Monday, April 17, 2017

Does Lunar resource exploitation make sense?

Hello loyal reader(s). Although I haven't blogged now for a few months, that doesn't mean I've been doing nothing. On the contrary, I have been advancing several super cool projects and today I'm going to write about an aspect of one of them.

Every few years (roughly coinciding with congressional budgeting schedules) NASA gets antsy and proposes some new ideas. Recently, they have included the asteroid capture mission, the Europa lander mission, and all sorts of other cool concepts. On the crewed side, however, NASA is (and has been) stuck in an organizational quandary, wherein it is allocated just enough $$ to do what it has been doing, and not quite enough to make a solid start on any of its mandated new programs, such as the Mars mission.

I have written extensively about crewed Mars exploration in the past, and a distillation of much of that is kept at . The main problem with Mars exploration is that there is no way of doing it with existing rockets. Developing new rockets is expensive, large rockets particularly so, and so the hunt has always been on for finding smarter ways of getting more mass to (and from) Mars using rockets that fit, somehow, within the current budget. This is a conceptual mistake, in that huge new rockets are certainly expensive, but they are cheap compared to the programmatic costs incurred by having a rocket that while undeniably huge, is just not quite huge enough. I am reliably informed that similar cost inefficiencies can occur in other areas too!

This blog post deals with one particularly baroque proposal, namely the installation of a robotic fuel mining base and "gas station" on the Moon, to refuel spaceships on their way to other places. This proposal has been floating around for a while but has recently gotten a lot more attention than is, perhaps, warranted, hence this blog. The topic is quite arcane so I will do my best to keep the writing both concise and precise. First, I will summarize the results, then delve into entirely inappropriate levels of detail.

Much of space exploration advocacy is performed by way of analogies. Unfortunately there is no good analogy for this particular proposal, so instead I have used math to compute some best case cost estimates for Lunar resource exploitation, and compared them to the alternate method (Earth-launched resources) computed using median case cost estimates. This biases the comparison toward Lunar fuel, but will it be enough?

This table shows the per year cost for a program designed to deliver 100 metric tonnes of cargo (such as water) per year to various locations in cis-Lunar space. It also estimates the development and deployment time to reach rate after program start.

Earth originEarth originEarth originLunar originLunar origin
Δv (km/s)
Cost ($m/year) expendableCost ($m/year) reusableTime to reach rate (years)Cost ($m/year) reusableTime to reach rate (years)
Earth surfaceKSC0NA0.150.05NA>15
Low Earth orbitLEO9.43001202>1000x5>15
Geosynchronous transfer orbitGTO2.446002402>1000x5>15
Trans-Lunar injectionTLI0.687503002>1000x5>15
High lunar orbitHLO0.147503002>1000x5>15
Low lunar orbitLLO0.689003602>1000x4>15
Lunar surfaceLS1.7318007205>1000>10

The most optimistic cost estimate for the robotic Lunar port suggests costs of $1b/year for 15 years to reach rate, and that's what I've used in this graph. I think all reasonable experts would agree it's highly unlikely to cost less than that, or to reach rate (100T/year delivery to some location) faster than that. The xN quantities encode the fact that moving fuel from the Moon to other locations uses >75% of that fuel in delivery. So if $1b/year for 10 years is enough to produce 100T of water a year on the Moon, additional time and tech and fuel and money is required to move that fuel to, say, Low Earth Orbit.

In contrast, we see that using today's technology at today's prices, the same quantity of water (or any cargo) can be delivered from the Earth to all the same locations at a fraction of the cost and a fraction of the time. Employing reusable rockets, such as those currently being pioneered by SpaceX, may reduce costs even further, to the point that the cheapest, fastest way to get even raw materials on the Moon is to launch it from Earth instead of mining it locally.

Before I dive into the nitty gritty, it is worth stating that a similar analysis focused on the use of Mars' atmosphere (rather than the deep frozen heavy metal-laced dirty snow of the moon) for propellant production shows a clear advantage over launching all the fuel required for the Mars-Earth trip from the Earth. 

Now I can dive into the nitty gritty. First I'm going to write about the why, then I'm going to write about the how.

