Quick Airlock Question

Toby Frost

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This is just as to how the details of a spaceship airlock would work in an SF novel. The actual technology doesn't really need explanation, as it's just a man's observations as he uses the airlock.

He's inside the airlock, in his spacesuit. The inner doors close behind him. I assume that the air is sucked out of the airlock while he is standing there. Then the outer doors open. At this point, as it's depressurised and there's no gravity, he won't get pulled out of the airlock and can just calmly emerge into space. Is that right?
 
Yes, except that "sucked" is probably the wrong word. The air would be happy to escape as soon as some kind of valve is opened. There wouldn't be any need for a mechanism to remove the air.
 
Gravity has nothing to do with air pressure. If there was gravity or centrifugal force holding the character to the floor before, it would still apply. You are right as to there being a lack of force pushing the character outward if all or almost all the air has been evacuated. Note that if this is a spinning station, then stepping out of the airlock will result in the character being flung outward and the airlock door spinning away.
 
Gravity has nothing to do with air pressure. If there was gravity or centrifugal force holding the character to the floor before, it would still apply. You are right as to there being a lack of force pushing the character outward if all or almost all the air has been evacuated. Note that if this is a spinning station, then stepping out of the airlock will result in the character being flung outward and the airlock door spinning away.

Good point. If it is a rotating space station, and exit is at the circumference, then it would be a bit like stepping into a bottomless well. You would part from the space station at a tangent (so the dropping into a well analogy isn't quite right as there would be no acceleration....just constant velocity). Even if you used a rope it would be difficult to climb back to the space station airlock door. If artificial gravity is 1g at the circumference then it will be even higher at, say, 10m outside. Maybe a winch would help. If you exited at the hub things might be easier. Presumably you are going outside to do some work on the space station? Climbing away from the hub is obviously easier than climbing back to it.

If the spacecraft in Toby's story is non-spinning and has some kind of artificial gravity, then perhaps it could be turned off in the airlock to make exiting easier.
 
He's inside the airlock, in his spacesuit. The inner doors close behind him. I assume that the air is sucked out of the airlock while he is standing there. Then the outer doors open. At this point, as it's depressurised and there's no gravity, he won't get pulled out of the airlock and can just calmly emerge into space. Is that right?
airlock pressure equalisation
 
Yes, except that "sucked" is probably the wrong word. The air would be happy to escape as soon as some kind of valve is opened. There wouldn't be any need for a mechanism to remove the air.

Wouldn't you need a pump if you're conserving air? If the pump moves the air back into the habitat, sure you could repressurize the airlock by just letting the habitat's air flow into it.

This is just as to how the details of a spaceship airlock would work in an SF novel. The actual technology doesn't really need explanation, as it's just a man's observations as he uses the airlock.

He's inside the airlock, in his spacesuit. The inner doors close behind him. I assume that the air is sucked out of the airlock while he is standing there. Then the outer doors open. At this point, as it's depressurised and there's no gravity, he won't get pulled out of the airlock and can just calmly emerge into space. Is that right?

Gravity and pressure are different things. An incandescent lightbulb is near-vacuum but the filament still falls when it burns out. So basically you need artificial gravity that can be shut off in the airlock. Firefly probably has the sort of shot that you're looking for.

Though if the whole sequence is programmed, the gravity shutting off could be the signal that the air-pump is done and you can embark. (Gravity is turned down as the air-pressure drops, giving a tactile indicator in case their visor is covered in paint?)

Or maybe the airlock never has gravity. Someone steps in, they're suddenly weightless. They hover while the air is being cycled out.
 
Wouldn't you need a pump if you're conserving air? If the pump moves the air back into the habitat, sure you could repressurize the airlock by just letting the habitat's air flow into it.
Yes, maybe. You would need quite a powerful pump to extract all the air back into the vessel (creating a vacuum in the airlock prior to opening it). But it might be worth it if air is a valuable resource.
 
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If the spacecraft in Toby's story is non-spinning and has some kind of artificial gravity, then perhaps it could be turned off in the airlock to make exiting easier.

