Space Infrastructure for Lunar Tourism
by Dave Dietzler
Many noble reasons for space industrialization exist. Telecommunication platforms, solar power satellites, intercontinental power relays, helium 3 fusion fuel, microgravity manufacturing of perfect ball bearings, new vaccines and medicines, exotic alloys, precious metals from asteroids, astronomy, SETI, technology spin offs, space tourism and more beckon to us from the space future. Space tourism, particularly lunar tourism, is one of the most enticing rewards of outer space. Life must consist of more than just satisfying animal needs like eating, drinking, getting intoxicated, sleeping and such. There must be more to reach for and experience. Lunar tourism for those few people who could afford it might not seem as noble as providing the world with clean energy from outer space but where would the world be if there were no higher rewards to strive for?
Dreams and Nuclear Rockets
Science fiction dreams could come true if reusable nuclear thermal rockets that use liquid hydrogen for reaction mass were perfected and built in substantial numbers in factories the way air liners are built. A single stage atomic rocket with a specific impulse of 850 to 1000 seconds could fly directly from the surface of the Earth to the surface of the Moon. It could refuel on the Moon and return to Earth; a feat that would require aerobraking and a very sophisticated thermal control system like the heat shield tiles of the Space Shuttle. Atomic rockets were not perfected during the space race so less efficient chemical propulsion was used. Unfortunately, research on nuclear rocket propulsion died with the conquest of the Moon and the end of the space race in the sixties. Given the present political climate and public opinions about nuclear power it is highly unlikely that anyone will allow the creation of a lunar tourism industry based on atomic rockets blasting off and landing as frequently as jet airliners because of fear. Flying nuclear reactors would scare the hell out of the public. If an atomic rocket crashed chances are that the reactor would bury itself under several tens of feet of earth and anyone close enough to be exposed to radiation and get cancer, become sterile or have mutated offspring would be killed outright by the impact. However, what if an atomic rocket burns up in mid-air during reentry let's say and large numbers of people are exposed to radioactive material? People are fed up with industrial disasters and many of them believe we are all doomed by climate change. If there is going to be lunar tourism it will have to be based on something other than nuclear thermal rockets.
Propellant from Space
The first step on the long road to lunar tourism is the creation of a manned reusable space plane that can put a person in low Earth orbit for a few tens of thousands of dollars. It would probably be powered by liquid hydrogen and use a combination of jet engines and rocket motors. Such a vehicle could fly people half way around the world in less than an hour as well as carry people up to space hotels in low and medium Earth orbit. Space hotels would probably be made of inflatable modules. Small vessels based on inflatables with heat shields could be fueled up in orbit with hydrogen and oxygen from Earth's surface. These vessels could fly around the Moon and aerobrake into Earth orbit upon return. This is how lunar tourism would begin. Overflights will not be enough for some adventurers. Landings might take place in open cockpit vehicles.
Growth of the industry will eventually require more propellant obtained on the Moon and from near Earth asteroids. High thrust rockets will be needed to travel from low Earth orbit to a space station at Earth-Moon Lagrange point one in just a few days time. Solar electric spacecraft would spend weeks even months spiraling out to the Moon. Although such craft would use very little reaction mass, life support constraints and radiation exposure in the Van Allen Belts would make it impossible to use low thrust electric propulsion with manned vessels. Cargoes would be sent to the Moon and bases would be “bootstrapped” using local resources. It is not likely that the Moon will be colonized and industrialized for tourism alone. It is more likely that lunar resources will be tapped for the construction of solar power satellites and/or helium 3 mining. Tourism would ride in on the coat tails of the space energy industry. Mass drivers on the Moon capable of launching millions of tons of raw material into space every year could supply propellant to depots at Earth Moon Lagrange pt. 1 and in LEO. Hydrogen and oxygen would come from lunar polar ices at first and later from asteroids.
The infrastructure needed to support a commercial space travel industry will be very impressive. A multitude of lunar industrial settlements where humans and robots work together to extract metals and oxygen from Moon rocks and regolith will be needed along with bases on polar crater rims that send robots down into the darkness of the crater bottoms to mine for ice. Dozens of near Earth asteroids will be mined for water, carbon compounds, metals and oxygen. Large tankers will move mammoth quantities of rocket fuel and oxidizer through space.
