After Sunday's first glide flight of Virgin Galactic's SpaceShipTwo, one of the first thoughts going through the head of test pilot Peter Siebold after coming to a stop on the runway was that it all went by too quickly. He and co-pilot Mike Alsbury had been released from the mother ship, Eve, just 13 minutes earlier at 45,000 feet.
The flight was an overwhelming success with the airplane out performing many of the pre-flight expectations. And with the hard work of a first flight behind him, Siebold wanted to make the flight again for the pure joy of flying an airplane he and the team at Scaled have spent the past several years developing.
"After we landed, I looked over to Mike and said, 'can we do that again?'"
Unfortunately with no motor, Siebold, Alsbury and SpaceShipTwo were stuck in the middle of runway 30 at the Mojave Air and Space Port waiting for a tow back to the hangar. The flight was the second time Siebold had piloted the first flight of a new aircraft design in the past two years. In December of 2008 he piloted SpaceShipTwo's mother ship, Eve, on its maiden flight.
The glide flight of SpaceShipTwo also marks the second time in less than a decade the team at Scaled Composites have developed and flown a privately funded vehicle designed for suborbital space flight. And in preparation for Sunday's flight, much of the experience gained from developing and flying SpaceShipOne was put into use.
Right off the start Siebold says the experience paid off and they were able to get straight to work.
"We released at 45,000 feet, it was very clean release, much less negative g [force] than we had anticipated, a very comfortable release" he says of being dropped from the mother ship. "We trimmed the airplane and the first maneuver we performed was an approach to stall."
Even though it was the first time SpaceShipTwo had been released and flown on its own, that first stall maneuver and everything that followed was old news for Siebold, Alsbury and the entire flight test team.
Like other first flights, the team at Scaled has relied heavily on simulators for designing, preparing and practicing just about every conceivable aspect of the SpaceShipTwo program. Scaled uses both ground based and flight based simulation to prepare the entire flight test team. A complete cockpit replica can be connected to the mission control team on the ground to simulate complete missions including a wide range of potential emergencies. NASA similarly relies heavily on ground based simulators for every space shuttle flight.
And in the air, the mother ship Eve can be configured to fly the same approach as the spacecraft.
"The mother ship has turned out to be a phenomenal in-flight training vehicle that can fly the same descent profile with its speed brakes and a combination of engine power settings, where we can descend at the same air speed and flight path angle as the space ship does" says Siebold.
Siebold says flying the descent profile in Eve helps the pilots become comfortable with the descent angles and the out the window views for the various landing approaches.
"Getting those subtle cues, the seat of the pants feedback, the g [force] that you'll experience from low key to final, it really gets you the rest of the way to the 100 percent ready for the flight."
In the lead up to Sunday's flight, the flight test team at Scaled at completed 19 sim sessions in the ground based simulator, each of which included multiple flights.
"We're talking at a least 100 simulated flights with the full crew participation, and that doesn't count what we've been doing with Eve."
Siebold says he logged more than 25 practice approaches and landings in Eve before the first glide flight in SpaceShipTwo on Sunday.
Because of a recent landing gear issue with Eve that is still being resolved, the Scaled team elected to fly the mother ship with the gear down and locked throughout the entire flight. This limited the release height to 45,000 feet, the approximate ceiling of the aircraft in the gear down configuration at the weight flown on Sunday.
After being released from 45,000 feet, the 13 minute flight to the ground for SpaceShipTwo was mostly filled with gathering vital performance data for the future space plane.
The first maneuver, the approach to stall, is important to establish the lower speed end of the flight envelope according to Siebold.
"On a first flight you're only objective really is to get on the ground safely and all the test points we do leading up to that are to verify and understand the airplane that we have and learn very quickly whether we have any issues" he says. "And the most important maneuver we do on a first flight is an approach to stall because a landing is in many aspects the exact same profile except you touchdown quite a bit earlier, before the wing does stall."
Siebold says SpaceShipTwo performed the way they expected in the stall and gave them a minimum flying speed for the light weight configuration flown on Sunday. The airplane did not have the rocket motor installed, and was flown at a relatively low overall weight.
Immediately following the stall, Siebold and Alsbury continued the flight, marking off the test points as the glider quickly descended.
