The cislunar gateway with no gate, revisited
by John K. Strickland
|As the obvious and practical location for a gateway to the Moon, Mars, and the asteroids, a cislunar logistics base is the first component we need in place.|
Operational plans that only include cislunar bases are being proposed, as well as plans which call for only lunar surface bases to be supplied directly from the Earth, in addition to the more modern, cislunar resource-supported lunar base scenarios. These plans and designs are all like pieces of a very important jigsaw puzzle, but one that, due to the current circumstances, forces us to start with the individual pieces, instead of a whole original image. Our mission, if we can manage it (politically, fiscally and technically), is to try to create a functional whole—a cislunar transportation system—out of some or most of these pieces.
As the obvious and practical location for a gateway to the Moon, Mars, and the asteroids, a cislunar logistics base is the first component we need in place. I am not as concerned about which orbit any cislunar station is placed in compared to the base components, but it is still clear that the Earth Moon L1 point has an advantage since it is always in the same general area, and can be reached from any location on the moon in about 12 hours at any time without waiting for an orbital position to match. A station placed in the “near-rectilinear (lunar) halo orbit” (NRHO) proposed recently would actually be in a high, elliptical, lunar polar orbit (HELPO) that takes more propellant and time to reach from or to than most other options. The best orbits to support lunar operations have a short and relatively unchanging transit time from or to the lunar surface, a lower delta-V per trip, and which can be reached from most places on the surface at almost any time. According to one article, even the Russians recently questioned the possible selection of the NRHO orbit for the base.
From my point of view, any implementation of a cislunar base as a “no gate” scenario is still a very undesirable one, which could result in lunar exploration and development being sided-tracked for half a decade or more, and push Mars exploration (at least by NASA) even further away than it is currently. I am most concerned with which base components are currently being proposed for inclusion (or not) as the crucial parts of a cislunar gateway.
What the base needs to truly serve as a gateway are all the logistics components. Logistics may seem unglamorous, but if you add logistics capabilities, all of a sudden you can do important things that are otherwise impossible or impossibly expensive. In original space concepts, a space station or base was supposed to be a transit point or node, a point midway in terms of delta-V between different locations in space. The current space station is being used as a space laboratory, with essentially no logistics capabilities save for those allowing resupply from the ground. Unlike the large space station shown in the movie 2001, there are no scheduled flights to lunar bases or other locations from ours.
What are the logistics capabilities we need at any cislunar base? Any base that supports reusable spacecraft must be able to handle passengers, cargo, and propellant. For passengers, that means a place of refuge and for transfer to another vehicle. For cargo, that means transfer and/or storage, and for propellant, it means transfer, storage, and dispensing via propellant depots. It is inarguable that you want a propellant supply where the reusable spacecraft are operating. You also need extensive docking positions: pressurized for passengers, and non-pressurized for external cargo, propellant, and propellant depots.
|It should be clearly understood that a cislunar base without a lunar base has no real purpose, and a lunar base without a cislunar base to support it is fiscally unsupportable, too dangerous, and unable to support much work.|
Cislunar bases that support lunar and Mars operations chop the delta-V burden for a given mission into much smaller segments. The resulting reduction in fuel mass fraction per trip segment makes reusable single-stage in-space vehicles practical and affordable. As has been said many times before, fuel is cheap but rockets are expensive. If you compare the small delta-V segments using LEO and cislunar bases with Apollo, which needed a staggering delta-V total of about 18 kilometers per second for a round trip (9.3 to reach orbit, 6 to reach and land on the Moon, and 2.75 to return to LEO), the advantages of the former become very obvious.
The version of the cislunar base that has recently been proposed at a February 2017 meeting in Tsukuba, Japan, has all of the standard parts you would expect to want for a crew, including habitation modules, docking systems, an airlock, life support, communications, power and thermal control, a cupola, and a robot arm for re-supply operations. According to a recent article by Anatoly Zak, “the outpost would be under construction and operation for much of the 2020s”. What does this do to the timetable for actual lunar operations? Since the current intent is to launch components for the new station with the SLS, there would likely be no NASA funds left over for any lunar operations.
However, several critical base components were not mentioned. In a cislunar or lunar orbit, far outside the magnetosphere, a crew is constantly exposed to the same flux of interplanetary solar and cosmic radiation that a crew would be exposed to on a trip to Mars. There have been many claims that such trips are impossible due to the radiation hazard. A cislunar base crew would be exposed to this hazard indefinitely, not just for eight months at a time. Without good radiation protection, a crewmember would rapidly accumulate a lifetime radiation dose and be forced to retire.