To get an idea of how much propellant will be needed, let's envision a thousand spaceships each amassing one hundred tons empty and able to carry one hundred passengers. They would take three days to reach the Moon, or more accurately the transportation hub station at EML1, spend twelve hours refueling and being checked out, then travel back to LEO in three days where another twelve hours is devoted to servicing them. This means one round trip per week per ship. If these ships spend two weeks of every year in “dry dock” so that they can be refurbished with new engines and other equipment, they can each make fifty flights per year. One thousand of them could move five million tourists every year. That's about as many people that travel by air every day globally at the present time.
If these ships use nuclear thermal rocket engines, a risk that might be acceptable if the ships never enter the Earth's atmosphere, with a specific impulse of 1000 seconds and the delta V from LEO to L1 is 4100 meters per second then 52 tons of LH2 will be required for one leg of the flight. If chemical propulsion using LH2 and LOX is used, 153 tons of propellant will be needed. If silane and LOX with a specific impulse of 340 seconds is used then 242 tons of propellant are required. The interesting thing about this is that chemical propulsion will use more propellant overall but less hydrogen than nuclear propulsion will. With LH2 and LOX, an Isp of 450 seconds, and a six to one oxidizer to fuel ratio, 22 tons of hydrogen will be necessary. With silane only ten tons of hydrogen is needed.
Propellant Demands for 100 tons ship and a delta V of 4100 m/s (4.1 km/sec)
Nuclear rocket Isp 1000 sec. Exhaust velocity = 0.0098(1000) = 9.8 km/sec e^(4.1/9.8) = 1.52
152/100 = 1.52 152-100= 52 tons LH2
LH2/LOX rocket Isp 450 sec. Exhaust velocity = 4.41 km/sec. e^(4.1/4.41) = 2.53 253/100=2.53
253-100= 153 tons propellant using a 1:6 fuel/oxidizer mixture 153/7 = 22 tons LH2
|SiH4/LOX rocket Isp 340 sec. Exhaust velocity = 3.332 km/sec e^(4.1/3.332) = 3.42 342/100=3.42
342-100=242 tons propellant SiH4+2O2==> SiO2+2H2O Si = 28 H4 = 4 2O2=64
28+4+64=96 4/96 X 242 = 10.08 tons hydrogen
It is likely that hydrogen will be the “pinch point.” Silicon and oxygen are abundant in Moon rocks, regolith, and C and S-type asteroids. There is enough solar wind implanted hydrogen on the Moon to provide water for early Moon bases but not enough to fuel rockets. There are huge amounts of hydrogen in polar ices but what will it cost to mine that ice? And do we want to waste such a precious resource that could be of immense value to future lunar civilization? It seems the smart thing to do would be to use less powerful silane and LOX to conserve hydrogen. This also eliminates nuclear dangers. Even greater efficiency might be had if silane is used as a carrier fluid for metal powder fuels (aluminum, magnesium, or ferrosilicon) in bipropellant rockets.
A fleet of a thousand rocket ships with nuclear thermal motors would use 5,200,000 of hydrogen every year. With LH2 and LOX propulsion they would use 15,300,000 tons of propellant overall but only 2,185,000 tons of hydrogen. With silane and LOX they would use a total of 24,200,000 tons of propellant but only one million tons of hydrogen. This will demand a lot of mining and material processing and transportation.
Annual Propellant and Hydrogen Demands for 1000 Ship Fleet
1000 ships X 100 flights (50 round trips) X 52 tons LH2 = 5,200,000 tons hydrogen
1000 X 100 X 153 = 15,300,000 tons propellant 1/7 of that is 2,185,000 tons hydrogen
1000 X 100 X 242 = 24,200,000 tons propellant 4/96 of that is 1,000,000 tons hydrogen
Carbonaceous chondrite asteroids contain 3 to 22% water. If an asteroid that is 10% water is found then ninety million tons of asteroidal material must be dug and processed every year to provide one million tons of hydrogen for silane. If that material contains oxygen and silicon in percentages similar to the Moon then there will be more than enough oxygen and silicon for propellant. A quantity of rock several hundred meters wide would have to be mined. In ten years a whole mountain of an asteroid would be mined up. This task would be daunting.
