The stall occurred as the crew was diving at a steep angle to set up for a specific test point, and it happened “because of a specific maneuver we would not normally encounter during operations,” says Scaled test pilot Pete Siebold. The change is designed to provide greater pitch authority and better longitudinal control at negative angles of attack. It will not only help during tests at unusual dive angles but “will provide added margin in regular operations,” says Siebold. “The basic aircraft has been performing very well,” he adds.
The upgrade, although relatively minor, is the second notable aerodynamic modification following the addition of nose strakes last year. These were initially tested in late May 2011 to “make the ride a little less oscillatory” in feather mode, says Shane, noting that “we're happy with that so far.”
With the resumption of glide flights, Scaled plans to go back to reexplore parts of the envelope expansion phase which originally began with the first glide flight in October 2010. “We've got to reclear the envelope and make sure the modifications we made are what is required,” says Shane. These will build up in terms of weight and varying center-of-gravity positions to gradually clear the unpowered flight envelope.
“We don't expect many differences. We shall reevaluate longitudinal stability and show that it matches predictions. We will only go and reinvestigate areas of the envelope where the strake change might impact,” adds Siebold.
With the envelope recleared, the first components of the Sierra Nevada Corp. -developed MR2 rocket motor will be installed and the buildup process to suborbital testing will begin, says Shane. Initially, glide flights will clear as much of the low-speed envelope as is practical with the aircraft ballasted with weight to represent the rocket after the fuel has been spent.
The remainder of the envelope expansion requires rocket power. “The program team has put together a plan for differing burn durations to achieve various Mach-number and altitude conditions,” he adds. The test plan will follow the example set with Scaled's SS1, the pioneering vehicle that in 2004 won the original $10 million Ansari X Prize by becoming the first private manned spacecraft to exceed 328,000 ft. twice within 14 days. Initial flights will see shorter-duration burns rocket SS2 to speeds likely in excess of Mach 1, while the feathering system will be tested later, with longer-duration rocket burns at higher speeds and altitudes above 200,000 ft. “For the first five flights, we don't need a full-duration motor,” Shane says.
“The plan is still being finalized, and although we know it will begin with short-duration burns, the exact times will be based on how we proceed in ground testing and how the rocket motor is performing. We still haven't finalized the specific goals of airspeed and altitude for each of the flights,” says Siebold. Unlike SS1, where the rocket-burn duration for the first powered flight was limited to 10 sec. to avoid going through the FAA licensing process, no such barrier exists for the already approved SS2. Previously, Scaled has indicated a target minimum burn of around 15 sec.
Testing of RM2, a hydroxyl-terminated polybutadiene and nitrous oxide-burning hybrid motor, is also accelerating with 13 full-scale, full-duration, rocket firings completed on June 26. During the five tests conducted so far this year, the motor has run over durations of 40-55 sec. Full-duration firings of 55 and 58 sec. were also accomplished in the second half of 2011. The latest test checked out pressurization, the valve/injector design, fuel formulation and geometry, nozzle, overall structure and performance.
The RM2's job is to propel SS2 and its two crew and six passengers or mission payload to altitudes over 100 km (62 mi.) with margin to spare. SS2 tests recommenced with a series of taxi tests at Mojave on June 1. Since ground and flight tests began, however, the propulsion system has become heavier than expected, resulting in an increase in operating empty weight over early assumptions. “For flight tests, this warranted looking at the overall capability of the landing gear,” says Siebold.
Unlike SS1, which was aimed at grabbing the X Prize, SS2 is designed to be a fully reusable transport. “We want a system that's robust,” Siebold stresses. “We therefore made changes to the wheels and brakes to get it closer to a production configuration.” On June 1, the spacecraft was towed behind a truck down the runway to conduct four tests of newer, higher-capacity brakes at 30-65 mph with four different pilots: Virgin Galactic chief pilot Dave MacKay and Scaled test pilots Siebold, Mike Alsbury and Clint Nichols.