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Appendix 5I:
LMF Solar Canopy Power Supply

The solar canopy provides electrical power for the entire lunar factory complex described in section 5.3.4. The canopy consists of many sections of automated (active or passive) solar energy collection devices. Mobile robots begin erecting the canopy after a useful fraction of the LMF base platform has been laid down and the central computer system installed in a depression near the hub. The canopy is just a simple framework of lightweight vertical metal/basalt struts snapped into universal connectors bolted into the heavy basalt foundation of the LMF. Horizontal wires or thin crossbeams support the solar panel mechanisms. The solar canopy is designed to be broken into relatively small sections for ease of assembly, installation, maintenance, and repair.

5I.1 LMF and Solar Canopy Geometry

One of the major constraints on LMF shape is the necessity for solar energy collection. The LMF may be visualized geometrically as a very broad, squat cylinder with some net density dL (kg/m3), mass (exclusive of platform) M, radius R, and height H. All factory energy is gathered using a "rooftop" surface area approximately the same size as the underlying foundation platform, so the fundamental constraint on factory size may be expressed by the condition M/πR2 = dLH <= MPs/P, where Ps is the usable energy delivered to the LMF by its solar collectors (roughly 150 W/m2 for high quality photovoltaic devices at 45° angle of incidence) and P is the total power required by the initial lunar seed (about 1.7 MW; see sec. 5.3.4-5).

For a factory mass M = 105 kg, R >= 60 m, the figure used elsewhere in this report. Estimates from O'Neill et al. (1980) that solar power systems (SPS) in the 100 kW range can be assembled for 8 kg/kW suggest a total mass for canopy collector panels (1.7 MW) of 13,600 kg, although this figure was derived from space-based SEPS and SPS design studies. A mass of 22,000 kg was adopted for the canopy, which includes transformers, diodes, cabling, and other necessary support devices. Since dLH = 8.8 kg/m2, the LMF in fact will be quite "roomy" inside - a "typical" population of 1-ton factory machines would be separated by an average distance of 2[103/πdLH)]1/2 = 12m.

Another major factor in determining basic factory configuration is the degree of isolation desired from the external surroundings. There appear to be few compelling reasons for solid massive walls enclosing a fully automated lunar manufacturing facility. Inclement weather, cleanliness, provision of human-habitable volume, protection from the dangers of seismic activity, noise/pollution abatement, and theft prevention are the usual reasons for heavy walls on Earth, yet these factors should have little if any impact upon factory construction in space or on the Moon. Further, rigid solid walls hinder growth and might delay reconfiguration as the LMF expands in size. The cleanliness problem in an open factory is expected to be minor, as mobile robots are designed either for external or internal operation but not for both (though in special circumstances MARR machines can be towed to external sites by mining robots).

The simplest solar canopy configuration is a web-like metal structure overlaid with flat solar panel assemblies. These cells are suspended from a series of crossbeams spaced at regular intervals along chords of the circular LMF. These crossbeams may be as thin as wires if adequately supported by strategically placed vertical columns. Calculations of stress reveal that a 1 mm radius aluminum rod (typically 108 N/m2 tensile or compressive strength) should be strong enough to support a 22-ton canopy structure with a loading safety factor of about 5. Support posts are 1-cm diameter aluminum/basalt columns placed at intervals of 10 m across the factory floor and anchored with universal connectors and several braces and struts for stabilization. These posts have an overload factor of more than 100, hence should be able to sustain low-speed accidental impacts by mobile robots without buckling. The total mass for the entire framework is well under 1 ton.

Ideally, all lunar operations should be conducted continuously with only scheduled maintenance shutdowns. However, continuous operation is possible only if continuous power is also available. A number of options for power storage during the lunar night have been considered in the context of a lunar base (Criswell, 1979; Vajk et al., 1979) in the literature. Possibilities include nuclear plants, volume heat capacity storage, chemical storage (batteries, fuel cells, exothermic reactants), capacitor banks, gravitational energy storage, pressurized gas, flywheels, and SPS transmission from orbit to lunar surface collection stations. These many promising alternatives, however, were not explored in depth. Without such an option, the baseline LMF must be placed on standby during the lunar night with one working year requiring two calendar years.

5I.2 Solar Canopy vs Lunar Igloo Designs

In the solar canopy LMF design the entire automated factory complex is erected on a fused basalt platform resting on the lunar surface. Above the factory floor is a relatively flimsy framework of solar energy collectors which provide system power.

The "lunar igloo" is an alternative in which geodesic domes of 120 m diam are constructed over each seed factory. Additional factory growth is accommodated by adjacent domes of similar size built with a network of connecting tunnels. Each dome is covered with at least 2-5 m of lunar topsoil which may be sufficient to permit the retention of an internal 0.3-atm oxygen atmosphere This configuration might be handy in preventing accidental vacuum welding and could simplify servicing and trouble shooting by humans during system failures. Light could be admitted to the underground LMF via a converging reflective geometry (Hyson, personal communication, 1980).

Since these models represent fundamentally different design concepts (see fig. 5.56), the team compared the two directly on a number of significant factors enumerated table 5.21. The conclusion was that the canopy model is possibly superior in the present fully automated self-replicating LMF application, but that the igloo model is not precluded in other scenarios.

Figure 5.56. -
Schematic of Solar Canopy and Lunar Igloo models of self-replicating or growing LMF.

Table 5.21.- Comparison Of Important Factors For Solar Canopy And Lunar Igloo Models Of Self-Replicating Or Growing LMF

Some important factors Solar canopy Lunar igloo
1. Maintain useful atmosphere? no yes
2. Maintain useful vacuum? yes yes
3. Prevent solar cell degradation? no no
4. Prevent external optics degradation? no no
5. Prevent internal optics degradation? no yes
6. System temperatures easily controlled? no yes
7. Low mass foundation structure? yes yes
8. Low mass total structure? yes no
9. System construction mechanically easy? yes less easy
10. Easy maintenance of system integrity? yes less easy
11. Internal lighting easily available? yes no
12. Human repairman accessible? yes yes
13. Human repairman habitable? no yes
14. Easy horizontal mass flow? yes yes
15. Simplicity of overall system design? yes less simple
16. Easy to expand LMF system size/mass? yes no
17. Waste heat easily rejected? yes no
18. Terrestrial manufacturing processes easily transferred? less easy yes

5I.3 References

Criswell, David R.: The Initial Lunar Supply Base. In Space Resources and Space Settlements, John Billingham, William Gilbreath, Brian O'Leary, and Beulah Gossett, eds., NASA SP-428, 1979, pp. 207-224.

O'Neill, G. K.; Driggers, G.; and O'Leary, B.: New Routes to Manufacturing in Space. Astronautics and Aeronautics, vol. 18, October 1980, pp. 46-51.

Vajk, J. P.; Engel, J. H.; and Shettler, J. K.: Habitat and Logistic Support Requirements for the Initiation of a Space Manufacturing Enterprise. In Space Resources and Space Settlements, John Billingham, William Gilbreath, Brian O'Leary, and Beulah Gossett, eds., NASA SP-428, 1979, pp. 61-83.