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I have noticed that astronauts in every launch I've found wear a spacesuit during launch, like this example from the shuttle.

enter image description here

I don't understand this. What kind of problem could cause loss of cabin pressure without compromising the whole vehicle? It seems like anything serious enough to depressurize a spacecraft would just blow the whole thing up.

Are there failure modes that cause loss of pressure but not rapid, unplanned disassembly? Or is there some other reason for the spacesuits?

I am interested in the topic in general, not just the shuttle.

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    $\begingroup$ Launch is a high vibration environment. Lots of things could shake loose to make an air leak without destroying the ship. And small capsules may be durable enough to survive rocket-destroying catastrophes. $\endgroup$
    – ikrase
    Apr 26 at 7:31
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    $\begingroup$ Redundant safety mechanisms are always the best approach. $\endgroup$ Apr 26 at 12:41
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    $\begingroup$ @ikrase: See for instance the incident a year or two ago, where a Soyuz rocket blew up upon launch: astronomy.com/news/2018/10/soyuz-rocket-failure $\endgroup$
    – jamesqf
    Apr 26 at 15:56
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    $\begingroup$ The suits also hold in place and conceal the diapers. I'm not even sure how much of a joke this is. (Apparently, judging by comments, they aren't strictly necessary for that.) $\endgroup$ Apr 26 at 16:02
  • $\begingroup$ @Peter-ReinstateMonica Diapers are necessary in case of long holds on the launch pad. $\endgroup$ Apr 26 at 22:06
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Are there failure modes that cause loss of pressure but not rapid, unplanned disassembly?

Yes.

The 3-man crew of Soyuz 11 died when a valve was jolted open, venting out all the cabin air supply. Soyuz was redesigned after that accident to carry two crew in pressure suits instead of 3 crew in shirtsleeves. (I believe they now carry three in pressure suits.)

More generally there are any number of failure modes that could depressurize the cabin of any crewed spacecraft without destroying the vehicle. It's common, for example, for crewed capsules to have hatches which can be opened via explosive bolts; on Gus Grissom's Mercury-Redstone 4 flight, the hatch blew open unexpectedly after splashdown. If a failure like that occurred at high altitude or in orbit, the cabin would quickly vent to space without any significant airframe damage.

In such a case, it would be unlikely that the spacecraft could reenter safely, since there would be thermal, drag, and center-of-gravity issues. A micrometeoroid puncture could likewise depressurize a spacecraft, and if small enough, it could be repaired and reentry would probably still be possible, depending on the location.

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    $\begingroup$ I'm almost tempted to ask a question about whether it would have been possible for Apollo, Mercury or Gemini to even be able to reenter successfully with a missing hatch... $\endgroup$
    – Moo
    Apr 26 at 21:46
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    $\begingroup$ It wouldn't, I'm fairly certain. If one was en route to a space station when they lost the hatch, survival might be possible, but otherwise not. $\endgroup$ Apr 26 at 21:50
  • $\begingroup$ @Moo They're in pretty close to vacuum during the hot time. They only need to protect themselves against radiant heat, not against hot air. $\endgroup$ Apr 26 at 22:08
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    $\begingroup$ If the hatch is open, you will have a significant drag imbalance, and the capsule will most likely tumble, and then subsequently disintegrate due to the non-heat shield side heating up. $\endgroup$
    – Nelson
    Apr 27 at 9:41
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    $\begingroup$ @Nelson Do you know that to be true or is that just an assumption? Because generally the leading edges matter but a gap further back is invisible unless is disrupts the airflow. $\endgroup$
    – JamesRyan
    Apr 29 at 12:16
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@Russell hit the most important point (cabin decompression), but there are some others:

  1. Protection from small fires or chemical fumes. These are irritants to the eyes and respiratory tract, and can hinder an astronaut's ability to function, if not injure or kill. The suits are fire retardant, so they will not burn like some clothing will. They provide some heat resistance that is useful for handling a small fire, although a large fire like Apollo 1 may still result in injury or death.

  2. G-force control. Shuttle suits were able to provide compression to the legs during high G-forces (particularly landing), helping to move blood to the brain instead of pooling in the legs. Fighter jet pilots sometimes call these "G-suits".

  3. Protection when ejection seats are used, particularly for the Gemini and first 4 Shuttle flights.

  4. Parachute. This was the normal landing method for Vostok flights, part of the emergency bailout procedure with Shuttle ACES suits, and also needed for the above-mentioned ejection seats.

  5. Water survival. Many suits have had flotation collars which keep the head above water, and neck dams which keep water out of the body of the suit. Some have also had an inflatable 1-person life raft built in.

  6. Survival gear. Knives, guns, survival food, and water rations have all been parts of spacesuits.

  7. Rescue. A person in a bright orange suit is easier to spot than other colors (the blue uniforms of the suitless Shuttle era were especially bad). Spacesuits also often carry radios, radio beacons, mirrors, flashlights, and glow sticks.

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  • $\begingroup$ Guns? Do they expect to be attacked by space bears? $\endgroup$
    – forest
    Apr 29 at 0:50
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    $\begingroup$ @forest: Link added. Here is another one. Yes, cosmonauts have been threatened by bears. $\endgroup$
    – DrSheldon
    Apr 29 at 1:52
  • $\begingroup$ Wow. Well, I learned something today. $\endgroup$
    – forest
    Apr 29 at 1:57
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The most recent Crew Dragon mission had a near miss with a piece of space debris on its way to the ISS

The 4 astronauts on board were instructed to put their suits back on in case of a collision

"For awareness, we have identified a late breaking possible conjunction with a fairly close miss distance to Dragon," SpaceX's Sarah Gilles told the astronauts about 20 minutes before the conjunction on Friday. "As such, we do need you to immediately proceed with suit donning and securing yourselves in seats."

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