Section 4.11 - Lunar Development

We developed our orbital infrastructure in earlier steps to avoid the overhead of a large gravity well. With that in place we can now go to and from the Lunar surface relatively easily. The advantages of using the Moon are that it is nearby and has very large available resources. So once the cost and difficulty of reaching it are reduced, we want to develop it as the next step.

Development Concept

We assume that thorough prospecting has been done via orbiting spacecraft and surface rovers before a surface location is selected for mining and development. The Moon has a different origin than the Near Earth Objects and has undergone melting and separation, longer exposure time to the space environment, and extensive mixing from impacts. So its composition will be different than the NEO population. Certain locations are special, such as craters that could contain water ice or places in constant sunlight, both in the polar area.

We deliver a combination of robotic, automated, and remote controlled machines to the Lunar surface ahead of large numbers of humans to start preparing the site. Once mining, chemical processing, and parts production are operating, and you have a stockpile built up, then you can set up habitats and start having people there in larger numbers. You continue to use the machines in parallel to humans, but now they can be controlled locally rather than remotely. Taking this kind of approach will leverage the limited early human population. There might be as many as 50 to 100 machines per person.

We don't know for sure what the best starter kit is for Lunar development, but as an example we will assume an Iron-based economy. Whatever items cannot be made locally at any given time are brought from elsewhere, with an increasing amount of local production as the facilities develop. Metzger et. al. ^[1] have modeled bootstrapping industry on the Moon, and found 12 tons might be sufficient for a starter kit. Under a fairly wide range of assumptions, that starter kit could grow to a much larger installation.

Below we discuss three items for Lunar development in a general way: a Starter Kit, an Expansion Kit, and bulk delivery of materials from the Moon to Lunar Orbit. These are only some of the industrial infrastructure for the Moon, and does not include other end items you might want to build there.

Starter Kit

The starter kit includes the equipment to mine and process Lunar iron, and fabricate and assemble items from it. This gives an immediate return on hardware delivered to the Moon.

Iron Mining

Iron-bearing meteorites have impacted the Lunar surface since Since it's origin. From Apollo mission rock samples we know around 0.5% of the broken material making up the Lunar surface, called the regolith, is bits of iron ^[2]. It is generally small particles attached to other minerals by melt glass formed during the impacts. This is in addition to 5-15% iron oxides in minerals, but metallic iron does not have to be chemically processed, which avoids complexity in a starter kit. All you need to extract the iron-bearing fraction is a magnet. So you send a solar-powered rover around the nearby lunar surface, sifting out the smaller rock particles, running them past a magnet, and saving the part the magnet attracts. Then carry that back to a central location and dump the collected iron enriched dust. Repeat as many times as needed.

Iron Extraction

The next machine has a large concentrating dish to focus sunlight, and some manipulator arms to smooth the ground and make slight depressions and grooves for molds. Spread your iron-enriched ore over them and focus the sunlight to melt it. This is essentially sand casting except the metal is melted in place rather than separately in a furnace and then poured into a mold. Iron is relatively dense, and will sink to the bottom of the mold, while the glassy and mineral parts will rise as a slag. It will end up as basic shapes like plates and bars, with slag stuck to it on both sides from melting. To prevent the mold from melting, you can optionally select minerals with high melting points as the mold material. The raw stock will need to be sand-blasted to clean off the slag. Fortunately there is no shortage of mineral grains on the Lunar Surface to do that with. You now have an inventory of iron stock to use for all types of construction and parts making.

Part Fabrication

Machine tools such as lathes and milling machines are designed to make metal parts from metal stock. Modern ones are computer controlled, and are themselves mostly made out of metal. Thus an initial set of machines supplied with raw stock can mostly make more machines. Given plans for a wider variety of machines, the initial set can increase in size in complexity. For example a forge press or rolling mill is useful for making some types of metal parts, but those are likely too heavy to bring to the Moon to start with. If you have a supply of raw materials, you can build them locally.

Assembly

Standard assembly methods like bolts and screws can use robotic equipment. For some items that need to be pressure-tight processes like welding are preferred.

