Cones

Introduction

Possible space habitat geometries that would give pseudogravity by rotation are discussed in Geometries. In addition to the shapes discussed there, another possibility is the cone. Cones have already been demonstrated to be effective pressure vessels for supporting life in space (e.g. Mercury, Gemini, Apollo command module). The following is a brief discussion of possible benefits and problems of conical (typically truncated conical) shapes as habitats. An example of what is meant is shown in figure 1.

Figure 1.

The slope would probably not be as steep as indicated in the figure (see later).

Possible problems

1. Pseudogravity would vary as an inhabitant moved along the habitat parallel to the rotational axis, because of the variation in radius. This is a feature shared by spherical and simple toroidal colonies, but is a disadvantage compared with cylindrical or axially extended toroidal colonies. The degree of problem would, of course, depend on the angle of the cone and the axial dimensions. A short section of an acutely angled (gentle slope) cone would be relatively undramatic in effect, a long section of a wider angled (steeper slope) cone would feature a greater variation in rotational radius and thus in pseudogravity.

2. Inhabitants would be living on sloping ground, which would affect ease of construction of dwellings, for example.

3. Unless the slope were very gentle, it would not be possible to use external maintenance rings (see What rings are for). A small variation in radius could probably be accommodated by variable ride height suspension on the ring.

4. Compared with a cylinder, it would be more complex to construct, with the varying radius of curvature along the axis, for example.

5. Depending on the slope, soil may have to be retained arranged to avoid slippage (some sort of herringbone ribbed pattern or permeable barriers may work). This would be an added structural complication

Possible benefits

1. Compared with a cylinder, there may be greater visual interest.

2. Compared with a cylinder, the recreational opportunities may be greater (slopes).

3. The presence of slopes may allow surface running water (streams). Provided you constrain the channels, water will flow without a sloping floor. The slope that matters is that of the water surface, so if you put more in at one end of a channel it will flow to the other end until the surface slope is removed. A section of canal between locks works like that. However, for various reasons, not least being a wish to reproduce one of the more pleasing aspects of the Earth, one might wish to produce a stream or small river rather than a canal. To do that relatively realistically, the floor would probably have to slope. The degree of slope needed for such water movement is uncertain*.

4. The presence of slopes may allow better drainage. Unless one wants the conditions associated with stagnant water (e.g. acid bogs or anaerobic marshland - which may be required for conservation reasons but which would be of less value for food production), it may be necessary for the floor to slope to allow drainage of subsurface water.**

5. Extending points 3 and 4, there would be one point along the rotational axis at which water would collect. This would also be the case in spherical and simple toroidal habitats, as shown in figures 2 and 3 and in certain artists impressions (e.g. see Artists' impressions 2 - O'Neill Bernal Sphere and Artists' impressions 3 - Stanford torus).

Figure 2.

Figure 3.

This would be of benefit as a point from which water could be pumped for recirculation, and may also increase visual interest and increase the habitat possibilities for other, water related, life forms. This contrasts with the situation in a truly cylindrical colony, for which drainage may be less easy because of the absence of slope (see also above) and for which the presence of any bodies of water would depend on their being in excavations from whatever soil were present. These excavations would have to extend below the water table of the soil, as shown in figure 4 (if there were room for a permanently saturated groundwater layer- see end note discussion).

Figure 4.

Thus any water bodies shown in artists impressions of cylindrical colonies (e.g. see Artists' impressions 4 - 'Island Three' paired cylinders ) would be of necessity rather shallow. Another possibility would be to increase the depth of the water collection point by extending it into the thickness of the wall, as described by Arthur Clarke in his Rama novels, but this would require very thick (and thus heavy and expensive) walls, and would mean building in a relatively weak point (again, a point made by Clarke).

Possible constructional arrangements of (truncated) cones

As well as single truncated cones as in figure 1., it would be possible to arrange them in pairs or greater numbers as shown in figures 5, 6 and 7, giving quasi cylindrical habitats. Collection points for water are shown in these figures.

