The Sacramento L5 Society, local California chapter of the National Space Society, has been engaged
over the last year in analyzing possible means for powering a Moon base through the two week long lunar night. One of the attractive means for accomplishing this appears to be the creation of a solar-powered laser platform based at EML1 (Earth-Moon Lagrange point 1). However, a NASA 2009 concept study ( http://www.nasa.gov/pdf/315858main_Cheng-yi_Lu.pdf ) suggests that a very large percentage of the total system mass of such a laser system would be composed of the apparatus necessary for "station-keeping" at L1. Further, the fuel for station-keeping would need to be continually replenished over time.
It was while exploring alternative station-keeping means that the idea of the Gravity Winch came into being.
The Gravity Winch is a concept that utilizes the purposeful movement of Gravity Anchors as a means of shifting or maintaining a platform's position at L1. A Gravity Anchor is simply a line or "tether" dropped down a gravity well, augmented with a mass at the end of the tether. L1 is uniquely suited to the use of a Gravity Winch, since it possesses two gravity wells down which Gravity Anchors may be lowered. In function, a tether is dropped down both the Moon’s gravity well and Earth’s. Such an arrangement could be composed either of two separate tethers, each with their own Gravity Winch, or it can be composed of a single tether sharing a single Gravity Winch. In the case of the single tether approach, lifting one Gravity Anchor out of one gravity well is matched by dropping the other Gravity Anchor down the other gravity well. (Note: For the time being, further analysis will be confined to studying the single tether approach at L1.)
There are long term and short term effects involved with the use of a Gravity Winch. Long term, moving
the Gravity Anchors with the Gravity Winch increases or decreases the pull of gravity from one direction or the other by increasing or decreasing the relative pull of gravity on the weighted end. That is, when a weighted tether is deeper in one of the gravity wells, it is at the same time shallower in the other gravity well. That creates a net increase in pull towards the gravity well the weight is deeper in. And since gravity continually acts, the pull is additive over time.
Short term, it seems intuitively clear that, with careful shifting of the Gravity Anchors from one side to the
other, the L1-based platform is able to be “balanced” between the two gravity wells, similar to the way a long pole helps a tightrope walker to balance.
In terms of station-keeping, both these effects can be thought of as removing the “z” vector (along the
Earth-Moon axis) from fuel-burning station-keeping consideration, thus allowing fuel-burning station-keeping to concentrate on the “x" and “y” vectors. That alone is a useful aspect of a Gravity Winch, and should be a real aid in reducing station-keeping requirements. Most importantly, a Gravity Winch allows the L1-based platform to be continually shifted in the z axis, without expending fuel, as the Moon (and the L1 point) moves farther from and closer to the Earth in throughout the Moon's elliptical orbit. Note that this continual movement of the L1-based platform may be accomplished without the need for expending fuel of any sort, although work, provided by a solar energy converter, will need to be expended via the Gravity Winch itself.
It is clear, therefore, that a Gravity Winch should be able to maintain station keeping along the z axis. Interestingly,
though, it may be that a Gravity Winch can also help to maintain station keeping along the x and y axes as well. Consider that two Gravity Anchors in effect place a tension on the connecting tether, and that at the approximate middle of that tether sits the platform being stabilized at L1. That tension connects the two weighted ends in a straight line pointed at the exact center of gravity of the Earth and the Moon. And any movement of a portion of the tether, as by an attached station moving in the x and/or y vectors, is thus countered by the force of opposing gravities that wants to keep the tether straight. Thus, a Gravity Winch may be able to keep the L1-based platform from "drifting away" in all three axes, thus obviating the need for any other form of fuel-burning station keeping other than the Gravity Winch itself.
There is one more interesting potential use of a Gravity Winch at L1. It may also be considered to represent the
starting tether of an eventual “lunar space elevator”. By lengthening the tethers in both directions over time, eventually the Moon side tether can be made to touch down directly on the lunar surface.
License: GNU Affero General Public License 3.0 (AGPL-3.0)"
Source Code/Project URL: https://github.com/L1-Society/L1-Society