# The impacts of the Moon's limited gravity on the design of a Moon city's transportation network

In the near future mankind could well permanently settle on the Moon.

Whether for research or mining or just good old fashioned novelty is irrelevant.

The colony includes:

• Large domed structures for farming and recreation. The largest dome on Earth (according to Wikipedia) is the Singapore National Stadium coming in at 310 m (1017 ft) in diameter. With the reduced gravity and potential advances in material sciences, I am going to place the diameter of the domes at 1000 meters (3280 ft). There are currently 20 domes, spaced roughly 2 miles apart (each is at least 2 miles from any other dome).

• Underground domiciles form the majority of living spaces. Most domicile spaces are 3.65M × 3.65M (12 ft × 12 ft) and house two people per room. There are 12,000 people on the Moon colony.

• A central command center, which also serves as a hardened evacuation site in the event of some catastrophic event – a meteor strike, perhaps. The command center cannot hold more than 2,500 people.

How would you manage the transportation of goods and people with this setup?

I am primarily looking for methods of travel that make use of the reduced gravity to move things differently than on Earth.

A more specific bonus question...could you make the tube system work for humans?

• Catapult :) You could lob robust stuff like canned foods and machine parts back and forth with catapults and a catching net system. But this is also taking advantage of the airlessness, and so will require airlocks on each end and that complexity and cost may outweigh any potential gains. But its cool – Innovine Feb 22 '17 at 16:41
• "could you make the tube system work for humans?" - do you mean Futurama like system? – Yuriy S Feb 22 '17 at 17:47
• (1) I appreciate your use of both measurement systems (English and metric).  I suggest that you use them both throughout, and choose one of them to use consistently as your primary one.  (2) Are you suggesting two-person apartments that are 144 ft²?  I realize that living accommodations on the Moon will need to be Spartan, but some homes on Earth have closets larger than that.  (3) And by the way, did you mean “Underground domiciles for the majority of living spaces” or “Underground domiciles form the majority of living spaces”?  … (Cont’d) – Peregrine Rook Feb 22 '17 at 18:41
• (Cont’d) …  (4) I find it a little odd that you ask about “the tube system” without having said anything about it (i.e., not even having stipulated its existence). (5) Could we make the tube system work for humans? I hear that London and a number of other Terran cities have done exactly that, long ago. What impediments do you foresee to doing the same on the Moon? – Peregrine Rook Feb 22 '17 at 18:42
• @PeregrineRook Thank you...I had intended to add a picture about the tube system but I forgot! I will clean up the math as well – James Feb 22 '17 at 18:54

The answer is: trains carrying containers, just like we do it on Earth. Trains might seem boring and not very sci-fi, but they are actually super bad-ass and very sci-fi for a number of reasons:

SIMPLE IS GOOD

The general rule with engineering is 'less moving parts'. Trains are extremely straight forward and very well understood. They run in one dimension only, forward and backward, reducing the complexity of transport compared to other vehicles by multiple magnitudes. Compare them to ground vehicles, that run in two dimensions, or air vehicles, that run in three, and you start to magnify the complexity of moving something from one place to another exponentially. Also Moon bases will likely be fairly static; its not like you can just go start a village somewhere, so the loss of freedom from moving on a single plane is not that much of a big deal.

The benefits of working on only one dimension cannot be stressed enough, it eliminates SO many engineering problems its not even fair to compare them to other transport types. At a basic level, trains are three components: a drive wheel, engine and battery. This simplicity makes repairs and maintenance easier and more accessible to more people, which is good in an environment like the moon where help (ie. Earth) is always far away.

Trains are highly reliable, efficient and can move huge volumes with minimal oversight and energy expenditure. While some deliveries on Earth require planes, drones or couriers to do last mile fast deliveries, 95% of transport is slow and planned weeks in advance, and being on the moon wouldn't change that. Food, supplies, oxygen and water needs to be able to be moved around in volume, over and over again. One of the rules of good design is 'don't solve problems you don't have', so trains fit the bill here quite well.

