Headlines > News > Pilots discuss SSO program at flight symposium

Pilots discuss SSO program at flight symposium

Published by Sigurd De Keyser on Sun Sep 19, 2004 9:00 pm
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chabot imageBy ALLISON GATLIN: LOS ANGELES – With breath-taking speed and in leaps and bounds, the first privately funded, manned space program achieved its program milestones of rocket-powered flight to 328,000 feet and a safe return, but not without coming up against unexpected problems.
In the year since they last updated their peers on the innovative program, the members of the SpaceShipOne team successfully flew four rocket-powered flights in the unusual spacecraft, the last one reaching suborbital space and making pilot Mike Melvill the nation’s first civilian commercial astronaut.

Melvill and his fellow Scaled Composites pilots – and would-be astronauts – briefed members of the Society of Experimental Test Pilots on the space program Saturday during the organization’s annual symposium, presenting the problems faced and lessons learned from the four rocket flights.

The brainchild of famed aircraft designer Burt Rutan, SpaceShipOne is designed to be air-launched from the White Knight carrier aircraft, then use a rocket engine to boost it to 100 kilometers – 62.5 miles – above the Earth, where the occupants experience weightlessness and see the blackness of space. Completing an arc, the spacecraft descends to a runway landing, much like a glider.

Like any developmental program, progress toward that final goal comes in steps.

SpaceShipOne began with flights secured beneath the belly of the White Knight, then unpowered glide flights.

The first rocket-powered flight took off Dec. 17, with pilot Brian Binnie at the controls. The 15-second rocket burn was the maximum allowed under Federal Aviation Administration rules without a space launch license.

Binnie traveled to 68,000 feet at speeds of Mach 1.2, making his first manned supersonic flight by a privately built, nongovernment-funded aircraft.

Video of that flight shows Binnie thrown back in his seat as the rocket ignites, a problem that was fixed on later flights. The trouble was that as Binnie was forced about six inches back in the seat, he also pulled back on the control stick, resulting in problems controlling the aircraft.

That and several other lessons learned from Binnie’s flight were addressed for the second rocket-powered flight, on April 8, with pilot Peter Siebold at the controls. Siebold traveled to more than 105,000 feet, with a 40-second rocket engine burn producing speeds of Mach 1.6.

Because the earlier flights were not planned for the maximum burn time for the rocket engine, the spacecraft did not carry a full load of liquid nitrous oxide. This led to a “sloshing” in the tank when the rocket ignites, which shifted the spacecraft’s center of gravity and affected stability and control, Siebold reported.

Siebold’s flight carried a full load of fuel, as well as sported a new pilot restraint system.

However, more challenges emerged in the second flight.

“It opened the door to as many questions as it answered,” Melvill said.

Controlling the spacecraft during its high-powered ascent and into supersonic speeds has proven to be a recurring challenge for the pilots. The spacecraft uses conventional manual controls, much like those found on general aviation aircraft, with additional electric trim control to help aid the pilot during the high-stress moments of the ascent.

“The workload after ignition is very high,” Melvill said, and is further complicated by wind shears that buffet the spacecraft.

The pilot-designed simulator allows the pilots to practice the skills they need to control the rocket flights, but the flight tests have shown that the simulator cannot precisely predict and mimic every scenario, and the pilots have to fall back on their own inherent skills, they reported.

In one example of this, Melvill had to rely on positioning relayed by radio from the ground when the cockpit display went out shortly into his first rocket-powered flight, on May 13. Using the black sky of space as his heading, Melvill reached 211,400 feet and the edge of space. The 55-second rocket engine burn produced speeds of Mach 2.5.

Despite a rough ride on the ascent, SpaceShipOne’s re-entry is “amazingly smooth,” Melvill said, requiring little work on the part of the pilot.

To re-enter Earth’s atmosphere and land, SpaceShipOne employs an innovative means of creating huge amounts of drag early on, so it lands at roughly the same speeds as a general aviation airplane.

To accomplish this, the spacecraft’s twin-tail section lifts until it is virtually perpendicular to the wings and body, generating drag and slowing the spacecraft.

This feathering of the tail causes the spacecraft to right itself for landing, similar to a falling shuttlecock. The spacecraft then comes back down level to the Earth, with thermal protection materials preventing the underside of the craft from becoming dangerously hot.

“Once the feather comes down, it’s much smoother,” Melvill said.

Once the spacecraft drops back below Mach 1, the ride becomes rough again, as evidenced in video of Melvill’s flight.

On his historic June 21 flight, Melvill strayed off the intended trajectory when he was first buffeted by wind shear at launch, then overcorrected trying to regain control.

“Wind shears can wreck havoc,” he said, in this instance costing him about 30,000 feet in altitude by changing the trajectory. He was lucky to reach the 100-kilometer mark, “succeeding by less than the width of a gnat’s eyelash,” he said.

That flight also was hampered by lopsided thrust from the rocket.

“I suddenly found myself 20 miles south of Mojave,” Melvill said. “I headed back to the airport as fast as I could go.”

To accomplish its space travel goals, Scaled Composites designed its own rocket motor, emphasizing safety and reliability by removing the riskiest elements.

Fueled by hardened rubber and nitrous oxide – laughing gas – the rocket “looks remarkably to me like a turkey baster,” Binnie said, with a bulb on one end holding the nitrous oxide and the long cylinder of the ignition chamber attached.

A simple design, the only moving part is the valve that allows the nitrous oxide to enter the chamber, he said.

Poway-based SpaceDev was chosen to provide the valve and other components of the hybrid system.

The rocket’s first ground test ended in flames and failure, however.

“We had visions of the Tier One (Scaled Composites’ name for the space program) literally going up in flames,” Binnie said.

Changes eventually led to a rocket motor that closely matches the optimum requirements for the suborbital flight, providing the right amount of thrust at the right moments of flight.

Too much thrust early in the flight wastes energy, and too much too late puts the spacecraft at risk of tumbling out of control in the thin air of the upper atmosphere.

The SpaceShipOne team is preparing for the two flights required to claim the Ansari X-Prize, an international competition to jump-start the commercial space tourism industry. The competition will award $10 million to the first privately funded team to successfully build and launch a spacecraft capable of carrying three people to 100 kilometers and safely return to Earth, then turn around and duplicate the feat with the same ship within two weeks.

The first flight attempt is scheduled for Sept. 29 at the Mojave Airport, with the second coming as soon as Oct. 4.

These flights will differ from the June 21 flight in that the spacecraft will carry more payload weight than before, the equivalent of two passengers for a total of 270 kilograms (approximately 600 pounds).

The pilots are working on their techniques in the simulator to minimize rolls as the rockets ascends to elicit maximum performance from the rocket burn, Siebold said.

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