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Safe harbor: A Soyuz (foreground) and Progress supply vehicle docked to the International Space Station in August 2007. (NASA)

United We Orbit

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Because these are problems imposed by the laws of physics, it’s no surprise that in the mid-1960s, U.S. and Russian engineers came up with essentially the same design for docking mechanisms. Both countries built systems that worked like this: The chase vehicle extended a long, stinger-like probe with capture latches at its tip. On the target vehicle was a cone-shaped receptacle. When the tip of the probe entered the wide end of the cone, it was naturally guided to the back, where another latch mechanism was waiting. The engagement of these two latches was called “soft docking.” The docking probe then retracted, drawing the two vehicles together so that facing rings could be latched together for a “hard dock.”

This was the basic design used for NASA’s Apollo lunar missions and for the Skylab space station. It also became standard for Soviet vehicles and, with one exception, has served all Soyuz, Progress, and science module dockings with Russian space stations to this day.

The inescapably “male” and “female” nature of the probe-drogue system has led to countless earthy jests by astronauts and cosmonauts over the years. The major drawback is equally obvious: Only mechanisms of different types can successfully mate. For short spaceflights this wasn’t a problem, since each vehicle could easily be outfitted with mission-specific hardware. But engineers knew that at some future point it would make sense to build spacecraft that could dock with any other vehicle in orbit.

The “androgynous” docking mechanism sprang from this anticipated requirement. When Nixonian détente thawed relations between Moscow and Washington in 1971-1972, the resulting plan for the symbolic Apollo-Soyuz orbital docking gave space engineers the opportunity to build and test an androgynous docking mechanism. The new design had an immediate political advantage: Neither the Russian nor the U.S. spacecraft would appear dominant. Though arguably for the wrong reasons, space engineers were enabled to do the right thing.

Based on preliminary sketches by virtuoso spacecraft inventor Caldwell Johnson (a self-made NASA engineer who had co-designed the Mercury capsule in the 1950s), as well as a symmetric ring-to-ring system designed at about the same time in Moscow, U.S. and Russian engineers—led by docking expert Vladimir Syromyatnikov— joined forces and came up with a new design. Each vehicle would have a docking ring with three open “petals” extending out from it. The petals were for alignment only: They fit slot-and-groove style between the petals of the other vehicle’s ring, so that the facing rings could fit together only in the prescribed way. During docking, the ring on the active vehicle (complete symmetry was sacrificed) would extend outward on shock absorbers and be rammed (slowly!) into the passive vehicle’s ring. The petals would then fit like fingers sliding together, and latches on the active vehicle’s petals would catch latches on the target docking ring. Finally, after the motion from initial contact was damped out, the extended ring retracted to pull the two vehicles closer together. At that point the heavy latches around both rings would engage to achieve a hard dock.

The new system worked fine on the one mission it flew (Apollo-Soyuz), and its advantages over the probe-drogue set-up immediately became clear. For one thing, the damping mechanism allowed it to handle much more massive vehicles. True, it demanded more accurate alignment from the pilots, but neither pilots nor engineers saw that as a problem.

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.

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