Another important factor is that, unlike newer technology, trains can be easily automated to run 24/7 without any risk of human error. This means trains can operate in a vacuum without any risk to drivers. Even the loading and unloading can occur automatically, in multiple airlocks to move from vacuum to pressurized without any risk to the integrity of the station. All of this is existing, proven technology, which is very important when attempting anything in a new environment; in a vacuum everything needs to be simple, clear, easy to use and understand. The less time you spend trying to invent new technology, and more time you spend coming up with weird ways your current tech could catastrophically fail and fix it, the better.

SAFETY IS PARAMOUNT

In terms of safety, trains also win hands down. Even on earth, the dominant factors in our choice of transport is safety and cost. If given a choice between a ride in a train to the center of your local city and hiring a helicopter to pick you up from your backyard, I know which one the average Earth resident would choose. If given a choice between hurtling at 700km/h across the moonscape in a tube, inches from a deadly vacuum and sitting safely in a cheap ass train, I know which one moon travellers will choose.

Humans are irrationally risk averse, and like to tell scary stories to themselves, even when the data demonstrates otherwise. The classic example is air travel, which is incredibly safe statistically, but lots of people are still afraid of flying. So they get in a car and drive it themselves, which is incredibly dangerous, but its not about what the technology IS, it's how its PERCEIVED. So while new transport methods might be very safe in principle, I find it hard to believe that humans will just get in board all at once, especially when there are safer modes available that give more PERCEIVED control over their own personal safety.

Then consider that every scrap of material up there either has to be transported from earth or synthesized from moon rock, and you end up with Occam's Razor cutting out almost everything by the cheapest, most efficient solutions that do the job and nothing more.

Also from what I have heard from astronauts on YouTube, the vacuum of space is freaking terrifying. It is like living in a world filled with sarin gas, and the slightest crack in a window or opening the door the wrong way will kill you and everyone you love. You can't ever, ever make assumptions or cut corners with technology, because then you die. This is a mantra often seen in divers or parachutists, because they live with real danger every second they are in that environment. I would be prepared to bet that on the moon that no-one will ever go anywhere in anything but the slowest, safest mode of transport possible, because they live in perpetual danger of turning into Total Recall Arnold.

INERTIA IS A BITCH

In close to zero G, the biggest danger is not going fast, its not being able to stop. On Earth, we can rely on gravity to provide natural fail-safes; if something is going too fast, then at some point it will fall to the ground and lose its inertia almost immediately through friction. If you stop providing power, gravity will hold your wheels to the ground for you. Even if you hit something, and break up, all those bits eventually hit the ground and stop moving. So will the things you hit, and so on.

This is not the case on the moon, without gravity or an atmosphere to slow things down, turning all moving objects into deadly weapons. For example, a vehicle or space craft which loses power en route is not just stranded in the middle of a moon wasteland; they are a bullet targeted straight at their destination with no means of stopping. There's not much you can do to deflect their path short of shooting them into pieces, which themselves might just become more bullets. The things those bullets hit become more bullets, and so on forever.

For the reason above, trains are the best solution. They are/can be stuck to rails on the ground through physical means, to prevent them going AWOL. Contact with the rails ensure they have a means of generating friction should something go wrong. Even trains on earth have dead-man switches, and trains on the Moon would be no different. Their fixed nature makes it trivial to put in guards or barriers to prevent shrapnel spreading in the event of a crash. When you put the brakes on, you are slowing down the whole train equally. And you always, always always know where they are pointed, and where all the bits might be able to go. That allows you to practice good design and set up entire transport routes and facilities to mitigate risk, reducing the risk of bulletisation to zero (a good number).

One of the most overlooked factor in the efficiency of transport is not the miles per gallon of the vehicle, but the time spent loading and unloading. Goods in a container might spend a week on a boat going from China to the US, but they spend another week or more sitting in docks, being shuttled around and stored safely. The energy invested in manning, operating and organisation the docks and workers on the ends of transport routes heavily outweighs the actual hauling costs. So you should design you transport solution to fit your logistics, not the other way around.

Containers on trains are one of the fastest, most efficient ways of getting goods on and off a vehicle; if you have ever played Factorio, you will see a basic example of how straight forwards. Train comes in, all containers replaced in ONE swift action, train leaves, repeat. Again, their one dimensional nature makes the entire operation technically simple, which is great.

This is also made easier by the fact that the Moon has little to none of the space and storage concerns we have here on Earth that can make higher density transport options more economically viable. You can just stick and stack containers anywhere in a vacuum, they aren't going anywhere.

