In next month’s issue of “Aerospace America”, there is a remarkable article by the renowned American astrodynamicist Robert Farquhar on the subject of an alternative architecture for lunar exploration. In this article, he contrasts the benefits of using the Earth-Moon L2 point as a staging location for the CEV and LSAM vs. the current plan to use a low lunar orbit. Here are some excerpts from the article:
A Halo-Orbit Lunar Station
By ROBERT W. FARQUHAR
In the summer of 2005 the President's Vision for Space Exploration led to the development of a comprehensive "Exploration Systems Architecture Study" for lunar exploration in the next decade and beyond. A key item in this plan foresees an orbital rendezvous between a lunar lander and command module (CEV) in a low lunar polar orbit. However, for reasons that will be discussed here, it may very well be better to locate the staging point in a "halo orbit" around the translunar libration point, L2.
The halo orbit has previously been considered as a possible location for a lunar far-side data-relay satellite. A relay satellite following a halo trajectory will always maintain line-of-sight contact with the Earth and the Moon's far side. Moreover, the entire halo orbit, when viewed from the Moon's surface, would subtend an angle of only 6.2 deg. Of course, station-keeping will be required to keep a satellite in halo orbit, but the control techniques are extremely simple and the annual fuel expenditure is quite reasonable (DV ~400 ft/s per yr). Detailed control analyses for halo satellites exist.
The Exploration Systems Architecture Study calls for two spacecraft, the Crew Exploration Vehicle (CEV) and the Lunar Surface Access Module (LSAM). In a typical mission sequence the CEV will be used to transport personnel and cargo from an Earth-orbit to lunar orbit. Staging in low lunar orbit would employ a conventional lunar transfer. Staging at the L2 position, on the other hand, would use a powered lunar-swingby trajectory of the type shown in the next illustration. This maneuver substantially reduces the DV requirements for braking to the vicinity of L2. After arriving at L2, the LSAM disengages from the CEV and descends to the lunar surface.
How do low-lunar-orbit and L2 compare in this strategy?
Operational Considerations: The most important reason for requiring a lunar orbiting vehicle in the future lunar program could be its function as a communications and control center for all lunar surface and orbital activities. The communications and control tasks would include
1. Control of rendezvous and docking operations for the LSAMs.
2. Monitoring and control of the ascent and descent trajectories of an unmanned LSAM.
3. Navigation and control of unmanned lunar surface vehicles.
4. Communications and navigational support for manned surface expeditions.
5. Control of unmanned remote-manipulator vehicles in the lunar vicinity. These vehicles require continuous communications and minimal transmission delay times for efficient operation. They would be used mainly for satellite maintenance and repair.
6. Command, control, and monitoring of all elements of the lunar program.
These tasks can be conducted very efficiently from a CEV at L2. It will give continuous communications coverage for all far-side lunar operations directly-without dependence on relay satellites. It likewise permits uninterrupted direct contact between the CEV and Earth. Moreover, by placing a single relay satellite at the cislunar libration point, L1, the CEV will always be able to communicate with almost every point on the Moon or in orbit about it. This type of communications and control network offers the additional advantage of being quasi-stationary with respect to the lunar surface. Finally, it should be noted that Earth stations already cover near-side lunar operations.
A CEV in a typical 100-km low lunar polar orbit, would be particularly ill-suited for the communications and control functions, for the following reasons:
1. A lunar lander on the surface would not have any direct contact with the CEV for periods as long as 11 days. Furthermore, the line-of-sight contact time would only be about 10 min per orbit even when the CEV passes over the base site.
2. Continuous direct contact between the CEV and the Earth would only be available for two 3-day periods each month. At other times, line-of-sight contact would be interrupted during every orbit.
3. The CEV would be almost completely dependent on satellite and/or Earth relay links for control of certain critical lunar operations (e.g., a surface rescue mission). Furthermore, two simultaneous relay links would usually be required and switchovers would occur every hour.
The halo orbit offers inherent operational advantages for logistics staging. For instance, the DV requirements for transfers between the halo orbit and the lunar surface are almost identical for any landing site, since plane changes can be made quite cheaply at the halo orbit. (The difference in DV cost is usually less than 200 ft/s.) It is also worth noting that, because of the quasi-stationary characteristic of the halo orbit with respect to the lunar surface, the launch window for transfers between L2 and the lunar surface is infinite.
On the other hand, with the staging point in a 100-km polar lunar orbit, the nominal staytime for lunar surface sorties would probably be constrained to 14-day intervals. Otherwise, due to precession of the polar orbit, a plane change would be necessary when the LSAM returns to the CEV. A graph on page 61 shows the DV penalty for this plane change as a function of surface staytime.
The differences in launch-window flexibility for transfers between the lunar staging point and an Earth parking orbit are not as clearcut as in the case of LSAM operations. Launch opportunities for economical TLS transfers are limited by certain varying geometrical factors. With a CEV based in low lunar orbit, these factors include Moon's position, nodal regression of the Earth parking orbit, and orientation of the CEV orbit with respect to the Earth-Moon line. Transfers to L2 would not be subjected to the third constraint, but the transfer times would be somewhat longer than those required for the low lunar orbit.
Another oft-stated argument in favor of a CEV based in low lunar orbit has it an ideal base for a rescue LSAM. As can be seen from the graph F-4 on page 61, however, the plane change DV penalty can become rather high when a surface rescue mission is needed at an inopportune time. Notice that the DV cost is not very sensitive to the maximum allowable transfer time. For a rescue tug stationed at L2, the tradeoffs are quite different, as the graph (F-7) just at left shows. From a DV standpoint, neither concept has a clear advantage for all rescue situations.
Finally, the station-keeping requirements of the two staging concepts should be considered. Although the normal DV costs for the two concepts are almost equal (~400 ft/s per yr), the CEV could remain in the vicinity of the L2 point (with some occultation) at a cost of only 100 ft/s per yr. Without orbit control, a CEV based in low lunar orbit would impact with the lunar surface in about four months.
Briefly, it has been contended that a CEV based in low lunar orbit would "provide a highly stable, safe, and flexible operations base." The factors just reviewed cast doubt on this claim.
The performance for a particular mission mode can be evaluated by calculating the normalized propellant weight. Normalized propellant weights for the assumed mission modes are given in the graphs above. Notice that the CEV in low lunar orbit is rather sensitive to plane changes at the lunar polar orbit. These results show significant performance gains with L2 staging.
Conclusions and Recommendations: A L2-based CEV could offer important operational and performance advantages compared to a CEV based in low lunar orbit in a lunar program. Therefore, it is recommended that the present strategy for the lunar-program portion of the ESAS be reexamined. Comprehensive tradeoff should be initiated of several mission modes for lunar systems using low-lunar-orbit and L2-based rendezvous. These studies would provide the information needed to select unequivocally the most effective strategy.
Now most of you readers who have been members of the AIAA since 1972 will of course by this point realized I have been pulling your leg. This article is NOT coming out next month in Aerospace America, rather it CAME out in June 1972 in “Astronautics and Aeronautics”.Here is the real article for your enjoyment.
But hopefully you will see that the implications of Farquhar’s results are just as pertinent today as they were in 1972, perhaps even more so.