Advanced Propulsion Systems
Fusion drives with specific impulses in the hundreds of thousands versus hundreds for chemical propulsion would use only comparatively tiny amounts of hydrogen for reaction mass. If controlled fusion turns out to be more practical than mining massive amounts of asteroidal and lunar material then fusion could be the way to make the space tourism business practical and profitable. There is another form of propulsion worth considering. Beamed power could make passenger ships with electric drives feasible. Ion drives can be clustered together to get more thrust but the real problem is that the solar or nuclear power plants become prohibitively massive. If a passenger ship is equipped with banks of electric thrusters using hydrogen or sodium vapor for reaction mass and a lightweight receiving antenna to capture power from large multi-gigawatt solar power satellites then high thrust electric propulsion that uses only a fraction of as much propellant as does nuclear thermal or chemical rocket propulsion would be possible. A fusion miracle might not be needed.
It would be very disheartening if at the end of this century the Moon is no more popular than Antarctica is today with a few tens of thousands of visitors every year. Most of us who were young during the Space Race looked forward to a 21st century with regular and frequent flights to the Moon. If it does become possible for five million tourists to visit the Moon every year, something that will require substantial habitable volume on the Moon perhaps in pressurized lava tubes, then over the course of a 75 year lifetime as many people as there are in the USA today could visit the Moon. It is hard to imagine lunar tourism for all of the Earth's nine to twelve billion people expected by mid-century. Spaceplanes the size of jumbo jets or larger and passenger liners the size of the Battlestar Galactica would be called for. Supplying the liners with propellant from asteroids would be a job comparable to the global coal mining industry today. Fusion or beamed propulsion start looking even better. It is also possible that in another century or more super materials will make it possible to build space elevators that can haul the masses into space as easily as we ride buses or jets today. Ships could be flung off the ends of the space elevator counterweights with no fuel and oxidizer at all. These ships could head towards the Moon or they could rendezvous with cycling stations to Mars. While a six month trip might not deter those with a passion for Mars it could someday be possible for tourists to reach Mars in just a few weeks time aboard ships propelled by gas core fission or fusion.
Propellant for Lunar Spacecraft
by Dave Dietzler
Lunar Ice for Rockets
Barring the creation of high thrust fusion drives with incredible exhaust velocities that allow spaceflight with only tiny amounts of reaction mass, passenger flights from low Earth orbit to the Moon will use vast amounts of propellant. Nuclear fission rockets would use less propellant than chemical rockets, but the role of nuclear fission in space will probably be limited by publicly perceived and real dangers. Propellant for chemical rockets that pass rapidly through the Van Allen radiation belts, unlike unmanned solar-electric cargo ships, could be obtained from the Moon and near Earth asteroids.
Lunar polar ice could supply liquid hydrogen and liquid oxygen for “Moon Shuttles” that travel between the lunar surface and a space station at EML1. However, it is doubtful that ice will be easy to mine in super cold craters. Conditions there will make metals brittle and prone to breaking. Ice mining machines will need onboard nuclear power plants or receiving antennas to get power from microwave beams transmitted from stations on the crater rims. Although millions of tons of ice are believed to exist in perpetually dark craters the ability to mine that ice and produce propellant from it in the future may be limited.
Extending Hydrogen Supplies with Silane
It should be possible to extend hydrogen supplies by combining it with plentiful silicon to make silane—SiH4. Simple calculations show that by using silane and liquid oxygen it is possible to effectively double hydrogen supplies.
Isp 450 seconds for LH2/LOX Isp 340 seconds for SiH4/LOX
Using online calculator at: http://quantumg.net/rocketeq.html it is found that for a 100 ton ship, payload and dry tank mass, and a delta velocity of 3200 m/s, enough to reach escape velocity from LEO, that 107 tons of LH2 and LOX would be required and 161 tons of SiH4 and LOX.
Running a hydrogen rich 1:6 fuel to oxidizer ratio like the Space Shuttle main engines, it is found that 107/7 = 15.3 tons of LH2 are needed.
Since SiH4 + 2O2 ==> SiO2 + 2H2O Si atomic mass is 28, H2 is 2 and O2 is 32
28 + 4 + 64 = 96 (4/96)X(161)= 6.7 tons of hydrogen with silane and LOX, less than half as much as is required with LH2 and LOX.
