Tunnelling on the moon may seem like it belongs in some far-flung future epoch of the human journey, but with a growing excitement in the space sector, the underground sector is starting to take notice and think about some of the problems that may be involved.

This is because the first step on the journey to deep space will probably involve lunar habitats. There is still some dissent in the scientific community – a number of interested parties would like to see humanity bypass the moon and go straight to Mars – but the lunar camp seems to be winning the argument. A lunar base will be built to support human space exploration and to exploit the resources offered by the lunar environment.

Tunnelling, or at least the exploitation of existing underground spaces (lava tubes, etc.) will be required for this owing to the hostile conditions of the lunar surface. Temperatures range from -170 to +130°C, and the lack of a magnetic field or atmosphere means that the surface is bathed in high levels of radiation, while small meteorites pummel the regolith (the layer of unconsolidated solid material covering the bedrock of a planet) on a daily basis. These impacts can be at tens of thousands of kilometres per hour and are almost impossible to effectively shield against.

“Because of this, you have to bury all of your structures,” says Rostami. “Underground spaces can be used for living, storage and growing plants. These areas can be connected using tunnel boring units.

“With cut-and-cover tunnelling, you need heavy and large equipment to dig down from the surface. But on the moon, heavy equipment cannot be properly operated because of the low gravity. Additionally, you cannot transport heavy machines to the moon as it costs hundreds of thousands of dollars per kilogram of cargo.”

Similarly, less-mechanised, conventional tunnelling techniques and drill-and-blast require a lot of manual operations, which means a lot of people. For Rostami, a base must be mined, and it must be mined with specialised lunar TBMs.

Such a machine would be designed on Earth and probably machined here too, but would need to be capable of being broken down and transported to the moon. There is still a lot of logistical discussion around this.

Locally sourced

What is clear though is that there will need to be a manufacturing plant for the repair and maintenance of parts on the moon. There will also need to be extensive facilities to manufacture spare parts owing to the prohibitive expense of transport from Earth. For this and other space manufacture, 3D printing is very attractive as it means that blocks of material can be transported into space before the end-needs are identified. Recently astronauts on the International Space Station 3D-printed a tool that they did not have access to on the station, and had to be designed on Earth on the same day and emailed to the printer. This is the adaptability of 3D printing.

Maintenance will be a problem due to the low lunar population and the lack of specialised labour. People will need to be trained to perform multiple tasks, but there will be technical assistance available from Earth. A good understanding of the core principles of several fields will be desirable. The same worker could have to deal with electronics, hydraulics, mechanical parts or even re-grinding tools.

There must be a huge level of automation for operations, expecting to use some robots to do some tasks, as well. If it comes to maintenance, it needs to be carried out by humans; they have to think about how to change parts of lunar TBMs. If you need to go outside the envelope of the tunnel to the cutting chamber, it could be vacuum. That means that you have to pre-grout the ground to be able to maintain the atmospheric pressure from the cutterhead side, as it is almost impossible to send crew in space suit into the cutterhead to do any maintenance. The system has to be designed so the air pressure can be maintained at atmospheric pressure and you can operate with normal gears in normal conditions.

In terms of material up to the moon, aside from meteorites, which will be found at the centre of every crater, what is available is the regolith. There are possibilities to make some structural elements from regolith using some sort of resin. There is a lot of research going on topic to use this lunar regolith for manufacturing of various things, from habitat structures to machine parts.

However, the regolith itself brings its own problems. This surface dust is incredibly highly ionised. So much so that it would be incredibly harmful to lung tissue if ever inhaled. It is also potentially damaging to any exposed electronics. The reason for this is the constant bombardment of the lunar surface with solar radiation, which itself can interfere with electronic and electromagnetic equipment. Thus, every unit should be radiation shielded. The electromagnetic radiation on the moon is much higher than on the Earth. There is no medium such as water or air to allow the discharge of electric charges of particles, so an ionised haze of charged particles floats just above the lunar surface.

“If we use metal equipment, it can take care of ionisation,” says Rostami. “One of the main issues from Apollo mission was the dust, which was getting into the electronic equipment and of course it could cause malfunctions. In addition to the radiation shielding, electronic equipment needs to have a system to avoid the dust interfering with the operational performance.”

There is also an ongoing discussion about basalt fibres as the material is basically alkaline and volcanic rock. Researchers are trying to make basalt fibres and hope to use them as part of the manufacturing scheme.

At the moment, no companies have taken it upon themselves to produce a full design of a lunar TBM. Rostami says that they are currently advocating the use of lunar TBMs and efforts must be concentrated to look at potential issues and put some design ideas together.

NASA has a concept called Technology Readiness Level (TRL), which goes from 0 to 9. ‘0’ means concept while 9 the actual unit, which is going to be deployed. The equipment goes from a concept level to preliminary design with an initial test and then the product can be launched. As the lunar TBM is somewhere between concept design and having some components, it is considered somewhere between TRL 2 and 3.

“We are still a long way from a machine being designed. There is ongoing research and now the attention is concentrated to launch cargo efficiently into space. Three companies are famously working on this area: Space X, Blue Origin and Virgin Galactic, and they are trying to improve the launch process to be more reliable and cost effective.”

There has been recent discussion around getting to the moon by 2024-25, although this timeline has been criticised as being accelerated for political reasons. However, there is still a lot that is unknown about the lunar subsurface as very limited crewed missions have been sent and most observations have been remote. Rostami says that to construct the lunar base underground, they need another 10-15 years. But they have to start thinking now to appreciate some of the issues for these machines.

