by Dave Dietzler
Earlier I described cycling stations for inter-lunar travel for large numbers of people in the future and taxis made of shuttle external tanks for moving people from Earth orbit to rendezvous with the cycling stations and from cycling stations to a large space terminal station at L2. More can be found in the Lunar Tourism pages. Peter Kokh has done some excellent work on inter-lunar cycling stations also. See: http://www.lunar-reclamation.org/papers/transitel.htm
Transportation to and from L2
A large rocket ship or "Moon Shuttle" is needed to move people from from L2 to and from the lunar surface. This rocket could use a mixture of abundant lunar LOX and metals for a monopropellant. This could consist of ultrafine particles of aluminum and silicon suspended in LOX which is somewhat viscous. Ultrafine particles will disperse better in the LOX and stay suspended. They will burn better too. In weightless space the metallic particles will not settle out. In lunar gravity and during acceleration and deceleration the particles might settle out. Magnetic stirring could keep the fuel well mixed and homogenous. Variations in monopropellant consistencies would lead to thrust fluctuations and control problems or even complete engine failure so this must be prevented.
Magnetic stirring involves magnetic impellers in the propellant tank that are caused to spin by electric motors outside the tank. There will be no "through the hull penetrations" this way. Magnetic stirring devices are common in laboratories and lab techs are familiar with the principle. Ultrasonic agitation which can even cause gasoline and water to mix might prove to be a more effective method. Future engineers will have to determine the best system by experimentation with unmanned rockets. Until the bugs are all worked out of this system rockets on the Moon will run hydrogen from polar ice or silane and oxygen in the early years of lunar industrialization and colonization.
Metallic Fuels from the Moon
To conserve the precious water of the Moon we must develop rockets that burn metals and oxygen if millions of people are going to visit the Moon in the late 21st and 22nd centuries and beyond; my favorite dream!
Aluminum and silicon both burn with about 13,000 BTU per pound and silicon actually forms a lighter exhaust product, SiO2 with a molecular mass of 60 atomic mass units while aluminum forms Al2O3 with a molecular mass of 102 a.m.u. so silicon, the second most abundant element on the Moon after oxygen may yield better rocket performance as lighter exhaust products travel faster. Silicon oxidizes in air at room temperature to form a nanometer thick protective coating of SiO2. It oxidizes rapidly above 1200 C. and rocket motors run at 3000 C. and hotter. Finely divided silicon is flammable. The stuff will burn fine.
Magnesium has a low ignition temperature (think of flash powder which is just magnesium dust) but cannot be used as rocket fuel because it is shock sensitive and will detonate. We might use a fuel mixture of 21 parts silicon, 15 parts aluminum, and 37.33 parts by mass oxygen because that's the mix we will get if we electrolyze anorthositic highland regolith that has had the ilmenite removed magnetically in a magma furnace. This way we work with nature and less work is required. Experimentation will determine the best monopropellant mixture in terms of performance, cost and reliability.
Metallic fuels turn the entire Moon into an ocean of rocket fuel that will never be depleted unlike the mere six billion tons of suspected ice at the lunar poles. Metals can be mined anywhere on the Moon so we could set up rocket fuel factories where ever it is convenient and/or profitable and this could be very economical when compared to asteroid hydrogen imports. Importing millions, even billions of tons of fuel from Earth is not even worth considering.
Refining those metals and extracting oxygen from moondust is the challenge. This writer has proposed numerous processes but a system based on magma electrolysis may be best. Molten metals could be sprayed from a nozzle to form fine droplets that cool off by radiation and solidify and these could then be placed in the grinders to make an ultrafine powder. Grinding the particles will probably be done with centrifugal grinders that don't have any abrasive wheels or grit to wear out.
Moon Shuttle Construction
Getting back to the rocket ship pictured herein, it could be made of a titanium frame. This metal is lighter than steel but as strong or stronger. The tanks could be made of aluminum-lithium alloy. This alloy has higher tensile strength than Duraluminum which is made by adding about 4% copper. The new lightweight Shuttle ET made of Al-Li weighs only 50,000 pounds as opposed to the old one made of Duraluminum at 59,000 pounds. The tanks would be spherical because this is the strongest shape. Since the rocket propellant will be pressure fed we must have tanks that can withstand pressure. Metallic fuels will probably jam up turbo-pumps with deposits so pressure feed is the way to go.
An aluminum scuba tank with air at 3000 psi is thicker than a steel tank with 2250 psi but it is lighter. High pressure tanks filled with gaseous helium will be made of Al-Li and these will drive the monopropellant from the tanks and into the rocket motors. There is enough helium on the Moon so gaseous helium is our choice for pressurant.