A really good rocket can launch about 4% of its initial mass into low Earth orbit (LEO). For the Saturn V (the most powerful rocket ever built), the orbital payload was about 140T. To get from LEO to the moon, Mars, or elsewhere, yet more fuel has to be burned. For LEO-Mars, around 25% of the LEO mass can be payload, the rest has to be fuel and oxidizer. At this point even the Saturn V can launch only 35T to Mars and that's not really enough to keep four brave astronauts alive for a three year mission and then bring them back.

Instead, the 140T in LEO can be the payload and spaceship with empty tanks. 3 more launches of the Saturn V can increase its mass to 560T, at which point it has enough fuel to fly to Mars with 140T of payload, which is much better.

Unfortunately, four launches of the Saturn V is much more expensive than one, and building a rocket 4x bigger than the Saturn V, while exciting, is not part of the solution space NASA is presently looking at, possibly because the manufacturing facility at Marshall Space Center in Alabama couldn't fit it through the door. 

If ~400T of propellant is needed in LEO, however, perhaps it could be obtained from the Moon? But how? Remember that the baseline expense case is three more launches of an already existing launch vehicle, so any alternate scheme should be some combination of cheaper, safer, faster, or more scale-able.

The best Lunar resource extraction architecture I've come across so far looks something like this.

The following new robotic vehicles are developed on Earth:
- A solar electric propelled orbital tug.
- A hydrogen/oxygen powered lunar orbital shuttle and lander, based on the Centaur upper stage.
- A solar powered fuel processing plant with some capacity for remachining or replacing worn out components.
- A Lunar orbital nanosat platform containing numerous guidable lead or steel rods.
- A battery powered combine harvester robot that ingests lunar regolith.
- A battery powered generic transfer truck with robot arms and useful tools.
- A solar powered deep space electrolysis cryogenic fuel depot. 

The lunar components (in sufficient numbers) are deployed near one of the permanently shadowed regions at the lunar pole, landing on the landing vehicle. The orbital nanosats deorbit cavalcades of dense metal rods to precisely impact the mine site, performing a kinetic drill and blast procedure. The combine harvesters scoop up the fractured regolith, physically process it for water and other volatiles, and transfer the ore to shuttle trucks while dumping the depleted material, which can also be used (eg sintered) to make roads or landing pads. The trucks shuttle the physically separated ore back to the fuel processing plant, which performs chemical separation and packages water ice in aluminized mylar coated pallets for transportation. It also performs limited electrolysis to make fuel for the lunar orbital shuttle's ascent flight.

The shuttle flies the water ice to low Lunar orbit, depositing it at one of the deep space fuel depots, refuels with electrolysed fuel from that depot, and returns to the lunar surface. That part of the operation has a mass efficiency of just 20%. That is, 80% of the extracted water is used propelling the shuttle to and from the Lunar orbital depot. Hydrogen boiloff may be mitigated by (eg) platinum catalysis and conversion back to water.

Non-hydrolyzed water ice is collected at the lunar orbital fuel depot and transported by solar electric tug back to low Earth orbit, consuming a relatively trivial fuel fraction but taking at least several weeks. Water ice is stockpiled at the low Earth orbiting depot(s), which must hydrolyse it all in time for the required launch to Mars, or wherever, and requiring huge solar arrays to do so. 

There are numerous other proposed systems which are less mass or time efficient, or have less overall benefit. As an example, it may be possible to fly a Mars vehicle to land itself on the Moon, refuel there, and then fly on to Mars. However, it would take less fuel to fly from LEO to Mars directly. Similarly, the mass benefit of any post-launch refueling drops off extremely quickly for any depot beyond LEO. Although the Moon has relatively low gravity, its lack of an atmosphere extracts a toll in both directions; launch and landing.

If the above scheme for mining propellant from the moon sounds complicated, that's because it is! In fact, of millions of potential failure modes, the net outcome is the same - not enough water delivered to LEO, or even none at all. To mitigate the programmatic risk for the crewed flight to Mars, a mechanism for the delivery of water from Earth to top up the LEO-based solar powered fuel depot must be provisioned for. At which point, of course, it is (by the table above) far cheaper and quicker to cancel the lunar program entirely and refuel the depot, or the Mars vehicle itself, using that same Earth-launched mechanism. 

I really do not believe there is much more to say about the Lunar-derived fueling concept. Here are some links to other resources if, for some reason, your curiosity is not entirely sated.