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I wonder what the most important potentials and problems are with folding out structures in space. The basic idea is to achieve large structures in space while reducing launch mass and launch volume, and also avoiding human space walking construction work in microgravity. What stress are different kinds of structures in space exposed to? Such as astronauts moving around in them, engines accelerating them.

I can think of three purposes with having big structures in space:

  • 1) Parabolic dish for radio telescopes or solar collectors.
  • 2) Living space for astronauts.
  • 3) Radial distance for an "artificial gravity" centrifuge.

Concerning #1 above, does the geometry of a parabola provide some easy way to "throw out" a foil to get the desired shape, or must it be painstakenly formed inch by inch? And would a thin metal foil work well as a radio telescope dish? Could even a useful mirror for optical telescope be unfolded? Russia has a 10 meter radio telescope in space, the Spektr-R interferometer. It must've been folded out in space.

On #2 Bigelow Aerospace already have two (uncrewed) expandable space stations in LEO and will attach one to the ISS next year. How are they unfolded? They use the term "expandable" instead of "inflatable".

On #3, rotation to create radial acceleration that simulates gravity, large distances are prefered in order to avoid unnatural shifts in weight as crew moves radially. At least a distance long enough that two meters, i.e. standing up, is a practically negligable fraction. I've seen a proposal to use "air beams" instead of wires. An "air beam" is like a hose inflated with air or other gasses so that it gets stiff. It would allow for (human) movements inside of it and doesn't get slack like a wire could. The only actual application of air beams I find today is for tents(!) Would it be a practical technology in space? Here's "some guy's" ideas about air beams for artificial gravity. Slide 10 has an image that demonstrates the strength an air beam can have.

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    $\begingroup$ There are more purposes. The Voyager probes had several folding and extending structures: the magnetometer boom is 12 m long extended; the RTG and telescopes sit on booms that were folded down alongside the spacecraft for launch. $\endgroup$
    – Hobbes
    May 4, 2014 at 13:03
  • $\begingroup$ The word 'folding' in the question title suggested a mechanically self-erecting system. The subject of interest seems more to be about inflatables. Both of these concept paths come together in the foam structures that have essentially two balloons, one inside the other, with the void between the two filled with foam. Goodyear Aerospace actually brought such a design to test article level four years before Apollo 11 landed on the Moon. $\endgroup$
    – MercuryPlus
    May 4, 2014 at 20:38
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    $\begingroup$ See also: Artificial objects longer than 40 meters in space and What are the practical engineering difficulties in building and launching large inflatable modules? $\endgroup$
    – user54
    May 5, 2014 at 13:33

2 Answers 2

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There have been folding structures used in space since the early days.

Many spacecraft have folded their solar arrays for launch; many further have used rolled up arrays on folding arms, so as to minimize launch volume (and thus launch fairing mass).

A folding parabolic antenna was used on a few missions; the most notable is notable for its failure to open: Galileo. The actual cause of the failure isn't truly known, but is thought to be vacuum welding. A prabola can be created by panel shapes, and then held open with springing; it's not quite as smooth as a casting, but is still doable.

Large multi-segment telescopes are used on planet; they are a practical way to get a large (≥5m) telescope into orbit. Any such telescope would likely also have additional components deployed to shadow and to protect the objective mirror.

Folding arms and even wheel-carriages have been used on rovers, including the manned rover for the Apollo landings. (http://www.collectspace.com/ubb/Forum29/HTML/000731.html shows the stowed rover being loaded into the LM. http://www.armaghplanet.com/blog/nasas-lunar-rover-everything-you-need-to-know.html shows part of the deployment process in one of the included videos.)

The primary considerations for habitat space include internal pressure; a folded unit needs very little structure when it can be held in deployed mode by its own internal pressure. Several current designs (as yet unflown) use a central structural core, with an inflated habitat surrounding it. The Central core includes the docking port for mating to existing craft/structures, as well as the power and data connections. Bigelow Aerospace is involved in the design of the ones to be tested in 2015. (http://www.nasa.gov/exploration/technology/deep_space_habitat/xhab/, http://www.nasa.gov/mission_pages/station/news/beam_feature.html)

The key benefits of all the expandables are:

  • reduced launch dimensions, which reduce fairing dimensions and thus fairing mass
  • reduced launch mass due to less need to support stress during launch.

The key drawbacks include:

  • mechanical failure of the extension motors
  • mechanical failure of the attachments during launch (eg: Skylab)
  • corrosion or vacuum welding rendering it fixed into the stowed position (eg: Galileo)
  • failure to deploy in correct sequence (eg: most parachute failures; major concern on the Apollo Lunar Rover)
  • packaging failing to release (common in halo equipment airdrops; a major concern with the Pathfinder, Spirit, and Opportunity rovers)

Some additional drawbacks noted for inflatables:

  • lack of hard shell for impact resistance
  • lack of metal shell for radiation reduction.
  • potential for excessive force upon inflation to damage the module
  • potential for damage in packing (a concern for the Pathfinder mission)
  • potential for damage in removal of packaging
  • potential for damage if deployed near sharp objects (also a concern mentioned in the Pathfinder data, albeit more as a failure post-deployment).

Some of those have potential solutions - a concrete & fabric inflatable has been developed that is airtight once deployed - http://www.youtube.com/watch?v=vv3SII568v0 is a brag video by the manufacturer. Likewise, foaming polymer compounds could be used to create rigid walls.

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  • $\begingroup$ Thanks for your answer! But two points: I note that Bigalow at least say their softness is an advantage at impact with space debris because it does not create a shower of secondary projectiles as a rigid wall would. And if filled with some hydrogen dense gas (maybe H2 or CH4), inflatables should help block radiation. Again with lower rate of secondary particles than heavier metals cause when hit by high energy cosmic radiation. $\endgroup$
    – LocalFluff
    May 6, 2014 at 7:42
  • $\begingroup$ @LocalFluff two layers of rigid with a buffer has been shown by NASA to be pretty close to ideal for micrometeor defense, tho for high impact risk, 7 layers of copper was what was used on one of the space probes. Further, a rigid shell is likely to deflect many microimpacts while a soft shell will deform and send much more of the energy into the atmosphere inside. As for radiation, yes, a layer of good old water is about ideal... but a rigid shell - water bladder - rigid shell, at low speeds, is better than soft-water-soft, where only the water protects. $\endgroup$
    – aramis
    May 7, 2014 at 1:08
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This is done by necessity, all the time, for solar panels.

ISS panels

There are also large deployable antennas used for communication satellites and soon for the Soil Moisture Active Passive mission.

SMAP antenna deployed on ground

NASA plans to test an inflatable habitat on ISS.

Bigelow inflatable ISS module

The separation of rotating structures for artificial gravity is likely more efficient using a tether than a deployed rigid structure. You don't need crew quarters on both ends. There's plenty of other non-crewed mass to put at the other end.

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  • $\begingroup$ Rotating artificial gravity is often critized because of what could go wrong with a wire because it is not rigid. A semi-rigid airbeam need not be too much heavier per meter and has the advantage of allowing indoors in-air transports between different levels of artificial gravity and with the counter weight. I suppose a ballon can be manufactured to any shape, such as a parabolic dish, and still be completely folded more easily than rigid elements. Parabolic segments aren't easily packaged compactly together. $\endgroup$
    – LocalFluff
    May 5, 2014 at 10:04

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