# What does “double vacuum pressure” mean in space-suit testing?

There was news of the new SpaceX space suit in Reddit and now a mention in Elon Musk's Instagram where he says:

First picture of SpaceX spacesuit. More in days to follow. Worth noting that this actually works (not a mockup). Already tested to double vacuum pressure. Was incredibly hard to balance esthetics and function. Easy to do either separately.

Question: What does "double vacuum pressure" mean in space-suit testing? How does one test a space suit to double vacuum pressure?

below: From The Verge's 08-Sep-2017 article Elon Musk shares another photo of SpaceX’s future space suits; Full-body this time. Credit: SpaceX/Elon Musk, original Instagram post.

The suits that SpaceX has designed are pressure suits, meant to be worn by astronauts while riding inside the Crew Dragon. They’re primarily needed in case there’s an emergency scenario during flight; if the spacecraft suddenly depressurizes, the suits will keep the astronauts alive until they can get to safety.

below: From last year's reddit post SpaceX suits look like they come straight from a scifi movie, appears similar to image in Musk's instagram post.

• @Uwe the image the question is an old and unofficial "leak" more than a year old and most probably shows a mock-up. The new one is on Instagram. – jkavalik Aug 23 '17 at 15:06
• The line after pressure is the one that confuses me. That seems to imply someone at some point considered making it less functional for aesthetic reasons. – user17699 Aug 23 '17 at 18:55
• First thought is that it's good for double the pressure of a vaccuum. ie. zero psi. Well, that's not very good at all! – Octopus Aug 23 '17 at 20:34
• Just to add to all the undocumented speculation, I'm going to guess that only the helmet is pressurized in this suit, just like the shuttle astros wore before the Challenger accident. They wore blue unpressurized flight suits and a pressurized helmet. cbsnews.com/network/news/space/51L/rw_51lphotos/… The orange pumpkin suits came after the Challenger accident. – Organic Marble Aug 24 '17 at 20:21
• @organic marble You said it. That’s why I chime in once in a while - it’s usually when I see bad data gaining a foothold. As you said, there’s enough of that. – JPattarini Jun 15 '19 at 18:31

A space suit could be tested to double operating pressure. This test can be done in a vacuum or in air. For the stability of the suit, only the pressure difference between inside and outside is essential.

To allow enough flexibility, the pressure difference (inside to outside) should be not greater than 0.2 to 0.4 bar. Breathing pure oxygen in the suit at such a low pressure is possible and healthy to the user of the suit. At higher internal pressure it would be difficult for the astronaut to bend a knee or ellbow.

• This is a nice, concise yet complete answer, and makes a lot of sense. Thank you! – uhoh Aug 23 '17 at 16:50
• Bend the knee? To Dany? ;) – NVZ Aug 24 '17 at 3:28
• Current US and Soviet suit designs are 0.25-0.35 bar; do we actually know that SpaceX's design operates with similar internal pressure? – Russell Borogove Aug 24 '17 at 3:29
• @RussellBorogove: It would be weird if it didn't. I mean, it doesn't contain any revolutionary solutions like exoskeleton to reduce the suit's resistance to flexing (so the lower the pressure the better) and 0.3 bar is a reasonable lower limit where the astronaut can operate without impact on performance. The pressure can be temporarily reduced to allow extra flexibility, but not for long as this causes oxygen shortage. So while the operational limits may differ (e.g. the suit may lack the venting capacity) the nominal pressure shouldn't. – SF. Aug 24 '17 at 10:39
• The look of the suit -- and I'm talking about the body here, not the helmet -- is different enough from all the suits I've seen before that I'm curious if it actually does incorporate a revolutionary solution of some kind, is what I'm getting at. – Russell Borogove Aug 24 '17 at 15:48

Pressure here refers to a relative measurement, not absolute, as we're comparing things (inside and out). "Vacuum pressure" here refers to the pressure generated by having a vacuum on one side, and 1 atmosphere of pressure on the other.

This suit has been tested to double that, meaning it had its internal pressure increased to 2 atmospheres, so that double the pressure was applied to it. Thus it was tested at double vacuum pressure.

• A space suit with 1 or even 2 bar pressure inside and vacuum outside would be useless, you can't close your hands or bend a knee or elbow against that pressure difference. Think of the pressure inside a car tire. To allow enough flexibility, the pressure difference should be not greater than 0.2 to 0.3 bar. – Uwe Aug 23 '17 at 14:27
• @ProPuke Do you have a reference to back up your assertion that the suit is pressurized to 1 atm? – Organic Marble Aug 23 '17 at 14:29
• @Bear it's not just a small inaccuracy. Pressure is a fundamental issue of space suit design and use. Wrong answers get down voted, that's how SE works. If they are edited so that they are no longer wrong, that opens an opportunity to reverse the down vote. This encourages the person who writes the wrong answer to fix the problem. Other readers stopping by use up and down votes to judge the accuracy of the post, so up voting a wrong answer misleads future readers and defeats the whole purpose of voting. SE is about good answers, and the votes help show if an answer is good or not. – uhoh Aug 23 '17 at 17:29
• @Uwe While 2 atmospheres of pressure would be a uselessly rigid suit, we are talking about a test. Certain tests exercise a single property to great extremes beyond normal operation, with disregard for other operational parameters. So I don't think we can assume that the operational practicality of the pressure precludes it from being used as a test focused on durability. You might test at extreme pressures as a safeguard. – AaronLS Aug 23 '17 at 18:47
• I downvoted it because there is no evidence the answer, which quotes specific pressure numbers, is not completely made up. Or pulled out of thin air, as it were. I would be happy to upvote it if a reference was given. – Organic Marble Aug 23 '17 at 23:25