Or orientated to make getting out easier. An artificial 'down' doesn't necessarily have to be consistent throughout the ship. Or even the same value of G throughout the ship. Areas of increased G - carefully fenced to stop people walking into them - could be used to circulate air or in pipes to move liquids around without needing pumps with moving parts, and if you've got easy cheap handwavium powered artificial gravity why not have it on the outside of the ship too? - or at least on clearly marked strips as walkways. This exterior artificial gravity wouldn't have to be on all the time - just when someone is doing an EVA.
 
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Or orientated to make getting out easier. An artificial 'down' doesn't necessarily have to be consistent throughout the ship. Or even the same value of G throughout the ship. Areas of increased G - carefully fenced to stop people walking into them - could be used to circulate air or in pipes to move liquids around without needing pumps with moving parts, and if you've got easy cheap handwavium powered artificial gravity why not have it on the outside of the ship too? - or at least on clearly marked strips as walkways. This exterior artificial gravity wouldn't have to be on all the time - just when someone is doing an EVA.

I love it! You would actually need antigravity though. On earth we have water tanks at a high places so that water is forced through pipes and out of a faucet at the end. If the artificial gravity was created at the faucet, would the water in the basin (sink in UK) not be attracted to it? Better to make antigravity at the tank. All of which is just minor detail really. I'm sure a bloke with a wrench and a pencil behind his ear could figure it out if someone were able to actually invent artificial gravity/anti-gravity.
 
I love it! You would actually need antigravity though. On earth we have water tanks at a high places so that water is forced through pipes and out of a faucet at the end. If the artificial gravity was created at the faucet, would the water in the basin (sink in UK) not be attracted to it? Better to make antigravity at the tank. All of which is just minor detail really. I'm sure a bloke with a wrench and a pencil behind his ear could figure it out if someone were able to actually invent artificial gravity/anti-gravity.

I would imagine an artificial gravity that cover the floor of any room the sink was in would, just like a kitchen or bathroom on earth, pull water out of a tap and down a plughole but getting it back up into the tank (after treatment to make it potable again) would require some kind of pump.
 
Wouldn't you need a pump if you're conserving air? If the pump moves the air back into the habitat, sure you could repressurize the airlock by just letting the habitat's air flow into it.
I would expect air (and water) to be conserved because it's going to cost to replace them, so yes, pump out the airlock before opening the door.


This is just as to how the details of a spaceship airlock would work in an SF novel.
How big is the spaceship?

From a practical point of view, don't make the airlock any bigger than it has to be, because the bigger it is, the longer the pumping out process is going to take, and the more energy it will use. If the ship has one airlock then size is going to be a compromise between making it as small as practical whilst large enough for any likely task. If there's more than one, then you might have different sizes - something small for a one-person excursion, something bigger for a group to exit, or move machinery out through.

From a safety point of view, the time spent doing the pumping out is also when your user can check that the suit is functioning properly and not leaking.

From a boredom point of view (and if you went to get really technical), your spaceman might need to be patient. Pumping air out of an airlock takes time. With current tech (OK, article is about 8 years old) you could be looking at 10 minutes plus, with the possibility of hours of prep time adjusting to a different atmospheric mix akin to diving.


The NASA/ISS checklist document linked in that article (International Space Station ISS EVA Systems Checklist) runs to in excess of 600 pages.

and another one referencing an incident on the ISS where the EVA team wanted back in in a hurry. Start to finish, about half an hour, with eight minutes taken to re-pressurise the airlock. o_O

 
From a practical point of view, don't make the airlock any bigger than it has to be, because the bigger it is, the longer the pumping out process is going to take, and the more energy it will use.

I think the book was Red Planet. The martian was named Willis, I think, and there was a cartoon in the 90's. Anyway, in the book they use a garbage airlock and it might be that they only fit because they were kids.
 
I would expect air (and water) to be conserved because it's going to cost to replace them, so yes, pump out the airlock before opening the door.

I gave up on the show The 100 after one episode for this very reason.

