Asteroids, Orbital Refueling, Electric Propulsion and Beyond.
By Dave Dietzler
Return to the Moon
Returning to the Moon with a plan to stay and industrialize it seems likely sometime in this 21st century. Falcon Heavy which can put over 50 metric tons in LEO (low Earth orbit) could be used. Given the shifting winds of politics there is no telling if the Space Launch System for returning to the Moon will ever “get off the ground.” SpaceX has also proposed a larger rocket that can orbit well over 100 metric tons for sending humans to Mars. Blue Origin is also working on large rockets. Huge chemically propelled rockets are fine for climbing out of Earth's gravity well, going into orbit and launching small payloads to other planets. Booster stages can be recovered and reused to reduce costs. Upper stages might be refueled in orbit and reused or dismantled for construction materials.
If really large cargoes are to be sent to the Moon and planets of the solar system spacecraft highly efficient electric drives like electrostatic ion drives, Hall thrusters, plasma thrusters or VASIMR will be needed. These use very little reaction mass or “rocket fuel” compared to chemical or nuclear thermal rockets, but they have low thrusts and take months just to reach the Moon. Slowly spiraling away from Earth means lengthy exposure to Van Allen Belt (VAB) radiation that could be deadly for humans. The VAB radiation could reduce the efficiency of solar panels and damage radiation sensitive cargoes. Some cargoes could be shielded and nuclear power plants might be preferable to solar panels.
High thrust chemical rockets will be needed to send humans to the Moon in just a few days time and limit time spent in the VABs. Chemical rockets could rapidly accelerate ships in LEO up to escape velocity and those ships could then activate electric drives and reach Mars faster. These rockets could be reused to cut costs. Spent upper stages might be pressed into duty. Perhaps upper stages with SpaceX Raptor engines that use liquid methane (LCH4) and liquid oxygen (LOX) could do the job. Liquid methane and liquid oxygen are "soft" cryogens unlike LH2 which is a “deep” cryogen that is more difficult to store in space for long periods of time. Orbital refueling infrastructure in LEO and possibly EML1(Earth-Moon Lagrange 1) or LLO (low lunar orbit) would be necessary. Propellant could be rocketed up to LEO from Earth and stored in orbtal depots. Refueling depots in LLO or at EML1 could be loaded up with propellant from the Moon in the form of hydrogen and oxygen from polar ices or metallic powders and LOX from regolith mined just about anywhere on the Moon. Eventually LEO depots will be filled with propellants from the Moon and/or near Earth asteroids.
Refueling in space is not just a simple matter of connecting a hose between two spacecraft. Cryogenic propellant will be transferred through corgurrated metal hoses like the flex pipe connecting stoves to gas lines. Plastic hoses would freeze up and crack. Spacecraft carrying propellant will have to fire ullage motors to shift the propellants to the bottom of their tanks or they would have to rotate to produce centrifugal force that pushes the rocket fuel to the tank bottom. Gas pressure would fill the tanks to push the propellants out. The fluids would move through the hose to a pump that drives them under pressure into the receiving spacecraft's tanks. As the super cold liquids are injected the warmer empty receiving tanks might cause them to vaporize, so precooling of the hoses, pumps and the receiving tanks will be necessary. This would have to be done behind foil solar shields. Some of the fluid might boil off and it would have to be recaptured and stored in large tanks made from upper stages or external tanks and reliquefied. It is doubtful that one load of propellant rocketed up to orbit will be enough to fuel an interplanetary space ship. It will be necessary to store up several payloads of fuel and oxidizer in tank farms at orbital refueling depots that have solar shields and boil-off reliquefying machinery. Once enough propellant is stocked up it can be transferred to a space ship shortly before it leaves LEO. Orbital refueling has yet to be perfected and no space depots exist today. This technology must be developed before any large scale manned exploration and industrialization of the Moon, asteroids and Mars can happen.
