Re: Technology: Rigid Airship

From: Ken Clements (
Date: Wed Apr 12 2000 - 13:57:16 MDT

Spike Jones wrote:

> I did some calcs on this today. Looks to me like the vacuum dirigible
> notion doesnt work out. If one uses only idealized assumptions regarding
> the strength of materials, and assumes assembling a spherical vacuum
> ship on orbit, ignoring how to lower the thing into the atmosphere, etc,
> then one sees that the mass of the atmosphere displaced increases as
> the cube of the linear dimension, but the mass of the sphere also increases
> as the cube, since the surface area increases as the square of the
> diameter and the thickness increases proportional to the diameter.
> Looks to me that even assuming a hollow diamond sphere, we
> still arent in the vacuum dirigible business. Dammit. {8-[ spike

Yes, your calculation holds for things that are small enough to discount the
change in air density from the bottom to the top. If the diameter is large enough
to put the top of the craft up in the thin atmosphere, then the wall thickness
decreases with altitude, and the mass is no longer a cube function (as you had for
constant wall thickness). The other critical asymmetry to consider is that the
weight of the material in the lower half of the craft wall tends to offset
implosion pressure, whereas the weight of the material in the upper half of the
craft wall *adds* to the implosion pressure, so I came to the conclusion that it
is best to just get rid of it. See:


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