Figure 5.

Figure 6.

Figure 7.

The slopes shown are exaggerated for clarity. It would probably be preferable to go for shallower slopes (see end note) to minimise variations in pseudogravity and ease constructional problems. More realistic slopes in a situation such as in Figure 7, for example, may appear merely as a slight ripple or be almost imperceptible to an external observer.

Comparing the three designs shown, an obvious point is that, for the same slope and the same overall colony length, the use of multiple cone sections would give less variation in radius than would the use of two cone sections. Thus the variation in pseudogravity ('Possible problem' number 1) between the 'top' and 'bottom' of the slopes would be less. The smaller variation in radius may also allow the continued use of maintenance rings ('Possible problem' number 3). However, the reduction in radius variation of the multiple section design would also reduce the maximum depth possible in the water bodies, which may affect their usefulness as habitats for other life forms (or their recreational possibilities). The figure 5 design may be the least complex from the point of view of having only one, central, circumferential body of water. The figure 6 design may be best for redistribution of water since the water would be collected near the end caps, from the centre of which it could perhaps be projected along the axis of rotation to disperse and fall as rain (see Rain). Figure 7's resemblance to a concertina suggests a possible collapse mode for these designs, compared with a true cylinder, under acceleration along the axis of rotation. However, this problem may not be that serious, given that the slopes would probably not be nearly as steep as shown and that the accelerations in question would have to be kept low for other reasons (see Pushing).

*What slope would be needed? The US Uniform Building Code states that for positive drainage, a flat roof should have a slope of not less than ¼" in 12" (i.e. 1 in 48). This equates to an angle of one degree and 12' from the horizontal. This is supposed to be adequate to prevent standing pools of water. However, it is also the case that much lower slopes can prove adequate for drainage. For example, the lower reaches of the Amazon flow rapidly with a slope of 3 inches to a mile. For a less extreme example, Roman aqueducts worked well with slopes of 1 in 1600, or ~1 metre in a mile. Note that the effective slope for moving water in a space habitat would differ from the geometric slope because of the effects of rotational pseudogravity. The degree of this difference would depend on the radius and rotation rate involved. Also note that the previous, Earth derived, examples are all therefore assuming Earth normal gravity. A lower level of gravity (or rather of pseudogravity) would require a steeper slope to gave the same drainage rate. A further point is that the drainage examples quoted above are for a free water path. Drainage along a slope through soil would obviously be slower than for a free path, for the same slope angle.

**It is possible for soil water to drain into collection points without a sloping floor, since, in a parallel with the situation described above for free water in canals, abstraction of water from one point in a body of soil (such as a well) would lead to a net flow of water towards that point. However, wells only work if they penetrate the water table, into the permanently saturated groundwater layer of the soil. Could (or should) a colony have sufficient depth of soil to allow for such a permanently saturated layer? To provide a suitable growing medium for plants, a soil typically needs an aeration zone of adequate thickness in which there is capillary water in a continuous film around particles, replenished by water moving under the influence of gravity (or pseudogravity). Gravitational water excess to that needed to replenish the capillary water moves down through the soil until it enters the permanently saturated layer, where such a layer exists. If the aeration zone is of inadequate thickness, the soil is described as waterlogged and reducing conditions and accumulation of organics predominate (bog or marsh). However, if the excess water percolates down through the aeration zone and then follows the slope of impermeable bedrock (e.g. on hillsides) a permanently saturated layer may not be present. A sloping colony floor could take the place of that impermeable bedrock, thus avoiding a permanently saturated layer and possibly reducing the total thickness of soil necessary. (Soil provides radiation shielding, but it also adds weight without strength. This is unlike the case with a thicker wall structure, which provides both shielding and strength, so for efficiency soil should probably only be deep enough to provide for plant growth ).

If any or all of the above discussion is rubbish from an engineering (or other) point of view, please e-mail and let me know so that I can remove or correct it.

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