It should also be possible to build rockets that burn aluminum powder and liquid oxygen in the form of a monopropellant slurry as demonstrated by Wickman (see: http://www.wickmanspacecraft.com/lsp.html ) This propellant combination is not very powerful and a tank full of monopropellant is like a loaded bomb. It might be more effective if Moon Shuttles use bipropellant rockets burning a fuel consisting of a silane and metal powder (aluminum, magnesium or ferrosilicon) slurry. This could greatly extend hydrogen supplies.
Cost factors will come into play. Water can simply be split into hydrogen and oxygen by electrolysis and these gases can be liquefied in Sun shielded space radiators exposed only to the cold of outer space. Ferrosilicon and oxygen can be produced by magma electrolysis. Magnesium can be obtained by silicothermic reduction of magnesia. Aluminum production by electrolysis may require carbon, chlorine, a flux of lithium chloride, or other elements not common on the Moon. Chlorine is also needed for silane synthesis but it can be recycled. Making silane is complex but the whole process can be automated and without labor costs prices can remain low. If the greater complexity of producing silane and metal powders does not spell higher prices this could be a better way to fuel Moon Shuttles based on the assumption that ice mining and hydrogen production will be costly and limited. Even so, propellant from space will be far less expensive than fuel rocketed up from Earth.
NEOs: Valuable Rocks
Spaceships traveling from LEO to a spaceport at Earth Moon Lagrange point one (EML1) will probably be filled with propellant derived from near Earth objects someday. Industrial bases on the Moon will be “bootstrapped” from a minimal mass of machinery that makes maximum use of on-site materials to self replicate and build mines and mass drivers. Lunar materials would be used to build space shipyards perhaps at L5 where artificially intelligent robotic asteroid mining ships and tankers are built.
Carbonaceous chondrite asteroids can be over 20% water and a few percent organics in material resembling kerogen. The stony component of these asteroids consists mostly of oxygen, silicon, iron, magnesium, small amounts of aluminum and calcium, and numerous trace elements. Once again, there is access to lots of silicon to make silane and extend hydrogen supplies. Asteroidal material would be mined, crushed up, and roasted with solar heat to drive off water and organic compounds. This dried out material could be treated with fluorine to displace oxygen in minerals and form tetrafluorosilane gas (SiF4, b.p. minus 86 C.). Oxygen and SiF4 could be separated with membranes. The SiF4 would be decomposed at 850 C. with solar heat to get silicon and recover fluorine. Alternatively, something like a giant mass spectrometer could separate all elements in the asteroid.
All that useless rock that was sought after for its water and organics now becomes valuable property. It becomes a source of oxygen for breathing and propellant and a source of silicon for silane and solar panels. Silica, SiO2, the main component of glass, obtained by re-oxidizing silicon or boiling it directly out of minerals in the vacuum with focused solar rays, can be used for construction in space. Glass fibers have more tensile strength than steel. Fiberglass made with a polymer matrix from asteroidal organics could find many uses. Iron can be combined with carbon via the ancient crucible steel method. Rods of iron are packed in carbon powder and brought up to red heat for a few days with free solar energy. Carbon dissolves into the iron and forms steel. Space stations and space colonies could be built with steel. Magnesium is a respectable metal compared to unalloyed aluminum and it will not catch fire in the vacuum of outer space. It is a good reflector. Sheets and foils of magnesium could concentrate solar energy onto silicon photovoltaics and increase their power output. It would also be wise to consider slurry fuels made of silane, iron and/or magnesium powders. Magnesium is shock sensitive in liquid oxygen and will detonate. Tanks of magnesium and liquid oxygen slurries might be ignited with electric sparks for blasting into asteroids.
Transporting all this material mined by unpaid robots that never sleep through space will be a challenge. Solar electric propulsion is very efficient and requires only meager amounts of reaction mass. Solar sails many kilometers in diameter or magnetic sails like those envisioned by Andrews and Zubrin might move mountains of raw material through space with no propellant at all. Finally, one must wonder if it will be more profitable to mine small numbers of large asteroids over a kilometer wide and just take the materials wanted and leave the slag behind, or bag and capture large numbers of small asteroids just tens of meters wide and use every last bit of them for propellant and space construction materials in Earth-Moon space?
See: Lunar Tourism 2
More in depth discussion of lunar tourism and space industry in the future.