In previous posts I’ve mentioned that when I first got to NASA I worked in the Propulsion Research Center, which was a fun place to work because you got to think about and try just about anything you wanted to so long as you could get funding, and there was this sugar-daddy at NASA named John Cole who would fund all kinds of crazy stuff. I never got any funding from John but my patron was Les Johnson, who was kind of like NASA’s “point-man” on tether technology. After about two years in the PRC, Les told me it was about time to quit fooling around and become a serious-type manager like him, and to come and join him in the newly forming In-Space Propulsion project.
So in the fall of 2002 that’s what I did, and before long I was writing NRAs (National Research Announcements) to solicit universities and corporations to bid on technology work for tethers. We put the first NRA out for tethers and got responses and had a meeting where a committee picked the winners in March 2003. After that things started getting serious. We had real money for the first time to do momentum-exchange tether work, and there were still so many unanswered questions that needed to be solved.
Sometimes fate or luck or serendipity drops things in your lap. In the summer of 2002, I met one of the most clever and hard-working people I’ve ever had the good fortune to meet–Dr. Stephen Canfield of Tennessee Technological University. The next summer he was down at MSFC and I was in the middle of trying to figure out the answer to a very thorny problem: if you have a tether that’s spinning, how do you keep the solar panels pointed at the Sun? My friend Kyle Frame and I would sit in my cubicle for long stretches of time with pieces of paper pretending to be solar panels and pencils and sticks standing in for the tether, trying to figure out some way to do it that wasn’t totally foolish.
One day Steve Canfield stopped in and asked us what we were up to. We described the problem and he asked a simple question:
“Do you care what orientation your solar panel is in so long as it is pointed at the Sun?”
I said no, we didn’t care, and then he showed me something he’d been working on since he was a grad student. It looked like this:
He called it a “Trio-Tristar Carpal Wrist Joint.” I thought that sounded like a real mouthful so I just called it “Canfield’s joint” and eventually everyone (except Canfield) began to call it a Canfield joint. It was kind of a crazy looking thing that you couldn’t figure out what to do with it unless you held it in your hands and started playing with it. Unfortunately, in a blog post I can’t reach out of your screen and hand you your own Canfield joint to play with, because if I could you’d figure out in a few seconds what I’m talking about, but the real magic of the Canfield joint is that you can point the joint anywhere in a hemisphere without winding up anything.
The joint has several parts. There’s the “base plate” which stays attached to whatever the joint is mounted to, like your spacecraft, and then there’s the “distal plate”, which points to whatever it is that you want to point at. There are six legs on the joint, in three units. The joint is called a “parallel structure” because there’s more than one load path for the loads to follow, and this is what gives it its potential strength. Where the legs mount to the plates is a simple revolute joint. I didn’t know what that meant so I asked Canfield and he said that it just meant that it was a simple, one-degree-of-freedom (one way to move) joint or hinge. Where the two legs come together you could have a spheric joint (a ball and socket with two degrees-of-freedom) or you could have three revolute joints in series. That’s what we usually do.
I asked Canfield what the joint was for. He said that he originally wanted to use it to replace the CV joints in cars, since if it had all revolute-joints then it wouldn’t need a boot. If I hadn’t had to replace the boot on the CV joint in my car when I was in college and dirt-poor, I wouldn’t have had any idea what he was talking about, but the loss of money was still burned in my mind, so I appreciated that application.
Well, to make a too-long story shorter, I learned how the Canfield joint worked and figured out how to solve my little problem on the tether. Tell me if you like the result:
Canfield Joint on MXER Tether
Medium View of Canfield Joint
Closeup of Canfield Joint
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