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Two different designs for passive catchers, although rejected for the baseline, were considered in some detail. Figure 4-28 illustrates one version of a passive catcher.


It consists of two major parts: a slowly spinning bag of Kevlar fabric, and a non-spinning rim which contains power, propulsion and other necessary systems. (Such "dual-spin" designs are commonly used in satellites.) To keep the mass of the catcher within reasonable limits, it is limited to 100 m in radius. Consequently the dispersion of the incoming payloads must be less than this. After the incoming masses arrive within the 100 m radius of the target area, they strike a grid of cables across the mouth of the catcher and break up, thereby releasing a spray of fine particles and small gravel-size rocks which fly inward. These particles strike the bag at up to 200 m/s (23 ply Kevlar stops 44 magnum bullets fired point blank) and come to rest against the surface of the bag where they are held by centrifugal force thereby preventing them from drifting free and escaping from the mouth. Uncertainty about achieving the accuracy of launching required for this catcher, and also some doubt about whether the material would break up and be contained as planned, led to the rejection of this alternative.

A second version of a passive catcher is a circular disk, 10,000 m^2 in area, of crushable material such as rigid foam or bonded, glass wool boards. A payload would penetrate this material dissipating its energy and lodging in the material from which it could be retrieved at a later time. Theoretical analysis shows that a typical payload would penetrate about 1.3 m into FR type polystyrene foam (density of 28.4 kg/m^3). The foam catcher could be foamed in place. After collecting for a period of time it could be melted down with a solar furnace; the desired material extracted; and the catcher refoamed in space. It has the advantage of being very simple in conception, but its 500 t of mass is a disadvantage, as is the fact that, at least initially, the plastics for making the catcher would have to come from Earth. Eventually it would be possible to use mostly lunar materials such as bonded glass wool. Like the other passive catcher, the foam catcher requires very high precision in the launchings.

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