This article discusses some of the issues to be addressed when considering the early use of Lava Tubes for habitat location. Early means the first few missions, and assumes that the first mission landing site is directly adjacent to a pre-surveyed lava tube.
I shall assume the following criteria for a pre-surveyed lava tube to be suitable in the context of the development issues to be considered in this article:
The development of the lava tube would occur in several stages, to be described in detail later:
Regolith is removed to the layer of rock strata surrounding the lava tube. The excavation will have gently sloping sides to prevent infill, and a cleared central area for mining equipment to be deployed.
This stage will also arrange for the creation of laydown areas for later equipment deliveries, largely consisting of the clearance of rocks in the immediate area.
Sintering equipment, consisting of a manually-controlled solar reflector, will allow the fusing of the top few centimeters of regolith to provide a working pavement. This sintering could also be applied to the excavated area to further stabilize the regolith. The size of the solar reflector should be a nominal 10m, and the equipment would be fully portable.
The crew habitat will be the standard first mission design. In addition, the mission will carry the sintering equipment and other lightweight equipment required for the first stage lava tube development.
A nominal eight-week period is alloted for this mission.
Once the rock strata have been exposed, construction of the access shaft will be via the use of explosives. This method is proposed over drilling due to the light weight of the excavation equipment, and the simplicity of the method. It will, however, require the additional construction of a blast shield for the crew habitat, to shield it from rock debris.
The aim is to blast a shaft a few meters wide, through the rock strata, and to create an entrance via the ceiling of the lava tube.
The first blastings are the most dangerous, since once a shaft has been developed, it becomes possible to direct the ejecta away from the crew habitat.
After each blasting, a certain amount of regolith and debris "muck-shifting" may be required in order to place further charges. If the charges can be made very small, and located in drill holes made by manually powered or electric drills, then almost all muck removal can be via the charges themselves. The site is not a confined area such as a tunnel, nor is the intent to remove as much rock as possible. Hence the purpose of the explosive would be to move the rock as far away from the site as possible, in a controlled manner.
Assuming that the blasting finally reveals an entrance, this is then stabilized through further sintering.
The next actvity is to undertake exploration of the lava tube by EVA. Mission crew would be lowered into the tube, together with a small amount of support equipment.
Their primary objectives are to survey the lie of the ground inside, and to test a small version of the inflatable habitat inside the lava tube.
The ground survey will determine the characteristics of the bedding of the lava tube. We need as smooth a surface as possible, and some ground clearance may be required in advance of the main inflatable delivery on the supply mission to follow.
The mission crew will also erect a small, protoype inflatable, made of one of the material options, in order to test the erection methods, the sealing designs, and the final stability of the structure.
The data gathered from the prototype inflatable erection will be used to finalize the manifest for the supply mission.
The first mission crew habitat is not suitable or desirable for lowering into the lava tube, since it must serve as the Lunar Orbit Return platform. Separate operations on this mission will bury the habitat for radiation and micrometeorite protection and for insulation during the lunar night.
The period of lunar night on this mission would be used to conduct lunar astronomy experiments, maintain crew fitness, and stablize the ecosphere of the habitat for long-term occupation.
This crew will return after their four-week stay, to be replaced by the crew of the supply mission.
The first mission crew return to lunar orbit, to rendevous with the LTV and supply mission. They will have left the hab in a fully working state for re-occupation by the next crew.
Mission 2 crew decend with the supply craft, which consists of supplies and a return vehicle. It carries a lava tube habitat consisting of an inflatable and its support systems.
Landing very close to the buried Mission 1 hab, the Mission 2 crew use winches to bring the return vehicle platform next to that of the Mission 1 hab so that it is easily accesible from that hab in case an emergency return is required.
The supply stage consists of consumables, the inflatable hab, and support equipment. The inflatable hab is protected from cosmic rays, solar flares, ultraviolet, and micrometeorite bombardment by a special lightweight casing. It needs to be moved into the lava tube for proper protection as soon as possible.
With the inflatable erected and pressurized, the Mission 2 crew now have a quite different accomodation from that which any astronauts have ever experienced. They are effectively living in a very thin plastic tent inside a cave on the moon. There are no windows with a view of anything other than rock or the kilometers of lava tube which stretch beyond the dimly lit area below the access hole in the lave tube ceiling, some 30 meters above their heads.
The biggest danger they now face is from themselves, rather than the hostile space environment from which they are protected by a layer of rock and regolith. The temperature outside is always stable, if a little cool, at -20°C. The main danger is that outside of the tent is zero pressure. Although the material is re-enforced with anti-rip webbing, the smallest hole in the fabric can spell disaster.
