Will the Air Force Finally Get a Spaceplane?
If Boeing's X-37 can maneuver politically as well as in space.
- By Ben Iannotta
- Air & Space magazine, January 2003
(Page 2 of 5)
“We were not off on the lakebed; we were coming in with all the other airplanes,” says Randy Hein, Boeing’s X-40 program manager. “They cleared the airspace, but we were coming in on an operational runway, which was a neat thing to be able to do.” As Hein speaks, the gleaming X-40 waits inside a storage facility at Boeing, like a Ferrari ready to be taken for another spin.
Hein and his team sweated out one moment during the first flight test at Holloman that captures the challenge of designing an aircraft to land without a pilot. “We had one touchdown where we lifted up and came back down—bounced, if you will,” Hein says. The vehicle was undamaged, and the team’s analysis quickly pinpointed the problem. When the X-40’s tires touch down on the runway, the main gear tires spin up from zero to about 195 mph, creating a force that pushes the nose down. Piloted aircraft show the same nosedown tendency, and in training pilots learn how to compensate to keep the nose up on landing. In the X-40, flight software must compensate, and on the first landing, the X-40’s automated control surfaces overcompensated (as more than one pilot has done), and the vehicle briefly lifted off. Boeing’s engineers revised their computer model of the forces created by the tires’ spin up and reprogrammed the flight software to increase the reaction rate of the control surfaces.
All subsequent landings were “nominal,” as engineers like to say, and the improved computer model will contribute to smooth landings for the more complex X-37. The X-37 is managed through a NASA program to test technologies in the propulsion, avionics, structures, and thermal protection systems of reusable launch vehicles. It is 25 percent larger than the X-40 and made of graphite/bismaleimide, a composite that can withstand higher temperatures than graphite/epoxy.
Like a Rock
The most daunting technological problem facing Boeing engineers in designing the X-40 and X-37—the problem facing any team designing a reusable spacecraft—is inventing a configuration that can achieve control through a range of flight regimes: reentry, hypersonic flight through the atmosphere, and subsonic approach and landing. “There are great debates” about the best way to land a returning spacecraft, concedes Randy Hein.
One approach to the problem is represented by an earlier NASA program. The X-38, designed as an ambulance for emergency oneway flights from the International Space Station, was a lifting body with a wedge-shaped fuselage that was slowed to land on skids by a massive 5,500-square-foot parafoil (see “Lifeboat,” Aug./Sept. 1998). The two-ton space shuttle takes another approach, gliding home on delta wings, slaloming nose-up through a series of S-turns to bleed off speed. The X-40 and X-37 are shuttle-like vehicles, with stubby fuselages and small wings, all sized so that the vehicles can survive the high temperatures of hypersonic speeds and produce the lift needed at landing.
“We kind of refer to it as a ‘lifting wing-body,’ ” says Arthur Grantz, Boeing’s chief engineer for the X-37. While the fuselage produces more lift during the high-angle-of-attack entry phase, the wings are more important at landing and generate 60 percent of the lift. “We’re more like a rock coming down than an airplane,” says Boeing engineer Dave Childers, who is one of the team’s experts for navigation.
For its descent through the atmosphere, the X-37 uses four flight control surfaces. Ruddervators, a combination of the words “rudder” and “elevator,” control yaw and pitch. “The functions are combined because a centerline rudder is ineffective at high angles of attack [the X-37’s attitude during most of its descent], and a horizontal tail and elevator would experience very high temperatures,” says Grantz. The vehicle is also equipped with a body flap, a surface beneath the main engine that supplements pitch control at very high speeds.
On the wings’ trailing edges, flaperons provide roll control and supplementary lift at landing. Finally, a speed brake is extended from the top of the fuselage to help control speed during the X-37’s approach to the runway.