View Full Version : Using pressure differences in space to 'eject'

03-18-2013, 02:05 PM
Hi All,

If I could take you back to that scene in 2001: A Space Odyssey, where Bowman ejects from his pod and propels through zero gravity into the ship. I have a similar scene where my character is inside an airlock and is propelled from it through zero gravity.
Can someone explain the dynamics of movement between low pressure and high pressure? At the moment I have him depressurise the air lock (would there be a button for this?), this causes the door to fly off and he is ejected forward out into a zero gravity tunnel. Would this work?

Thanks in advance.

03-18-2013, 03:26 PM
Hah, I write scenes like this! Without murderous computers.

If the airlock's pressurized and there's vacuum or a lower pressure outside, the atmosphere inside will vent when the hatch opens. In theory your MC would be vented together with the escaping atmosphere, as air friction drags him along. Anything he comes into contact with will slow him down and/or alter his trajectory.

I'm guessing that your MC would have to manually override any failsafes and/or has authority to blow the hatch's emergency explosive bolts, assuming there are smart control systems.

If external conditions outside the airlock aren't an urgent factor (as in, near-fatal temperatures or radiation, he has to move quickly) he could as easily hold his breath, depressurize the airlock, open the hatch, brace against a surface, and propel himself outside towards whatever safe environment he's moving to. Though that's not as visually dramatic as the emergency venting method. :)

Shrug, I'm probably over-thinking.


03-18-2013, 03:35 PM
I'm pretty sure holding your breath is the last thing you want to do, as the air in your lungs will expand in the lower pressure environment. Empty your lungs first--and don't try to breathe.

03-18-2013, 07:18 PM
Being ejected into hard vacuum without a suit is not a pleasant experience. Attempting to hold your breath will result in your lungs explosively decompressing - if you're lucky they'll vent back up your windpipe; if not then they'll rupture into your chest cavity somewhere first, and you risk getting bubbles in your bloodstream. You'll instantly vent any gas in the lower part of your intestine, too; if your story is of that sort of tone then there's a free scientifically-sanctioned fart joke for you.

The more problematic part is the effect of vacuum on the rest of your body. Any exposed liquid will vaporise. If your eyes are open, this will hurt. There's also the potential for soft tissue damage in your nose and mouth; not massive damage but enough that you'll want to close your mouth and hold your nose. After you've emptied your lungs, of course. Don't clamp your lips too tight though; let gas escape if the pressure builds, or you'll injure something. There's some danger to your ears too, but it's tricky to plug your nose and ears at the same time while still leaving yourself at least one hand to feel where you're going because your eyes are closed. Ears are usually a little drier than nose and mouth, so maybe you can risk it.

Worse still, your flesh will expand. Even the weediest astronaut will suddenly look like a steroid-pumped bodybuilder. This will be painful. Tight clothes will help a little, but unless you're wearing something like a mechanical counterpressure suit (http://en.wikipedia.org/wiki/Space_activity_suit), it'll hurt.

Worst of all, there's decompression sickness to consider - known by divers as "the bends". Nitrogen usually held dissolved in your blood suddenly forms bubbles as the pressure that held it there disappears. Any part of you that contains blood will hurt. A lot. This causes unconsciousness within about 10-15 seconds, and death within roughly 90 seconds. That's quite aside from the fact that the lack of oxygen will be causing hypoxia; normally a healthy adult human can live for about 3 minutes without air, but that's on a full lungful. With empty lungs, and with your bubbly blood less able to distribute the little oxygen you do have, you're in real trouble; you've got 10-20 seconds before you're likely to pass out.

When you add up the pain, the inability to see, the lack of breath, and imminent unconsciousness, you're probably not going to get far without a suit. Your best bet is to line it up before you blow the airlock, breathe out as it blows while pinching your nose and closing your eyes, shut your mouth as you run out of breath, and have someone to catch you at the other end. Perhaps you could throw yourself into an airlock which automatically closes and pressurises as the computer detects your predicament.

Of course, if you have a suit, it's all good.

03-18-2013, 07:50 PM
The character dies in a bomb explosion seconds after being ejected anyway, but useful to know what you wrote!
I just wanted to make sure that depressurising the airlock would expel him out in there is a vacuum on the other side.

03-18-2013, 08:40 PM
Out of interest, how accurate is the depressurisation scene in Event Horizon? Most of what's been mentioned above seems to have been covered by in that scene. Given the general standard of science in the film I'd be pretty surprised (but impressed) if it were reasonably accurate. It's a good fun film either way.

spice chai
03-19-2013, 07:17 AM
I would just like to add that people have survived rapid depressurization accidents of several seconds duration, so there are precedents. If you are looking for some interesting textural details, you can feel the sizzle of the saliva boiling off your tongue (at low pressures, body temperature is higher than the boiling point of water). http://www.aerospaceweb.org/question/atmosphere/q0291.shtml

03-21-2013, 02:08 PM
I just wanted to make sure that depressurising the airlock would expel him out in there is a vacuum on the other side.

Most probably. Full sea-level atmospheric pressure in the International Standard Atmosphere model is 101325Pa, which means an airlock door with an area of 1 square metre will experience a force of 101325 newtons - the same force you'd feel from a 10.3-metric-tonne object sitting on you; WolframAlpha tells me that's 5.5 times the average force experienced by something being hit by a baseball bat. Most spacecraft and stations in real life are pressurised to the equivalent of a few thousand metres above sea level, so the pressure can be anything down to about two-thirds of that, maybe less, but in the future we may have upped the pressure to help the astronauts' health, aided by futuristic materials and techniques that help make the spacecraft bodies withstand the pressure difference without bursting, or popping like balloons when punctured (which is mostly why we use a lower pressure in real life). A standard door has an area of a little under 2 square metres. My back-of-the-envelope calculation suggests that a human silhouette is somewhere around 1.2-1.6 square metres, probably. Now, you wouldn't get the full force of that air pressure because it's acting inward on you in all directions; what actually shifts you is mostly the rushing wind as the air vents. The closer you get to the door when it blows, the stronger the force will be; if you're standing up by the airlock door when it disengages, provided the door gets clear rapidly enough (explosive bolts to blow it off in an emergency?) then you'll move like you've been given a firm shove or a kick. If you're quick and strong and there's a convenient handle you might be able to hang on. Further back it'd be easier; more than a few metres away and you'd feel a very strong wind and would have a second or so to grab a last short breath and then exhale as the pressure dropped low enough to endanger your lungs as previously mentioned; that might gain you a couple seconds of consciousness. So to answer your question, get your character up to the airlock - maybe he looks through the window before slamming the EMERGENCY VENT button by the door - and yeah, he'll get thrown clear. Not bullet-from-a-gun kinda thing, but in space with nothing to slow him down, he'll be moving at a fair enough speed.