Building Infrastructure in Space: The Lunar Mass Driver
Most of the discussion of space exploration revolves around getting to Mars or the Moon, establishing a colony, and then the story sort of trails off… The actual infrastructure of what is needed to develop human presence in space goes curiously unremarked. There seems to be a tacit assumption that, if you establish a Mars Colony, the forces necessary to supply and sustain it, along with all the necessary technologies, will simply come. But I think we can do better.
We should envision an actual economy in space, a logistical system, that is capable of powering ambitious exploration. If you want to build something at scale, you don’t start with the end product; you have to collect the right tools and infrastructure first.
One of the key problems of space exploration and development is the massive cost of sending mass into orbit from Earth. Efforts to reduce this, such as by SpaceX, will help. However it is a bit strange; there is plenty of mass already in space. Why not harness resources already in space, rather than launch them? Which resource do we need most badly in space?
Rocket Fuel.
A space business that merely parks rocket fuel in high Earth Orbit would create massive value and open up new opportunities. Critically, the fuel mass should not originate from Earth, it should be sourced from elsewhere in the Solar System.
One excellent option for rocket fuel is ice, such as from the Moon’s surface. If it could be launched from the Moon into Earth Orbit, the water could be broken down into Hydrogen and Oxygen to power chemical rockets in future exploration. This fuel would come in handy in many ways. They could power long-distance vessels to Mars or the asteroid belt. They could fuel high-thrust engines necessary for landing on the Moon or Mars. They could even fuel rockets returning to Earth, sparing the need for expensive and heavy heat shielding. With enough Delta-V, heat shielding is not required; one can merely decelerate before hitting the atmosphere.
The real distances in space are measured in Delta-V, not actual distance. If fuel beyond Earth orbit becomes very cheap, then everything is effectively much closer.
The mathematical advantage is very clear. Consider two ships, both launched from Earth to land on the Martian surface. Ship A carries all the fuel it needs to reach Mars. Ship B refuels in Earth Orbit with Hydrogen/Oxygen from the Lunar surface. Ship A will need to be exponentially larger and more expensive than Ship B. Sure Ship B needed all that infrastructure to fetch ice in orbit. But that system is mostly reusable; thus the cost can be amortized over many ships.
Lunar ice is potentially much easier to source than rocket fuel from Earth. This may seem surprising at first. However consider the physics. The Moon has far less gravity than the Earth. It also has no atmosphere. One of the greatest difficulties of launching payloads from Earth is the thick atmosphere. But on the Moon, we could simply build a large linear accelerator (otherwise known as a mass driver). Built over miles, it could accelerate a payload into Lunar escape velocity, and to enter Earth orbit. Since the payload is launched via traction with the Moon itself, it is exponentially more efficient than a rocket launch, and can simply be powered with local electricity. Once the payload is launched from the Moon, it would enter high Earth Orbit. Further positioning of the payload into a customized Earth orbit would require a dedicated spaceship, which must be returned to the Moon. But the process could be automated and require minimal fuel compared to Earth launch.
The linear accelerator on the Moon could come in several forms. A massive cannon could launch a large block at high-g’s. A longer track, spanning many kilometers and accelerated via a maglev, could provide a gentler acceleration that would also be survivable by humans. Building such a device, and installing the requisite infrastructure to power and maintain it, would be a huge up-front cost. But it’s the right approach because Physics ultimately makes it easier to transfer mass in low-gravity, vacuum environments. Earth is really one of the hardest possible places to operate in; unfortunately we start the journey here.
Depositing water ice into high Earth orbit would be a game changer. It could simply be sold on the open market. A ship from Earth would simply rendezvous with the payload and process it into Hydrogen/Oxygen rocket fuel. This process requires energy. But energy can be efficiently transported into orbit, whether in the form of solar panels, or nuclear fuel. To achieve space propulsion, you need propellant mass and energy density. Usually these concepts are united, but under this scheme, they are disentangled; the energy density comes from Earth efficiently, the propellant mass comes from the Moon efficiently.
Materials besides water ice could similarly be deposited into high Earth orbit, where they would remain stably for centuries. Basic materials, like iron sheets, or 3D-printed structures made from Carbon, all extracted from the Lunar regolith, could be deployed via a Lunar mass driver en masse.
Once fuel and basic materials can be manufactured in space, the opportunities grow exponentially. We could send an ever-decreasing number of advanced elements directly from Earth: only those that we are not yet capable of manufacturing in space. Unlike the economy on Earth, the priority for establishing manufacturing capability in space would go by the mass of the item produced, in addition to its complexity. My guess is that the heaviest parts of spaceships we will want to build will be those of relatively low complexity: rocket fuel and structural elements. The complex, hard to build parts, like computer chips, are generally lightweight and will be launched from Earth.
Rather than travel directly to Mars, it could make sense to first establish a rudimentary manufacturing capability in space. We can utilize the unique advantages of easily launching basic but heavy materials from the Lunar surface. The Moon would only be the beginning. Things traveling in deep space can be transported much more efficiently than direct Earth launch, despite the vast disparity in distances. Our human mind is trained to associate distance with effort; but in space this is simply not so.
Establishing manufacturing capability in space for our bulkiest items, like rocket fuel, is crucial to making our exploration efforts affordable and scalable. We should take advantage of the unique delivery options available to us once we are in space, like a Lunar Mass Driver. Creating this sort of infrastructure is likely the highest return-on-investment activity we can achieve for furthering space exploration, and eventually settlement.
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