ASI W9700532r1.0
#95 May 1996
Section 6.9.3.2.095.of the Artemis Data Book
by Larry Jay Friesen
MMM#95 - May, 1996
[A companion paper, "Lagrange Point Staging for Lunar and Planetary Flight" appeared in last month's MMM.]
Introduction
It will greatly ease the long-term economics of supporting a lunar base to produce propellants at the Moon. These would be used for flights between the lunar surface and any near-Moon space stations, and from there back to Earth. It has even been proposed to supply lunar propellants to low Earth orbit (LEO) to be used for Moon-bound ships. This will come as no surprise to long-term students of lunar base proposals. The major reason is that traffic models for lunar base show that by far the largest budget item in mass being moved around between the Earth and Moon is rocket propellant.
Lunar propellant could also be used to launch inter-planetary space flights. This would be especially advantageous if those flights were launched from a near Moon staging base, such as the L1 Lagrange point space station proposed in the preceding article [MMM #94, April '96]. I am going to argue that the combination of an L1 base and lunar propellants would make a powerfully synergistic combination for suppor-ting both lunar and interplanetary ventures.
The most frequently proposed lunar-derived propellant is liquid oxygen extracted from the oxides and silicates that make up lunar rocks. This would be burned with hydrogen provided from Earth. One attraction of this is that the oxygen/ hydrogen combination provides one of the highest specific impulse values available from chemical propellants. Specific impulse is a performance measure for rockets somewhat analogous to miles per gallon. It is often given in units of seconds, meaning the number of seconds that one pound of propellant could produce one pound of thrust, before it is consumed. The few combinations known that produce higher specific impulse: (a) produce only slightly higher, not grossly higher, specific impulse; (b) are composed of more expensive materials; and (c) are more corrosive and difficult to handle.
One disadvantage of this, if one is trying to minimize mass lifted from Earth, is that the hydrogen will probably still have to be supplied from Earth. Hydrogen is extremely rare on the Moon [Ed. in general. We can hope that Lunar Prospector will confirm indirect indications from the Clementine mission that there is economically significant ice in the permashade areas at the lunar south pole. We should know by early '98, latest.] A minute amount is found implanted in lunar soil by the solar wind. It is conceivable that this can be extracted in amounts adequate for life support. However, the amounts of material that would have to be processed to extract enough hydrogen to support a reasonable amount of traffic to and from the Moon are far larger than I, for one, would find attractive.
Other propellant combinations based on lunar materials have been proposed. Silanes would stretch the terrestrial hydrogen by combining it with lunar silicon to make compounds analogous to methane and ethane. This would increase the proportion of the [total] propellant [combination] supplied from the Moon. However, it would also reduce specific impulse. Specific impulses of silanes burned with oxygen are roughly similar to those of hydrocarbons burned with oxygen, or in the range of 300+ seconds rather than the 400+ seconds of hydrogen and oxygen.
Advantages of Lunar Oxygen & Aluminum Together
A particularly appealing propellant combination is lunar oxygen plus lunar metals, especially lunar oxygen and lunar aluminum. Aluminum and oxygen alone will provide a specific impulse somewhat lower than most hydrocarbons. Brower et al. expect a value of 285 seconds [1]. However, this should be quite adequate for lunar landing, lunar liftoff, and departure for Earth from an L1 station using a lunar swingby trajectory. Lunar escape velocity is only 2.4 km/sec, so we don't need an enormous specific impulse for operations in the lunar vicinity. A big advantage of this propellant combination is that no terrestrial material at all is required. Keeping down the mass we have to lift from Earth is likely to be a major factor in keeping down the operational costs of our missions.
One means of enhancing the performance of lunar oxygen and aluminum could be to combine them with terrestrial hydrogen in a tripropellant engine. Andrew Hall Cutler [2] estimates that [with] an H:O:Al mass ratio of 1:3:3, such an engine would have a specific impulse exceeding 400 seconds - only slightly poorer than hydrogen and oxygen alone. This ratio also manages to decrease slightly the proportion of hydrogen that has to be brought from Earth, compared to the approximately 1:5 combustion mass ratio of H:O for Shuttle main engine technology. Brower et al. [1] expect that with an H:Al:O mass ratio of 1:2.5:2.75, a specific impulse of 475 seconds can be achieved. This would increase performance, but at the cost of bringing more hydrogen from Earth.
It might turn out that, for instance, lunar aluminum and oxygen alone would be best for a lunar lander flying back and forth between the lunar surface and an L1 staging base, while lunar aluminum and oxygen combined with terrestrial hydrogen would be more advantageous for a space-to-space lunar transfer vehicle (LTV) flying between the L1 station and LEO. Trade studies are needed to decide for what flight phases is is most advantageous (i.e. what minimizes mass launched from Earth) to use lunar aluminum and oxygen alone, and when it is best to add terrestrial hydrogen. How much hydrogen should be added to the propellant mix, weighing the cost of mass launch against performance gain?
