How Fast Does Air Escape from a Pressurized Suit into Space?

AI Thread Summary
Air escapes from a pressurized suit into a vacuum at speeds that can reach Mach 1 due to choked flow conditions. The specific exit velocity depends on the internal pressure and temperature of the suit, which can be approximated using compressible flow equations. For a suit with a fixed volume of air, the mass flow rate and velocity calculations must account for changing temperature as air escapes. While Bernoulli's principle may not apply in this scenario, understanding choked flow is crucial for accurate calculations. Utilizing resources like NASA's articles can provide further insights into these fluid dynamics concepts.
Carlos PdL SdT
Messages
6
Reaction score
3
I have a weird question here. At what speed does air flow from a pressurized container at one atm into vacuum (rupture in ISS for example).
I was watching the Martian and I wanted to see how much deltaV Mark Wattney can get by puncturing his suit, so I did my homework and saw that the spacesuit held two liters of liquid nitrogen and one liter of liquid oxigen (about 2.6kg total propellant mass). The mass of a person with a spacesuit is 100kg as estimated by NASA in the bok, so I only needed to know the specific impulse, or the exit velocity. I applied Bernoulli and came to an answer of about 400m/s which is way more than Mach 0.3 so Bernoulli doesn't work.
How do I get the air escape speed if it is above Mach 0.3?
 
Engineering news on Phys.org
It depends on what the internal pressure and temperature are. In most cases, the flow will also be choked (it will always be choked if the external pressure is vacuum), so it will be moving exactly Mach 1.

Are you at all familiar with compressible flows?
 
  • Like
Likes Carlos PdL SdT
boneh3ad said:
It depends on what the internal pressure and temperature are. In most cases, the flow will also be choked (it will always be choked if the external pressure is vacuum), so it will be moving exactly Mach 1.

Are you at all familiar with compressible flows?
Not much. Till now I've only seen incompressible flow in class. But then I guess I'd only have to plug in sqrt(1'4*R*T) and make a relation between T and the amount of air that remains within the suit. Or assume it's always the same which is easier to do.
 
It will be a poor approximation if you assume the amount of air is not changing in the suit. The air will be escaping fairly rapidly and the suit doesn't have that much volume. I'd suggest doing a Google search for choked flow (though I am not a huge fan of the Wikipedia entry, the NASA article on it is pretty good). That will hopefully give you a sense of what you are dealing with here. You can then probably move on from there to calculate the mass flow rate and/or velocity or whatever else you need. Just keep in mind that the temperature is going to change as the flow accelerates so you have to account for that.
 
  • Like
Likes Carlos PdL SdT
boneh3ad said:
It will be a poor approximation if you assume the amount of air is not changing in the suit. The air will be escaping fairly rapidly and the suit doesn't have that much volume. I'd suggest doing a Google search for choked flow (though I am not a huge fan of the Wikipedia entry, the NASA article on it is pretty good). That will hopefully give you a sense of what you are dealing with here. You can then probably move on from there to calculate the mass flow rate and/or velocity or whatever else you need. Just keep in mind that the temperature is going to change as the flow accelerates so you have to account for that.
Thank you again, but as I understand it in this specific case where the air is held in separate tanks I could assume a constant pressure and temperature. It'd change however if I used my first example of a hole in the ISS. Anyway I looked at the NASA site and I feel like it's going to become one of my best friends for the next years
 

Thread 'Query on Isentropic relationships (such as PV^gamma = constant)'

Hi all, I have a question. So from the derivation of the Isentropic process relationship PV^gamma = constant, there is a step dW = PdV, which can only be said for quasi-equilibrium (or reversible) processes. As such I believe PV^gamma = constant (and the family of equations) should not be applicable to just adiabatic processes? Ie, it should be applicable only for adiabatic + reversible = isentropic processes? However, I've seen couple of online notes/books, and...
View full post »

Thread 'Dry sump pump issue'

I have an engine that uses a dry sump oiling system. The oil collection pan has three AN fittings to use for scavenging. Two of the fittings are approximately on the same level, the third is about 1/2 to 3/4 inch higher than the other two. The system ran for years with no problem using a three stage pump (one pressure and two scavenge stages). The two scavenge stages were connected at times to any two of the three AN fittings on the tank. Recently I tried an upgrade to a four stage pump...
View full post »

Hot Threads

Recent Insights

Back
Top