They are living in a low-gravity environment where the dangers of puncturing the tent through negligence are very great. As such, restrictions on the use of equipment, such as electric tools, which could cause flying debris, must be observed. The floor of the inflatable habitat is very susceptible to damage, which is why it is first covered with a kevlar mat (so that it is sandwiched between this material where it contacts the floor of the tube, or people walking on it). All equipment must be very carefully grounded using padding so that sharp objects cannot be pressed through the matting. The crew also wear special footwear, and the carrying of any sharp object must be strictly controlled.
Such is the danger of compromising the security of the inflatable habitat, that most initial work must be to further protect the membrane, to allow more normal working and living conditions inside.
Additional protective netting must be installed to provide a capture barrier between the free space inside the inflatable habitat, and the membrane surface. This will stop flying objects from puncturing the hab, and personnel guard-rails must surround walkways to keep personnel from accidentally falling against the walls.
The living space must be considered as more like that of a river-village on stilts than a campsite. Walkways supported by contacts bolted to the rockfloor of the lava tube lead to platforms mounted in the same way, which provide safe living and working areas.
With the moonbase crew now living in a stabilized environment, and able to work effectively on the surface with the promise of more friendly accomodations in the lava tube, they can prepare for the consolidation of the moonbase with the construction of a more durable habitat.
It will be called "RockHab1." Rockhab1 is not an inflatable. It is a four-storey high building made out of solid rock, and constructed inside the lava tube. It provides living accomodation, workshops, recreation rooms and early tourist accomodation in a full 1ATM pressurized environment.
This is landed as a single item. No return craft or consumables, just the equipment to make the next habitat. See the notes on RockHab1 construction for the details.
These leaves us at the end of Stage 3 with a considerable resource--a full-pressure living environment that provides excellent shielding from the environment of space, and with the structural integrity to allow living conditions to rise to about those to be found in a large modern submarine, but with far more space.
We now have a living facility that requires much in the way of outfitting. In short, a great deal of rather empty roomspace. One of the first options could well be to provide full medical facilities, to offset the need to ship people back to Earth for treatment. With the use of communications links and medical robots, it is feasible that operations could be performed in this environment, although the standard operating procedures would have to be adapted to Lunar gravity.
Perhaps a good architectural model to adopt would be that of the Zen style of minimalism, blended with as much plant life as we could cultivate, and the full use of media communications to provide guests with entertainment. The habitat is now structurally sound enough for the abolishment of prohibition on the moon, although additional technology would be required to manage the deployment of this resource to crew at the right time. Positive testing for alcohol whilst on duty would result in being thrown in the brig and shipped home, whilst operation of some key systems would be subject to a breath test (e.g., opening certain airlocks).
The provision of lighting and color in the habitat would be an early requirement. The lighting can come from solar sources during the 14 days of lunar day, but lunar nights of equal duration would require the provision of daylight-mimicking light sources for both humans and plants.
Providing color coatings to the incredibly bland grey walls and ceilings of the RockHab could be via the use of special chemical dyes that can be added to regolith to produce a water-based paint. The dye can be supplied in powder form, and mixed on Luna.
Additional structural improvements may extend to corridors between the RockHab and the access shaft site, where a pressurized lift shaft takes personnel to the surface lock. Indeed, there is no reason why rock plate construction need stop, other than the requirement to get them down from the surface away from sources of damage.
The lava tube can be gradually populated to allow more and larger living and working spaces--right up to the (perhaps several-hundred- meters high) roof of the lava tube is possible. The main constraint will be on the ability to produce enough atmosphere gasses and outfit these huge volumes. Provided that we can produce large amounts of oxygen and water, there is no reason why we cannot cultivate large areas of vegetation to provide food, recreation and atmospheric processing.
This article builds on the work of other contributors to the ADB, by proposing that lava tubes can and should be treated as a primary target for the first missions, simply because they offer a very attractive living space with excellent protection from the main hazards on Luna, and plenty of room for expansion. A means of quickly and easily constructing large habitats is proposed, and further study into the basic methods and techniques introduced as to their viability is encouraged. Inflatable habitats can only be considered as temporary structures. They are simply too vulnerable for long-term use, even below the surface. The dangers come from the very occupants themselves as well as outside sources, and the large volumes normally envisaged are just that--empty. The main tasks to address are the simplest means of using the raw materials available to produce both structures and an atmosphere.
Inside the lave tube, lighting will have to be provided. The most ready source during the lunar day is the Sun, and simple sun-tracking mirror systems and dispersers can be used to provide multiple-source lighting of the immediate area below the access point.
