Explosive Decompression and You

Ashtagon's homebrew rules set.

Explosive Decompression and You

Postby Ashtagon » Sun Dec 13, 2015 8:38 pm

There are a number of bad tropes associated with losing pressure (tropes and more tropes).

The first one is a question of how long it actually takes to decompress. While the actual formula is much more complicated, a simplified version that can be used for gaming purposes is that the amount of air lost is equal to the speed of sound (1126 feet per second) multiplied by the cross-sectional area of the hole. This means that a one-foot-square hole will de-pressurise a single 10-foot cube every second (okay, 1.126 such cubes, but close enough for gaming purposes). At typical aircraft cruising altitude (40,000 feet), external air pressure is about 20% sea level, so it will take proportionately less time to equalise to that pressure (4/5 as much time at that altitude, or alternately 1.4 10-foot cubes per second).

Star Trek's utility shuttles share one useful fact in common with a Learjet -- they both hold 7 passengers. As a Learjet also has a cabin volume of 368 cubic feet, that means a typical bullet hole through the hull (1/2-inch diameter) will result in all air being lost in 818 seconds (136 rounds, or a little over 13 minutes). In the event of an extended fire-fight, divide that by the number of bullets that go through the hull. While this will certainly be a mind-focusing situation after the fight, during the fight it is very unlikely to be an issue, even when you consider that air pressure will be uncomfortably low about about a quarter of that time.

What if a window gets blown out from our not-a-Learjet shuttle? Call that a square foot of external hull. It will take 0.326 seconds to go from standard pressure to vacuum. That's fast enough to cause pulmonary barotrauma, or burst lungs.

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Another major trope is that the wind forces will suck or blow you out.

There's actually two major issues at work here. First, the air rushing out due to de-pressurisation. Second (and this applies to vehicles moving inside an atmosphere only), the external wind causing internal turbulence and causing people to "fly out". D&D currently lacks any rules to model the external wind turbulence issues. "Fighting from inside effective wind speeds of 550 mph (typical jet-liner cruise speed)" is a little outside the norm. The SRD only defines wind speeds up to 300 mph (as "tornado").

The second question -- whether the explosive de-pressurisation will push people out of the vehicle -- is a definite no. That is entirely cinematic. Ever helpful, Mythbusters (http://gadgetopia.com/post/2606) went on to prove that the wind speed won't blow you out (although you might be injured if you are sitting next to the new hole in the hull).

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Re: Explosive Decompression and You

Postby Big Mac » Wed Feb 24, 2016 12:03 am

Ashtagon wrote:There's actually two major issues at work here. First, the air rushing out due to de-pressurisation. Second (and this applies to vehicles moving inside an atmosphere only), the external wind causing internal turbulence and causing people to "fly out". D&D currently lacks any rules to model the external wind turbulence issues. "Fighting from inside effective wind speeds of 550 mph (typical jet-liner cruise speed)" is a little outside the norm. The SRD only defines wind speeds up to 300 mph (as "tornado").


D&D does have some rules for flying vehicles travelling through tornados...in Spelljammer. There are rules for landing and taking off in bad weather. But Spelljammer has open-decked ships, so I'm not sure there are rules for air rushing out and causing problems.
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