It seems technologically possible to produce a space based solar power system for Earth from lunar materials, but the economics cause difficult constraints and the current geopolitical situation is very difficult.
Investment in costly infrastructure is necessary to take full advantage of the potential low cost of achieving orbit from the moon. Launches to orbit for a large customer are necessary to pay for the infrastructure. So, committing to infrastructure and the building of SBSP should be a package deal. One or the other by itself or half-way measures do not make much sense.
Industrial production of oxygen on the moon with depot storage should be a first step. Then depots orbiting the Earth and moon. This technology is difficult, possible, and certainly possible to get wrong. Then there is the recycling of rocket exhaust into rocket fuel by having the acceleration to orbit on Luna occur in a tube that is horizontal along the equator in a tube in a semicircular ditch in the lunar regolith with an air-lock door on the tube. The air-lock door must be closed after the rocket, rocket upper stage or rocket-sled payload leaves the tube to allow the rocket exhaust to be captured by vacuum pumps. The regolith should be built up enough so the craft exiting the acceleration tube misses any landscape features. A rocket-sled can go through the tube with the payload stage ejected at orbital velocity and the rocket-sled moving on to a deceleration track and recycled or a rocket can free fly down the center of the tube with guidance from RFID embedded in the walls. The guidance of a free flying rocket would need to be about as accurate as is achieved in acrobatic formation flying of jet airplanes. The choice between rocket-sled and free flying rocket is a matter of which technology is most easily verified by development of models on Earth.
If acceleration in the tube averages about 30 meters per second squared then the tube on Luna needs to be about 30 miles long. A rocket-sled can use one of various deceleration techniques to be recycled. A free flying rocket can optionally continue on orbitally to an orbiting depot where another tube would exist for delta v to return to the moon with the decelerating rocket exhaust recycled to rocket fuel on the depot. The orbiting depot would need large, high specific impulse electric thrusters with low thrust to weight ratio which are possible with various technologies. The mystery to me is why these technologies have not been already employed, since they were all available since 1985.
Of course remotely controlled equipment would be necessary to mine the moon, separate oxygen which is 44 percent of the moons regolith, store oxygen in tanks, separate the regolith into constituents by electrolysis in a potassium chloride or potassium fluoride bath with the potassium and chlorine or fluorine recycled, take the iron sponge from the bottom of the electrolysis bath and purify it by a carbonyl process, form the iron and aluminum into pigs, alloys, and bar and sheet stock, form sifted regolith into sintered brick and fiber glass, build buildings, the orbital acceleration tubes, sheltered and shaded East-West roads and North-South roads, each type by its proper techniques, make the solar cells and ship products out. Astronauts doing any of those things on the moon by any means other than remote control simply could not be economically competitive. Eventually there should be enough infrastructure built up to be able to support human workers on the moon doing tasks suitable for human beings in vehicles with suitable life support systems and in buildings with recycling life support systems. When people come to the moon they should be called passengers, not astronauts.
A zeroth step in building a space based solar power system is verifying the technologies. Step 0.1 is committing to all that is necessary for the whole chain of steps to work and finally start producing revenue. Within step 0.1 there are agreements among nations to share the financing, engineering, hardware building, electrical power sales arrangements, and revenue.
For high specific impulse, large, low thrust to weight ratio thrusters on the moon orbiting depot, the reaction mass should be oxygen plasma since oxygen is readily available on the moon. The low thrust to weight ratio is not something that one needs to strive to achieve. It is the natural result of a lack of weight constraint and the use of every available method to increase the exhaust velocity of an electric thruster.
For East-west roads on the moon the pavement could be graded regolith or sintered and perhaps glazed bricks separated by filtered regolith. There could be an East-West awning over the road held up by a row of pillars and made of aluminum sheet or aluminized glass sheet or material of suitable alloy containing some proportions of aluminum, silicon, magnesium, calcium, titanium or whatever available material is found to be most economic for the use. The pillars would separate the Northern lane from the Southern lane. There could be solar cells for charging batteries that are swapped, spent for charged, by passing vehicles. The vehicles might be walking vehicles that wear space suits holding one percent of an Earth atmosphere pressure of nitrogen thus eliminating the need for a gas tight rotary seal around wheel axles that would otherwise be necessary to prevent wheel lubricant from evaporating into the vacuum. Alternatively, wheels could be outside of the pressure containing suit and supported by magnetic bearings with only electric wire connections to the inside of the vehicle. The key to economic remote controlled equipment on the moon is long lived equipment.
The North-South roads could be sometimes two lane roads with a wall between the lanes and an awning hanging out over the lanes on both sides of the wall and sometimes a three lane road with two walls separating the center lane from the Eastern lane and the Western lane. The two walls would support an awning covering all lanes. When the sunlight comes from the East, the Western lane would be used. When sunlight comes from the West the Eastern lane would be used. Where three lane stretches meet two lane stretches there is a provision for cross over as necessary to stay in a shady lane. Spurs going off to the East or West under Awnings would provide the battery exchange stations where the spent batteries are charged.
People have complained that so much industrial development would ruin the pristine nature of the moon but people need to dig to get the scientific truth of the moon's composition. Where there is soil dug up and pushed around it will be first photographed then analyzed as much as is necessary to get a good idea of the moon's nature. This is exploration. There are more than 9,370,000 square kilometers of lunar surface. A few hundred thousand square kilometers reserved as parks here and there might be reasonable, but not the whole 9,370,000 square kilometers.