During the lunar night, operations inside the lava tube may be suspended, although there are some advantages in terms of the warmer temperatures to be expected inside the lava tube, than on the exposed lunar surface.
Systems need to be provided for equipment and crew access, plus backups. The equipment can be simply winched down with a friction-braked system, which the crew would tend to. Later, demand for a two-way mechanical facility will grow. This may be via an electric winch, together with at least two manual systems which allow self-winching whilst wearing an EVA suit. All equipment can be standard mountaineering issue, but with the extra solar radiation and temperature considerations accounted for. An emergency fast-egress system could consist of a fixed and stressed steel cable in the centre of the access hole, to which a crew member requiring fast-egress (e.g., suit failure/leak problems) could attach himself and use a small rocket pack to ride back to the surface along the guidance wire.
The main inflatable would have a diameter of five meters, and a length of twenty meters. The internal pressure would be less than 1ATM, but sufficient to allow the crew to breathe without any additional life support. Since the working pressure would be akin to high-altitudes on Earth, most physical work inside the inflatable would require the addition of small oxygen bleed sets to enable the crew to perform manual tasks more easily.
The proposed inflatable consists of a pre-assembled membrane construction, which requires un-stowing from its protective shield once it has been lowered to the floor of the lava tube.
Before the main pressure container is unpacked, the tube floor needs to be covered with a protective sheet to stop penetration of the pressure walls by rock. This sheet has to be very lightweight, but puncture-proof. A material similar to bullet-proof vests would be appropriate.
Before the inflatable is pressurized, it requires connection to an airlock, and careful layout along the floor of the lavatube. It must be located sufficently close to the access point in the lava tube, but far enough way so as not to impede equipment deliveries or be in danger of pressure loss from damage due to activities at the access floor area.
Until further supply craft provide better materials, the inflatable habitat will be used for crew accomodation and farming only. The main purpose of the crew is to live in the base to perform daily work shifts on the surface--primarily getting the lunar oxygen plant up and running.
The combination of the surface-buried hab and the lava tube inflatable habitat allow the moonbase to establish the main requirements for long-term habitation. Namely, a good supply of consumables (such as oxygen and water) plus the ability to grow food and provide a more natural habitat in the form of the large containment of the inflatable habitat.
The next priority should be the construction of a more secure inflatable habitat, since this would form the basis of a lunar hotel. Prior to that, further construction of access shafts further along the lava tube will expand the living area potential, and small electric vehicles can be used for long-distance travel under the lunar surface in relative safety and convenience.
The second lava tube habitat will be a dramatic improvment on the first. Four 2.5-meter floors high, and 100 meters long, it is assembled inside the lava tube and is fully sealed.
The second supply mission lands four regolith processors. These take regolith in one end, and compress it down a ceramic tube. In the middle of the tube is a region heated by a solar furnace reflector. The screw compressor produces a continuous feed to the furnace, and the molten rock is then fed to a die to be formed into cubes which are sliced off as the cooler rock, minus its volatiles, is spewed out.
Each cube is 25mm by 25mm and collected by an assembly robot which welds them together (using its own solar reflector) to make 1m by 1m by 25mm rock plates. It takes approximately 40 minutes to make each plate, and the volatiles from the process are collected for later processing and use.
The above assumes that no annealing of the cubes is required. The material produced would be similar to glass, and may have hairline fractures which could fail under temperature stress after the process. Annealing would make the material suitable for transportation to the different temperature environment of the lava tube.
One thousand square meters of plates, representing one floor area of the RockHab, takes 4000 minutes, or 67 hours, to produce. But we have four of these processors, so that amount of material is ready in just 1000 minutes or 17 hours
We have a crew of six who oversee the operation, and work two shifts of 12 hours to make the best of the lunar day. The RockHab requires a total of 8000 square meters of plate, so with 17 hours per 1000 square meters, the job is done in 17 x 8 = 136 hours. With two 12-hour shifts per working day, 11 shifts are required from the crew. So in just over five days, we have all the rock plates to make the habitat.
These have to be transported down into the lava tube one by one, where they are assembled and welded together. Computer optimization produces a construction sequence which allows all the sealing welds to be performed from outside the structure. Again, annealing may be required, but abundant solar energy can be piped or reflected down from one of the access shafts for this purpose.
Once the shell of the structure is complete, the outfitting may take much longer. In fact, the outfitting constraints may require that the RockHab be built over a much longer time frame than theoretically possible, so that it can be put to use as crew accomodation as soon as possible.
The addition of internal airlocks and windows would require additional materials, but with careful design, these could be restricted to specialized fittings which are best shipped up from LEO. Many of the fittings can probably be produced using robot grinders working with solids constructed by the surface robots joining the standard cubes together.