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 of all of this 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.

There were many comments questions and answers at the meeting.
One attendee asked what propellant was imagined for the example rocket launching.
The speaker said that methane and oxygen at 3 kilograms oxygen to one of methane were imagined. This is less oxygen than needed for stoichiometric burning of the fuel but the rich mixture provides about 2500 meters per second exhaust velocity.
One question was about the launching. The rocket would launch from an electric catapult giving it about 4% of mission delta V and providing ullage thrust for ignition.
It was suggested that the 4% electric thrust could be increased incrementally until the rocket was brought to orbital velocity completely by electric thrust. The speaker claimed that Gerard K. O'Neill's mass driver launching of cargo to L5 was limited to one kilogram sized packets because at orbital velocity one kilogram requires as much electric power as is practical for such a device so orbiting a ton-and-a-half cargo to orbital velocity all electrically would be 1500 times impractical.
One participant suggested that the impact of the rocket exhaust on the tube would be considerable at the moment of ignition. The speaker suggested that as a flame trench leads exhaust away from rocket launches on Earth, rocket exhaust would be led to the rear during a tube launch. The tube would extend not only down range but also in the opposite direction where it would have a larger diameter. The difference is that on Earth the exhaust is thrown away, with a tube launch it is kept. The participant noted that the volume in the up range direction from the launch point need not be cylindrical but spherical or any shape would do.
Compartmentalizing the tube with a number of doors that would close behind the rocket as it passed in tube launched was suggested to preserve the high pressure near the launch point to help the vacuum pumps remove exhaust in this relatively high pressure area. A separate port for drawing off exhaust would serve each compartment. There is some theoretical benefit to this but cooling the exhaust in the whole tube might provide a better distribution of thermal management load if the tube were left as only one compartment. One port for pumping out exhaust would be enough even if more than one pump were attached to the port to provide the most efficient pumping as the conditions of temperature and pressure changed as the last portion to be pumped out was reached. There is only one port for putting air into a car tire or letting it out. That works.
Why silicon steel for the inner corrugated steel tube? There is more silicon than carbon available on the moon to use.
How will the exhaust be changed into fuel? Carbon dioxide and carbon monoxide mixed with hydrogen at the right temperature with a catalyst produces methane and water. Electrolysis of the water gives back the hydrogen so it can be used on the next batch of fuel.
Can exhaust recycling be done only on the moon? No. It could be done at an orbiting fuel depot also. So, a rocket making a trip from the moon to orbiting depot then back and repeating could use the same fuel over and over again. Exhaust lost in recycling would need to be made up from another source.
A suggestion was made that the launching tube could be shortened to something less than thirty miles, putting the door at the end of the shortened length and making no other change. This would result in the rocket proceeding to orbit passing the end of the tube while the rocket motor was still burning. If the rocket passed out of the tube with three

out of fifty-six seconds burn time left then the launching tube would be more than two miles shorter (and cheaper to build) and three seconds of rocket exhaust generation would be wasted. Meanwhile, the launch tube construction robot crew could be occupied building another tube.