United We Orbit
It's a story of spacecraft meets spacecraft.
- By James E. Oberg
- Air & Space magazine, January 1997
(Page 3 of 5)
By the time the Russians were designing the Mir space complex in the mid-1980s, they needed exactly this kind of system for the Buran space shuttle, which was to mate with the station. The shuttles were too massive for the probe-drogue design, and the Russians would now be using a variety of different docking combinations—Soyuz to Mir, Soyuz to Buran, and Buran to Mir. The androgynous system was the only one that could satisfy all these requirements.
The Russians called their design “APAS,” for “androgynous peripheral aggregate of docking” (“docking” in Russian is stykovka). They improved the Apollo-Soyuz design in several significant ways, most visibly by turning the guide petals inward rather than outward. The system was perfectly designed for the Buran-Mir dockings. But the Russian shuttle was scrapped before the system got the chance to prove itself.
Meanwhile, U.S. space designers had been developing their own docking mechanisms for the shuttle and the Freedom space station. The only principle guiding this complicated, clumsy system seemed to be that it not look like the Apollo-Soyuz design. By the early 1990s, however, the political winds had changed, and it was no longer unacceptable for Americans to acknowledge Russian space expertise. After a brief review, the Russian system designed for Buran-Mir was adopted for shuttle-Mir and the space station, with Rockwell and RSC Energia doing the modification work.
When Gibson and Precourt were tapped to fly the first docking mission, they knew they were in for a challenge. No space shuttle docking hardware had ever worked properly on its first attempt in orbit. The highly public embarrassments of failed first attempts to dock with the Solar Maximum satellite in 1984 and the Intelsat satellite in 1992 (both of which involved hardware carried by spacewalking astronauts, not vehicles), as well as several less publicized but equally frustrating failures with other space hardware, reminded everyone how easily things could go wrong.
Even after all the hardware had been analyzed and tested piece by piece, experienced engineers knew they weren’t finished. At the insistence of veteran space docker John Young, NASA added a special program for “end-to-end testing” at the Kennedy Space Center’s Orbiter Processing Facility, where the shuttle is still horizontal. The docking assembly was installed in the shuttle’s payload bay with all the flight hardware in place. Test engineers rigged up a mockup of the passive mechanism on Mir and lowered it by crane at docking speeds of only inches per second. They verified in the cockpit that the instrument panel performed as advertised throughout the whole sequence.
One value of these tests was to raise the crew’s comfort level with the post-contact damping process, the time between initial capture and hard docking, when the two giant vehicles would be only loosely joined together. During this time, Mir’s attitude control system is switched off so as not to introduce motions that could bend the docking mechanism. But even in this “free drift” mode, the Russians had worried that random twisting of the two large masses might never settle down. Noted Precourt: “This would prevent us from drawing the ring back in.”
Based on the ground tests, the crew came up with a solution. Precourt explains: “We interrupted the auto[matic] sequence at the first point we saw ring align, stayed there about a minute, waited until motion stopped, and then we retracted.” With the rings on Mir and the shuttle perfectly parallel, the hard dock could proceed.
Even though their hardware was different, the shuttle-Mir dockers knew they had much to learn from the previous generation of astronauts. Precourt spent time chatting with six-time spaceflight veteran John Young, now a special assistant to the Johnson Space Center director. Precourt was especially interested in the difference between simulation and reality. “In a simulator, a lot of the sensations aren’t there, but in flight you are subject to a lot of distractions,” he says. Young told him to trust the simulators, which was good advice—the crews who’ve docked with Mir say they are extremely faithful to the actual experience. If anything, says Precourt, the real flight “was a lot smoother than most of the sims, in terms of everything working.”