This affects ship design. I am afraid that gaseous helium will warm up the LOX in the monopropellant and cause it to boil and over pressurize the tanks. We don't want to vent over pressure if we don't have to because then we will be without propellant and that could be disastrous. Conversely, gaseous helium might be cooled by the LOX and contract thereby causing pressure loss. I solve this problem by using two tanks-one for ascent and one for return descent, each with the right amount of monopropellant for each burn or series of burns.
The rocket with its spherical tanks would never fly on Earth where streamlined cylindrical rockets are used to knife through the air at high speeds, but in the vacuum of Luna and space this doesn't matter. We could make a rocket shaped like a box if we wanted to, although a box shaped tank wouldn't be as strong as a spherical one. Without aerodynamic concerns we can take liberties in designing the shape of our Moon based rockets. There will be no accidents due to skins flying off or nose cones or even ET insulation or solar panels blowing off as happened to Skylab caused by high speed air flow and that will help us all rest easier. The tanks will be coated with some polyurethane made from H, C and O harvested from regoilth. They will also be wrapped in aluminum foil to protect them from solar heat. We don't want the Sun to heat up our tanks and cause the monopropellant to boil as that would really cause a tank overpressure problem! Aluminum foil will not be blown away in the vacuum as would happen to a foil covered rocket ascending from Earth so once again we find some advantage.
The Rocket Motors
The steel or titanium rocket motors would be lined with a refractory material like titanium dioxide or magnesium oxide ceramic. The linings will be replaced after every so many flights. A metal cooling jacket with LOX in it would hot corrode rapidly but a nozzle lined with a ceramic will not. Cooling jacket passages could clog up with metallic fuel particles. We must also wonder about silicon nitride and silicon carbide for rocket motor and nozzle linings. These have high tensile strength and resist creep and are being used in natural gas combustion turbines now. So much work remains to be done in the future by rocket engineers.
The rocket ship will be steered by thrust modulated steering motors instead of gimbals or exhaust vanes although gimbals are a possibility. The upper descent tank stage will have rocket motors of its own so that should there be a main engine failure during return to the lunar surface it can fire some explosive bolts and detach from the heavier ascent stage and land with its own retros on gas bags. The gas bags could use some helium, nitrogen or just oxygen to inflate. It might not make the best landing but our spacesuited passengers could survive. If the main engine failed during ascent the back up motors on the upper descent tank stage might propel the passenger module to LLO or just make a suborbital hop and descent. Unfortunately we cannot simply deploy parachutes on the airless Moon and make an emergency landing or glide back on wings.
Dual Tanks
Lest there be some confusion by terms like 'upper stage' let me clarify this. The whole rocket ascends to LLO or L2 with "fuel" from the large lower ascent tank and descends with fuel from the smaller upper tank fed down to the main engines and steering motors under ordinary circumstances. During descent the larger lower tank is empty so the rocket is much lighter and descent requires less "fuel" hence the upper descent tank on the rocket ship is smaller. We cannot afford to throw away rocket stages except in emergencies. The rocket ship must be fully reusable. I just want to make it safe.
Attitude Control
Finally, the ship will use electric flywheels to control its attitude in space and it will use thrusters powered by silane and oxygen to maneuver. The specific impulse of the rocket motors will not be that high, perhaps 250 seconds, similar to solid rockets, but the mass ratio will be very high thanks to titanium and aluminum-lithium and lightweight construction because it will not have to stand up to high gravity or intense Gee forces. It will only need to perform a delta V of about 3.2 kilometers per second to reach LLO and return. With a ISP of 250 seconds a rocket only needs a mass ratio of 4.35 to ascend to LLO and retro back down and we will have no trouble getting a mass ratio of 20. See: Making Monopropellant Air friction losses will be non-existent and lunar gravity is low so that gravitational losses will be low, thus a Moon Shuttle could almost reach its theoretical velocity as determined by the rocket equation.
Boarding & Disembarking
For those of you who have read this article and looked at the pictures the question may arise,"How do people on the Moon get in and out of that rocket? It's enormous!" Yes, the rocket will be about thirty stories tall as I envision it. A mobile gantry with elevators that crawls on huge treads similar to the crawler-transporter at the Kennedy Space Center will be used to allow 200 passengers to board and disembark from the giant rocket ship. It will have electric motors powered by a nuclear powerplant similar to the ones in submarines. Large lunar spaceports will have several of these giants machines at work. They will have airlocks to dock with the rocket and pressurized double decker buses on the Moon. See: Moon Ports
Fast Forward
Obviously, this could only exist when industry was very large on the Moon in the more distant future. Lunar industries like helium 3 and platinum mining from asteroid impact craters and solar power satellite building will exist for decades before enough development on the Moon (lava tube cities, monorails, etc.) and enough money is made to make tourism for large numbers of people possible.
