Re: Lighter than Air.

From: Ken Clements (
Date: Thu Mar 16 2000 - 23:01:33 MST wrote:

> How about a vacumn dirgible. That is...instead of pressurizing a
> ballon....evacuate a pressure vessel. If the weight of the volume of the air
> displaced exceeds the weight of the pressure vessel it should float..
> Neal Stephenson alluded to such devices in "diamond age". Are they possible
> now?.....using geodesic type structures and hi tensile material?
> Can such a structure be made light enough?

I have thought about this for some time, and wrote about it on sci.nano a few
years ago. I believe that there are some researchers working on it. Josh Hall
noted that in order to have a lift capability better than hydrogen, a vacuum
flotation diamondoid shell a meter in diameter would have a thickness of only
about a micron. Some implosion stability was questioned.

I became interested in this subject, not for vacuum replacement of simple
balloons, but because it leads to the possibility of designing stratospheric
stations that are "ships of the sky." This is a very large paraboloid structure
that floats in the high atmosphere the way a boat floats on water. Just like an
ocean going ship, the hull gets thinner as you go to higher altitude (lower
pressure). The top edges of the structure are molecularly thin, at distances
from earth where the pressure is in the millitorr range. Being open to space, it
makes a nice place to put observatories, or mass drivers that are going to throw
things up to a space hook. However, being open, it will always have a flow of
molecules falling down from the edges that need to be pumped out by the "bilge"
turbopumps (solar powered).

Here is a thought experiment to explore this. Suppose you could build a giant
steel disk in space and lower it toward the earth. This disk is so big that the
edges are still out in space when the center encounters the curvature of the
atmosphere. Suppose this disk is 52 cm thick at the center. Steel has a density
of 7.85 g/cm^3, so (close to the earth) the weight of the disk at the center is
about 4 N/cm^2. Well, the pressure of air is about 4 N/cm^2 at an altitude of
5000 meters. This means that the weight of the disk is supported by the pressure
of the air below it. As we move from the center of the disk to the outer edge,
the disk would be made thinner (thus lighter) because the altitude of the point
is higher in the atmosphere until we get to the edge of space. This can only be
a thought experiment because at some point you cannot make the steel any thinner
and air would rush in, and as you move away from the center the resultant vector
of pressure is no longer normal to the disk, creating stress. However, the
interesting part is that there is no stress on the central, heaviest area of the
structure, so almost any construction material would work (thickness adjusted for

By changing the shape from a disk to a paraboloid (more like a boat), the
structure can be made smaller while still getting the edges out to an area of
micropressure. The first tradeoff is that now the central part of the structure
must sustain hoop stress (as does the hull of an ocean ship), and hold up the
weight of the extending edges, but with nanotech materials this starts to look
very attractive. The second tradeoff is that structures that do not extend so
far mean less weight (less stress on materials), but then more molecules fall in
per second and the turbo pumps need to work harder. I am interested in designs
that start at about 20000 m above the earth, so they can be serviced by a
conventional balloon, but then give access to space.

The trick is to be able to build (grow) the station in place. I envision a
structure that is many km in diameter, and (although made of nanotubes) is so
fragile that it could not be made anywhere on earth, or survive retro
acceleration descending from orbit. It is a project on my back burner, and may
never be practical in the sky of earth, but is interesting for places such as


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