Powering the Lunar Base

NASA’s selection of SpaceX’s Starship within the Human Lander System (HLS) program was both surprising and exciting for space nerds all over.

Previously I have written about how Starship’s ambitious approach could transform the Artemis Program, particularly since Starship’s excessive cargo payload capacity creates a lot of opportunities that were previously curtailed by the harsh reality of razor thin Lunar mass budgets.

As a rough rule of thumb, conventional approaches to Lunar transportation put cargo costs at upwards of $100m/T, while Starship should be able to get as low as $1m/T without any miracles, and perhaps as low as $100k/T long term. 100-1000 times cheaper transport costs is the sort of logistics improvement that creates possibilities, and much of my Twitter feed recently has been full of die hard space nerds coming to terms with this fact.

Of course, if Starship works it won’t be long until other companies and countries replicate its success – there’s nothing magical about it. So when we think of Starship right now we think of SpaceX’s prototypes blazing a trail on a south Texas beach, but long term we should think of it as any launch system that is big, ambitious, fully reusable, readily manufacturable, and above all cheap.

Lunar Starship, c. 2021.

What does a Starship moon base look like?

— Casey Handmer, PhD (@CJHandmer) April 18, 2021

Of course we’ve had these lovely renders from SpaceX over the last few years showing what a Moon base could physically look like but once the sci-fi elements are subtracted we’re left with the basics. A bunch of pressurized spaces covered in dirt.

A Starship-enabled Lunar base is not some dismal single room tuna can like Jamestown Base from Season 1 of “For All Mankind”.

Jamestown Base from “For All Mankind”

After all, a single Starship could arrive with more pressurizable volume than the entire ISS and ~215 T of cargo. Instead, a Lunar base could look and feel more like McMurdo “Mactown” Station in Antarctica, the only Antarctic station with a ATM machine.

McMurdo Antarctic Station

Such a base would contain plenty of habitation volume to enable occupation without being cramped, dedicated facilities for sleeping, eating, working, and playing, logistics, transport, etc. Unlike a Mars city, it wouldn’t necessarily be oriented around the audacious goal of locally replicating a complete industrial stack, but it should be capable of comfortably housing more than a thousand astronauts at any one time, from any nation on Earth.

The habitations will necessarily be pressurized, meaning most likely vaults and cylinders with few sharp corners. They will also be covered in a layer of Moon dirt to provide protection from micrometeoroids, temperature swings, and cosmic radiation. Even so, there are ways that natural light could enter the structure, particularly because at the Lunar south pole the sun will never be more than a few degrees above (or below) the horizon.

In Lunar gravity (~0.16 g), roughly 9 meters of Moon dirt would be needed to counter a 0.5 bar working atmosphere, so some kind of tensile pressure-stabilized structure could be employed to enclose large volumes, albeit without the transparent “open sky” permitted by Mars’ thin atmosphere.

But enough of architectural speculation (for now), let’s focus instead on the core of where the battle is won or lost: logistics. And in particular, electricity supply.

A reliable source of electricity is also a challenge for the Antarctic Stations, which generally use diesel generators and occasionally supplement with wind power. Antarctica is a cold, windy, and often very dark place. Indeed, when my wife Christine returned from her winter-over at the Amundsen Scott South Pole Station she did have a slight scent of diesel. Needless to say, getting diesel to the south pole is non-trivial, and in ages past it was flown in by C-130, though three gallons was burned for every gallon delivered. More recently, giant tractors haul it from McMurdo (which is generally supplied by ship) over the ice, taking several weeks to reach the pole. Still better than walking dragging a sledge, as the early explorers did.

Way back in the ancient times, McMurdo even had a nuclear power plant! Though after 8 years of sporadic operation the reactor, by then dubbed “Leaky Poo”, was dismantled and shipped back to the continental US along with 9000 cubic meters (!) of contaminated soil.

This is all a primer for the core problem: powering a station of any size on the Moon is non-trivial.

