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Earth-Moon L1 Considerations
This is a paper that I wrote a couple years ago, and that I never really found a place to publish. But then, I remembered that I'm a bloggist, and have a place to publish things that no one else wants to publish. It seems like it's a useful discussion right now, given that NASA has announced their lunar architecture plans, and things seem to be somewhat in ferment.
The choice of transportation node location is strongly driven by both near-term and longer-term architecture requirements, which are in turn driven by overall exploration program goals and their phases. More specifically, the choice of whether and how to utilize the Earth-Moon L1 location is largely driven by our reasons to go to the moon. For this reason, it's not possible to recommend a specific location for lunar transportation staging operations, but we can do analysis that can help NASA or private entities make such a decision in the context of other agency choices as the program evolves.
There are at least four schools of thought on the purpose of a human return to the moon prior to human expeditions to Mars.
In the first two views, any visits to the moon are not for the purpose of long-term operations or eventual settlement, and so any investment in infrastructure to support them would be wasted, when it could be invested instead in getting us on to the actual, eventual goals, Mars and beyond.
In the third view, the infrastructure would be invested in, but emplaced on the Moon itself, so orbital activities, not on the lunar surface, would be superfluous, and again a waste of scarce resources.
It is only in the fourth view, in which the Moon is both a place that we will be using as an ongoing research and development test bed, and as a source for new resources (particularly propellant resources), that orbital nodes related to it become of interest. From that standpoint, EML1 turns out to be a very interesting location.
In addition to the role of lunar exploration in choosing transportation nodes, there is another consideration, which is the degree of desired reusability of transportation elements. This is in turn a function of the cost of propellants at various nodes. In general, due to the nature of the rocket equation, the lower the cost of propellant at any given location, the more likely it is that reusability of elements operating out of that location will make sense.
This is because, currently, the cost of propellants at any location in space is a function largely of the rocket equation, because they must be delivered to those locations all the way from the earth, using chemical propulsion. In a sense, from the standpoint of propellant delivery, a vehicle can be considered a tanker, in which its propellant payload is whatever is left over after expending the much larger amount of propellants needed to provide the change in velocity necessary to get it to its destination. Thus, as a result of the rocket equation, the cost of propellant at any location in space goes up roughly exponentially with the amount of velocity change required to get it there from the production source. This means that, for locations far beyond earth orbit (such as geostationary orbit, lunar orbit, EML1 or the lunar surface), the effective cost of the propellants required to return a vehicle from that location can be higher than the cost of the vehicle itself, rendering vehicle reusability pointless from an economic standpoint.
Establishing propellant production in one of those distant locations can change this logic, perhaps dramatically. (Potentially) cheap propellants manufactured on the lunar surface transform a reusable LSAM, whether to low lunar orbit or some other intermediate point (most notably EML1) from an expensive proposition to an attractive one, at least for one reuse. In turn, using the LSAM as a tanker to deliver the propellants to that staging point could perhaps dramatically reduce the costs of propellants in that location as well, relative to the cost of delivering them there from earth, because the velocity change required to do so is much lower. This in turn potentially makes a multiple-reuse LSAM viable. The LSAM is used to deliver both propellants and cargo, and the number of economical reuses of a vehicle would be driven by the LSAM design itself, rather than propellant logistics issues per se. The issue then becomes—what is the staging location that best utilizes this scenario, with the two obvious choices being low lunar orbit (LLO), and EML1?
A Place In Space
Lagrange points are to the Moon as geostationary orbit is to the earth. They are the places where gravity and momentum are in continual balance, and from which the view of the moon (due to its tidal-locked situation, in which it never rotates with respect to the earth, or the earth-Moon system itself) never changes, because they rotate around the system with the Moon.
The one that Dr. Lagrange designated L1 is of particular interest, because it is the closest one to the Moon that is visible from both earth and the earth-facing side of the Moon, being located continuously between them on the line connecting their centers. For this reason, it is one of the only few places, and the most convenient place in what the late Congressman George Brown called “Greater Metropolitan Earth” that can be reached from the lunar surface at the same cost, any time one wishes, and with no launch windows—there is essentially no relative motion between it and any point on the Moon (ignoring slight variations resulting from the eccentricity of the Moon's orbit and other minor perturbations).
Its advantage lies not only in its location with respect to earth and Moon. It is also a reasonable location for a spaceport to the rest of the solar system. It is sufficiently far from earth to be high up in its gravitational bucket, vastly reducing the amount of propellant needed to escape the earth-Moon system from there. Such an escape trajectory is a minimum, and necessary condition to other solar destinations, whether to Mars or near-earth objects and other inner and outer planets. The location, unlike the earth’s or Moon’s surface, or even low earth orbit, is in a little gravity “dimple,” rather than a deep gravity well. Yet it is close enough to be a convenient trip from both earth and the lunar surface, in terms of trip time.
