Big Gyros (canceling or controlling precessional effects)

Introduction

Spaceships or colonies that rotate to produce artificial gravity will effectively be large gyros. As anyone who has played with a toy gyroscope will know, that can have an interesting effect. Any attempt to turn a gyro about an axis perpendicular to the axis on which it is spinning will produce a turning effect about a third axis perpendicular to both of the others. It's called precession (but if you're bothering to visit sites like this you probably knew that already). Anyway, the following considers ways of getting around problems associated with precession.

The first way, for a colony on solar orbit, would be for the axis of rotation of the colony to be perpendicular to the plane of the orbit. Such arrangements have been proposed, with sunlight reflected into the colony via an arrangement of mirrors including one, not rotating with the colony, turning the light through 90 degrees onto another, circular arrangement of mirrors, from which it would be reflected into the colony. This has been suggested for toroidal or axially extended toroidal colonies (Figure 1).

Figure 1.

Aside from this single case (of only attempting to turn the ship or colony about the axis on which it is already rotating) the approach to controlling precession is that, essentially, something spinning could be coupled with something else spinning on the same or a parallel axis but in the opposite direction. The 'something else' could be identical (other than spin direction, of course) (i.e. homologous) or different in either mass, rotation rate or diameter, or rather in a combination of these (i.e. heterologous).

Homologous precession control

O'Neill's idea for L5 Lagrange point cylindrical colonies envisaged that they would collect light via mirrors that required the rotational axis of the cylinder to be kept pointing towards the sun. Obviously this would involve one complete rotation of the cylinder about an axis perpendicular to the rotational axis in one year. The plan was, therefore, to couple together pairs of contra-rotating cylinders situated one 'above' the other (if the plane of the ecliptic is taken to be the horizontal). The cylinders would be joined by a compression tower at one end and a tension tower at the other, this application of a couple generating in its turn the necessary gyroscopic turning forces for the sunwards pointing (Figure 2). It should be noted that this arrangement may only work for rotation of the cylinder pair in one direction and in one plane. Rotation in the opposite direction could collapse the tension tower, which would now be in compression, and would test the tensile strength of the compression tower. Any external forces which caused rotation of the pair in another plane could cause forces that would make the colony pair skew relative to each other, which if taken far enough could cause them to touch (with whatever unfortunate consequences).

Figure 2.

Alternatively, homologous precession control could be achieved with the rotating portions sharing a common axis of rotation. An example of this would be axially extended tori 'on a stick' (Figure 3). For example, if four such identical tori were used, with numbers 2 and 3 rotating in one direction and 1 and 4 in the opposite direction, there should be net cancellation of gyroscopic forces. One disadvantage of this design is that it would require some ingenuity to get sunlight into the innermost tori. It may be best suited to artificial lighting in an interstellar ark type of design. In such a case the common axis could also serve to reduce the amount of impact shielding required at the 'front' and of engine radiation shielding required at the 'back', compared to a side by side pair of cylinders, because of the reduced 'frontal' area (always assuming that it was traveling along the line of the axis of rotation). As a further example, the 'Explorer Class' 'fast' interstellar ship design study suggests contra-rotating segmented tori as habitations, to control torque effects on the ship as a whole when the habitations are spun up and down for different (accelerating and non-accelerating) portions of the proposed flight.

Figure 3.

Heterologous precession control

Generally this approach can allow more flexibility than is possible with homologous designs. An example of this would be the 'Sunflower' type colony, with a torus and a short cylinder sharing a common axis (Figure 4), the outer torus providing agricultural space for the inner living area. The contra-rotation of these could be used to allow an appropriate arrangement of mirrors to maintain sun-pointing orientation.

Figure 4.

The arrangement shown above may work in an in-system colony, but it would share the problems, mentioned above, of a side by side pair of cylinders if used for an interstellar ark, in that the frontal area to be shielded may be unnecessarily large. As for the homologous colonies the preferred approach in this scenario may be to make all of the habitats the same diameter as well as rotating them about the same axis. One design with this arrangement would be to take the four tori 'on a stick' already mentioned and join the middle two together, giving a larger central colony section (Figure 5).

Figure 5.

However, it may be considered preferable that instead of adding complexity by having three habitats, the central habitat could be balanced by contra-rotating masses at each end, rather than habitats. This seems at first sight wasteful of energy and materials, particularly in the case of a habitat that needed to be accelerated and decelerated like an interstellar ark. However, in such a case erosion/collision shielding would be needed in 'front' of the habitat and probably radiation shielding at the 'back' of the habitat to protect against the effects of the engine (assuming something like a fusion or antimatter engine), so the mass may be in place anyway and could be spun without too much extra complication (Figure 6).

Figure 6.

A further approach, involving shielding, to controlling gyroscopic effects would be that of making the main colony cosmic ray shielding independent of the pressure vessel that forms the habitat. This has been suggested for earlier 'Island One' type habitats such as, for example, the NASA/Ames 1975 Summer Study proposal of a torus rotating inside a 'tire' of cosmic ray shielding or O'Neill's Bernal Sphere design with an outer shield of lunar slag between two thin spherical shells. O'Neill suggests rotating the colony by reaction against the shield, which should give automatic balancing of precessional effects for the colony as a whole (Figure 7). Similarly the Summer Study suggests using the shielding as reaction mass for rotation of the main colony, noting that the shielding, given its much larger mass, would rotate much more slowly (and would have to rotate more slowly given its intrinsically lower strength, it being constructed of mechanically fastened slag blocks, and because its great weight would tear the colony apart if it were to rotate as one with the colony). One particular feature of contra-rotating colony and cosmic ray shield is that they would be moving past one another at a separation of a few metres at some hundreds of miles per hour, typically, which would require some care in avoiding 'scraping' of one against the other and which could make maintenance of either a somewhat hazardous procedure

For larger colonies the structure needed to provide pressure and rotational strength should also be thick enough to give cosmic ray shielding, so the option of rotating against an outer, concentric, contra-rotating shield would not provide the relatively economical and elegant solution that it could for smaller colonies.

Figure 7.