Recycling Rocket Exhaust Presented at Mare Cognitum Chapter Meeting

At the Zoom meeting of the Mare Cognitum chapter of the Moon Society on the 16th of June, 2022, a speaker gave a talk roughly reproduced below:

     I will explain potential advantages of recycling rocket exhaust into rocket propellant on the moon.  Then I will suggest likely difficulties in that recycling.  Then I will offer possible means of addressing those difficulties.  
     The propellant on the moon from recycling rocket exhaust will be worth money.  Just how much is difficult to figure exactly.  The cost of the Apollo program divided by the number of pounds that reached the moon yields about $2.8 million per pound which includes the cost of shipping it all to the moon.  So, the cost of shipping to the moon must be less than $2.8 million per pound.  
     Starting with the cost of a recent Israeli moon mission I figure that the cost must be less than $300 thousand per pound.  Working from an estimated cost of at least $6,000 per pound for achieving direct lunar transfer orbit, without the cost of developing a payload, I get at least $13 thousand per pound shipped to the moon.  So, the cost of recycling rocket exhaust into propellant should be less than that to make it worthwhile.  How much less is a good question.  If the cost of recycled fuel can be made low enough it could make transportation from the moon for materials for building space based solar power (SBSP) stations practical.  That is the really big draw.  Once the facilities for manufacturing SBSP are in hand modifying them to produce space habitats would be a possibility. Solar sails manufactured in the as deployed condition to go along with the space habitats is another possibility.  That is, if economically successful, recycling rocket exhaust could open the solar system from Mercury to Jupiter to economic exploitation.  
      It is the large demand for materials shipped from the moon for building SBSP that makes the problems with recycling rocket exhaust worth looking at to find a solution.  Recycling would require launching rockets horizontally on the surface of the moon instead of launching in the traditional vertical way.  The rocket would need to travel about 30 miles downrange within a tube on the surface of the moon for the rocket exhaust to be captured if [the rocked] accelerates at about 30 meters per second squared on average.  The tube would need to be quite straight along the intended trajectory to accommodate the rocket reaching orbital velocity of about 1680 meters per second.  Thermal management would be a concern in capturing hot rocket exhaust and converting it to cold fuel.  Tanks for various process fluids would be needed.  Building infrastructure for all needs would need to be done without ambient air for internal combustion engines on construction machinery.  Lubricants tend to evaporate in the ambient vacuum of the moon making various bearings difficult to design.  

     For example, I choose a tube with a diameter of 12 feet.  The rocket would be sized to remain about three feet away from the tube walls because the practice of flying aircraft in tight formation on Earth has shown that airplanes can routinely maintain such a distance from one another and rockets have maneuverability comparable to airplanes.  I imagine RFID devices would be embedded in the wall of the tube for sensors in the rocket to determine position and velocity as the rocket flies through the tube.  A computer would control maneuvers to maintain the proper position in the tube.  
     The rocket launch tube itself and a considerable array of radiator tubes should be constantly shaded from the sun to help maintain a adequate temperature. This is possible for a tube stretching East and West near the moon's equator by having a shade suspended directly overhead by pillars running with the tube and radiator array in an East-West direction.  I propose an anchoring external tube of sintered regolith brick supporting an inner corrugated silicon steel tube for containing the exhaust after the rocket leaves the tube and a door closes.  The line of the corrugations of the silicon steel would run circumferentially about the tube so that the steel could thermally expand without changing the length of the tube.  The support from the anchoring outer tube would allow the tube to change diameter and shape slightly in response to a change in temperature while maintaining the same length and position.  Ethylene glycol and water could be used as a cooling radiator fluid.  Other cooling fluids would also be used to help compress and store exhaust awaiting recycling.  The final choice of radiator fluid remains for the engineers who fix the specifications.  
     Silicon, iron, regolith and many other materials are available for building these things on the moon so only a small percentage of materials needed to build a rocket exhaust recycling system on the moon would need to be imported from Earth. To build all of this from lunar materials requires remotely controlled industry on the moon to produce the sintered regolith bricks, glass fiber cables, steel and other metals; grade the foundation for the tube and assemble it.  The machines necessary to do this would come from Earth with components of the industrial machinery actually made on the moon wherever that is practical.  The remote control from Earth should go on with three eight hour shifts of controllers per day.  Electrical batteries can power mobile machines and the batteries could be exchanged spent for charged at charging stations.  Mobile machines could walk so that all of their bearings could be enclosed in space suits that would retain a nitrogen atmosphere and prevent excessive lubricant evaporation.  It is hard to put the wheels of a wheeled vehicle inside a space suit and retain their function.  The outside layer of the space suit would not be woven fabric but a plastic film on a smooth surface designed to flex for the necessary motions of the machine.  Accordion folds over joints is a possibility.  The outer film would wash off and be separated from the dust it collected.  Then the outer coating would be reapplied.  
     The relation to human space flight is that the enterprise would build a destination that would be worth a human visit.  A habitat on the moon with recycling features, shielded from radiation by lunar material and containing a centrifuge to provide necessary exercise for people who come to do indoor work on the moon.  No work for humans wearing space suits would be available.  
     This would be a vast enterprise to produce a vast solution of SBSP for any people on Earth that will accept it.