IN SUMMARY - TRAINS ARE SICK

All in all, old fashioned rail trains are the perfect vehicle for an environment where safety, reliability and efficiency is everything. You might be tempted to try and impress your audience with fancy transport options, but like so many thinks in life the stuff we use right now is METAL AS F**K when you really drill down into it. It's up to you to sell the story to your audience in an engaging way. I am sure than when we end up going to space properly, we would build nothing but trains everywhere if we could.

EXTRA PERSONAL OPINIONS

As a species we are driven primarily by a desire for efficiency and safety; how much can we get done with our limited brains and muscles, without getting ourselves killed, so we can chill out as much as possible. I think good science fiction should reflect that sentiment and some of my favourite sci-fi books are the ones where there are no flying cars, magical transport tubes or fancy AI bullshit that just makes things happen. They are filled with technology that seems designed to fulfill a function, not impress the reader. They are a vehicle for the desires and needs of the characters. I am more impressed by an author that seems to have understood her setting and done the research to fit it than those who want to talk about killer robots and then McGuffin their way to it. That's lame.

• Welcome to WorldBuilding.SE Ucinorn! Cool answer. If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! – Secespitus Jun 27 '17 at 9:36

# Maglev Trains

They wouldn't worry about a piffling thing like gravity

The principle of magnetic levitation is that a vehicle can be suspended and propelled on a guidance track made with magnets. The vehicle on top of the track may be propelled with the help of a linear induction motor. Although the vehicle does not use steel wheels on a steel rail they are still referred to as trains as by definition they are a long chain of vehicles which travel in the same direction. This is the definition of a MAGLEV train.

So, being magnetically linked to the rail (and anchored) prevents gravity from being an issue. Lower gravity should make them more efficient, I expect...

Interior view:

• Are you suggesting that passengers in a maglev vehicle would experience zero gravity while in the vehicle?  Why?  They aren’t magnetic; they aren’t being levitated. And P.S. You should probably attribute the source of the vomit comet image as well. – Peregrine Rook Feb 22 '17 at 17:40
• When designing a maglev, you do indeed have to take gravity into account in order to calculate the balance of forces on the train, so the body is in equilibrium. – HDE 226868 Feb 22 '17 at 18:06

# No need for rails on the moon.

One option is a suborbital mass driver.

You take a mass driver that can accelerate a payload very precisely.

You could use it to get payloads into orbit. But another option is to fire payloads to a destination.

You don't need to worry about atmosphere or wind since the atmosphere is extremely thin, barely there. So there's almost no drag.

You can then "catch" the projectile as it enters the mouth of another mass driver at the destination.

Of course you would have to trust your engineers a great deal. One mistake and you could find yourself even slightly off target and on course for the side of a mountain. it's possible that a system like this might include some emergency system in the craft with a small stock of propellant for correcting their course if they're slightly off target. Or it might be used for cargo but not humans.

Depending on the distance you might need to accelerate your cargo up to something a little under escape velocity like 2 km per second.

Healthy Humans lying flat can withstand something like 10g for a few seconds. To accelerate up to close to 2 km per second at 10g, that means your mass drivers would need to be a few km long at either end.

Think of it as a little like a maglev train only without any tracks for most of the journey and with hellish acceleration at the start and end of the journey for about 20 seconds.

• I was going to suggest that vehicles travel in circular orbits with a radius equal to the radius of the moon plus epsilon.  Of course you would need to be very careful that your trajectory not impact a mountain.  It makes a lot more sense to spend a few more cents on energy to achieve an eccentricity of 0.01, so you wouldn’t come so close to collision.  On the other hand, with that approach, you would need to be very very sure that the “catching” apparatus is operational.  The circular orbit is fail safe; if you can’t land, you can keep on flying without needing to do anything.  … (Cont’d) – Peregrine Rook Feb 22 '17 at 19:22
• (Cont’d) … Regarding course correction, it occurs to me that each catching station should have an electromagnet array to adjust the vehicle’s trajectory laterally — although, with the projectile traveling faster than 6000 km/hr, I don’t know how much they could deflect it if it was off-course. – Peregrine Rook Feb 22 '17 at 19:22

If you are looking for a means of transportation inside the domes, then bicycles is the right call. If trains are already overlooked, bikes then are nearly completely neglected. They are relatively cheap to produce and basically don't need infrastructure. They also don't need energy to operate and if the dome is only 1 km in diameter then everything is really close. The reduction of friction also helps, and the people are kind of forced to exercise, what is good in space, people in low G really need to exercise. They are also really small, something good in the crowded place that the people live.

