All The Socks That Need Darning

George Turner in reply to Michael Cain says:

Well, you wear the socks the 99% of the time your not on an EVA.

As an aside, I had an idea for a force-free suit joint that I haven’t had time to prototype.

You stitch up a typical sleeve, with the usual gathers around the elbow (like a wooly worm section), with two pieces of nylon that that keep the worm section from stretching out. That’s pretty much the standard pressure suit elbow or knee joint, as you’d have on a Russian or US suit. But since it’s not a constant volume joint, it wants to straighten out under pressure.

So you build one and measure it’s torque-displacement curve (you can see some of these posted for various designs). The more you bend such a joint, the harder it is to bend.

But for any particular joint, that torque-displacement curve is constant. So all you have to do is counter the torque by a fixed function of joint angle. To do that, you give the joint a natural elbow. For an elbow you use some short rods and a bearing, or just a couple of rods joined with a piece of nylon webbing, or you use solid pipes for the straight sections away from the joint, with the upper and lower sections, looking much like a medieval armor.

Then you attach a tendon cable, which will act just like the tendon connecting your bicep to your forearm. The tendon cable could attach to a spring, somewhat counter-acting the natural tendency of the suit joint to straight under pressure.

But then you have the spring pull on a cord that goes around a pulley, while a second sheave of the pulley is cut as a cam that’s based on the natural torque-angle curve of the suit joint. That should allow you to completely cancel the joint’s natural stiffness.

But instead of a spring, you could use an air piston, with one end open to vacuum and the other end connected to the suit pressure, so that the force applied to the pulley/cam system by the piston is directly proportional to the pressure differential of the suit. As an interesting aside, what this mechanism ends up doing is making a constant-volume suit joint, with the motion of the air-piston exactly compensating for the change in internal volume caused by flexing the joint. Thus no work is performed in bending the joint.

At some point maybe I’ll try to build a little prototype. When I was last looking at doing it, I had a piece of nylon tent cloth sewn into a tube, and was trying to find a good sealing compound. There are quite a few polyurethane coatings used for inflatable rafts that might work well.

If you got really fancy with the cams and cables, I think you could make a 3-D joint on the same principles, for a neck, a torso, or possibly a hip.Report