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The following image is a technical diagram of the international Space Station's RMS. One (or possibly two) of these units will be included in the LEO Assembly Node.
End effector: For this kind of work, we want to use the RMS end effector that the Canadians designed for the Shuttle RMS. That gives us interoperability with NASA and the Russian Space Agency. At this level of rendering, it just looks like a can open at the business end. Inside the can are three wires which grapple the post on the grapple fixture. That gives us automatic centering and fine alignment. We especially need the fine alignment to mate the power and data connectors as we inch-worm around our little space station.
Joints: In-line joints won't allow the arm to fold up, and the booms can't rotate past each other. Offset joints provide both these features. We can get a much larger arm into our launcher's payload fairing if we fold it up for launch. Allowing the booms to rotate past each other gives us more operational flexibility and avoids joint singularities. That can be especially important if we have to maneuvering around in tight spaces.
SPAR used in-line joints on the Shuttle RMS because they had to squeeze the whole arm into the relatively tiny volume next to the payload bay sill. Those same requirements forced them to add a complex and heavy mechanism to roll the whole arm out to clear any payloads in the payload bay before it could begin operation. On the International Space Station, and on our Artemis Project assembly fixture, we can launch the RMS as a separate unit.
With the yaw and pitch joints, we can get the end effector to any location in space that the arm can reach. We need the yaw joint to deal with situations where the orientation of the grapple fixture we're after is out of the plane of the pitch joints once we get there.
Besides the large illustration showing how the pitch joints work, there are three more little pictures in the image:
(1) RMS folded up for launch. This is in the background. Note that how the offset joints allow the arm to fold up to a fraction of its extended length. This is a great advantage over the Shuttle RMS, where the arm can only be as long as the Orbiter payload bay.
(2) RMS extended for work. Show what these things might look like when they're extended. Unfortunately the illustration is looking from perpendicular to the plane of the roll joints. I just plain forgot to roll the arm. (Besides, I'm using a 2-D drawing program, so adding a roll could get really tricky.)
(3) End view of End Effector. We're looking into the business end of the End Effector. There's a rotating mechanism inside the End Effector which tightens those wires in a spiral around the central post of the grapple fixture. (See rmsparts.gif for a sketch of the grapple fixture.) As those wires tighten, they pull the End Effector down on the grapple fixture. Those triangular wedges on the grapple fixture are canted up toward the center, and guide the End Effector into perfect alignment.
Once the grappling is complete, a little motor drives home the electrical connectors for power and data. (I showed that mechanism as a rectangular box in rmsparts.gif, partly because I'm lazy and partly because it's a rectangular box on the real thing.)
The electrical interfaces are reversible: the RMS can either get power and data from the grapple fixture, or provide power to a payload through the interface. This is one of the bits of existing technolgy we'd really like to use for the Artemis Project because designing all those mechanisms and electrical interfaces, and writing the software to control it, could become very expensive.
We also have the luxury of having to launch the RMS only once, so that the mass of the equipment is not quite as critical as on the Shuttle. Launch to low earth orbit is expensive, but it gets really expensive if you have to pay for dead weight every time you light the boosters.
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