The cislunar gateway with no gate
by John K. Strickland
|A cryogenic propellant depot at L1 or L2 provides a critical piece of the space transport puzzle: a guaranteed source of high-energy fuel.
Now, there seems to be an attempt to replace the best new set of coherent ideas for effective in-space transportation, the cislunar concept, with what is essentially a fake version of it. This may sound like a harsh judgment to many, but let us look at what a real cislunar transportation system would provide us with, and what is totally lacking from the proposed L2 “Gateway” station that has been reported in the media recently. To be clear, this is not an argument about government versus private space, but instead about making rational choices on expenditures to enhance our ability to efficiently use and explore space in and beyond low Earth orbit (LEO).
Cislunar space is the area around the Earth extending out to just beyond the Moon’s orbit, and including all of the five Lagrangian points that are stable in position in reference to the Earth and Moon as they rotate about each other. For transport purposes, the two L-points close to and in line with the Moon, L1 and L2, are the most important. L1 is always in front of the Moon and L2 is always behind the Moon, each by roughly 60,000 kilometers. Being in what are essentially zero gravity locations, no reentry shells or landing legs are needed for vehicles to visit a station placed there. Even large objects placed there, in gentle halo orbits, can be kept there with minimal propellant use for station-keeping.
In a previous article, I examined the two most critical aspects of the new in-space transport system: reusable vehicles and cryogenic propellant depots. A reusable spacecraft cannot be reused if it has no source of propellant in space. If you ran a trucking company in California, you would need gas stations for your trucks to operate, and you would not build them in New York. Likewise, it makes sense for propellant supplies to be where the spacecraft need them. This means depots both in LEO and at either the L1 or the L2 point. Many favor the use of L1 due to the direct communications and the importance of maintaining the back side of the Moon as a radio-quiet zone for radio astronomy. L2 has a very slight energy advantage, but objects can be easily moved between L1 and L2.
A cryogenic propellant depot at L1 or L2 provides a critical piece of the space transport puzzle: a guaranteed source of high-energy fuel. The depot becomes in effect a propellant accumulator, where the propellant can come from any source and be used to reach any destination. A depot would use a sunshade, multi-layered super-insulation, and a solar-powered cryo-cooler to keep all of the cryogenic propellants (liquid oxygen and liquid hydrogen) from boiling away. Propellant launched from Earth to an identical LEO depot could be transported leisurely and efficiently to the L1 depot by electric space tugs, which would then return to LEO empty for another load of fuel.
Trips in space are measured as much by the velocity change the rocket needs to make at the start and end of a trip as by the distance travelled, since no propellant is expended during the trip, unless the vehicle is using electric propulsion. From L1, you can leave the Earth-Moon system with only a tiny change in velocity. More importantly, you can land anywhere on the Moon with a velocity change (called delta-V) of about 2.6 kilometers per second. You can also return to L1 with the same velocity change. This allows a single stage vehicle, such as a reusable lunar lander, to deliver a payload to a lunar base and then to return to L1 for another payload. The trip takes about 12 hours each way. This saves the huge cost of building a new lander for every payload and the even larger cost of launching each new lander from the Earth all the way to the Moon. Instead, all that is needed is the propellant and the payload. You can also reach the orbit of Mars for less than 2 kilometers per second from L1.
This means that reusable cargo landers could initially bring cargo for a lunar base down to the surface, using propellant brought from Earth, and then, when a lunar propellant production system is operating, use and bring propellant from the base up to L1, where it could be used for Mars expeditions and for bringing additional equipment down to the lunar surface at a massively reduced cost. If the landers, propellant, and other base equipment were first launched and then transferred by reusable rockets, Moon and Mars exploration programs could be supported simultaneously on today’s NASA budget.
|A logical way to develop a waystation at L1 would be to first place a prototype depot using a spent Centaur stage outfitted with a docking and refueling port for a small robot lander, which could place multiple small scientific payloads on the Moon.
