There are many reasons for continuing the human exploration of the Moon. Whether to emplace advanced astronomical instruments, to gather information about the formation of the solar system, or to establish a "testbed" for advanced exploration technologies, the return of humans to the lunar surface represents a fundamental step beyond LEO. But is it worth the cost? When the subject of future exploration is mentioned at high government levels, the typical response is "it's too expensive, we can't do that." However, it seems that such a reaction has the problem exactly reversed and should be stated "when we find a less expensive way, we'll go."
Finding an economical method of lunar exploration is what our current study efforts are all about. Unsubstantiated arguments about the "best" way to return to the Moon are analogous to quarrels over deck chairs on the Titanic. Cost must be a primary engineering parameter which is constantly traded against performance, risk, and schedule. Perhaps most importantly, in this transitional period when new program starts are problematic, the limited dollars available must be invested in those technologies which hold the highest potential for exploration mission cost reduction.
What are the reasons that human exploration missions are perceived to be so expensive? The various "top-down" computer models that are used to predict program costs give us some clues. Not surprisingly, one of the most important parameters is the mass of the transportation systems. The bigger a spacecraft is, the more it is likely to cost, and since exploration missions are high-energy missions, they require lots of propellant, big tanks, and heavy supporting structure. Another factor is a somewhat vague concept called "complexity." This reflects the fact that a vehicle which must perform rendezvous, proximity operations, docking, and crew transfer is more costly than a simple crew module. Also, a system that is an upgrade of a proven design has lower development costs than a "clean sheet" approach. The final consideration is the supporting infrastructure required to complete the mission. If the total space budget is a priority, one program cannot be permitted to "off load" costs to another; for example, we cannot ignore the costs of building or modifying a space station if it is needed to support exploration. So, to summarize, the ideal technology for exploration missions is one that would enable the combination of low mass, simplicity, heritage from current systems, and independence from ancillary infrastructure - a challenging combination, but one which we think we have found with a mission concept we call "LUNOX," short for "lunar oxygen."
Most persons interested in lunar exploration know that there are vast reserves of oxygen locked up in the lunar soil. In fact, nearly half the weight of lunar surface material is oxygen! Dozens of potential extraction methods have been identified, several have been patented, and one has been successfully tested on small amounts of returned Apollo samples (see Beyond LEO Vol. 1., No. 2). Oxygen on the Moon would be valuable for many different purposes, but using it as spacecraft propellant oxidizer for the return to Earth is the application that holds the highest promise for reducing mission costs. Eighty-five percent of the weight of the propellant of a hydrogen-oxygen-powered spacecraft is oxygen, and if the return oxidizer does not have to be carried to the lunar surface, the size of the spacecraft and launch vehicle required is drastically reduced. For example, when compared to Apollo, or the "First Lunar Outpost" strategy (see Beyond LEO, Vol. 1, No. 1), the weight that must be injected toward the Moon can be reduced by nearly a factor of three! This is enough to avoid the development of a large, new booster or the necessity of on-orbit assembly at a space station - a single launch of a Shuttle-derived vehicle ("Shuttle-C"), long-known for expected low development costs, is sufficient. A supply of lunar oxygen also makes the lunar-orbit-rendezvous technique utilized by Apollo obsolete. Since the penalty of taking the Earth-return oxidizer to the lunar surface is eliminated, there is no need for a second spacecraft to remain in lunar orbit, and norequirement for any kind of rendezvous at all. We've therefore met all of our cost-cutting strategies for transportation: less mass, simpler systems, launchers which are modifications of existing designs, and a reduction of required infrastructure. The key to realizing these advantages is the establishment of the oxygen production capability prior to sending people back to the Moon.
It is not possible at this point to advocate the "best" extraction technique for LUNOX production. Trades involving system mass, power requirements, process complexity, oxygen yields, reagent resupply, and equipment maintenance must be made. However, a design study has been performed for a plant utilizing the "hydrogen reduction" process consistent with production quantities required to return a crew of four from the Moon twice a year. This concept uses the combination of a small teleoperated front-end loader and regolith hauler to mine and transport feedstock and dispose of process tailings, a three-stage "fluidized bed" reactor to produce the oxygen, and a 40 - 60 kilowatt nuclear power supply. In addition, small oxygen "tankers" would be needed to transport the liquefied oxygen to the waiting crew vehicle, where it would be pumped into the empty oxidizer tanks.
The LUNOX production equipment and associated surface vehicles must be transported to the Moon by dedicated cargo flights. If the Shuttle-derived vehicle is used for both piloted and cargo missions, a delivery capability of approximately 12 metric tons to the lunar surface results. Our design studies indicate that all of the required lunar oxygen production infrastructure could be delivered in only two flights, at which point teleoperated oxygen production would begin. Human flights would not take place until a sufficient cache of oxygen had been extracted, liquefied, and stored, and the propellant transfer equipment had been operationally verified.
The piloted flights would begin with the launch of a man-rated Shuttle-C from the existing Shuttle pads at the Kennedy Space Center. After a short pause in Earth orbit for proper phasing, the crew would be injected into a 4-day, translunar trajectory. After another short pause in lunar orbit, the final descent and landing would occur. Upon completion of the surface mission, the crew would refill the lander's oxidizer tanks with ten tons of lunar oxygen for the trip home. The crew module would reenter Earth's atmosphere in much the same way as the Apollo Command Module, but modern navigation and control capabilities should allow touchdown at prepared landing sites, therefore eliminating the expense of mid-ocean recovery forces.
While much of the hardware in the LUNOX scenario is expended, there is an opportunity to reuse the most costly element - the crew module.
From this description of the LUNOX strategy, it is apparent that its feasibility relies heavily upon the development of key technologies: high duty-cycle electric surface vehicles for regolith and oxygen transport, automation and robotics for surface mining, and highly reliable chemical processing for unattended lunar oxygen production. This is somewhat of a departure from the technology expertise of the aerospace industry where the focus tends to be in the areas of vehicles and propulsion systems. However, this can be viewed as an opportunity to broaden the relevancy of government-sponsored space technology to the commercial sector. In addition, the programmatic risk associated with the oxygen extraction techniques can be mitigated by proof-of-concept precursor missions involving small lunar landers or rovers equipped with process demonstration packages.
When cost estimates of this strategy are compared with those of a more "traditional" approach, the leverage of space resources becomes apparent. The development costs of the piloted spacecraft and launch vehicle are reduced by more than 55 percent, and the unit costs by more than 60 percent! Even when the expected development, production, and launch costs of the oxygen extraction equipment are included, this investment is still less than the total development costs of the traditional approach, and the per flight savings accrue from the first piloted mission.
It appears that the use of space resources is the single highest payoff technology yet identified for renewed human exploration of the Moon.
Joosten, B.K. and Guerra, L.A. (1993) Early Lunar Resource Utilization: A Key to Human Exploration. AIAA Paper 93-4784, AIAA Space Programs and Technologies Conference, Huntsville, AL.
Zubrin, R.M. (1992) The Design of Lunar and Mars Transportation Systems Utilizing Extraterrestrial Resources, IAA Paper No. 92-0161, 43rd Congress of the International Astronautical Federation, Washington, D. C.