] an equal weight to passengers as we build up, as well as the rocket motor and its components.” Full-up weight for SS2 is 30,000 lb.
Unpowered glide testing is being used to refine the vehicle’s aerodynamics and low-speed handling qualities. Building on the incremental envelope-expansion approach established with the SpaceShipOne (SS1), the next test phase will involve higher-speed subsonic flight with a short burst of power from the RM2. A follow-on test phase, using longer rocket burns, will open up the supersonic, higher-altitude corner of the SS2 envelope.
Initial rocket trials will be preceded by cold-flow tests in which all the propulsion system components will be exercised, including the flow of oxidizer, without ignition. In the SS1 trials, “this tested the basic functionality of the propulsion system and was a great dress rehearsal for the first powered flight,” Siebold notes.
At the time, FAA regulations limited initial powered burns to 15 sec., but was still sufficient to enable SS1 to go supersonic. “We’d anticipate a very similar progress, with changes as required consistent with the differences between SS1 and SS2,” he adds. On SS1, apogee increased from 67,800 ft. on the first flight to 105,000 ft. after a 40-sec. burn on the second flight.
Right now, the focus remains on lower-speed handling-qualities assessment and performance. Flutter tests, which verify that the aircraft had good structural damping as the speed increases, are yet to be completed. “There’s more to do, but we’ve got some new instrumentation that allows us to make real-time calls on flutter readings. Demonstration of a flutter-free vehicle is one of the main objectives,” Siebold says.
Other evaluations are also on track. Initial approach to stalls were flown on the first three missions. Described by Siebold as the most important maneuver on a first flight, the approach to stall defines the low-speed end of the envelope. “We’ve increased the maximum angle of attack on each flight and decreased the minimum speed, and we’re meeting all the anticipated results—so no surprises to date.” Although Siebold says there “are some small areas where we’d like to make improvements in the long term, they’re just subtle changes [related to] handling qualities. But I’m really pleased with what we came out of the box with.”
Visibility and pilot situational awareness are “far better” than on SS1, and “from the cockpit it’s a very comfortable descent. This aircraft has plenty of control power for descent, and plenty of power for the flare.” Although exact numbers are withheld, approach and touchdown speeds for SS2 are “not substantially different” from those of SS1 at around 130 kt. and 100 kt., respectively. A belly-mounted speed brake provides good control of the glide ratio, says Siebold, providing an additional option to using the turn radius of the approach to control the descent rate and helping pilots to achieve more accurate touchdowns consistently to within 1,000 ft.
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