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.
Son of Shuttle
When Boeing absorbed space shuttle builder Rockwell in 1986, it inherited Rockwell’s years of experience in studying the space shuttle’s landing profile. “We know the shuttle’s characteristics,” says X-37 program manager Al Santana, “and that helps us correlate our data and devise flight control algorithms.” Santana worked on the shuttle’s guidance, navigation, and control systems when the vehicle was being designed by Rockwell.
In the 21 years since the shuttle’s first flight, according to Santana, the important advances for reusable spaceplanes have taken place in composite structures, thermal protection systems, and avionics. Boeing engineers are experimenting, for example, with a new heatresistant composite material: carbon/silica carbide. Carbon/SiC, or C/SiC, can be used to form lightweight, thin control surfaces, Grantz says, that don’t require the additional external insulation of ceramic tiles. “You can get aerodynamic surfaces with smaller radiuses and thinner airfoil sections,” he adds.
Grantz expects the X-37 to withstand reentry temperatures even higher than those that the shuttle’s ceramic-tiled skin protects it against. The surface of the X-37 will heat to 2,700 to 2,800 degrees Fahrenheit, compared with shuttle temperatures of 2,400 to 2,600 degrees, he says.
The trailing edges of the X-40’s wings are an awkward-looking two inches thick because engineers assumed the X-37’s trailing edges would require ceramic tiles. Now that engineers plan to use C/SiC for the flaperons and control surfaces on the ruddervators, the wing and ruddervator trailing edges will be only one inch thick.