It would also be worth doing trade studies to answer the question: would it be advantageous to ship lunar aluminum and oxygen propellants to LEO? When the overall mass flow in the system is considered, will that reduce mass launched from Earth? Further, would aerobraking for LTV return to LEO be useful in a transportation scheme making heavy use of lunar propellants? Or would the propellant used in hauling the aerobrake around exceed the propellant saved when braking into LEO?
If one wants to extract aluminum as well as oxygen from lunar materials, it means re-examining the extraction techniques. Reduction of ilmenite, for example, an often cited approach, will not do. Ilmenite reduction starts with a[n iron and] titanium-rich mineral found in lunar mare soil and produces oxygen, but no aluminum. Other oxygen extraction methods that do not produce aluminum will not do, either, at least not without steps added to get aluminum metal.
This also means carefully considering where on the Moon to go for raw material. Lunar maria are high in iron and titanium, but tend to run low in aluminum, only around 7%. The highlands, in contrast, are rich in an aluminum-rich material called anorthosite, and highland soil tends to be about 13% aluminum by weight [1]. If aluminum and oxygen are both target materials, the lunar highlands are the better place to go for feedstock. [Editor's qualifying comment follows article.]
Lunar Propellants for Interplanetary Flights
Lunar derived oxygen and aluminum propellants could also be used to aid the departure of interplanetary space flights, if those flights were launched from an L1 base on gravity slingshot trajectories as described in the preceding paper [3] [MMM # 94, April '96] If a "triple thrust" departure is used to go to Mars, for example, using both a lunar and an Earth flyby, the total velocity change or delta V (DV) needed to depart the lunar vicinity is only 350 meters [0.35 km] per second. DV gives a measure of the amount of energy and propellant needed to accomplish a maneuver, if you know the performance of the propellant combination your ship is using. The additional DV needed at perigee to place the ship in a Mars-bound trajectory is only 790 meters [0.79 km] per second.
It is quite conceivable that lunar settlers may one day produce oxygen and aluminum propellants for departure stages of planet-bound spacecraft, maybe with Earth-derived hydrogen added for extra performance. Lunar oxygen could well fill the oxidizer tanks of the interplanetary craft for the subsequent maneuvers in its itinerary. If the interplanetary ship designers select a tripropellant propulsion system, the ships may carry lunar aluminum as well, with only a small admixture of terrestrial hydrogen to boost specific impulse. Using lunar propellants combined with launching and retrieving interplanetary flights at L1 [3] could significantly reduce costs of interplanetary travel.
Ways to Implement Lunar Aluminum/Oxygen Propellant Usage
How are we to implement the use of lunar oxygen and aluminum propellants together?
One way would be to pump aluminum powder as we do fluids. In this case, it will probably be necessary to use a carrier gas along with the powder to keep the aluminum grains from vacuum welding or sticking together from electrostatic forces. Here we could use lunar hydrogen implanted in soil grains by solar wind, because only a small amount is needed. The hydrogen for this function does not have to be a significant fraction of the propellant.
Another technique is a hybrid rocket engine using solid aluminum and liquid oxygen. A conceptual design for such an engine was proposed by Brower et al. [1]. Their design calls for a hexagonal array of aluminum bars the length of the combustion chamber. Liquid oxygen would be fed down the bars for regenerative cooling before reaching the flame at the bar tips. The engine could use oxygen and aluminum only, or could use tripropellant operation with hydrogen.
References
[1] Brower, D., Adams, T. Kelly, C. Ewing, and T. Wiersema, "Conceptual Design of Hybrid Rocket Engines Utilizing Lunar-derived Propellant", AIAA paper 90-2114, delivered at AIAA/SAE/ASME/ASEE 26th Joint Propulsion Conference, Orlando, FL, (July 16-18, 1990).
[2] Cutler, Andrew Hall. "Aluminum Fuelled Space Engines for Economical Lunar Transportation". Lunar Bases and Space Activities of the 21st Century, W.W. Mendell, ed., Lunar and Planetary Institute, Houston, (1985) p. 61.
[3] Friesen, Larry Jay, "Lagrange Point Staging for Lunar and Planetary Flight", Moon Miners' Manifesto, issue # 94. April 1996, Milwaukee, WI.
LJF
[EDITOR'S COMMENT: MMM thanks Larry Jay Friesen for this excellent and enlightening pair of papers. However, his remarks on lunar base siting demand comment.
Those who do not foresee (or do not wish to speak to) the industrialization of the Moon, and are only trying to brainstorm the economics of renewed exploration activities, may indulge in the thought that efficient oxygen production is the only determinant of consequence when it comes to picking a site for a lunar base. But those of us who look forward to real, industrially self-supporting communities on the Moon must take a wider view. Lunar settlers will need access to all economically producible elements. From a lunar development point of view it is clear that we ought to put our outpost in a location from which both suites of materials (aluminum rich highlands, iron rich maria) can be accessed with equal ease - the highland-mare "coasts"! This solution is so self-manifest that the very continuance of this debate exasperates! Those who come from different disciplines must talk to one another!
PK
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