"The series is set 97 years after a devastating nuclear war wiped out almost all life on Earth. The only known survivors are the residents of twelve space stations in Earth's orbit prior to the war. The space stations banded together to form a single massive station named "The Ark", where about 2,400 people live. Resources are scarce and all crimes no matter their nature or severity are punishable by death"

The death penalty is carried out by having the offender thrown out of an airlock. So scarce are the resources keeping them alive they can afford to throw away a huge airlock's worth of air and a whole body's mass of recyclable meat every time someone breaks the slightest rule?! I decided these people were too stupid to be worth caring about.
 
The death penalty is carried out by having the offender thrown out of an airlock. So scarce are the resources keeping them alive they can afford to throw away a huge airlock's worth of air and a whole body's mass of recyclable meat every time someone breaks the slightest rule?! I decided these people were too stupid to be worth caring about.

This bothered me about Wall-E. I get that there was a story-purpose for it, but it was still jarring.
 
Yes, maybe. You would need quite a powerful pump to extract all the air back into the vessel (creating a vacuum in the airlock prior to opening it). But it might be worth it if air is a valuable resource.
To get to lower levels of air pressure, i.e., closer to a vacuum, I believe it would be pumping time, not pump strength that matters. The importance of that is in determining the length of time one has to sit inside an air lock before going into space. Of course, the reverse process is much simpler and would probably be limited to minimize effects of sudden repressurization.
 
To get to lower levels of air pressure, i.e., closer to a vacuum, I believe it would be pumping time, not pump strength that matters. The importance of that is in determining the length of time one has to sit inside an air lock before going into space. Of course, the reverse process is much simpler and would probably be limited to minimize effects of sudden repressurization.

How about this? The air isn't pumped out of the airlock at all. Let's assume an airlock big enough to turn around in and have some kit with you. Instead of ALL the air being pumped out to leave a vacuum, it's pumped into something like airbags in the walls of the airlock which inflate - pushing air out of the lock as they do so which (virtuous circle) makes it easier to pump till the guy in the suit is right up against the door - or, more likely, where he needs to be to open the door since it would seem logical that any spaceship door would open inwards. Internal air pressure would keep it closed without having to need any kind of locking mechanism. (An outward opening door would need some hefty mechanism to stop it being blown open.) Only when the air bags are fully inflated then do they have to pump the (much reduced) remaining space to vacuum.
 
How about this? The air isn't pumped out of the airlock at all. Let's assume an airlock big enough to turn around in and have some kit with you. Instead of ALL the air being pumped out to leave a vacuum, it's pumped into something like airbags in the walls of the airlock which inflate - pushing air out of the lock as they do so which (virtuous circle) makes it easier to pump till the guy in the suit is right up against the door - or, more likely, where he needs to be to open the door since it would seem logical that any spaceship door would open inwards. Internal air pressure would keep it closed without having to need any kind of locking mechanism. (An outward opening door would need some hefty mechanism to stop it being blown open.) Only when the air bags are fully inflated then do they have to pump the (much reduced) remaining space to vacuum.

It has promise. Except the pressure in the room would remain at 15lbs per square inch and it would be impossible to open the door due to the pressure differential. So, it has to open outwards. At which point the remaining air would rush out and the air bags would quickly expand giving the spaceman a hefty push in the direction of space. Alternatively, I think a valve to allow the remaining air to escape slowly (also reducing any sudden stress on the bags) would be good. And then the door could be opened inwards.
 
he pressure in the room would remain at 15lbs per square inch and it would be impossible to open the door due to the pressure differential.

Agreed but it would be a much smaller volume to evacuate if you wanted to conserve all the air. It would be fairly easy to construct airbags that had a limited size so wouldn't expand further than a certain point. Some sort of bellows arrangement?
 
[Due to the inflated airbags] the pressure in the room would remain at 15lbs per square inch and it would be impossible to open the door due to the pressure differential.
Once the air lock proper has been largely emptied of air, simply reverse the pump direction for the airbags and deflate them. The airbags do not need to be fully deflated, so this could minimize pump time and allow free space for the door mechanism to operate. I think there is a very workable idea here that could minimize air loss and reduce the pump time to evacuate the air lock.
 

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