Asteroid defense should not be neglected in favor of a lunar development program and missions to Mars. The same orbital refueling infrastructure needed for a lunar industrialization and Mars exploration could be used to fuel robotic asteroid deflection spacecraft. Programs to create protection from potential large impacters, lunar industry and Mars missions should proceed simultaneously. Orbital refueling infrastructure would be the logical first step for all these objectives.
A mass driver on the Moon could supply mass for a gravity tractor that could deflect threatening asteroids if there is enough time for warning in advance. Another real "pull yourself up by your bootstraps" idea would be to capture a small asteroid say 10m diameter that can be reached with a delta V of less than 500 m/s and outfit it with some kind of propulsion system and use it as a gravity tractor. It may also be possible to send a robot to a threatening asteroid that digs up some of the asteroid to build up mass for a gravity tractor then start thrusting away. In the worst case a small spacecraft could deliver a nuclear bomb to the oncoming asteroid and detonate it. If this or any other venture beyond Earth orbit is to work there will be a need for nuclear-electric propulsion (NEP) or solar-electric propulsion (SEP) and plenty of reaction mass. Some rather smart AI and weightless excavating equipment would also be required.
EROs: Easily Retrievable Objects
Fuel and oxidizer payloads would be rocketed into LEO at first. Eventually, fuel and oxidizer will come from the Moon and near Earth asteroids. There are at least a dozen near Earth asteroids that can be propelled to Sun Earth L1 (SEL1) or Sun Earth L2 (SEL2) with a velocity change of 500 meters per second or less. These Easily Retrievable Objects (EROs) are about 2 to 20 meters in diameter and there are probably more out there that have yet to be detected because of their small size.
See article about EROs: http://www.ibtimes.com/new-class-easily-retrievable-asteroids-could-be-captured-rocket-technology-found-1382529
If these EROs are composed of rock, we can foresee based on the study of meteorites that they will probably be mostly silicates containing iron and magnesium. Oxygen will be the major component and oxygen is the major component of rocket propellant. Eight weights of oxygen combine with one weight of hydrogen. The Space Shuttle main engines used a ratio of six weights of LOX for every single weight of LH2. A methane/LOX burning rocket engine would use about four weights of oxygen for every weight of methane. Metal powder base fuels might also be used. Experiments have shown that aluminum and LOX in a monopropellant slurry make an effective combination with a specific impulse of about 270 seconds. According to Peter Kokh, aluminum powder burns better if it is mixed with calcium powder. Stony asteroids will contain some aluminum and calcium but much more silicon, iron and magnesium. Iron powder could make a low performance fuel. Magnesium powder in LOX has been shown to be shock and vibration sensitive and will detonate. Magnesium might not make a good rocket fuel but it could make a good explosive in the nitrate poor environments of outer space and the Moon. Silicon burns with as much heat per weight unit as aluminum, but there is little or no data on silicon/LOX slurry monopropellants. Silicon can be combined with hydrogen to make silane, SiH4. Silane burns with oxygen at a mixture ratio of 1:2, but much less hydrogen is needed if silane is used. Some simple stochiometery and the rocket equation can be used to determine that the silane/LOX combination yielding about 340 seconds Isp will require only about half as much hydrogen as the LH2/LOX combination. Asteroids would supply an abundance of silicon. Manufacturing thin film silicon solar panels and integrated circuits in space won't demand very large amounts of silicon. The best thing to do seems to be to send hydrogen up from Earth and combine it with asteroid silicon for silane which is also a “soft” cryogen like LCH4 and LOX. Iron (steel), magnesium, aluminum and calcium (for electrical conductors) could be reserved for space construction purposes.
