[The following is an excerpt from a letter to Allen Meece]
[Updated 4 September 2002]
Using a commercially available, pre-cast PET tank for the crew
hab requires us to use a tank with 1.25" walls or thicker to be sure that we
have an adequate pressure vessel. Tanks like this are built to hold >60
tons of liquid under one or more atmospheres of pressure for pumping, so we'll
know that it's capable of holding less than an atmosphere of pressure in
vacuum. However, a 3.5m x 6.5m tank of this type weighs 2500kg+ and a 2.5m
x 2.5m tank weighs 800kg+ for the tank walls alone. Metal tanks capable of
handling similar pressure weigh even more.
Structurally, these tanks are overbuilt for our purposes,
being capable of withstanding many times what we would ever ask of our VBP
hab. With the addition of an internal framework to hold flooring and
equipment, they have far more wall strength than we need. They were also
never intended to be used with vacuum on one side of the tank wall, so they lack
an important feature: double pressure walls.
We know that a simple Spectra (or even even double Dacron)
ballute lined with polypropylene will hold atmospheric pressure against a
vacuum. Double its strength, and it would even be marginally suitable as a
cabin wall. Double the polypro layer, and it is effectively double
hulled. Using two sequential gas barriers allows leaked gas to build up
between them, spreading the pressure differential across two layers. It
also guards against catastrophic failure of one layer.
By covering a hard-walled tank with a flexible ballute,
we would create a double-walled hull with the benefits of both hard and cloth
hulls. It would provide a firm anchor for the hab's pressure doors and
interior structure, could resist blows that would crack a hard-walled tank from
end to end, and -- most importantly -- be made of much thinner and lighter
plastic. We could probably build a hab of this type using less
than 1.5 ton for both module walls, rather than >3.3T. This
represents a considerable savings in weight without any significant loss of
structural strength or endurance.
