Re: pressure containment: not a major issue?


Joe Strout wrote:
I've been thinking some more about containing air pressure in a space
habitat, and concluded that I (and most authors) have been making too
much of this. Please check my reasoning and let me know if I'm
overlooking something.

I was sipping a can of soda last night, and it occurred to me to look up
the internal pressure of an unopened soda can -- it turns out to be
around 2-4 atmospheres. Yet this is contained quite neatly by a very
thin aluminum wall. Pressure doesn't scale with size; 1 ATM is 1 ATM
whether it's a soda can or Rama. So, even that soda can is a couple
times thicker than the wall of a space habitat would have to be.
Suppose for a good safety margin, we make our pressure walls ten times
thicker than that -- it's STILL going to be a very tiny amount of mass
compared to the shielding and structural mass.

So, it seems to me that we really shouldn't be worrying about pressure
containment when choosing a geometry; containing 1 ATM is easy and won't
add much to the mass budget, no matter what shape we choose. (Of course
I realize that in the case of shapes that can buckle, the buckling
problem probably DOES scale with size, but I suspect that accounting for
this still leaves a much smaller amount of mass than that needed for
shielding and weight support.)

Any thoughts?

I remember doing lots of calculations for moon domes.

If you take a dome of 10m diameter, at 1 bar, the internal force is:

pi*5^2*10E5N = 7.5MN.

The hoop length is pi*10 = approx 30m, so each metre of length needs to
support 250KN.

Quite a few fabrics can work at 250MPa (=200 milllion N/m2), including
a safety factor. Hence every metre, 1/1000m2 is needed. Therefore the
fabric needs to be 1mm thick.

The area of a 10m dome is pi * 10^2 = approx 300m2. So the volume is
0.3m3. Assuming the density is about 2, the mass is a mere 600kg. So
the pressure is no issue - of course, you need a safety factor and
meteor and radiation protection.

Taking just the pressure change. If we go for 100m diameter, the area
is increased by 100, but the circumference by 10. So the thickness
needs to increase 10 fold. The area has increased 100 fold, so the mass
has increased 1000 fold, to 600 tons. Here it becomes a problem.

If we go to Kevlar, we could assume its 10 times stronger, so the mass
would be 60 tons.

.

Relevant Pages

  • Re: pressure containment: not a major issue?
    ... I was sipping a can of soda last night, and it occurred to me to look up ... Pressure doesn't scale with size; ... times thicker than the wall of a space habitat would have to be. ... compared to the shielding and structural mass. ...
    (sci.space.policy)
  • pressure containment: not a major issue?
    ... I've been thinking some more about containing air pressure in a space ... I was sipping a can of soda last night, and it occurred to me to look up ... times thicker than the wall of a space habitat would have to be. ... compared to the shielding and structural mass. ...
    (sci.space.policy)
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    ... mass of structural material for the whole ship and making it cube root ... And the mass does not scale as the cube if you are comparing a "solid" ... ship or habitat to a cylinder. ... If you break up a planet and use the materials of it to make far more ...
    (rec.arts.sf.science)
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