Folding and Unfolding Large Thin Sheets

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Sometimes, bigger really is better, especially when you’re building spacecraft and you’re dealing with quantities that come per unit area: electromagnetic radiation, atmospheric drag, interstellar hydrogen, whatever.

Want more power from your solar panels? Get solar panels with more area. Want more drag as you’re entering a planetary atmosphere? Get a parachute with bigger area. Want more solar photons to push your solar sail? Bigger sail. Want to reflect more radio waves? Bigger antenna. You get the picture.

Bu there’s a problem: as you make things bigger, you also make them heavier. Heavy is bad. You pay for every kilogram you carry on a rocket, and you pay a lot. (Point of reference: every kilogram of payload costs $4000 on the SpaceX Falcon 9, and that’s on the cheap end of launch vehicles.) So you keep the mass of these big things down by making them thinner. And so these structures start to look like large, thin sheets.

There’s another problem: these structures need to fit inside the rocket for launch. This wouldn’t be an issue if our rockets were super big (so many challenges of space exploration would be resolved if our rockets were arbitrarily big), but they’re not. The biggest rockets we have right now (that would be the Ariane 5 and the Atlas V) can fit things that are no bigger than 5.4 m in diameter. That’s about the distance from the short line to the front wall of a squash court. And if you’ve never played squash, let me tell you that that’s tiny.

People figured out the solution a while ago: build jacks-in-the-box (jack-in-the-boxes?) that fold up and compactify for launch, and spring open when in space. An astounding variety of mechanisms have been developed to unfold, inflate, or unfurl these structures. (In the business, these are known as deployable structures.) And as engineers began to think about folding these large thin structures, they discovered one very important thing: it is much easier to package structures along one dimension than to package them along two dimensions.

Let me be more precise: given a sheet of some length and some width, it is almost always easy to stuff it into a box of smaller length but same width. You can wrap up the sheet (like toilet paper, aluminum foil, saran wrap, wrapping paper, paper towels, etc.) or you can accordion fold it (like an accordion). But if the box has smaller length and smaller width than the sheet, well, life gets tricky.

Last year, my fellow researchers and I came up with a way of compactifying large thin sheets by first accordion folding, and then wrapping them. Now this may seem like an obvious thing to do, but if you actually try and do this with standard creases, you’ll run into issues: the sheets will crumple and buckle along the crease lines as you wrap. We use very non-standard folds that allow for some sliding along the fold line, and with these folds, we can package sheets very tightly and without this crumpling. (For more details, check out our paper on the subject.)

Once packaged, we need to make sure we can unpackage these sheets in a controlled way. (Obviously, when you’re unpacking these sheets in space, you don’t want things flying around chaotically.) The video below shows one of our deployment tests of these sheets (or membranes) that has been packaged using our new method.



Our plan is continue developing methods like these for different classes of structures, but it’s heartening to note that our ideas work for simple sheets at small scales.

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