The Moon Shuttles will be mighty sights to see. As they lift off silently they will emit bright white hot flames and clouds of smoke like the aluminum burning SRBs (solid rocket boosters) of the Space Shuttle. The smoke will dissipate rapidly in the vacuum, the particulates will cool and settle back down to the surface of the Moon and rejoing the powdery regolith from which they came.
1. The Space Shuttle External Tank has a titanium frame and aluminum/lithium skin. It amasses 50,000 pounds (25 english tons) and holds 765 tons of LH2 and LOX as well as supporting the heavy Orbiter. This thing has a mass ratio of 30 to 1. It may be possible to make the big Moon Shuttle with a titanium frame and Al/Li tanks like the E.T.
Titanium is available on the Moon and can be alloyed with lunar aluminum and manganese. The FFC process may reduce the price of titanium from $14 per pound to $3 or $4 per pound. We should have no trouble getting a very high mass ratio with this metal. As for the lithium which constitutes 10% to 15% of Al/Li alloy it will probably be worth up-porting to the Moon if a source of lithium cannot be found on the Moon. The high pressure helium tanks and passenger section could be made of an Al/Li frame and skin. The heavy Shuttle Orbiter has an aluminum frame and it's strong enough.
2.Steering motors could serve as the OMS engines for small delta Vs in space required for maneuvering, changing orbits, and rendezvous with space stations. The steering motors and main engine will have electrical spark ignition systems for starting and restarting. They will be stopped by shutting off the fuel flow control valves.
3.Nozzles for the motors will be expanded for improved specific impulse in the vacuum. Since the aluminum burning SRBs get 270 seconds in Earth's atmosphere we just might get 285 seconds with our aluminum burning motors in space.
4.Spheral solar panels consist of silicon micro-beads sandwiched between layers of aluminum foil [1]. They are very light and flexible. The tanks could be wrapped in this instead of plain aluminum foil to provide power for the ship which also will use alkaline fuel cells powered by hydrogen and oxygen. Water from fuel cells will be stored and electrolyzed later to recover precious hydrogen.
5.It seems like one big spaceport where cycling station taxis and Moon Shuttles dock and undock will be needed at L2 to handle millions of travelers every year, but L2 is not really a point in space. It is more of a region. Numerous spaceport stations could circle around L2 in halo orbits. They will use ion thrusters powered by solar energy with abundant lunar magnesium for reaction mass for station keeping. Magnesium is more plentiful than lunar sodium which has been proposed for ion drives and it has a boiling point lower than lithium which has been used in experimental ion drives. There will be no trouble generating magnesium ion vapors for the drives. It only takes about 10 meters per second per year to stay on station at L2 [2].
Spaceports might also use mag-sails and ride the solar wind without any reaction mass at all to stay on station. Cycling stations might use magnesium ion drives and mag-sails to correct their orbits which will be perturbed over time by the influence of lunar gravity and the gravity of the Sun and other bodies in the solar system. With solar sails the cycling stations could take advantage of solar light pressure and even respond to microwave beams from powerful stations on the Moon and Earth to stay on course. The spaceport stations in L2 halo orbit might do that too.
The solar sails could be unfurled when needed and rolled up when not needed like the sails of wooden ships of old. They could be attached to the staying lines of a 50 kilometer diameter mag-sail loop. Mag-sails can be energized when in use and discharged when not needed.
These systems-ion drives, solar sails (which won't add much mass to the stations) and mag-sails could be used for adjusting the orbits of interplanetary cycling stations to Mars as well as interlunar cyclers and L2 space stations. The same ET based taxis that serve interlunar cyclers might be adapted to work with cyclers to Mars by adding some bigger fuel tanks and putting only 50 to 100 Mars pioneers aboard since it might take days to reach the interplanetary cycling station. Lunar technology and Mars technology work together. It all seems rather romantic to me. Bon voyage!
Ref. 1) http://www.spheralsolar.com/
Ref. 2) visit: http://www.cds.caltech.edu/~shane/papers/
The ill fated Genesis probe required a mere 10 m/s/yr. to stay on station at Earth-Sun L1. Unfortunately it crash landed, so landing seems more dangerous than slipping off station at a Lagrange point and "falling out of the sky" or drifting into deep space.
Earth-Moon L1 and L2 have been described as "saddle shaped." If a station drifts to far along its direction of travel it moves back to its original location. If it moves to far perpendicularly along its direction of travel, towards Earth or the Moon, it will continue to drift. I don't think that stations would fall straight to Earth or the Moon as some may fear; they would spiral into lower orbits as they do have some lateral velocity. So there is no immense danger at EM L1 or L2.
