The boom is at least semi-rigid in 6 DOF (perhaps least so in torsion around its axis, at least for STEM types). But even with a lightweight connector on the end, you won’t be able to move it laterally very quickly once you have it out to 100 meters, and stopping its whip oscillations would be an interesting control problem.

So it looks to me like you want one long stiff boom (from one side only) which can be shot toward the approximate location of the target spacecraft. Then, you want a much lighter-weight and shorter boom from the target spacecraft. Once contact is made between the lightweight boom and the stiff boom, the lightweight boom can be retracted all the way to its spacecraft, thus making a stiff connection between the stiff boom and the target spacecraft.

(In an extreme version of this, the lightweight booms wouldn’t be booms at all, but ropes with free-flying remote-controlled microthruster pods. But I digress.)

If the stiff boom is on-orbit permanently, then its length does not need to be variable. It can simply be left extended, pushed back and forth through a gimbaled ring for 3DOF positioning. At that point, you can think about using something like NASA’s SAILMAST for the boom: http://nmp.nasa.gov/st8/tech/sailmast_tech1.html

Sailmast is impressive: very stiff in 6 DOF, and 35 grams per meter. If you had a direct spacecraft-to-spacecraft connection with Sailmast, you probably would not need a second boom. Which is good, because if you have trouble stopping relative rotation with one boom, you’ll probably have even trouble with two similar booms. Once you have two booms connected, if you don’t stop rotation within the first half-spin, then the booms will twist up in a very weak and probably destructive configuration. (Don’t cross the booms!)

A permanently-extended boom would, of course, extend beyond the docking adapter, but it wouldn’t need a whole lot of angular room to swing in. Or, it looks like a partially-deployed Sailmast is stiff for all but the last meter or so, so you might be able to extend and retract it usefully, with the force-transfer points a meter from the coil, as long as you don’t extend it fully and let it snap into its locked-extended configuration.

A separate question: I don’t understand why it would be useful for the main tether to be able to pull with more force than it can push. If you start the spacecraft moving with a forceful pull, you’d better be able to stop them with a forceful push. In theory, you could pull hard on a long boom, then wait till it got close, and when the boom was short enough to be stiff, give it a hard push to stop it. But that seems overly risky. (I’m remembering the line from Heinlein’s _The Rolling Stones_: “He started it with a pull; he thought he could stop it with a shove. They had to amputate both his legs, but they saved his life.”)

Chris

As far as the distal probe ends of the booms are concerned — you end up thinking about male-vs.-female geometry questions and whether you want “keyed” geometries so things can mate in only one orientation or attitude.

It occurs to me that you may want a ferro-magnetic fluid clutch/brake on a volumes of revolution geometry male/female pair of parts for the distal end of the first boom pair — to dampen torques in a gradual process controlled by the receiving “dockee’s” docking operations attitude control computer. Once the relative rates of rotation have been dampened to a minimum through the single made rotary joint — the second boom can be used to complete the “summation” of the combined inertias — with only a small residual inertial delta for the dockee attitude control to deal with.

This definitely is a promising alternative to what is effectively 50-year-old SOP for rendezvous and docking — if for no other reason than the kinetic physics of it.

~Jon

The probe firing mechanism may need to move left-to-right and up-and-down. If the probe attaches to the side of the hatch a method of performing the final aligning the two spacecraft will be needed. It only has to work over say the last 6 inches, it can use a conventional electric motor but may need to rotate the spacecraft through +/- 180 degrees.

~Jon