Work on developing the concepts for depots has been going on for years, while the realization of the usefulness of the L1 and L2 points for transport has become known more recently. The intent of a cislunar transportation system would be to use all reusable vehicles to support Lunar and Mars bases via space waystations, both in LEO and in L1 or L2, and later in low Mars orbit. Although round trips between the Moon and L1 take slightly more propellant than between the Moon and lunar orbit, vehicles can depart at any time as waiting for orbit synchronization is not needed. Note that the existing International Space Station has few of the components needed to make a true waystation. It has no propellant storage or transfer capability, and no area of multiple docking ports that would allow cargo to be transferred in vacuum from a space tug to a lander.
A logical way to develop a waystation at L1 would be to first place a prototype depot using a spent Centaur stage outfitted with a docking and refueling port for a small robot lander, which could place multiple small scientific payloads on the Moon. The prototype depot could also include radiation monitors in different locations, to measure the radiation hazard there and how best to minimize it. All of the L-points are far outside the protection of the Earth’s magnetosphere, so unlike the ISS, any crew there would be fully exposed to solar radiation storms and would need storm shelters.
If the small-scale depots, robot tugs, and landers worked well, a serious lunar development effort would include the placement of full-size waystations in LEO and at L1. They would include room for storage of at least 100 tons or more of propellants, multiple docking ports, refueling ports, a crew refuge, and robot arms on rails able to move cargo between vehicles. The L1 station would also need a temporary storm shelter for transiting crews. Initially, no crews would be stationed at the waystation. Crews on their way to and from the Moon would do this work. A permanent crew would be assigned only if traffic became heavy enough to warrant keeping a crew there to manage the station full-time. Since most astronaut time on the ISS is needed for maintenance, a lot of work needs to be done on developing self-managing stations and remote station management systems similar to those used for the Mars rovers.
The combination of the two depots, plus reusable cargo, crew, and tanker vehicles moving between way-stations in LEO, L1, and the lunar surface, provides the vision of a robust human expansion into the solar system beginning in the early 2020’s. Now let us return to the sad reality of late 2012.
The currently-proposed L2 station trial balloon concept seems to be partly based on a NASA document released in June titled “Voyages – Charting the Course for Sustainable Human Space Exploration”. The document itself includes a reasonably rational progression of capabilities, leading eventually to human Mars expeditions. It is rather vague about which technologies and components would be developed and where and when they would be used.
The document also reveals what may be an ideological struggle within NASA. On page 11 it pointedly avoids mention of depots, which are perhaps still out of favor. The team of people who wrote the document state: “advanced cryogenic propulsion stages will initially provide high-thrust transportation throughout cis-lunar space and ultimately operate on long-duration missions without propellant loss from cryogenic boil-off.” This is an exact description of a cryogenic propellant depot, but they were not allowed to call it that, and in addition, the quote seems to refers to propellant tanks being taken on a mission, rather than tanks used to store fuel for a mission departure. The idea of a portable depot system as part of a space vehicle is actually new, and is a perfectly good idea, but why not just call a depot a depot?
|This scenario, where an L2 station habitat is emplaced before the development of reusable tugs and ferries, makes no sense. What is being proposed for L2, instead of a real waystation, is just a smaller copy of the existing ISS, possibly using old station components.
We often talk about the exploration of space, but there is actually nothing to “explore” at L1 or L2. These are simply very valuable locations in space that we want to use. There is no material or objects there except atoms and ions and the occasional passing particle of comet dust. So discussion in the NASA Voyages document about exploration of cislunar space is a complete distraction from the plain hard development work which needs to be done in order to properly exploit either of the lunar L-points for transport purposes.
The big problem is with the order of events and the description of the initial station as conveyed to Mark Matthews of the Orlando Sentinel by NASA sources. The article covers one aspect of what is in the Voyages document, the one emphasized by the sources, a so-called L2 Gateway station. This scenario, where an L2 station habitat is emplaced before the development of reusable tugs and ferries, makes no sense. What is being proposed for L2, instead of a real waystation, is just a smaller copy of the existing ISS, possibly using old station components. A crew would apparently be placed at this tiny L2 station habitat without first developing any of the depot equipment and re-usable vehicles which would enable the station to be a functional way-station, a true gateway to cislunar space. The Voyages document states: “A long-duration habitation facility could support a Lagrange gateway at EM L1 or L2”. By itself it could not, and in reality, it would probably be the last portion of a real gateway to be added to an L2 waystation complex, which would need to be fairly large.
