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."
Before mission STS-71, the astronauts "flew" over 200 approaches in a variety of simulators. Docking with Mir requires a very slow closing speed - barely more than an inch per second during the final approach. It also demands great precision. The docking rings have to be parallel within two degrees in each axis, and the targets have to be aligned within three inches of each other. The astronauts have various tools to help them measure the alignment. A metal "stand-off cross" extends on a rod above and parallel to a black painted cross on the Mir target. If the crosses appear in TV views to line up perfectly, the pilot knows he's on track. The TV cameras also have grid markings to make it easier for the astronauts to check their alignment.
One concern had been the disorienting view caused by the camera's being at a distance from the pilot's eyeballs. "You're not looking at the real world," explains Precourt. "It's not like landing an airplane with a view straight out the front windshield." It's more like closing your eyes, holding your hands out, and trying to touch your fingertips, he says. But even though it took some getting used to during training, it turned out not to be a problem.
Gibson and Precourt, as well as every docking crew after them, learned in the simulators to hit the marks every time, even when jets and instruments and computers failed. On the STS-71 docking, the angular errors were measured in tenths of degrees, almost too small to be noticed. The arrival time was nearly perfect too: They were only two seconds off.
Experience has shown that on-time arrival doesn't matter all that much. "I always argued against getting hung up on the docking time as if it were critical," says Kevin Chilton. "I wanted to dock a minute later or a minute early just to show it's not important." He ended up docking "pretty much on time" anyway.
In fact, so far every docking has been a model of precision. "When you think about it," says Precourt, "it's pretty amazing that you'd have two vehicles flying in space that are subject to bending and moving, yet the relative position of the docking ports can be precisely known when we arrive."
With at least five more shuttle-Mir missions planned, and with dockings to the international space station scheduled to begin in 1998, orbital docking is finally becoming, if not routine, then at least no cause for great anxiety. Engineers working on the space station have come up with a few modifications to the shuttle-Mir design but not many. They plan to fine-tune the orbiter's damping mechanism to further reduce the energy transferred to the station at contact. The station also will have a few of the old-style probe-drogue ports, since a variety of Russian, American, European, and Japanese vehicles will have to dock with it.