RECYCLING ROCKET EXHAUST

This is a concept for lunar industrial development.

Concept description

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. Many launches to orbit for a large customer are necessary to pay for the infrastructure. So, committing to infrastructure for launching to orbit 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 with the 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 leaves the tube to allow the rocket exhaust to be captured by vacuum pumps. The regolith on which the tube rests should be built up enough so the craft exiting the acceleration tube misses any landscape features. The rocket flies free down the center of the tube with guidance from RFID tags mounted on the walls. The guidance of a free flying rocket would need to have about the accuracy that is achieved in acrobatic formation flying of jet airplanes. Jet airplanes have routinely flown as close as three feet from wing-tip to wing-tip while in formation flying. The tube launched rocket on the moon would have three feet clearance from the walls. The choice between rocket-sled cargo launching 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 continues on in orbit to an orbiting depot where another tube would exist for providing delta v to deorbit and return to the moon with the deceleration rocket exhaust recycled to rocket fuel on the depot. For orbital stabilization 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. The low thrust to weight ratio for the orbital maintenance thrusters on the orbiting fuel depot would not be something people would strive to achieve. It would be a natural result of releasing a weight constraint in design and using every means possible to increase exhaust velocity.
• The horizontal acceleration of a rocket in a tube should start with electric acceleration of a movable launching pad for the first 4% or so of orbital velocity. The moving start of the rocket prevents the rocket exhaust from having too much erosive effect on the tube, prevents excessive pressure build-up behind the rocket, prevents the rocket from flying in its own exhaust, provides ullage thrust for the start of the rocket engines and a portion of mission delta v. A larger diameter section of tube to collect exhaust near the launch spot may also be desired. Six feet larger in diameter than the rocket should be about the minimum diameter for the launching tube to provide the clearance to avoid the rocket smashing into the tube wall. Rocket scientists can calculate whether the tube needs to be larger at some parts of the tube to accommodate the volume of exhaust. As the rocket continues down the tube at increasing speed, the mass of exhaust gas deposited in the tube per unit length decreases. So, the diameter of tube needed to accommodate the exhaust gas decreases with distance traveled in the tube to where it is less than the diameter needed to avoid crashes into the tube wall.

Mining the propellent

Of course, remotely controlled equipment would be necessary to mine the moon, separate oxygen which is 44 percent of the moon's regolith, store oxygen in tanks, separate the regolith into constituents by electrolysis in a bath of calcium chloride, potassium chloride or potassium fluoride; with the potassium and chlorine or fluorine recycled; take the iron sponge from the anode 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; not in space suits. When people come to the moon they should be called passengers, not astronauts.

• The carbonyl process The reason for the carbonyl process in purifying (and perhaps extracting) iron is that it can separate the iron from the nickel that is naturally in lunar iron that comes largely from meteorites. Nickel carbonyl and iron carbonyl plate out of vapor at different temperatures. The nickel is needed to put a corrosion resistant coating on the inside of the corrugated silicon steel tube that catches the rocket exhaust.
• 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, thrusters for orbital stabilization of the moon orbiting fuel depot, the reaction mass should be oxygen plasma since oxygen is readily available on the moon.
• For East-west roads on the moon the pavement could be graded regolith or sintered and perhaps glazed bricks separated by sifted 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 (four or more legged) 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 (and all other lubricants inside the space suit) from evaporating into the vacuum.
• Alternatively, wheels could be outside of the pressure containing suit and supported by magnetic bearings, instead of a typical greased axle bearing, 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.
• Roads can be involved with bringing necessities to the tube construction area, such as sulfur to aid in sintering in a sulfur dioxide atmosphere. It would be possible to construct a solar power grid active in lunar day and lunar night by connecting distant spots on or near the 87th latitude North or South. Electric power by wire could flow from this grid following a road to where a fuel recycling depot is being built. If nuclear power is available at the construction site, a polar electric connection might not be necessary. Road construction could be delayed until necessary. If it is found to be most expedient to build the polar power grid and forgo nuclear power, then for three circumpolar points with always one of the three in sunlight, a road to connect point A to point B and point B to point C would be about 240 miles long. Nuclear power or no nuclear power there would be much construction activity before a 30 mile long tube to collect the exhaust of a rocket launch to orbit could be built.

• 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 for industrial development, it will be first photographed then analyzed as much as is necessary to get a good idea of the moon's nature. The opportunity for exploration will not be missed. 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.
• As an alternative, a rocket upper stage or rocket-sled payload could leave the acceleration tube at orbital velocity leaving the first rocket stage or the rocket-sled to proceed to a deceleration track. This could accomplish a special purpose but it would introduce extra engineering considerations to be dealt with.
• Sintered brick is a possible material for building a tube to recycle rocket exhaust on the moon but it might be decided that an all metal tube is better.
• After the acceleration tube and fuel depot on the lunar surface are completed they can be helpful in constructing the exhaust collecting deceleration tube for the lunar orbiting fuel depot.
• People who were intent on using mass drivers to build space habitats as suggested in "THE HIGH FRONTIER" by Gerard K. O'Neill need not give up hope. Recycling rocket exhaust to make it possible to build SBSP and space habitats does not preclude perfecting better mass drivers for space transportation. The more that extraterrestrial resources become available, the more they can be used to develop more advanced technology.
• If the rocket for which exhaust should be recycled burned liquid methane and liquid oxygen, then the Sabatier reaction could be used to add hydrogen to the carbon dioxide at the proper temperature with a proper catalyst to produce methane and water. That water along with the exhaust water could then be subjected to hydrolysis to recover the amount hydrogen added previously and the amount oxygen that originally burned the fuel.
• oxygen gas could be used as a heat transport fluid for taking the heat from the captured hot exhaust and transferring it to shaded radiators extending from east to west. Oxygen is not the best heat transfer fluid on Earth but on the moon we might take what we can get most cheaply.
• It is essential that peaceful use of the moon be guaranteed with treaties forbidding any weapon based on the moon or in space that would reasonably have potential for damaging targets on Earth. Treaties must include a means of verification by inspection with robots for the inspecting nation given access to a reasonable environment and electrical power sold at rates equivalent to what it costs the operator of an industrial establishment to provide this for its own robots. The plans for industrial establishments on the moon must not be allowed to be secret.