Rutan aims for space: A look at SpaceShipOne
by Jeff Foust
|“The program is a lot like the X-15, but we had a minor annoyance: we had to build our own B-52,” Rutan quipped.|
After passing through its peak altitude, SpaceShipOne will quickly descend, again on a steep trajectory. To cope with the heat of reentry the spacecraft has a unique feature: the trailing edge of its wings, and the twin tail sections attached to them, rise from the horizontal to nearly the vertical position. This reconfiguration puts the spacecraft into a stable, “carefree” orientation, making it less susceptible to errors in the angle of attack. This is a far cry from the X-15 and space shuttle, which must be carefully oriented for reentry. “We go straight into the atmosphere for reentry without ever touching the controls,” Rutan said. In other words, SpaceShipOne is less a space shuttle than a shuttlecock.
During reentry the vehicle reaches its peak speed of about Mach 3. However, due to the low dynamic pressures on the spacecraft, the vehicle never exceeds an “equivalent” airspeed of 285 kmph. The heating on the spacecraft is also limited during reentry—the stagnation temperature at peak heating is just under 600°C—meaning the spacecraft needs only a limited thermal protection system (TPS). “We have designed the structure so that the crew is not in jeopardy if the TPS fails,” Rutan said, although he added that the vehicle would require repairs if the TPS failed.
After reentry, at an altitude of about 24,000 meters, SpaceShipOne lowers its wings and tails back to the horizontal position and becomes a glider. It is designed to glide for distances of up to 65 kilometers, allowing it to land back at the runway where it and the White Knight took off from even if there was a significant error in the angle of its initial trajectory. For at least the planned series of test flights, the takeoffs and landings will occur in Mojave, although the rocket flight portions will take place in restricted airspace over nearby Edwards Air Force Base.
Despite the generally radical concept of building a reusable suborbital spacecraft, Rutan is taking a conservative approach to development. Many, if not most, of the subsystems on SpaceShipOne have already been tested on White Knight, which has flown fifteen times since August 2002, racking up 20 flight hours at altitudes of over 15,000 meters. For example, both vehicles have virtually identical crew cabins with similar controls. The cabins are distinctive for their bulbous shape, with over a dozen portholes in place of a conventional windshield or canopy. That design allows Scaled to pressurize the cabin to near sea-level conditions, permitting the crews to work in a shirtsleeve environment rather than wear bulky pressure suits. Rutan said that decision was based on his work with Proteus, a high-altitude aircraft for which White Knight is a direct descendent. “What we did here is put the spacesuit outside the vehicle,” he said.
|During reentry, SpaceShipOne is less a space shuttle than a shuttlecock.|
While Scaled is developing most of the vehicles’ systems in-house, one key exception is the propulsion system for SpaceShipOne. “We’re not rocket scientists here,” Rutan explained. He said he looked at off-the-shelf systems, such as the solid-propellant engine used on the third stage of the Pegasus launch vehicle, but found those options “totally unaffordable.” Seeking instead to develop a new hybrid-propellant engine, he looked at a number of companies. The “primes”, or major aerospace companies, were also unaffordable, he concluded, and while a number of smaller companies were cheaper, he was concerned that it was too risky to put the propulsion system into the hands of a single, small company.
Rutan’s solution was to hold a competition. He selected two companies, Environmental Aerosciences Corporation (EAC) and SpaceDev, to each develop and test a hybrid propulsion system using nitrous oxide (laughing gas) oxidizer and hydroxy-terminated polybutadiene (HTPB, or rubber). Florida-based EAC has flown sounding rockets using nitrous oxide/HTPB engines it developed since the mid-1990s, while California-based SpaceDev has an active hybrid propulsion research program based on intellectual assets it acquired from the former American Rocket Company (AMROC), which developed a hybrid-powered rocket in the late 1980s. The competition between the two companies is ongoing, and Rutan did not indicate when Scaled would select a winning design.