If by some freak accident like a spaceship slamming into a station at EM L1 or L2 the station was pushed it would spiral down to a lower elliptical orbit instead of "falling from the sky" and could be relocated at L1 or L2 by ion drive tugs if it wasn't damaged beyond repair. Hopefully I am correct.
Computer simulations of stations at Lagrange points to pre-determine the effects of solar wind and even spaceship collisions will be called for someday. The insurance companies of the late 21st and 22nd centuries that cover these mammoth space tourism industries will base their premiums on such accident analysis. We could locate experimental satellites at L1 and L2 and play around with their orbits for concrete proof. Chances are it will take more fuel for reboost of LEEO space stations where slight air drag does exist than it will for space stations at L2.
Mass Details
Let's be generous and give each passenger 2 square meters per couch for 16 on each deck plus a toilet. It will be a 12 hour flight between the lunar surface and L2. The passenger section is 10m wide and has 14 decks each 7.5 ft. high including the control deck. It is thus 10m (33 ft.) by 32m (105 ft.) high. It is roughly they same size as the ET LH2 tank that amasses 26,000 pounds or 13 tons.
If we determine its area and allow 2 grams per sq. cm we get a mass of about 23 tons. If we allow another 8 gr./ sq. cm. for polyethylene radiation shielding for a total of 10 grams per square meter of shielding, enough to get solar flare dose down to 20 REM, we have about 93 tons of shielding.
Let's estimate the mass of the whole passenger section at 140 tons when LSS, couches, etc. are added. With 200 passengers at 20 tons that's 160 tons.
If we base our guesses on the Moon Shuttle as drawn, we can est. the descent tank at 13m diameter. It has a volume of 1150 cubic meters. Since the monopropellant is about 1.54 tons per cm it carries 1771 tons of "fuel." It has a volume about equal to the 13 tons ET LH2 tank. Let's make it 20 tons.
The ascent tank is 21m wide and has 4846 cubic meters of volume and carries 7463 tons of monopropellant. Since its volume is twice that of the 25 ton ET we can guess it amasses 55 tons.
Now we have 160+20+55=235 tons. If the motors, landing gear, high pressure tanks and piping amass 100 tons we have 335 tons for the ship's dry mass. Since it carries 9234 tons of monopropellant it has a mass ratio of (335+ 9234)/(335)= 28.6 That's incredible.
The weight of the fuel will only be 1539 tons in lunar gravity and the entire ship loaded will be 1595 tons. The 550 cubic meter LOX tank in the SSET holds about 650 tons of liquid oxygen and amasses about six tons. So it has a load to tank weight ratio of 650 to 6 or 108 to 1. Our descent tank in Earth's gravity has a load to tank weight ratio of 1771/20= 88 but in lunar gravity the load will be much lighter. Perhaps we could beef the descent tank up from 20 to 40 tons for a ratio of 44.
The ascent tank's load/weight ratio is 7463/55=135. We could beef it up from 55 to 110 tons for a ratio of 67. These tanks include the ship's support frame. Each tank would have a load to tank mass ratio of about 44 to 1 and 67 to 1 in Earth's gravity. Although the fuel will be lighter in lunar gravity and so will the tanks, the tanks will be just as strong in terms of tensile strength. In lunar gravity they will have the equivalent then of about 7.3:1 and 11.2:1 so those will be stout fuel tanks.
Now the rocket mass is 410 tons. If the motors and other equipment amass not 100 but 200 tons the rocket's dry mass is 510 tons. We still get a MR of (9234+510)/(510)= 19.1 At this mass ratio the rocket travels 2.95x faster than its exhaust gases or 7.23kps at 250 seconds and 8.24 kps at 285 seconds. That's more than enough for ascent to L2, braking into L2 and return to the lunar surface.
With 200 people per flight and 30 million per year in the 22nd century, we make 150,000 flights, 200 people up to L2 and a new 200 tourists down from L2. We must produce 1.39 billion tons of monopropellant every year just for the Moon Shuttles. This is a job for replicating AI robots! See: Mining for Propellant.
All that moondust would have to be processed chemically and electrically to get the fuel, a job that will require immense energies. Since the Moon has so little internal heat we could dig deep. It might be wise to mine inside of lava tubes in order to widen then and bore deep into the Moon. When we feel we have mined deeply enough we could pressurize the massive tunnel with oxygen and build cities within it. The same could be done for mine shafts all over the Moon. For comparison, about four billion tons of coal is mined every year on Earth.