The crew would thus have less to do at the L2 habitat than the crew on the ISS. There is no discussion of having a propellant depot there to support lunar ferries that could land robotic payloads on the Moon for the crew to supervise. There is no discussion of reusable vehicles at all, even in the Voyages document. Supporting a crew at L2 using the currently-proposed expendable NASA rockets would be several times more expensive than supporting a similar-sized crew at the space station. NASA has admitted, and other sources have confirmed, that the support rocket proposed, the SLS, cannot be launched more than about once a year (see “The SLS: too expensive for exploration?”, The Space Review, November 28, 2011). This would seem to be another great example of the classic NASA behavior of putting the cart in front of the horse. However, what is proposed as an initial L2 station crew habitat would work very well as part of a busy L1 or L2 waystation further in the future.
A lot of the current discussion regarding Mars missions seems to be focused on how to get the crew to Mars as fast as possible to avoid solar radiation exposure, yet this L2 proposal plants a crew right out in what is essentially interplanetary space with continuous exposure as long as the crew stays there. In the future, some want to put huge space colonies with thousands of people in solar orbit or other locations, which would be fully exposed to solar radiation storms. This would require the use of millions of tons of asteroidal material for radiation shielding. We do not have the capability to move this kind of material right now and will not for at least several decades. To provide a permanent crew at L2 with a substantial solar storm shelter would probably require hundreds of tons of mass and would be very expensive, to say the least. No details of what kind of storm shelter would be used are discussed.
|NASA officials need to clarify their plans and say what other technology and vehicle development work would be funded under the L2 Gateway project.
So, why is this idea being proposed at all? Matthews put his finger right on a probable reason in his article: “It gives purpose to the Orion space capsule and the Space Launch System rocket, which are being developed at a cost of about $3 billion annually”. This purpose is explicitly mentioned in Voyages: “This facility would provide a destination for exploration systems like the Orion MPCV Spacecraft”. The SLS and Orion funding is, in part, being taken out of the commercial crew and technology development programs, which could have accelerated development of propellant depots and reusable spacecraft. Thus, the proposal will attempt to tie support for presumed cislunar “exploration” of an artificial destination to more support for the SLS. In addition to the L2 gateway concept, NASA announced recently a slew of SLS-related scientific and engineering studies whose existence will also depend on the SLS program continuing. In addition, there are currently hard to believe claims being made that launching a giant and expendable SLS rocket would cost only 500 million dollars. So all this seems to be part of a pro-SLS publicity blitz in advance of possible huge budget cuts next year.
What would the effects of choosing this path be? Since most of the launches would be on expensive NASA rockets, the program would progress very slowly, if at all. No significant funding would be provided for reusable spacecraft or depot development, keeping us chained to using expensive expendable spacecraft for another generation. The cost of such a program would be very high, possibly beyond the capacity of the NASA budget even a decade in the future. There would thus be no near-term ability to place humans or larger equipment on the lunar surface, slowing the development of any lunar rocket propellant source, and delaying its use.
NASA officials need to clarify their plans and say what other technology and vehicle development work would be funded under the L2 Gateway project. Even though this current argument is not about government versus industry, private companies could play a role in it. If they could accelerate announcements of proposed reusable spacecraft and boosters, even if they could not start development of them immediately, it might make the NASA proposals look poor in comparison to the commercial ideas, and reduce the chances of funding for them. If a company offered to develop a reusable tanker capsule to refill a depot, maybe NASA would actually build a depot for them to fill. Reality tells us that, unfortunately, due to the interplay of bureaucracy and job-oriented politics, it may be a long time before we have a rationally planned or efficient government-led space program.