I remember reading somewhere that bikes are being used in those big airports here on earth because of those features, and that they are a really smart way of moving if we ever go to space, but I could not find that report (maybe it was a video, possibly vsause or something like that) anywhere, sorry for that, if anyone finds that out, please show me, it makes a long time I've seen it.

The biggest problem with trains is the cost to make the rails and the amount of metal you have to put on them, something that is still not a lot compared with other means of transportation. If you need to have these big resources coming and going, the trains are the best option indeed, but if what you look for is people's mobility, bikes are just fine.

The thing with using bikes to dome-to-dome transportation is how far would you have to travel and the fact that you would need to make highway-like bike path. But since you only have 20 domes 2 miles apart each, if you put the in line and run through end to end, you would have to go through 82 km (38 miles = 62km, plus the distance inside the domes), a lot but still possible. If you were not to put them in a line but instead in any other shape (two lines, three lines, triangles, hexagons, ...) the maximum distance you would have to travel would decrease a lot, although you would have to build more roads. You can find the mid term that is best for you.

Other answers have very nice ideas, I just want to point out a few things that may not be obvious (and a quick perusal of previous answers didn't show them up).

• Low gravity means low friction, a "conventional" train wouldn't work because mass (and thus inertia) is still there, but friction is proportional to weight, and thus would be very much reduced; wheels would skid without transmitting much impulse to the convoy. Some linear motor, maglev or double-wheel with compression would be necessary.
• Same applies to curves in any direction, you need the same centripetal force, but using steering wheels simply wouldn't work.
• The same applies also to passengers; being less anchored to ground they would feel any speed variation much more.
• In this condition i think (relatively) small bubbles suspended to cables (or, better, to a suspended rail) are the best solution as they would swing freely and thus the resulting force would always be toward the floor.
• Stairs are something quite dangerous and should be avoided at all costs; even in Earth gravity trying to run down a ramp can have "interesting effects"; on the Moon it would be very easy to run straight into thin air following a parabolic trajectory which is guaranteed to intercept with the stairs (if they are straight), but possibly many meters down.
• Similar effects are expected for any vertical change of direction, all effects of any bump on the way are going to be greatly amplified, so special care to avoid them should be taken.

Giant ramps/chutes.

I would have ramps going from the top/sides of domes to the base/recieving station at the other domes.

It's low tech, but it has several advantages. It isolates all the moving parts/intelligence in sending dome, with nothing to break down on route. These ramps can easily go over craters/etc, and ignore the geography below. Received objects (at least if not alive) can be queued at the recieving dome, and automatically will move to the next unload spot.

The lower gravity helps make it more practical than on earth. Its easier to built a strong ramp since there are fewer forces. It's going to take less energy to bring the "to be sent" objects up to the top of the chute. And earth doesn't have a lot of places that a 2mi jump to the next waypoint makes sense.

The chutes don't even have to be built that tall -- if you expect to send wheeled vehicles down the chute, it would need to drop 1m for every 500m of travel to move just with gravity. If you slide things down a teflon coated chute, it needs to drop 20m for every 500m of travel. Both of these are easily achievable.

Since gravity is overcoming friction already, you can add a nice impulse at the beginning to quickly move objects.

The chutes can be open (airless, less friction, probably automatic sterilization) or closed (allows transport of air, airlock free)

As a bonus, they can shaped like Futurama tubes (with Teflon coatings) and the descent shouldn't be noticeable.

To connect even more domes, you could have chutes that start in dome A went by dome B (and could have things snagged from it/added to it) and then continue to dome C, etc. That could be stretched as far as you want as long as it drops as specified above. Or just build the 380 ramps to have a direct dome to dome ramp for each pair.

• Welcome to WorldBuilding.SE Jesse! If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! – Secespitus Jun 29 '17 at 6:27