intended base substrate to be utilized as well as of the products which the SRS must manufacture from that substrate.
Following a method suggested by the work of Freitas (1980), the "extraction ratio" R (see sec. 5.3.6) is defined as the total mass of raw substrate material which must be processed (input stream) to obtain a unit mass of useful system output having the desired mass fractions of each required element (output stream). Consider the significance of the extraction ratio to the problem of materials closure. An R = 1 means that 1 kg of lunar regolith contains exactly the mass of all necessary LMF elements to manufacture a kilogram of desired output product. R = 10, on the other hand, means that 10 kg of lunar regolith must be processed to extract all of the elements required to make 1 kg of final product (see sec. 5.3.6).
For the purposes of the present study the team chose a trial value of R = 40 kg/kg. This choice is based on information from previous studies which suggests that 40 represents a good intermediate value between low closure and high complexity SRS materials designs.
On the one hand, for R < 10, the available mass fractions of certain critical but relatively rare elements such as H, C, B, and Cl fall too low to remain credible for a system requiring 100% closure. The missing materials must be imported as "vitamins" or the entire SRS must be redesigned to eliminate chemical processing and electronics using these elements. Examples of low closure models include the lunar processing factory designs proposed by Ho and Sobon (1979), R = 1.7; O'Neill (1976), R = 1.7; Phinney et al. (1977), R = 1.2; and Waldron et al. (1979), R = 1.1.These systems are capable of extracting only half a dozen of the most abundant lunar elements and are not expected to achieve more than 60-90% materials closure.
On the other hand, for R > 100 the problem lies not in extracting rare elements but in processing them fast enough to meet a T = 1 year replication time deadline. For instance, Freitas (1980) gives an example of a high complexity system which could extract 84 elements from asteroidal material. For R = 26,800 the replication time is 500 years. It appears that 10 < R < 100 is a plausible condition for 100% closure and 1-year replication in SRS. The maximum recoverable mass from lunar soil for each element assuming R = 40 is estimated in table 5.13. The question remains whether or not these quantities are adequate to achieve quantitative materials closure.
| Element | Typical global lunar abundance, kg element/kg soil | Maximum extractable mass, kg (Rch)(A)(100 tons) |
|---|---|---|
| Fe | 0.10 | 400,000 |
| Al | .07 | 280,000 |
| Mg | .05 | 200,000 |
| Ti | .02 | 80,000 |
| Mn | .001 | 4,000 |
| Cr | .002 | 8,000 |
| C | .0001 | 400 |
| Si | .2 | 800,000 |
| O | .4 | 1,600,000 |
| P | .0005 | 2,000 |
| B | .000001 | 4 |
| F | .0001 | 400 |
| N | .0001 | 400 |
| H | .00005 | 200 |
| Na | .003 | 12,000 |
| Cl | .00002 | 80 |
| Ca | .07 | 280,000 |
| S | .001 | 4,000 |
| (Inert gases)? | .00001 | 40 |
Certainly 100% closure exists for the six primary structural elements Al, Ca, Fe, Mg, O, and Si. Even if the entire 100-ton seed were comprised entirely of any one of these there is enough available of each. A similar argument may be made for Ti, since 80 tons in theory can be extracted. Steels and other alloys typically have 1% Mn, 0.2% Cr, and 0.1% C or less, which limits the total steel mass to 400 tons, 4000 tons, and 400 tons, respectively. Hence, alloy production will not be materials-limited by these three elements.
Carbon is also used in the boron and phosphorus production cycles. The mass of boron is so low that the carbon requirement is negligible in terms of mass. In the phosphorus cycle, 10 atoms of C are needed to cycle 4 atoms of P. Phosphorus is required as a dopant in silicon microelectronic chip manufacture and in phosphoric acid which is used as a photolithography process chemical and which also appears during the halogen recovery cycle. At most, 40 kg of phosphorus are required, necessitating a carbon budget of 100 kg. This leaves more than 200 kg of carbon to account for losses and special uses such as CO2 gas lasers.
Boron is used solely as a microelectronic silicon dopant; 4 kg of B can produce perhaps 103-104 kg of chips, more than enough for the 100-ton seed. A few kilograms of phosphorus (though high purity is required) will suffice for the same purpose, and the use of P as a process chemical should be more hydrogen-limited than phosphorus-limited because of the relative abundance of P in the lunar regolith.
According to calculations by Waldron et al. (1979), about 63 metric tons of H2, F2, and Na, half of which is F2, are needed for an HF acid leach extraction facility having a total mass of about 823 tons. According to Criswell (personal communication, 1980) this model may
