NASA SP-413 Space Settlements - Saint Ann's School

NASA SP-413 — SPACE SETTLEMENTS — A Design Study
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Figure 4-3 — Shelf thickness as a function of radius for spheres, cylinders, and toruses spinning to produce 1 g.
4) is shown, as are space stations of Gray (The Vivarium)
(ref. 8), Von Braun (ref. 2), and Tsiolkovsky (ref. 9).
Obviously only systems with radii of rotation greater than
895 m can lie on the line g= 1 below 1 rpm.
An aluminum cylinder like C-3 would weigh about 42,300 kt
and have a projected area of 55 X 10 6 m 2 , enough to hold
800,000 people — rather than the 10,000 people of the
design criteria. Similarly a sphere of radius 895 m would
hold 75,000 people and weigh more than 3500 kt if made of
aluminum.
A dumbbell shape has the advantage that the radius of
curvature of the part holding the atmosphere can be made
small while the radius of rotation remains large. However,
in this configuration people could only live on the cross
section of the spheres, and to hold 10,000 people with
670,000 m 2 of projected area the spheres would have to be
326.5 m in radius. Together they would weigh about 380 kt.
A torus also permits control of the radius that contains the
atmosphere separately from the radius of rotation.
Moreover, the torus can distribute its habitable area in a
large ring. Consequently, the radius needed to enclose the
670,000 m 2 of projected area can be quite small, with a
correspondingly small mass — about 150 kt for a torus of
major radius 830 m and minor radius 65 m (where the mass
of internal structure is neglected). The advantages of the
torus compared to the sphere and cylinder are discussed
further in appendices B and C which define some criteria
and parameters useful for such comparisons. The important
point is that for a given radius of rotation about four times
more mass is required to provide a unit of projected area in a
cylinder or a sphere than in a torus of small aspect ratio.
Thus, among the simple, basic shapes the torus is clearly
superior in economy of structural mass.
If minimum structural mass were the only concern,
composite structures would be the choice. Twenty-five pairs
of dumbbells would supply 670,000 m 2 with spheres 65 m in
radius and a total mass of 72 kt. The spheres could be made
smaller still and formed into a ring to make a beaded torus.
Alternatively, the toruses themselves could be made with
quite small minor radii and either stacked and connected
together to form a kind of banded torus, or built separately
to form a group of small, independent habitats.
However, as pointed out in the previous chapter, it is
desirable to compensate for the artificial and crowded nature
of the habitat by designing it to give a sense of spaciousness.
Composite structures are rejected largely on architectural
criteria of environmental perception. Not only would they
be more difficult to build than the simpler shapes, but also
their short lines of sight, little free volume and internal
arrays of closely-spaced cables and supporting members
would produce an oppressive ambience.
If the colony were composed of a number of small
structures, there would be problems of communication and
transport between them as well as the drawbacks of small
scale. Nevertheless, as table 4-1 shows, multiple structures
(and composite ones too) offer substantial savings in mass,
and it might well be that some of their undesirable aspects
could be reduced by clever design. It would be an attractive
option to be able to build up a colony gradually out of
smaller units rather than to start off with an initial large scale
structure. The subject of multiple and composite structures
is worthy of more consideration.
Chapter 4 — Choosing Among Alternatives