Space Settlements - A Design Study 1977 - Page 27

Soil is scooped and carried to the processor by two scooper-loaders (refs. 18-20). Ore is carried from the mining area on a conveyor system. At the launch area it is compacted to fit into a launcher bucket, and then fused.

The site for the base is in the Cayley area at 4° N. 15° E. where Apollo 16 landed. This site was selected because of richness of lunar ore, suitably flat terrain for the launcher, and the near-side equatorial region gives a suitable trajectory to L2. Apollo samples had an aluminum content between 4.5 and 14.4 percent, the highest percentage being from this site. The Apollo missions did not provide any evidence of rich ore veins below the lunar surface.

Lunar bases have been the subject of many design studies (refs. 18-20). The total mass of housing and life support equipment is approximately 2000 t brought from Earth to accommodate the construction crew of 300 persons. During the mining operations, there are only 150 persons at the base of whom approximately 40 are support personnel. Consumables of 4.95 kg (including 0.45 kg for losses) per person-day are supplied from Earth. The mass imported each year is given in table 5-13. Almost all activities are in a "shirt sleeve" environment within the shielded structure. A large area is provided for repair work. The mass and power required for these operations on the Lunar Base are summarized in tables 5-14 and 5-15.

The Mass Launcher

Critical to the success of the entire system of colonization of space is the ability to launch large amounts of matter cheaply from the Moon. There are two aspects: launching the material from the Moon by an electromagnetic launcher using the principle of the linear induction motor, and gathering the lunar material in space by an active catcher located at L2.

Each second the mass launcher accelerates five 10-kg masses of lunar material to lunar escape velocity of 2400 m/s. Errors in launch velocity are kept within 10⁻⁴ m/s along the flight path and 10⁻³ m/s crosswise to it.

The masses are carried in an accelerating container or "bucket." Built into the walls of each bucket are liquid-helium-cooled superconducting magnets which suspend it above the track. The buckets are accelerated at 30 g over 10 km by a linear electric motor running the length of the track. The bucket then enters a drift section of track, where vibrations and oscillations lose amplitude enough for the payload to be released with great precision of velocity. The velocity of each bucket is measured, and adjusted to achieve the correct value at release.

During acceleration the payload is tightly held in the bucket, but when lunar escape velocity is reached and the velocity is correct the payload is released. Since the bucket is constrained by the track to follow the curve of the lunar surface, the payload rises relative to the surface and proceeds into space. Each bucket then enters a 3 km region where a trackside linear synchronous motor decelerates it at over 100 g. It is returned to the loading end of the track along a track parallel to the accelerator.

At the load end of the track the liquid helium used to cool the superconducting magnets is replenished, and a new payload loaded. Then the bucket is steered to the start of the accelerator for another circuit. Figure 5-19 shows the mass launcher schematically. More details are given in appendix F.

TABLE 5-13 — ANNUAL MASS IMPORTS

| Item | Mass (t/yr) | | :--- | :--- | | Food | 250 | | Water | 50 | | Nitrogen | 20 | | Spare parts | 100 | | Total* | 420 |

- The same mass is also transported from Moon to Earth.

TABLE 5-14 — LUNAR BASE EARTH-SUPPLIED MASS

| Component | Mass (t) | | :--- | :--- | | Housing | 1200 | | Life support | 300 | | Mining equipment | 200 | | Mass launcher | 300 | | Total | 2000 |

TABLE 5-15 — LUNAR BASE POWER REQUIREMENTS

| Operation | Power (MW) | | :--- | :--- | | Mining | 5 | | Compacting | 10 | | Mass launcher | 180 | | Life support | 25 | | Total | 220 |

With a 70 percent duty cycle, this system can launch 1.1 Mt/yr. To assure this duty cycle during lunar night as well as lunar day, two complete mass launchers are necessary. A nuclear power plant rather than a solar plant is required so the operation can continue through the lunar night.

Power and Supply

Several nuclear reactor single-cycle helium-Brayton plants of 10 to 50 MW each are used instead of a single big plant because the smaller plants can be transported assembled and become ready to operate by use of space shuttle main engines. The redundancy of several smaller systems is attractive, especially since the plants need to be taken off-line for refueling every year or two.

The total capacity is 220 MW and the total mass is 9900 t, including a 10 percent design factor.

The mass of the power plant is estimated using the value of 45 t/MW, which is projected to be applicable to nuclear plants within the decade.¹ Shielding will be provided by lunar material.

¹ Austin, G., NASA-Marshall Space Flight Center, personal communication, June, 1975.

Figure 5-19 — The mass launcher.

TABLE 5-20 — CROP YIELDS

| Crop | Yield (kg/ha) | | :--- | :--- | | Rice | 15,000 | | Sorghum | 12,000 | | Soybean | 6,000 | | Corn | 14,000 |

Unit weights Rice 60 lb/bu Sorghum 56 lb/bu Soybean 60 lb/bu Corn 56 lb/bu

Conversion factors 1 ha = 100 X 100 = 10⁴ m² 1 kg/ha ≅ 1 lb/acre 1 bu = 35.24 liters 1 ton = 0.906 tonnes (t)

TABLE 5-21 — PHOTOSYNTHETIC PRODUCTIVITY ENHANCEMENT

- Controlled by quarantine.