Near-Term Benefits of EML1 Utilization
As previously discussed, if we are to utilize lunar resources for propellant production, having some place in orbit, but off the lunar surface, would be a convenient location for a depot to store the propellants delivered from the Moon. For lunar operations itself, this propellant would have two potential uses: delivery of payloads back to earth, and delivery of payloads (including a reusable LSAM) back to the lunar surface, either for use there or as a return trip to get more propellant.
LLO could be used for this activity, but it has several problems. First, like LEO, LLO is not a single, unique orbit—there are an infinite number of them with varying altitudes, eccentricities and inclinations. The orbital characteristics will vary over time, meaning that the amount of velocity change to get there from any location on the lunar surface will vary (perhaps greatly) over time as well. The only exception to this would be a polar orbit, accessed from one of the poles (which is a possibility, given that this is viewed as one of the most promising locations for propellant production). However, the node of a lunar polar orbit will also vary with time, making it inconvenient and expensive to return to earth from it much of the time. Furthermore, any low lunar orbit tends to be unstable over time, due to mass concentrations on the Moon, which means that any permanent facility in an LLO will eventually crash into the lunar surface absent continuous active station keeping.
For all of these reasons, LLO is probably a poor choice for a propellant depot (or construction hangar—another potential application for a cis-lunar staging area). It also would require communications relay when on the far side of the Moon, if continuous communications with earth is required or desired. Its use as a safe haven (or in fact any application that implies permanent infrastructure in that location) would probably be precluded by these considerations as well.
EML1, however, for reasons already discussed, will be relatively stable (it needs minimal station keeping to remain in place), is always the same distance, in time and velocity, from any point on the Moon (providing flexibility in lunar surface activity locations unavailable from a fixed LLO). In addition, it will always be the same distance (again, in terms of time and velocity) from the earth.
This stability allows it to be used for all of the things for which permanent facilities might be desired. Of course, any discussion about propellant depots or construction hangars at EML1 or, for that matter, at any off-planet location raises the specter for some of “another space station,” with all of the attendant concerns about costs and program complexity. However, as with the Shuttle program, in which many assume that its shortcomings have somehow “proven” that reusability is a mistake, it is dangerous to extrapolate any general conclusions about building space facilities from a single programmatic data point, such as ISS.
There are many reasons that ISS has turned out as it has, and few, perhaps none of them are intrinsic to building space facilities. Without getting into a detailed critique of the program that would be well beyond the scope of this article, a key point that should be understood is that the ISS had many purposes, the most important of which were political, rather than to actually do anything useful in space (including providing continuing employment in key congressional districts, providing foreign aid to Russia without dipping into conventional State Department budgets for such things, justifying the Shuttle development, etc.). It also suffered from “mission creep,” with requirements evolving and changing over time. Most of its true program requirements could, in fact, be satisfied without ever launching hardware into space, as evidenced by the fact that it survived for well over a decade (much longer if one counts all of the concept studies of the seventies and early eighties) without doing so. Most importantly, it was not part of anything larger—it became, and was, an end in itself.
A facility at EML1 (or anywhere else as part of the VSE) would not suffer from these problems, at least not intrinsically. It would simply be another development as part of a much larger activity (establishing a base on the Moon). It would not be the single focal point of human spaceflight development, as ISS became, and would be less prone to hijacking by other interests, allowing the focus to better remain on the development itself rather than which centers and congressional districts get the biggest slices of the pie. If it’s actually necessary as part of the overall infrastructure (in ways that neither Freedom or ISS ever have been) it will have a much better chance of success in terms of meeting its program and schedule goals. It makes no more sense to programmatically fear another “space station” in orbit than it does to fear a lunar base, or a Mars base, all essential things to developing robust space capabilities.
There are, of course, disadvantages as well for this location. As an intermediate point, EML1 adds both velocity and time to the total trip from LEO to the lunar surface, increasing total propellant requirements for the mission. However, this wouldn’t necessarily increase the mission costs, if the reduced propellant costs make up the difference. In fact, there is actually a benefit to the increased velocity change, in the sense that doing so increases the size of the LSAM, increasing the total amount of cargo able to be delivered to the lunar surface in a single flight (assuming that the LSAM is sized for the crew mission).
