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In an answer to my previous question, OrganicMarble posted a video:

The booms like through the center of ISS solar panels are quite common in space use - supporting magnetometers, keeping modules away from the core craft, etc.

I always thought they extend by "scisoring", pantograph mechanism action, whole boom getting slimmer but longer simultaneously. Apparently this is not the case - in the video you can see the still extended part of the boom remains "immutable", same unchanging structure. So what happens to the part of the boom that is being pulled into the assembly? Vanishing into a wormhole? Getting disassembled into atoms by nanomachines, to be reassembled later? How does that extending/retracting mechanism work?

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tl;dr - each bay of the mast collapses as it is pulled into the cylinder.

The ISS solar array mast is a truly ingenious mechanical structure which can be retracted into a remarkably small canister. It was developed by ATK and is called a Folding Articulated Square Truss (FAST) mast.

This picture shows a FAST mast without arrays. enter image description here

The mast is composed of aluminum longerons, stainless steel wire rope diagonals, aluminum rigid battens, and flexible fiberglass battens.

Every second batten ring is flexible. Therefore, the battens are able to buckle and two cells of the boom can be collapsed.

enter image description here

The deployment is driven by the strain energy of the buckled battens and is controlled by a retaining mechanism in the container. This mechanism is a rotating nut with four helical grooves. These helical grooves engage roller bearings located on the corners of each mast section in a manner similar to that of a screw being pulled through a rotating nut. It requires two revolutions of the nut to fully extend one mast bay.

enter image description here

This image shows a cross section of the deployment mechanism. It's a bit complicated, but you can see the nut at the top (gray area), the collapsed truss sections at the bottom, and the drum/lanyard/cradle mechanism that feeds the sections into the bottom of the nut.

enter image description here

References

ADVANCED DEPLOYABLE STRUCTURAL SYSTEMS FOR SMALL SATELLITES

Static Stability of the Space Station Array Fastmast structure

Static Stability of a Three-dimensional Space Truss

Personal Notes

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