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

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Discussion Overview

The discussion revolves around the speed at which air escapes from a pressurized suit into a vacuum, particularly in the context of a rupture scenario similar to that depicted in the film "The Martian." Participants explore the implications of air flow dynamics, including the effects of pressure and temperature on escape velocity and the concept of choked flow.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant questions the escape speed of air from a pressurized suit, initially calculating a value using Bernoulli's principle but finding it inconsistent with expected results.
  • Another participant suggests that the escape flow will be choked, indicating it would move at Mach 1 due to the external vacuum pressure.
  • A participant expresses limited familiarity with compressible flows but considers using temperature relations to estimate air dynamics within the suit.
  • Concerns are raised about the assumption of constant air mass in the suit, with a suggestion to account for changing conditions as air escapes rapidly.
  • Participants recommend resources for understanding choked flow and its implications for calculating mass flow rates and velocities.

Areas of Agreement / Disagreement

There is no clear consensus on the assumptions regarding air mass and temperature changes during the escape process. Participants present differing views on the applicability of constant conditions versus dynamic changes as air escapes.

Contextual Notes

Participants note the importance of considering changing temperature and pressure conditions as air escapes, indicating that simplifications may lead to poor approximations of the actual flow dynamics.

Carlos PdL SdT
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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?
 
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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?
 
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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.
 
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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