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Posted on 10/06/2003 4:58:21 PM PDT by Young Werther
Pitch hangup will result in delay and changes to design
SpaceShip1 Hiccups
Scaled Composites is planning changes for its SpaceShipOne rocket glider, following a pitchup incident in flight test that showed the current configuration is not safe for the planned flight to 100 km. altitude.
The glider was starting an aft center-of-gravity (CG) stall test on its third flight on Sept. 23 when it pitched up uncontrollably at about 85 KIAS--15 kt. above the 70-kt. forward CG stall speed demonstrated on the prior flight (AW&ST Sept. 1, p. 19). The results were reported by the company three days later at the Society of Experimental Test Pilots' 47th Symposium held in Los Angeles.
The nose hung at about 20-deg. pitch attitude and, despite full nose down elevator and nose down trim, would not drop. "The pitchup was not huge, but it didn't go down," said SpaceShipOne pilot and Scaled General Manager Michael W. Melvill. "That was upsetting; I really wasn't expecting that." SpaceShipOne has manual primary controls and electric trim in all three axes.
Melvill recovered by rolling to the left with the rudder, which caused the nose to slice toward the ground.
In operational use loaded with hybrid rocket propellants, the craft will weigh approximately twice as much and a pitchup and recovery could be more dangerous, particularly when the rocket is firing. Scaled officials are poring over the flight test data, first trying to understand what happened and then how to fix it. "It may be a small fix, it may be a big fix," Melvill said.
The program is on a rapid pace to reach the 100-km. (328,000-ft.) goal by the end of the year. Changes will take time, and the affirmation that danger may be lurking as the flight envelope is expanded could cut into the planned big jumps in performance between each test flight, particularly as the rocket is fired and the craft rapidly goes supersonic. SpaceShipOne was designed primarily by computational fluid dynamics.
The glider was dropped from the White Knight carrier at 46,600 ft. and 115 kt., and Melvill spent 5-10 sec. trimming it in roll. He then began the stall entry that led to the pitchup. The recovery placed the aircraft inverted, and after airspeed had built, a 2g pullout put SpaceShipOne in a secondary departure to the left. Rudder and stick weren't stopping the departure, so Melvill added right roll trim. That halted the departure, but caused further oscillations until the roll trim was removed.
"I had several iterations of overcontrol," Melvill said. "There is no feel on the stick at high altitude. Things happen slowly, but there's not much control with the stick alone. The trim has a huge amount of control, intended for supersonic flight." The departure occurred above 40,000 ft., where there is less damping of body rates than at low altitude. "Eighty-five knots in denser air could have been OK," Melvill said.
The aft operational CG had not been flown before and was achieved by bolting about 170 lb. of steel plates by the dummy nozzle at the fuselage end. They were not jettisonable. There was a tank of liquid ballast in the tail that could be dumped, but it was empty. The rocket engine's nitrous oxide oxidizer and rubber fuel are placed fore and aft of the CG, respectively, so there is little change in balance during the engine firing, which is nominally at a mid-CG.
However, there is an abort case that would result in a CG even farther back than the aft operational limit--the case where the motor doesn't light. The nitrous oxide must be dumped to avoid overloading the landing gear but the fuel remains onboard, shifting the CG aft. "Maybe you'd just fly back and land, and not try any stalls," Melvill quipped.
Scaled officials decided to abandon the planned "feather" maneuver on the sortie, which pops up the tail to put the craft in a deep stall. Melvill experimented with various speeds and trim positions on the way back to the Mojave, Calif., airport, and decided to fly the approach and touchdown at 110 kt., instead of a normal 100-105 kt. approach and 75-kt. touchdown.
Deployment of feather mechanism was made on the prior, second, flight of SpaceShipOne and recorded by a video camera on the right tail boom. Pneumatic actuators push the booms and aft wing up 65 deg. for a high-drag atmospheric entry configuration. At first the booms continue to fly straight and the body pitches up, but as the craft slows the nose drops, giving a 60-deg. descent angle at 60 KIAS (note boom angle).
The final surprise of the day came when the landing gear was extended on final approach to Runway 30 and the craft sank like a rock, compared to little change on prior flights. The glider was about 200 ft. high and Melvill put the gear out a little earlier than before, particularly since the prior touchdown on Runway 12 was roughly 3,000 ft. long.
But the gear had been changed since the last flight--forward doors had been attached to the gear legs that acted as flat plates when extended. There were no forward doors on prior flights. The extra drag had been estimated, but the actual was at least twice that, making the lift-to-drag ratio drop to 4.8 from 7 in the gear-up configuration. The higher approach speed may have exacerbated the effect.
The result was a steeper descent than expected, and instead of hitting an aim point of about 1,500 ft. past the threshold, the glider touched down before 500 ft. "If I hadn't been 200 ft. high, I might not have made it," Melvill said. The plan is to extend the gear closer to the ground in the future. Powered by a spring, it deploys in about 0.5 sec., and there is no backup procedure to go through that would argue for a high extension altitude.
The 3,000-ft.-long touchdown on the second flight resulted in a new approach technique. That landing was long mainly due to shifting winds, and clouds that blocked final approach turns, coupled with the runway being out of sight part of the time. The high key, which is the pilot's first aimpoint at the airport, was directly over the approach end of the runway at 8,000 ft. MSL, flying upwind (field elevation is 2,787 ft.). But the pilot could not see the runway through SpaceShipOne's small portholes until he had turned 180 deg. and was downwind abeam the runway.
Now the high key point is displaced 0.5 mi. to the side of the runway and the pilot can see it from that point. Flying upwind, the pilot turns in a 15-deg. bank and crosses the runway at 90 deg., is downwind abeam on the opposite side, and tightens the turn to roll out on short final, keeping the runway in sight the entire time. There are high key points on both sides of the runway.
The new scheme was devised by Scaled test pilot Brian Binnie, a former U.S. Navy test pilot. Pilots flew numerous approaches in the White Knight in a high-drag configuration mimicking the SpaceShipOne glide path to refine the procedure, and found they could fly the approach more accurately using outside references and basic instruments than looking at an internal display with an altitude predictor line.
I have full confidence that Burt Rutan will solve the problem. I'd volunteer to be the passanger
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Yeah, Burt has always been a wiz at html.
Translation: "I couldn't recover from the stall, so I put it in a spin and recovered." Sheesh! One more reason that I respect the pilots of test aircraft.
/john
If what these people are doing doesn't inspire young minds to work hard to achieve a dream, I don't know what will.
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