The first choice for space power, solar photovoltaic (PV) panels, are challenged by the Moon’s long night. Over most of the Moon, the surface enjoys 14 days of uninterrupted sunlight followed by 14 days of night. During that time, another power source must be available to power station operations that can’t be arbitrarily shut off for 2 weeks. Indeed, environmental control (ECLSS) will have to provide heat during the cold winter night, in addition to atmospheric regulation, lighting, and a million other things. Due to the demands of ECLSS on the Moon, combined with a likely profusion of varying circadian cycles and shift work, I think it’s fair to assume that energy demand on the Moon will be much more regular than on Earth, where demand typically plunges by a factor of 3 or 4 at night.

As a back up system, batteries can work on Earth where nights are short, the air is breathable, and most people are asleep. On the Moon, for any given baseload demand, about 80 times as many batteries would be needed. And while the relative mass of batteries and solar panels doesn’t matter much on Earth where both are typically fixed to structures with copious static capacity, getting a bunch of heavy batteries to the Moon eats into payloads for other things.

One possible way around this problem is to build a base on a mountain at the South (or North) Pole, which due to the Moon’s low axial tilt enjoy much more sun than average, just as the adjacent craters are permanently shadowed regions (PSR) and contain some frozen mud. These “peaks of eternal light” are not quite eternal, however. There are multiple peaks and, particularly during the Lunar winter, they tend to eclipse each other. Even the prime locations on or near the rim of Shackleton Crater endure nights as long as 7 days with shorter periods of darkness during the daily cycle. This is a factor of 2 improvement over the generic case anywhere in the non-polar Lunar regions, but it’s still a huge challenge.

To put this into concrete terms, space-rated solar panels are routinely made incredibly thin and as light as 50 grams per square meter – similar to a sheet of newspaper. If the entire 200 T payload of a Starship was consumed with solar panels, they could produce a peak of 800 MW, assuming 20% efficiency. This is enough power for a small city on Earth.

In contrast, really good lithium-ion batteries can achieve about 100 kWh/T, so a 200 T payload of batteries would store 20 MWh. Attached to the solar panels they could charge completely in about 90 seconds, and after night falls, at the same power draw, would discharge in the same period. 90 seconds is a lot less than 7 days.

Alternatively, if 20 MWh had to last for 7 days, then average power draw would be 120 kW, about the same as the ISS. The panels necessary to provide this would weigh just 60 kg, a rounding error on the total payload.

Arguably overnight power storage could be accomplished by electrolysing Lunar water, then capturing the exhaust from fuel cells. Even using Lunar water, the mass of the cells, condensers, electrolysers, power electronics, storage tanks, and heat exchangers is comparable a Lithium-ion battery, while the round trip efficiency is much lower. And that’s not even including the mass overhead for mining lunar water! For the sake of argument, say a closed cycle Hydrogen energy storage system achieves 1 MWh/T. In this case, the above scenario results in power loss after 900 seconds, or 15 minutes. Still a lot less than 7 days.

One potential approach is to position solar panels in a number of carefully chosen locations around the rim of Shackleton crater and to wire them together, such that at any given time most of them are in sunlight. On the plus side this reduces the odds of a power outage. On the minus side it requires building solar farms many kilometers apart, linking them up with improbably robust, cheap, and light electrical cable that won’t break down due to Paschen’s Law, and doing it all before the winter arrives.

I also looked into mounting solar panels on a very tall tower, similar to a radio mast. These are strong, light, and tall, so perhaps there was some trade between aluminium tower sections and panels that could put the panels high enough that they’d never be shaded, even in the depths of Lunar winter, and thus avoid more batteries than the 2 hours or so necessary to endure a Lunar eclipse.

Indeed, the global trade looks like this.

Diagram showing three way trade between solar power towers, panels, and batteries.

Unfortunately, it turns out that the minimum altitude necessary to avoid any shading is 2750 m, rather taller than any structure ever built on Earth.

Left: Map of the Lunar south pole showing minimum altitude of shade-free space above any given point.
Right: Crop of Shackleton Crater showing regions where shade-free space is less than 3000 m above the surface.

Since I believe powering a Lunar base should be less exciting than exceeding previous antenna height records by a factor of four, there must be a better way.

The lunar surface is subjected to large temperature fluctuations. During the day, it can reach 127 C while at night it can plunge as low as -173 C, a total range of 300 C. In the PSRs it can be even colder, as low as -185 C.