The suit is not designed for space walks. It is meant to protect the user in case they are exposed to vacuum. Therefore I find it strange that the accepted answer assumes the suit would be depressurized. It is not intended to be operated in a vacuum. In fact, the suit will be worn inside the capsule at 1 atm.

Furthermore, seems unlikely that pressurizing a suit to 1.8 atm would be considered rigorous testing (0.8 differential, double max operating pressure). I find it far more reasonable to assume that double vacuum pressure means they pressurized the suit such that the differential was double what it would be if exposed to vacuum at 1 atm. This would be a 2 atm differential, as many downvoted answers have suggested.

EDIT: Here is a fact sheet which has the Dragon capsule's internal pressure at 13.9-14.9 psi

• References....? Not assumptions....? – Organic Marble Aug 24 '17 at 18:26
• A good stackexchange answer should be more than "I find it far more reasonable..." because a dozen people can post a dozen things that they find reasonable. If you can reason this through, carefully and at least somewhat convincingly, from an engineering perspective, that might make up for not having any verifiable links, but that would be hard. It would be much better to show some verifiable backup to your thinking. Welcome to stackexchange, and if you haven't already, take the tour to learn about what makes a good SE answer. – uhoh Aug 24 '17 at 18:34
• Here we go, a Dragon data sheet, which has the internal pressure at 13.9-14.9 psi: spacex.com/sites/spacex/files/pdf/DragonLabFactSheet.pdf – DKu Aug 24 '17 at 19:11
• Using a 1/3 atm suit is accepted practice (on shuttle, and ISS) and doesn't require references. Using a 1-2 atm suit isn't, and does. It would be wonderful if someone made a usable 1 atm suit, eliminating all annoying prebreathe protocols. I just don't believe that it has happened yet. – Organic Marble Aug 24 '17 at 20:14
• @DKu : the Dragon data sheet gives a value of 13.9-14.9 psi for normal pressure inside the capsule. But there is no specification for the pressure inside the suit in case of emergency when the capsule has lost its atmosphere. – Uwe Aug 24 '17 at 20:21

Double vacuum pressure would mean two atm of difference from inside to outside. Others have remarked "you can't close your hands or bend a knee or elbow against that pressure difference": that's why space suits are normally designed with joints that operate without changing the inside volume and thus are not susceptible to stiffening under the large pressure difference.

That's what makes them bulky. The picture shown here is nothing like that. I wonder why.

• Do you have a reference to back up your assertion that double vacuum pressure for this suit is equal to two atmospheres? – Organic Marble Aug 23 '17 at 18:31
• It is not true that double vacuum pressure would mean two atm of difference from inside to outside. That is only a wrong interpretation of a unclear and misleading statement. Designing the suit with constant volume joints does not help the astronaut to close his hand against pressure in the glove. But it is true that the pictures do not show any indication for constant volume joints. – Uwe Aug 23 '17 at 20:19

I concur the Double vacuum testing is probably testing to twice bursting strength in vacuum.

See http://www.astronautix.com/s/spacesuits.html for a fascinating look at various spacesuits. They have been made out of pure metal, fabric, mixed materials, at 1 atm, at 0.2 atm, or at no atm simply mechanical counter-pressure. They have used "tomato worm" limbs (constant volume convoluted tubes - the better with to bend at pressure) among other solutions. All of the solutions work. The question is what question are you asking; "get me down" suit, survival suit, open space/outside vehicle suit, lunar surface, martian surface? What the constraint of this particular suit is will be fascinating.

• Do you have any references to support your assertion that the suit was tested to twice its burst pressure? Please note that the suit would certainly have been damaged in that case, since burst pressure, is after all, the pressure at which it would burst. – Organic Marble Aug 23 '17 at 23:28
• Testing to twice the burst strength seems to be self-contradictory. Imagine Musk saying "Already tested to double the burst pressure." The only way this could be true is if the burst pressure were zero; 2 x 0 = 0 or more nicely put "Twice nothing is still nothing" – uhoh Aug 24 '17 at 6:31
• You want to rise pressure until the suit bursts and then double the pressure inside the suit? But there is a hole now in the suit and rising pressure is not possible with a big hole. – Uwe Aug 24 '17 at 16:38