For a start, performance requirements in metres per day are not required. Instead, operation as close to full autonomy as possible is the aim, combined with the flexibility to cope with different ground conditions.

For example, many operations cause wear on parts. Cutters being the obvious components. Being lightweight and wear resistant is a challenge, but these two properties will need to be worked on and balanced for lunar equipment. “At the moment we use iron and steel, which work fine but they are heavy,” Rostami says. “In some cases we use tungsten carbide, which is harder and work better. Poly crystalline diamond is also used as cutting tool. This material is a bit more expensive, but for moon applications ‘being expensive’ is not the main concern.”

The main problem of carbide and steel is the weight. On the Earth Synthetic Polycristalline Diamond (PDC) is used, and it is very resistant towards wear and abrasion. The down side of PDC is related to temperature. If it reaches temps higher than 500°C, there are problems because diamond converts to graphite, essentially, it will just disintegrate.

“Working conditions at such temperatures might be problematic with PDC,” he explains. “Definitely we have to look deeper into the wear materials for cutting tools.”

Heat Dissipation and Power

Heat exchange is a major problem because a medium-size TBM on the Earth takes several thousands of kW of power. On the moon, this power requirement will have to be reduced. It is a fact not widely-appreciated that getting rid of heat in the vacuum of space is actually a major challenge as a vacuum is an excellent insulator. In fact, heat can only be dispersed by radiation as there is no contact medium. “The issue is that we are deploying a large amount of heat in a limited space and there is no flushing medium or no possibility of cooling the heating and equipment by air, which we use on the earth,” he says. “Down there you have to actually reduce the level of heat exchange. I haven’t seen any in depth discussion on heat exchangers that can dissipate the heat. The same issue applies to flushing medium and cleaning material if we drill a hole.”

Currently all the design for the drilling is based on how to take the material out of the tunnel and there aren’t any plans on how to use air or water or any other flushing medium for cleaning drill holes, as there is no flushing medium in a lunar tunnel environment.

“On the Earth we send compressed air or water to clean it up but on the moon we don’t have water or air to flush the boreholes,” he says. “The same discussion comes with cleaning the equipment or mucking when the flushing medium is not there to be used to maintain differential pressure between the face and the tunnel interior.”

What we can say is that most of the design work done on lunar excavation equipment assumes solar power. However, if you are in a permanently dark area for mining or a shadowed crater for better access to water ice, ground-based solar panels will not be an option. It is likely solar energy will need to be captured in orbit and beamed to the surface with lasers. Small, 1MW nuclear reactors several metres in size have also been considered. But this is not a question to be answered by the tunnel engineers, instead work must be done to redesign components to use less energy.

Low pressure and gravity

Low pressure can cause the spoil expand or ‘fluff’ after excavation. This is true on Earth, but will be exaggerated on the moon. Although there will be no shortage of dumping sites.

Additionally, we can’t work under vacuum conditions on Earth but that would be a likely prospect for lunar tunnelling. “Today, on the Earth,” says Rostami, “we often tunnel under 7-10 bar of external pressure, while the crew inside the tunnel work at atmospheric pressure. The maximum pressure that we have managed is about 17 bars. On the moon, it’s going to be only 1 bar of differential pressure (inside the tunnel working at atmospheric pressure and the outside at vacuum) so compared to the 7-10 bars that we have manage on the Earth, it’s not that bad.”

The TBM environment will need to be sealed to prevent air bleeding out of the tunnel and being wasted. Similarly, conveyors will need to be specially designed. They will need to pass through airlocks to take material to the surface, or some other solution will need to be developed.

Hydraulics will work fine in low gravity and pressure, but there are two other issues to be considered. Firstly, you have to deal with hydraulic oil and it’s relatively heavy and you have to maintain it on stock inside the tunnel. So it is an expensive proposition to move hydraulic oil to outer space for use on the related equipment. Secondly, if you need to use the equipment on the surface, that could be problematic because the temperatures can reach +120°C during the day, so hydraulics might have problems. While if you reach -120°C at night, the hydraulic oil can freeze and some components like cylinders can burst and so on. Most of the design should be focused to reduce reliance on hydraulics as much as possible. Ironically, protection from this extreme temperature range is one of the main reasons to go underground.

Lunar lining

Ground support requirements would be less stringent in low gravity. The design of lining has to do with the type of load that comes against the structure of the tunnel. It would be primarily a function of what the ground is going to look like. It is going to be unstable soil or stable rock.

Rostami says that in rock conditions we probably won’t need a lining, especially at shallow depths. All that is needed is to excavate the rock, take the muck out and maybe spray a membrane to make sure that the air doesn’t escape.

Soil is a bit different or in case the tunnel runs into a break in the material and it may need to have a shell lining, which itself could be composed of regolith and some resin, then cast into segments.

One of the concerns is the seismic activity on the surface of the moon. There have been seismic events recorded on the surface of the moon. This might be caused by meteorites hitting the surface or even by moonquakes. One thing that should be considered in the design – especially in unstable soils – is that the lining may face seismic or dynamic loading.

Shooting for the Moon

There is a huge international interest in going back to the moon. Some companies are already working on their own plans for space exploration, but deep space projects are primarily being led by national agencies, at least for now. In the future, it is likely that these efforts will have to be more organised, with mechanical engineers developing this new field of construction equipment to work on moons, planets and even asteroids.

The early space treaties and international conventions are not sufficient to ensure effective and peaceful cooperation, so a legal framework will need to be developed. Although it is still very early, converging technologies mean things are moving more quickly than many people realise.