Processing asteroids to get useful materials will be a challenge. Electrical and chemical methods for extracting useful elements from lunar regolith and rock could be applied. Solid rock would have to be broken down into a powder for various extraction methods to work. Perhaps a small asteroid could be placed in a huge Kevlar bag. Holes could be drilled into it and explosives planted. The asteroid could be blasted into a pile of rubble. Some asteroids already seem to be floating rubble piles resulting from collision with other asteroids. Those rubble piles could be surrounded with Kevlar bags and spacecraft with electric drives could haul them to SEL1 or SEL2. Rubble piles, natural or man made, in Kevlar bags could be rotated by applying small amounts of thrust to generate centrifugal force to drive the pieces of rock into jaw crushers and then through several stages of centrifugal grinders to powder them. The powder could then be processed in various ways to get oxygen, silicon and metals. If the asteroid is carbonaceous it will be possible to roast out water and carbon bearing tarry material.
If the asteroid is metallic it will consist of mostly solid iron and nickel with other metals like platinum. These asteroids won't supply oxygen, silicon, magnesium, etc. They will still be very valuable. Iron and nickel could be combined with carbon from C-type asteroids to make steel. This would involve packing iron rods in carbon powder and bringing them up to red heat in solar furnaces for a few days. Asteroid metal could even be processed in something like a huge mass spectrometer to separate all the elements. If this proves to be impractical iron powder or briquettes could be treated with hot carbon monoxide gas to form gaseous carbonyls of iron and nickel that can be piped off and decomposed with intense heat in solar furnaces to get pure iron and nickel and recover CO gas. The residue of metals left after treatment with CO to remove iron and nickel will be enriched in cobalt, platinum and other metals. Breaking down solid iron into smaller pieces and even powders cannot be done with explosives, jaw crushers and grinders the way rock can. High energy electric sparks or plasma torches might be used to cut up solid metal. Concentrated solar energy might be used. Lasers seem like the best idea. High power lasers will soon be deployed on aircraft, tanks and ships for weapons. In a decade or two lasers that are up to the job of cutting up metallic asteroids are likely to exist. Pieces of metal can be melted down and sprayed thru nozzles with an inert gas to form powders. Spinning metal rods can be melted with electric sparks and particles will be thrown off by centrifugal force. The particles cool and powder forms. This second method does not need an inert gas that has to be recycled somehow.
Electric Drives for Fuel Efficiency and Speed
Electric propulsion is needed to overcome the “tyranny of the rocket equation.” Even so, electric drives have drawbacks. They are slow. The require massive power sources. They endure more gravitational losses than chemical or nuclear thermal rockets do. Not much can be done about gravitational losses. Highly efficient solar panels and nuclear power plants for space are in the works. Beamed propulsion is another possibility. A massive solar power satellite could beam energy to a lightweight receiving antenna on the spacecraft. It might take months to reach the Moon or even Sun-Earth Lagrange regions with electric drives but robotic cargo ships won't be deterred by that. For journeys to Mars, Mercury or the outer solar system spacecraft with electric drives would take quite a while to escape from LEO but they can apply constant low thrust for months and reach higher velocities than chemical rockets to reach distant worlds like Mars or the moons of the Gas Giant planets faster. With fuel and oxidizer from captured asteroids and orbital refueling infrastructure manned spacecraft with electric drives could be rocketed quickly out of LEO and up to escape velocity, spend very little time passing thru the Van Allen Belts, and reach other worlds in the solar system faster than chemical, nuclear thermal or electric propulsion alone.
Asteroid mining, orbital refueling and electric propulsion combined can open the door to the solar system. Intelligent robots will pave the way. Some near Earth asteroids might require less delta V to reach than the Moon, but launch windows to and from these objects can be years apart. Keeping humans alive and sane in space for years without resupply from Mother Earth will be too difficult. Robotic asteroid mining spacecraft will be needed to do the job. Some asteroids contain water and hydrocarbon compounds. They could be a source of hydrogen and carbon as well as oxygen. Lunar polar ices could supply hydrogen and oxygen too. When these asteroidal and lunar materials are available it will no longer be necessary to rocket them up from Earth at great cost. Stony asteroids and the Moon contain an abundance of silicon which can be used with hydrogen to make silane and extend the hydrogen supply. The first step will be establishing orbital refueling stations and stocking them up with propellant from Earth to fuel up the great ships of discovery, robotic as well as manned.