TABLE 5-22 — INCREASED PRODUCTIVITY FACTORS* IN VEGETABLES (ref. 24)

| Vegetable | Factor | | :--- | :--- | | Tomato | 10 | | Cucumber | 12 | | Lettuce | 8 | | Radish | 6 |

- Under greenhouse conditions.

APPENDIX D: PRODUCTIVITY

Cost engineers in industry use a very simple model based on factors of man-hours of labor per unit of production. The factors are based on experience. The use of predetermined time and motion studies requires a finished engineering design and a methods study.

Let

e = man-hours per unit of the process, p, being performed, in Earth environment
F = factor by which a will be increased or decreased for the process, as a function of location
p = manufacturing, extracting, or erecting process being evaluated
W = quantity of the end product in production units
l = location at which the process will be carried out
E_p = man-hours required to produce W units of production by the process, p
E = man-hours required for some finished structure or unit.

Then for any process and any location,

and for any finished structure or unit,

The model makes possible a labor estimate at any stage of design, simply becoming more detailed as to operations and factors as the design becomes more firm.

APPENDIX E: MASS SHIELDING

There are three mechanisms that are important in mass shielding. First, a charged particle excites electrons for many hundreds of angstroms about its trajectory. This excitation extracts kinetic energy at a roughly constant rate for relativistic particles and acts as a braking mechanism. For relativistic protons in low-Z matter this "linear energy transfer" is 2 MeV/g-cm⁻² of matter. If the thickness of the mass shield is great enough a particle of finite kinetic energy is stopped. This is the least effective shielding mechanism in matter for relativistic particles.

The second mechanism is nuclear attenuation. For silicon dioxide the average nuclear cross section is 0.4 barn (10⁻²⁴ cm²). Thus if a charged particle traverses far enough in the shield (composed of silicon dioxide) it collides with a nucleus and loses energy by inelastic collisions with the nuclear matter. The measure of how far a particle must travel to have a substantial chance of nuclear collision is the mean free path, which for silicon dioxide is 106 g/cm². This mechanism is an exponential damper of primary beam particles.

Opposing the beam clearing tendency of nuclear attenuation is the creation of energetic secondary particles. For each nuclear collision there is beam loss from nuclear excitation, and beam enhancement (though with overall energy degradation through the increase of entropy) from the secondaries emitted by the excited nuclei. These secondary particles are, of course, attenuated themselves by further nuclear collisions with roughly the same mean free path as the primary particles.

The calculation of a mass shield can only be properly done by Monte Carlo simulation of the various pathways that the interactions can take. Two approximations often used either assume that secondary particles supplement nuclear attenuation well enough that only electron excitation stops the beam (the "ionization" approximation); or that secondary particle creation is negligible, resulting in beam attenuation by both nuclear attenuation and electron excitation ("ionization + exponential" approximation). Figure 5-26 plots the exposure rate versus all three formulations of shielding effectiveness for a cosmic ray spectrum incident on a copper shield. As the shield thickness becomes greater than approximately one mean free path length (for nuclear attenuation) the Monte Carlo result begins to show behavior that parallels the slope of the ionization + exponential approximation. The remaining difference is that the Monte Carlo result is a scale multiple of the ionization + exponential curve. This behavior then provides the following new approximation:

F = exposure rate,
N₀ = initial exposure rate of primary beam,
λ = mean free path for nuclear collision,
x = distance (g/cm²) of shield traversed.

Since the secondary particle production factor for a high-Z nucleus like copper is greater than for low-Z nuclei this approximation should be conservative using the factor of 5 for secondary production. Because thick shields (x >> λ) are being considered, this equation's sensitivity to error is greatest in the value of λ used, which fortunately can be accurately determined (106 g/cm² for silicon dioxide).

To calculate doses behind a shield the result is used that 1-rad in carbon is liberated by 3 X 10⁷ cm⁻² minimally ionizing, singly-charged particles. Within a factor of 2 this is valid for all materials. Assuming a quality factor of unity for protons, and an omnidirectional flux of protons at 3/cm²-sec (highest value during the solar cycle) the dose formula is:

dose = (16 rem/yr)e⁻ˣ/¹⁰⁶

x = shield thickness (g/cm²).

This is a conservative formula good to perhaps a factor of 2 in the thick shield regime. A spherical shell shield is assumed with human occupancy at the sphere's center. If the dose to be received is set at 0.25 rem/yr, to be conservative with the factor of 2, a required thickness of Moon dust (silicon dioxide) of 441 g/cm² is derived to protect the habitat.

As a final point, note that an actual shield will generally not have spherical symmetry. To handle this case the shield geometry must be subdivided into solid angle sections which, due to slant angles, have differing effective thicknesses. Equation (3) then has to be integrated over all the solid angle sections to calculate the received dose.

Figure 5-26 — Cosmic ray exposure for different approximations for a copper shield as a function of thickness. (a) Solar maximum, thin tissue (τ = 0.21); (b) Solar maximum, thick tissue (τ = 1.0); (c) Solar minimum, thin tissue (τ = 0.21); (d) Solar minimum, thick tissue (τ = 1.0).

Image

APPENDIX F: THE MASS DRIVER

The baseline lunar material transportation system involves a magnetically levitated vehicle, or "bucket," accelerated by a linear synchronous motor. Acceleration is at 288 m/s² along 10 km of track. The bucket is accelerated through use of superconducting magnets at 3T; bucket mass is under 10 kg.

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