"We flew some dynamic stability test points and then went into our envelope expansion where we increased the airspeed and verified through flutter test points and stick raps that the aircraft had good structural damping as the speed increased."
Similar to the flutter testing that took place on the 787 Dreamliner, Siebold says the 'stick wraps' were simply a matter of hitting the controls with a bit of force. The pilots then wait for the word from the engineers monitoring the data on the ground that the oscillations induced by the abrupt control surface movements decrease in magnitude.
After the flutter testing, Siebold performed some higher g force pull up maneuvers and some low g force maneuvers over the top of the roller coaster like flight profile during this section of the test. Once the high and low g force maneuvers were complete, SpaceShipTwo was put through a more aggressive simulated flare to landing, this time using the speed brake to simulate the proper landing configuration.
Unlike its predecessor, SpaceShipOne, this space ship has a belly mounted speed brake that can be used to control the glide ratio of the descent portion of the flight. SpaceShipOne had a lift to drag ratio (L/D) of 7:1 during the descent. The only way that could be changed was by lowering the gear, which was done just before touchdown as there was no way to retract the gear once it was down. With the speed brake, pilots of SpaceShipTwo will be able to adjust the glide during the descent to account for changing winds and other variables Siebold says.
"It gives a tremendous amount of flexibility where you can change the L/D in real time during the descent and not only use the turn radius of the approach."
Like its predecessor, SpaceShipTwo uses a modified version of a standard engine out approach used by the military. After flying over the intended landing point at an altitude known as 'high key,' the pilot makes a constant turning 360 descent to line up with the runway and touch down. By adjusting the turn radius of this circling approach, the pilot can adjust the descent profile as needed.
Though no specifics were given, at the light weight flown on Sunday, SpaceShipTwo's approach speed was "similar" to that of SpaceShipOne at around 130 knots according to Siebold. Though he adds the new space ship will fly a bit faster in its normal configuration than the original.
Once the final simulated flare to landing at altitude was finished, Siebold says they had completed all the items on the flight test card and were in the enviable position of having some extra altitude. SpaceShipTwo's L/D ended up being better than anticipated and the reward was some extra time to perform some clearing turns and a chance to hand over the controls to co-pilot Mike Alsbury for a short time.
Siebold says as an airplane, SpaceShipTwo flies extremely well. The relatively long and slender airframe and the narrow, swept wing exhibits little adverse yaw and is directionally very stable.
"The roll performance is pretty spectacular, it's quite agile in roll and allows you to turn pretty rapidly with relatively light stick forces, so it's a very maneuverable airplane."
Pitch and roll on SpaceShipTwo are controlled by the stabilator/elevons on the twin tails. The entire "stab" control surface can be trimmed through an electromechanical device during both subsonic and supersonic flight. When trimmed up or down together, the stabs control pitch, when moved independently, they control roll.
For the first glide flight a trim setting of -10 can be seen in the pictures which is a relatively high nose up trim setting because of the weight and center of gravity during the flight. During subsonic flight, elevons at the back of the stab are directly connected to the stick in the cockpit via cables to control the pitch when moved together and roll when moved independently.
With the end of the glide flight fast approaching, the pilots were enjoying the extra altitude. Based on all of their simulation sessions, the Scaled team expected the entire glide flight to last 12, not 13 minutes. Roughly 10 minutes into the flight the extra altitude was gone with the minute of extra turns and sightseeing. They were now at the proper altitude to begin their landing approach.
"We ended up in the traffic pattern approximately 10,000 feet over the landing point and flew the approach to touchdown" Siebold says.
Three minutes later they were on the ground. Discussing the landing, Siebold again laments over how quickly the entire flight went by, pointing out it won't add much time to his logbook.
"I haven't entered the flight in yet, 13 minutes, that's 0.2 on the hour meter for flight time. Probably my fastest first flight ever."
Siebold says both aircraft performed flawlessly during Sunday's flight with no post flight squawks. Scaled has a long standing policy of not commenting about future test flights, but with their customer, Virgin Galactic, talking about flying passengers to space within 18 months, it's a safe bet that a rigorous flight test schedule, including powered flights, will begin soon.
Photos: Virgin Galactic
