United We Orbit
It's a story of spacecraft meets spacecraft.
- By James E. Oberg
- Air & Space magazine, January 1997
(Page 2 of 5)
Docking problems frustrated Russia’s first space station mission in 1971 and nearly aborted NASA’s first Skylab mission two years later. When the Russians added the Kvant science module to the Mir station in 1987, an errant trash bag got stuck in the docking interface, preventing an airtight seal until spacewalking cosmonauts removed it. Other failures and close calls convinced both U.S. and Russian space engineers that nothing about space docking would ever become routine.
Bumping two large masses together in orbit without damaging or breaking anything makes for a tricky physics problem. Vehicles docking on Earth have at least some of their motion already constrained at the time of contact. Freight cars move along rails, ships float on water, even aircraft have aerodynamic stability. But in space, position and orientation can vary in all three dimensions, and can change at different rates. All these variables—Precourt calls orbital docking an “eight-degrees-of-freedom problem”—have to be controlled simultaneously to make sure the final contact happens within the mechanical limits of the docking hardware. On Earth we also encounter natural damping forces—friction, air and water resistance, the restraining forces of rails or cables. In space, all the energy has to be absorbed and damped out within the vehicles themselves.
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.