One further consideration of whether or not to use EML1 as a staging point is whether or not propellant depots based on lunar propellants are economically viable at all, relative to earth delivery. This will be a function of the cost of propellant production on the lunar surface, the cost of mission turnaround of an LSAM, and the cost of operations at the orbital depot, including propellant storage. An analysis should be performed to determine this, but it is highly sensitive to a number of inputs about which we presently still have too little understanding.
Far-Term Benefits of EML1 Utilization
As previously discussed, EML1 is a potentially interesting departure point for Mars and other points beyond the earth-Moon system. In addition to the delta-vee advantages already described, it could also be a safe place for a quarantine facility. Vacuum makes a good firewall, and it's better to have a hundred thousand miles of it than a couple hundred (as would be the case in LEO).
If it turns out that propellants can be delivered more cheaply to this location from the lunar surface than from earth to LEO, it may be a more cost effective means of doing outer (and inner, such as Near-Earth Objects) exploration than staging from LEO. The answer to this will not be known until the propellant-production technology requirements and designs are better understood, as well as the economics and degree of feasible reusability of surface-orbit tankers from the Moon, subjects beyond the scope of this article.
However, in general, if we are to become a truly space faring civilization, including the capacity to explore, mine and perhaps move NEO objects that could become a danger to us, we will at some point have to develop the capability to fuel and service spaceships off planet, including gathering extraterrestrial resources with which to do so (perhaps from those same objects). At some level of activity, this approach will reduce costs of operations, particularly marginal costs, an issue to which we tend to devote far too little attention. The sooner we start to develop such a capability, with all of the learning and technology development involved, the sooner we will attain that status. If we are to use the Moon as more than a brief foray on the way to Mars, or redoing Apollo, and if we want to get a head start on utilizing extraterrestrial resources, an EML1 base appears to be a logical way to do this, early on. However, it is also conceivable that initial forays to the Moon could be direct, until we understand more about lunar operations, at which time we could transition to an EML1 architecture as we understand more about the economics of surface operations, while still providing potential savings for missions beyond the earth-Moon system. As previously noted, it depends largely on just what we're trying to accomplish.Posted by Rand Simberg at December 05, 2006 10:18 AM
Also a good place for a Lunar space elevator fulcrum.Posted by Sam Dinkin at December 5, 2006 11:45 AM
Could you actually make one work from the south lunar pole, or would you have to transship to the lunar equator to board one?
One thing to remember is that the geometry of the Earth-Moon system changes as we pass through the 18 year Saros cycle.
In a nutshell, the apogee and perigee of the Moon relative to Earth varies a bit over time, with the minima being around 352,000 km (maybe 354, I can't remember exactly) for perigee and the maxima being around 404,000 km for apogee. 384,404 km is just the average.
While the gravipotential field will warp or stretch a bit over time, the d-V values should remain about the same. What changes is the absolute distances between the Moon, SSL-1, and the Earth.
The difficulty is how do you have the elevator fulcrum sliding up and down the thread as the absolute location moves in and out over the course of a month. Taking a thumbnail 16% of the difference of the above figures, that's over 8,000 km. (Actual results may vary...)
In any event, I've added this great post to the Lunar Library, and encourage readers to stop by the EML-1 section of the Library to find further reading along these lines:
This is a very important topic and critical not only for the Moon and other VSE destinations, but commercial cislunar development as well. EML-1 doesn't need the Moon to be a great place to set up shop and do business.Posted by Ken Murphy at December 5, 2006 04:48 PM
I thought that EML1 (as well as L2 and L3) were unstable Lagrange points? If so, any station placed there would require continuous use of propellant (admittedly not much of it) just to keep the station in place.
L4 and L5 don't have this disadvantage. In addition, they are also further out of Earth's gravity well. They are also visible from Earth and at least part of the Moon.Posted by Fletcher Christian at December 6, 2006 12:31 AM
We discussed “Option 4” in Era 4 of our AIAA Space 2006 Paper. In meetings at NASA HQ in Feb 2005 we suggested they also seriously look at utilizing the EML1 point as a major node for integrating multiple lines of space activities. Their response was not well thought out.
A complete synopsis of our approach to VSE can be found here.
In direct contrast to NASA current management we are always looking for good ideas for incorporation and would appreciate any feedback.
"A Place In Space" should have included the MEL1 Clarke Station and that of the MEL1 Chinese LSE-CM/ISS, or rather one or the other, but not both.
The GUTHVENUS and LSE-CM/ISS related web pages have been available for nearly 7 years.Posted by Brad Guth at December 27, 2006 12:38 AM
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