Yet there is no mention of any additional mass for radiation protection at such a base. A minimum mass of protection against both cosmic and radiation might be about 150 tons of water, which would provide a 50-centimeter water blanket around one crew module. However, while this provides adequate protection from solar radiation and storms, it only reduces the cosmic radiation flux by no more than 25 percent. For a permanent ci-lunar base that needs little or no station keeping, a layer of at least three to four meters of water or equivalent fully surrounding the crew would be needed. Such a large volume of water would be easy to move from a lunar polar mining base to the cislunar base, if the lunar base existed with a means of transport. To try to move many hundreds of tons of water from Earth would be ruinous and wasteful.
Of course, it only makes sense to start building a lunar base if you already have support for it, both political and physical. You cannot move lunar water for shielding to a cislunar base without having a lunar mining base. You cannot easily move spacecraft down to the lunar surface and back again without a cislunar support base. While current small lunar science packages can be landed on the Moon by direct flights from Earth, this would be impossibly expensive for a continuing, human-scale lunar operation, with all or most of the space vehicles expendable. The cislunar base and the lunar polar base, linked by fully reusable vehicles, would directly support each other.
Thus, to make a good selection of the various proposals currently on the table, we need clarity of thinking, hopefully unhindered by political or any other considerations besides economics, safety, and practicality. So it should be clearly understood that a cislunar base without a lunar base has no real purpose, and a lunar base without a cislunar base to support it is:
So what should be done to prepare for the cislunar era? Many of the jigsaw pieces mentioned above already exist, or soon will. Someone, such as NASA or a National Space Council, needs to suggest a comprehensive plan to make use of them. It has been suggested that when the commercial crew program becomes operational in a year or two, the development funding for it could be applied to some other purpose. It is likely that existing Congressional forces may attempt to capture all of that funding for the SLS and Orion. If we want an effective lunar and cislunar program, that money needs to be put into what would amount to a COTS-like program for space vehicle and equipment development to:
|So what should be done to prepare for the cislunar era? Many of the jigsaw pieces mentioned above already exist, or soon will. Someone, such as NASA or a National Space Council, needs to suggest a comprehensive plan to make use of them.|
The emphasis needs to be on filling specific needed capabilities, not just demo missions. As a good example, a standard reusable lunar ferry that can move 10 metric tons can do most of the required lunar transport (crew, cargo, fuel) work needed. Such a ferry does not need to weigh more than about 36 tons wet, with a 10-ton payload, 20 tons of propellant, 2 tons of fuel tanks, and about 4 tons of other structure. Since the Moon has no atmosphere, most of the current launchers can launch such a vehicle into Earth orbit dry, where it can be refueled for the trip to the cislunar base. Once at the base, it can be refueled again to support lunar operations with round trips to the lunar surface and back, or from the lunar surface up to the cislunar base and back.
The basic ferry would consist of a common propulsion system with engines and landing legs, and could carry several alternate payloads on its flat top. These could include large cargo items like a hab module. It could also carry tanks with about 9 tons of propellant as a tanker variant, or a 10-ton crew cabin with self-rescue propulsive capability. The logistics base should have at least two copies of each of the three versions (crew, cargo, tanker) of the ferry system. A ferry would be able to complete a round trip with a one-way cargo item. If fuel is available at both ends of a trip, cargo can be carried both ways, or five tons of cargo could be carried both ways with fuel available at only one location.
Then, to support all these reusable spacecraft, we really do need at least two cryogenic propellant depots that can each store and transfer at least 100 tons of propellant, preferably without needing to move or rotate the depot. We also need places for the depots and other cislunar vehicles to dock, some at pressurized ports and others at non-pressurized ports. A docking truss would provide both the physical structure and room to allow the non-pressurized docking at low cost. The robot arm, if mounted with rails on the truss, can move cargo delivered to the cislunar base from Earth and place it onto the lunar ferry.
|The road to space development and settlement is open if we can just choose the right path to get on that road.|
It is also very important that any new LEO base and the cislunar base be designed to be expanded in multiple ways, as it is probable that they would be used later to support Mars expeditions. Additions to the space station at times has been impeded due to the muddled idea of “design complete in orbit.” Only space historians should think in such terms! If the lunar polar deposits prove out, we could need to store up to 2,000 tons of propellant at the base. This would require ten 200-ton capacity depot modules, plus the ten berthing slots to dock them in.
To complete an efficient cislunar transport system, we need ferry vehicles that can reach the cislunar base from LEO, and return to LEO using single pass aerocapture and an orbit trim to save propellant. These can be very similar to the lunar ferries, but with heat shields instead of landing legs. For cargo, we may also be able to use electric tugs if system masses drop sufficiently. All these vehicles need to be developed quickly since the cislunar base cannot support lunar operations without vehicles and equipment being delivered to it. The image at the beginning of this article shows what a fully-developed cislunar logistics base might look like.
The road to space development and settlement is open if we can just choose the right path to get on that road.