Welding - One process that may work on the Moon, since it is in vacuum, is to use articulating mirrors to direct a beam of concentrated sunlight at various angles. That could heat and weld parts to each other to make larger structures like habitats. If that does not work, then standard methods like vacuum plasma arc welding can be used.

Expansion Kit

Power and Heating

Electric Power - Solar power is abundant on the Lunar surface. While initial power devices will be brought from elsewhere, we can rapidly expand it using our metal supply. Since mass is not an issue for a stationary facility, you can deliver closed cycle generators based on a thermal cycle, and use concentrated sunlight to power it. The mirrors can be made locally of sheet metal with an evaporated coating of aluminum to make them more reflective. Later generators can be made entirely locally, but in the early stages making just reflectors is fairly simple. To supply power during the Lunar night, you can take a container filled with Lunar rock and a little gas for heat transfer, and surround that with more rock and insulation. Vacuum serves as a good insulator by itself. During the day you heat the container with concentrated sunlight, and at night run the thermal generator from the stored heat.

Process Heating - Many industrial processes need heating in some form. For this you can also use concentrated sunlight without an electrical generator.

Chemical Processing

Unprocessed Lunar rock can be used for things like shielding, and with melting can be cast into blocks for roads or landing pads, but for other elements and compounds some chemical processing will be needed, such as for reducing oxide minerals to metals and oxygen. Some elements are rare in Lunar rock and would have to be brought from elsewhere.

Shelter

Some items need protection from daily temperature variations, radiation, meteorites, and human-caused hazards such as lander rocket exhaust. Most of that can be met with a structural metal shelter covered with sufficient Lunar regolith either in natural form or sintered into blocks.

Bulk Delivery

A catapult system was mentioned earlier in the Space Elevator section as a method to deliver bulk Lunar basalt. This might be used to make high strength fibers for the Lunar Skyhook. With the Skyhook in place a bulk delivery method might not be needed any longer, but we will leave that choice to later analysis. Other bulk materials produced on the Moon could be delivered this way, but as a starting point for design, the calculations below assume basalt is the cargo.

Delivery Orbit - Any object thrown into an orbit from the surface will intersect the launch point one orbit later. Therefore your cargo needs a propulsion unit to raise the orbit, or you need something already in orbit to gather the cargo and change its orbit. One device in orbit is probably less expensive than one on every cargo, so for discussion we will assume a small Skyhook with a catcher device hanging down some number of kilometers from the main core. The offset allows the catapult to throw slightly slower than orbit velocity to match trajectory with the catcher. Missed catches will land somewhere else on the Moon instead of coming back to the launch site and doing damage. An equatorial location allows launching at about 108 minute intervals.

Tip Velocity = 1680 m/s - We don't yet know how much less than orbit velocity we will need, so for simplicity we assume the catapult reaches full orbit velocity.

Cargo Mass = 20 kg - Using basalt fiber as the main cable material, the Lunar Skyhook would have a nominal mass ratio of 20:1. If we want to deliver 10 ton payloads to the Lunar surface, that requires 200 tons of fiber. Assume we lost 1/3 of the basalt to processing, so the starter mass is 300 tons of raw basalt. We allow 10% extra time for maintenance and missed catches, so the average interval is 2 hours per cargo. If the catapult launches 20 kg at a time, that will be 43.5 tons per year if it only operates during the Lunar day, so about 6.9 years to launch enough cargo for the Skyhook construction. Larger versions would work proportionately faster.

Containers - Bulk Lunar rock will not hold together when tossed at high velocity. Two options are iron containers from Lunar rock, and Kevlar bags which are delivered and recycled.

