What happens if the inner door of a depressurised airlock fails?

What are the consequences of the INNER door of a spaceship airlock system suddenly failing (for whatever reason) when the airlock itself is depressurised? (The ship itself is in space/vacuum and not earthside)

Would the outer door provide some protection from the main effects of air meets vacuum, or would there be knock-on effects , such as turbulence that could potentially destroy the outer door/hull as well - leaving the ship completely open to the vacuum of space...

EDIT Is there anything else besides the volume air displacement and changes in temperature that need to be taken into account? Is there some turbulence/structural damage component I need to take into consideration if there was a sudden breach between the air in the space ship and the "internal" pocket of vacuum.

• Just to clarify I'm well aware that there is no difference between vacuum and an unpressurised airlock vacuum. I'm just trying to figure out if there are additional complications to an inner breach besides air volume displacement and air temp. Commented Dec 3, 2020 at 16:21
• As the two answers show, airlocks are designed to 'fail safe'. What you're talking about is some sort of explosive decompression similar to the bomb in The Martian movie, and if the airlock is at vacuum then I'd expect the result to be similar for any part of the spacecraft, airlock or not. Commented Dec 4, 2020 at 10:44
• Yes. I don't think people who have read the question realised that. I was talking about a sudden breach (for whatever reason), not normal expected /designed use. Commented Dec 4, 2020 at 19:26
• Please feel free to explain why you think the question needs a downvote. I can't attempt to improve/clarify it with no input! Commented Dec 6, 2020 at 12:45

Both doors of the airlock should resist the full pressure difference between vacuum outside and spaceship air pressure inside. During normal use of the airlock both doors are exposed to both the full pressure difference as well as the same pressure on both sides, 100 % of operating pressure and 0 %.

The pressure to the outer door is equalized only when the chamber is evacuated. The inner door is equalized only when the chamber is filled with air of the same pressure as inside the ship.

If the inner door is closed, the airlock is evacuated and the inner door fails, the outcome depends on the condition of the outer door.

• If the outer door is closed it will hold the air inside the ship.
• If the outer is full opened, the air will flow out of the ship.
• If the outer door is opened just a little bit (the door opens to the inside as airlock doors should) the airflow and the pressure will smash the door to the outside on the seals and will stop the air.

If both doors open to the inside, it is impossible to open both doors simultaneously as long as there is air in the ship. A door could be opened only when pressure on both sides is equal. If pressure in the ship is 1 bar, the force to a door of 2 square meters area is huge, 10 Newtons per square centimeter and 200,000 N (200 kN) to the door.

When some of the crew are outside performing spacewalks, the outer door is open and the inner door is closed. A structural failure of the inner door at this point would result in an explosive decompression.

When the crew is inside, at least for the US side of the ISS, the inner door is open and the outer door is closed.

There is a brief interval during airlock depressurization and repressurization when both doors are closed. I do not know if it has been studied, but I cannot see why a structural failure of the inner door at this point would cause the outer door to fail.

• Thanks for your answer, but it doesn't really provide any new information/clarification etc. I am particularly interested in the breach in the "air - airlock vacuum - space vacuum" effect in the small space of the airlock as I have a larger scenario in my head. I want to find out the physics of a breach in this particular case to figure it out / formulate another queston etc. Commented Dec 3, 2020 at 10:31
• @EveryBitHelps I should have asked for clarification about what you meant by 'the main effects of air meets vacuum' - it doesn't really make sense to me, so I figured you needed the basics. I don't know what you mean by ' breach in the "air - airlock vacuum - space vacuum" effect ' either, maybe someone else will understand your question better. Commented Dec 3, 2020 at 12:58
• Basically I know the effect of air meets vacuum of space. What I want to figure out is what would happen if air suddenly meets vacuum but in an enclosed/encapsulated area that is itself surrounded by the vacuum of space. So region of air > small buffer of vacuum > large region of vacuum. Is the result any different, if so how? The airlock situation seemed the smallest/simplest definable problem to help me figure out what happens. Commented Dec 3, 2020 at 13:16
• @EveryBitHelps The air fills the vacuum? It would cool down some as it expands. I can't see how what surrounds the airlock matters in the slightest. I'm still not getting it. There's no diff between airlock vacuum and space vacuum. Are you thinking of some kind of water hammer effect or something? Commented Dec 3, 2020 at 13:18
• Thanks. That confirms my theory. Just to clarify. I'm well aware there is no difference between the type of vacuums. The difference in names was just a naming issue so that I could describe the areas in question. That is why i was asking if it is just a volume displacement issue of air into vacuum, like you have repeatedly explained, or if there were any turbulence or structural impacts that I was unaware of that needed to be taken into account. Commented Dec 3, 2020 at 16:18