Why do EVA suits have legs?

Question

Planetary exploration suits (PES) obviously need legs. Spacesuits with legs are “a classic icon of human space exploration” and a requirement for mobility. https://www.nasa.gov/feature/a-next-generation-spacesuit-for-the-artemis-generation-of-astronauts .

But do extravehicular activity suits (EVA) need legs? It has been reported that EVA suits have very little knee mobility compared with PES suits (unsubstantiated) Judging by photographs of EVAs, legs on EVA suits provide little utility:

If legs were not a requirement for EVA suits, it would provide more design latitude.

Robonaut with mono-leg and bi-legs for comparison

I don’t think astronauts can just chop off their legs for EVAs. Just doesn’t seem like too safe really. — Topcode, Jan 14 '22 at 19:25
Ok. Good point. They can keep their legs. But does the suit need legs? Why not a "monopant"? It would simplify design and add design options at the same time — Woody, Jan 14 '22 at 19:31
@OrganicMarble --- There is an entire spectrum between softsuits + pressurized gloves and pods. Now we seem to be at the softsuit extreme. The fist step along the continuum is to get rid of the pants and boots. Like, what's with the treads on the boots? — Woody, Jan 14 '22 at 19:39
@OrganicMarble --- The same function would be served by a monopant with a grapple where the "heal" would be. It might be easier to couple, as well. The Canadarm operator could help with primary alignment and the coupler could do the rest. — Woody, Jan 14 '22 at 19:55
I suppose it down to two things. Firstly, "bi-panted" suits is what is currently done, there's experience with that. Let's keep doing what we know & it is potentially cheaper. Depending on the EVA mission are you advocating astronauts have two suits: a monopant & a bi-pant? The second thing is, I can vaguely recall, & I stand to be corrected on this, there were some EVA tasks where an astronaut had to, or it was convenient for the astronaut to place one or both feet on a space craft during a maintenance or repair procedure, in which case a bi-pant suit would have been necessary. — Fred, Jan 14 '22 at 21:36
Yeah, why not a sleeping-bag-esque both-legs-in-one-pouch setup? — Anton Hengst, Jan 14 '22 at 21:40
just asked in SciFi SE: Vintage book cover showing humanoid astronauts in upper body EVA suits but a "pod" or "monopant" lower half? and already nicely answered! See also https://twitter.com/pulplibrarian/status/906201906741215233 — uhoh, Jan 14 '22 at 22:54
@AntonHengst --- re: sleeping bag-esque Ya... like a sack race at Sports Day. — Woody, Jan 15 '22 at 00:16
EVA suits have many layers - at least the inner cooling tubes layer will want to be close to the skin. The "monopant" design would create a hotzone between the legs? Maybe this is part of the reason? — IronEagle, Jan 15 '22 at 01:30
@IronEagle ---- Good point. But temperature could go either way. Heat absorbed from the sun depends on surface area and the monopant has lower surface area. Heat conducted away is basically zero in a vacuum, so won't change going to monopant. The air volume could be larger if there is no need for suit flexibility, so air and heat distribution would be easier. — Woody, Jan 15 '22 at 03:00
@OrganicMarble ---- source:https://www.researchgate.net/publication/4150025_An_experimental_investigation_of_robotic_spacewalking/figures?lo=1 Actual Robonauts flown on the ISS are human-sized — Woody, Jan 23 '22 at 22:48

GdD · Answer 1 · 2022-01-15T15:20:30.053

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Your question assumes that legs aren't used on EVAs, however legs are actually important for leverage. The first EVAs did not go well, astronauts were unable to complete tasks partly because they couldn't exert enough force. The assumption was that legs would be useless, but part of the solution was to anchor the legs on different parts of the spacecraft structure. Legs have some of the strongest muscles in the body, for them to be most effective they need to be apart some so that they can stabilize the hips and hold the lower body rigid. With feet together the body tends to flex. You can see this for yourself here on Earth, try holding your feet together while you exert a force on something, you'll find it's much harder then if you have your feet apart.

Also, having legs together limits mobility. Tie your legs together and then try and reach for something to the side, it doesn't work so well.

It's also important to remember that astronauts have to practice on Earth which does have gravity. Although neutral buoyancy tanks mimic zero G they still have to get to it, and walking is an advantage. As the point of training is supposed to be as realistic as possible the suits need to be the same as in space. I'm sure they could get past that if they wanted to, but there'd have to be a compelling reason.

It's hard to see what advantages there would be to a single leg 'mermaid suit' design, maybe they could save some piping but it isn't worth losing the mobility. If anything astronauts would like more leg mobility rather than taking away what little they've got.

edited Jan 15 '22 at 15:20

answered Jan 15 '22 at 14:04

GdD

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Do you have a reference for "there's other type of training on models where the astronauts need to be able to walk."? – Organic Marble Jan 15 '22 at 14:22
I'm not sure how much still goes on these days @OrganicMarble, I'm thinking of the models that were used for Gemini, Apollo and Skylab where not all of it was done in the tank. – GdD Jan 15 '22 at 15:03
Just wondered what you were referring to. – Organic Marble Jan 15 '22 at 15:04
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I edited it out @OrganicMarble as I can't find what I'm looking for and it's not particularly important to the answer. Looking for EVA training outside the tank just brings more up about tank training. – GdD Jan 15 '22 at 15:22
I thought you might have been referring to the air bearing floor, but that's somewhat obscure. https://images.nasa.gov/details-s66-19284 – Organic Marble Jan 15 '22 at 15:29
I've never seen that @OrganicMarble, that's really cool. – GdD Jan 15 '22 at 18:02
@GdD --- Can you provide an example of EVA foothold or leg anchor being used? I could find many examples of footholds used inside the ISS but no examples for EVA other than the bracket for attaching boot to Canadarm grapple.. If a dedicated foothold is installed, it would need to be in a specific location – Woody Jan 15 '22 at 21:57
@Woody the EVA foot restraint is indeed often at the end of the arm, but it can be placed in many other spots around the ISS. Here's a picture from Expedition 43 showing the foot restraint on the truss. https://www.spaceflightinsider.com/wp-content/uploads/2017/10/exp43_eva45_101017.jpg – Organic Marble Jan 16 '22 at 00:26
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@OrganicMarble --- Nice photo. Is there a "map" of locations where footholds can be placed? And who places the footholds? It would be useful if hold points (hand or foot) were spaced regularly, like LEGO knobs so astronauts could anchor themselves at any location. – Woody Jan 16 '22 at 06:44
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@Woody they are called WIFs (Worksite InterFaces) and there are hundreds of them all over the ISS. Pick a random EVA picture (like this one https://preview.redd.it/lh7z3i7bgi751.jpg?auto=webp&s=b2209103f59c82c9d7ebbd37a27abdcc7d3d570f) and you'll see the little brass tubes. I highlighted one from the picture and show a list of the dozens just on the S6 truss segment here https://i.imgur.com/6i1Odgs.png Body stabilization is fundamental to EVA. – Organic Marble Jan 16 '22 at 14:30

Why do EVA suits have legs?

1 Answers1