Iron Containers:

These would be made from local metallic iron, which could be used in orbit as another building material. Iron also gives the opportunity to fine tune the trajectory after the catapult throws the container using magnetic coils. Together with the catcher mechanically positioning itself during the 30-40 minute ballistic trajectory of the cargo, that should ensure a high percentage of catches. Lunar regolith has a density of about 3 g/cc, therefore a container holding 20 kg will have a volume of 7 liters. A cone 13 cm in radius and 42 cm tall has about this volume. The reason for a cone shape is so there can be a single attach point to the catapult. We restrict the container mass to 10% of the cargo, or 2 kg. The load will be constant across the cylinder height, so thickness will vary to maintain the stress, and the bottom will be domed for optimum strength, so we can model the structure as a 50 cm tall cylinder with an area of 4000 cm^2. With iron density of 7.8 g/cc, that gives a wall thickness of 0.64 mm. The cross section area is 5 cm^2, and with a working stress of 125 MPa, allowed load is 62 kN.

Kevlar Containers:

Kevlar is a high strength material used on Earth. Bags made of Kevlar cloth with a capacity for 7 liters each would be delivered to the Lunar surface. A typical cloth ( 5 oz/yd^2, 650 lb/in strength ) of similar dimensions to the iron container would have a breaking strength of 21 kN, and a working strength of half that. We assume a 6 layer bag (or thicker cloth) to get the required strength, with steel wire mesh woven in for magnetic steering. Estimated mass is 0.45 kg/bag, or 2.25% of the cargo mass. Without an initial Skyhook the bags would need to be delivered with a lander, along with the rest of the catapult system. For 300 tons delivered, that would require 15,000 bags with a mass of 6.75 tons.

Radius = 1000 m - A force of 62 kN on a 22 kg loaded container implies a centrifugal acceleration of 2820 m/^2 ( 288 g's ). With a tip velocity of 1680 m/s, the radius is then 1.0 km, and it makes 0.267 rotations/s ( 16 rpm ). For accuracy and stability, the structure will need to be something like a truss frame, so it will support itself when not rotating. Added tension cables take up the acceleration load at full speed since cables can withstand higher stress.

Arm Mass = 92 kg - We used a working length of 126.4 g-km for carbon fiber in the Space Elevator section. We have an average of 144 g's over 1 km in the centrifuge, which produces a mass ratio of 3.1, and thus a cable mass of 2.1 times the loaded container. The centrifuge should be balanced so it does not move during operation, so a total of 4.2 times the container mass of 22 kg or 92 kg. The balance arm should be shorter, and release un-contained counterweight rock at the same time as the loaded container, but in the opposite direction, and aimed to hit a hill or travel far away ballistically.

Power = 11.5 kW - A 22 kg container moving at 1680 m/s has a kinetic energy of 31 MJ. If the catapult is operated once per 108 minutes when operating, it needs to supply 4.8 kW on average plus losses. This is doubled due to the counterweight rock. Additional energy is needed to spin up and down the rotating arm. If we use two catapults, the energy of one can be transferred to the other rather than wasting it as braking friction. The rotating arm has a mass of 4.2 times the thrown mass, and motor-generator conversions can be about 90% efficient. Thus we lose an additional 42% in energy in conversion losses. So the total average power supply needs to be 11.5 kW, and each motor-generator needs to transfer 30 kW when operating ( 40 hp ). An off the shelf 40 hp electric motor has a mass of 250 kg. Given the Lunar night, we need to double the solar array peak power to 23 KW. Modern arrays have a power level of about 100 W/kg, so the solar array mass would be 230 kg.

System Mass Estimate - Besides the bare rotating arm and power supply, there will need to be anchor structures, robots to collect the raw materials and put it in containers, the fine tuning magnets downrange, and communications and other support equipment. The total for this is unknown until more design work is done. For now, we will assume each of the two centrifuges will mass 1.25 tons total and total support equipment, including power, will be an additional 2.5 tons, for a total of 5 tons. Since the system can launch 168 times per lunar day, that means it can deliver 3.36 tons. Mass payback therefore takes 1.5 lunar days (1.5 Earth months).

References

1. ↑ Preprint obtained from author, Apr 2012, to appear in Journal of Aerospace Engineering
2. ↑ Morris, Origins and size distribution of metallic iron particles in the Lunar Regolith, 1980