Why do spacecraft with a small hull breach not decompress immediately

265 views

[ad_1]

I remember seeing several instances where spacecraft (i.e. ISS) had small holes in them and they were fine (NASA even didn’t wake astronauts once). Since the pressure difference is so great (space is a vacuum), how come there’s no explosive decompression (all the air is sucked out at once)?

In: Physics
[ad_2]

> pressure difference is so great

That’s the thing – it’s not. Atmospheric to vacuum is the same difference as sea level to 10 meters under water. It’s just 1 bar, or 1 kg per square centimeter.

For comparison, a municipal water system usually has a pressure of 3-5 bar, and a dripping tap also doesn’t blow your house up.

The pressure different isn’t that great. One atmosphere of pressure is around 15 psi. All things considered, that’s not very high (your car tires are 2-3 times that, and don’t blow up when poked). Then there’s the fact that small holes really restrict the flow of liquids and gasses through them.

Holes can be a problem for materials that tear easily, like how s balloon pops. Materials that are much stronger won’t rip apart like that, and can tolerate small holes no problem. You can even test this by putting a piece of tape on a balloon and then poking through the tape with a needle. You’ll just get a slow leak.

You are correct the pressure “difference” is great, but only in terms of ratio. In fact the ratio of pressures inside and out tends to infinity. But a phenomenon of fluid flows named “choking” puts a limit to how much of the air inside the craft can leave the hole per second, for a given size of hole.

As the pressure ratio grows between inside and outside, gas indeed starts to leave faster and faster. However for a high enough pressure ratio (only about 2 for air!) the gas will reach the speed of sound and you get something called a choked throat. Because any gas leaving the hole then goes faster than sound, it actually has no way to tell the air still inside to leave faster- the expansion wave telling air it hits to go faster just flies away from the spacecraft!

So, even though there is an almost infinite ratio in pressures inside and out, the mass flow is restricted by the usually small size of the hole. The hole doesn’t break open because the pressure difference (not ratio) is not enough to break the strong and rigid material. Spacecraft aren’t balloons!

Fill a compressor with air, a big one with a 20 gallon tank, at 100 psi. Open the drain valve, which is like punching a small hole. It’s going to take some time to empty the tank, maybe a minute, to equalize the 20 gallons of 100 psi with the outdoor 14.7 psi. You’ll notice it starts coming out really fast at high pressure, and then goes more slowly as the pressure differential between the tank and the air gets lower.

Looking it up, the ISS is effectively a 241,867 gallon air tank at 14.7 psi. You have to equalize the pressure of all that at 14.7 psi to 0 psi, which is going to take a long time with a small hole. The pressure difference is also not nearly as high, so the air won’t be pushed out nearly as fast as with the compressor early on.