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Welcome to the seventeenth Dev Diary entry for Space Station Designer! In our last entry, we covered the history and Newtonian physics behind the Space Elevator concept. Now, if the math works out perfectly, you might be wondering: ‘why don’t we have a Space Elevator on Earth right now’?
Today, we are looking at the complex logistics of such a megaproject and the peculiarities of tether dynamics. And most importantly, why choosing the Moon instead of the Earth actually makes many things easier.
Even in the "easier" case of a Lunar Space Elevator, we are still talking about a tether roughly 60,000 km long. The mass involved is staggering. We currently know how to manufacture advanced, super-strong materials like Carbon Nanotubes (CNTs) in laboratories and in limited quantities which makes them expensive, and assembling these into a 60,000 km continuous ribbon is an entirely different challenge. It would require an unprecedented industrial effort, and international cooperation.
Carbon Nanotube (CNT) macroscopic structure
More conventional materials like steel simply don’t work. They are too fragile to endure the forces involved and too heavy to be practical. In the case of the Earth Space Elevator, we simply currently know no material strong enough to endure the forces involved.
The physics of tethers in space is especially tricky because they tend not to behave as you might expect.
Anyone who has tried rock climbing or alpinism knows the struggle of preventing a rope from spinning. While skill and athleticism can solve this on a mountain face, you run into severe difficulties when your tether's length is measured in tens of thousands of kilometers.
The only real "tether in space" experiment we know of was conducted during the Gemini XI mission in 1966, and it was no walk in the park. Both spacecraft (the Gemini capsule and the Agena target vehicle) had to burn a considerable amount of fuel just to keep their attitude under control and maintain an extremely tiny artificial gravity (0.00015 g) while tethered together.
View of the Agena target vehicle from the Gemini XI spacecraft (NASA). Note the tether connecting them, not under tension at that moment.
You might have seen movies like Gravity or Alone on Mars where the astronaut reaches the ends of his lifeline and is brutally and uncontrollably pulled back in the opposite direction, while hopelessly spinning. This is a slightly dramatic but overall quite realistic depiction of how tethers tend to behave in space.
First, we have to remember that everything must be sent from the Earth up to space using ‘old-fashioned’ chemical rockets. And fitting 60,000 km of ribbon into a single fairing is simply impossible. Even if it was, no launcher would be powerful enough. So you would need to patiently launch the tether coil by coil, and carefully fuse those coils together in the vacuum of space without creating weak points in the ribbon. It means having a dedicated space station where these tasks can be conducted in a relatively safe environment.
The unique advantages of the Lunar environment
The Moon is extremely light compared to Earth, meaning the gravity binding us to the surface is much lower. We’ve all seen videos of Apollo astronauts hopping around, but few realize their suits were heavier than late-medieval knight armor (100kg+). That’s because the gravity at the surface of the Moon is only 0.165G, or roughly the sixth of what we experience on Earth.
For our elevator, this much lower gravity means that the mechanical stress applied to the tether is drastically reduced. While no known material is adequate for an Earth Space Elevator, Carbon Nanotubes could theoretically handle the forces involved in the context of a Lunar Space Elevator.
The Moon exists in a vacuum, which simplifies engineering a lot. On Earth, a tether would be subjected to atmospheric drag, and the wind would carry humidity (turning into razor-sharp ice crystals at high altitudes), volcanic ash, and corrosive human emissions. The Moon spares our tether from such hazards. The only real, unpredictable troublemakers are micrometeoroids that can punch tiny holes through the ribbon, making it weaker and more prone to snap. That’s why a redundant, parallel design of 3 ribbons is reasonable in terms of safety.
Three parallel tethers would ensure a good redundancy. They would together form ‘the ribbon’.
The Earth-Moon Lagrange Point 1 (aka. ‘EML1’) is a spot in space where the gravitational pulls of the Earth and the Moon neutralize each other. It is obviously an interesting location to anchor our tether and build a Space Station.
Sending payloads to EML1 is much easier than reaching Low Lunar Orbit (LLO) like the Apollo missions did. It requires less fuel because you aren't fighting your way deep into the Moon’s ‘gravitational well’ (or ‘Sphere of Influence’); you just have to reach its edge. The transfer from Low Earth Orbit to EML1 takes a relatively long and slow trip, but it is highly fuel-efficient. Once there, you simply dock at the station, unload your cargo, and send it down the elevator.
The Earth, The Moon, and the Earth-Moon Lagrange Point 1 or ‘EML1’ (not to scale).
From a gameplay standpoint, we wanted to reflect these logistical challenges without making the process tedious. In Space Station Designer, we decided it would be simpler to manufacture the Carbon Nanotubes directly in space.
Instead of launching completed ribbon segments from Earth, you will set up a dedicated production line in space, specifically in Geosynchronous Orbit (GSO). You will still need to manage the massive logistical chain of launching raw Earth materials to your space factory, and our design philosophy was that the more a material is ‘advanced’ or ‘complex’, the more ‘basic’ materials it requires (along with raw materials) to be produced. So as you get closer to the endgame, you’ll have to manage a ‘production pyramid’, which will require planning things in advance.
The in-game ‘recipe’ for manufacturing a single unit of Carbon Nanotubes.
But how to transfer all these ribbon coils (remember, there’s more than 60,000 km of ribbon!) to EML1 ? That would sure require a lot of rocket fuel, and that’s where we need something more efficient than conventional rockets. Hence the Nuclear-Thermal space tug, some kind of freighter which would be assembled in space and whose only purpose would be to ferry cargo between GSO and EML1.
Nuclear-Thermal Rocket (NTR) engines have been successfully tested in the 1960’s (see NERVA, KIWI…) so the technology is ‘off the shelf’. These are at least twice as efficient as chemical engines, as they heat fuel to a point that completely removes the necessity of using an oxidizer. Operating from GSO is also far enough from Earth to worry about the potential accidental contamination. Of course, such a spacecraft has to be serviced with care, and I can tell you its dedicated docking bay in the GSO station is quite a sight!
NERVA Nuclear-Thermal Rocket engine or ‘NTR’, NASA, 1970.
Enough for today! In the next and final diary of this series, we’ll dive into what this megaproject actually adds to the late-game experience, how the player interacts with it, and talk a bit about something you find almost only on the Moon and that makes the Lunar Elevator a potential gold mine: Helium-3.
Space Station Designer will be released on Steam the 18th of November! Wishlist it now: https://www6.slitherine.com/game/space-station-designer
--Ignacio, Mauricio, and Nicolas (Polar Motion)