In the lunar south pole region, a heat exchanger system between radiators positioned in shaded regions (even artificially shaded regions, such as in a pit surrounded by MLI) and absorbers placed in sunny regions could produce energy with a Carnot efficiency approaching 75%! Running the working fluid through a (very) large ice bath could ensure energy storage through the dark nights when the absorbers would be shaded.

To maintain a baseline power supply of 1 MW, a large supply of water could be converted to ice, generating 335 kJ/kg from the latent heat of fusion. There is little marginal benefit to preheating the water to near boiling or chilling the ice to lower temperatures. The total volume required is about 100,000 T, or 40 Olympic swimming pools. By comparison, *only* 2000 T of batteries would be required to do the same job, or a mere 10 dedicated Starship flights. So locally procured water would need to be found in vast, easy to access quantities. Still, having 100,000 T of thermal mass floating around is never a bad idea.

The coolant loop, operating between 100 K and 400 K would need something like pressurized supercritical ethane. Unfortunately a gravity fed heat pipe would be upside down in this case. I don’t know much about refrigerants, but using ethane as an example, it has a heat capacity of 2.28 kJ/kg K, or 684 kJ/kg over the 300 K temperature variation. At 75% Carnot efficiency, the coolant loop would need to move 2 kg/s to produce 1 MW of power. With a 16 cm (6.5″) diameter buried pipe and a flow rate of 1 m/s, the fluid would have to move about 5 km to transition between shaded and mostly sunny areas, for a total length of 10 km and a total fluid volume of 200 cubic meters, or roughly 200 T, just for coolant. (Or somewhat less if artificial PSRs can be constructed in a more convenient location).

Poiseuille’s Law enables an estimate of total pumping power at around 25 W for this point design.

Solar heating is fine but what if there was a more reliable way to provide the heat source fed to radiators in the PSRs? Some kind of magical metal that just emits thermal energy? And a stream of neutrons. A scaled up (or multiple) Kilopower reactor could deliver 1 MW of baseline power with less than 150 T of total mass, including 4 T of Uranium. Assembled in any convenient PSR, a nuclear reactor could supply power indefinitely, regardless of lunar phase. Indeed, if used away from polar regions they may be unable to provide much power during the day, due to the difficulty of radiating heat in direct sunlight. It turns out the Sun is also a nuclear reactor and it’s much bigger and hotter. And unlike Lunar water, refining Lunar uranium is probably easier than getting a launch permit for it.

The final crazy idea I’ll consider here is a variation on the space-based solar power trope. I have written more than once about how silly this idea is, but at its core is a concept that may finally have its day: remotely beamed microwave power.

Like peaks of eternal light, there are numerous places on the surface of the Moon, including close to polar regions, that are always in view of Earth. Indeed, anywhere on the Moon’s terminator there are places where the Earth appears to rise and set every few weeks.

Animation of scene depicted in “Red Moon” by Kim Stanley Robinson showing “Earthrise” from Petrov Crater.

Microwave antennas positioned here could receive power beamed up from the surface of the Earth from one of at least three stations such that one is always in view.

A 1 MW baseline could be safely transmitted at a power density of 100 W/m^2, implying a receiving area of about 200 m in width, once beam losses and conversion losses are taken into account. Achieving a beam that narrow (1/2000000 rad) from the Earth would require a phased array spaced out over a much larger area, at least 200 km at 5 Ghz (6cm). Each element of the phased array is a high power gyrotron with a steerable high gain antenna phase locked to a carrier transmitted from the Moon.

The advantages of this approach include the low cost of the surface infrastructure on the Moon and its relatively low complexity scaling. Increasing the power requires adding more gyrotrons to each transmitting site on Earth, which is much simpler than commissioning new nuclear power plants in places so cold that conventional metals used for robots get brittle.

The main disadvantage is that such an array is also a very effective anti-satellite weapon and could cause proliferation concerns.

Starship opens up the possibility of building a large, permanently occupied Lunar base. With this comes the unexpected but ultimate pleasant challenge of ensuring the base is adequately supplied with the electricity it needs to run its systems, no questions asked. How would you approach this problem?