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In a world geologically much like Dune, there's no less than 300m of sand above compacted sand-stone across the planet. An Earth example would be Rub Al Khali, where sands reach 300-350m above ground level.

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Cities ranging from 30,000 to 150,000 people are dotted across the planet. For purposes of travel, our protagonists need to travel under-sand, in order to avoid detection from orbit. Setting aside the technological issues related to the sustainability of the cities and the events going on, using our tech level, how can a crew of 30 people travel up to 50km at a time under sand?

EDIT: The detection is entirely by visible detection (no infrared, etc.) with a quality such as a 'live' google-maps.

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    $\begingroup$ Just because it's almost unbearably cute (and still relevant): have a look at this $\endgroup$ – Joe Bloggs Jan 18 '17 at 8:52
  • $\begingroup$ @JoeBloggs - Super cute; when I watch videos about our Earth and life, I think a lot of our Worldbuilding is right in front of us. $\endgroup$ – Mikey Jan 18 '17 at 9:41
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If they travel between big cities there could be tunnels conecting the cities and therefore allowing for travel between these cities.

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    $\begingroup$ This may be it. Any form of digging machine is going to leave a trail that can be seen from orbit. $\endgroup$ – John Jan 17 '17 at 19:58
  • $\begingroup$ I prefer them to move freely as they like, but if nobody can provide anything that would work, then I'll accept this as the best answer. $\endgroup$ – Mikey Jan 17 '17 at 20:31
  • $\begingroup$ @Mikey: They would not be able to move completely freely, but maybe you could make tunnels very common. So every city has at least one or two tunnels to other cities and there are nets of tunnels around the biggest cities. If you connect tunnels that would make for quite some movability. You might need to stretch "our technlevel" though, but they will focus on things like this if people lived in such an environment. $\endgroup$ – Secespitus Jan 20 '17 at 10:43
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Mechanical Sandworm

Edited to expound on motion as it's unclear

Basically, it's a drill that instead of pushing aside the sand, it pulls it through its body and this would allow for minimal displacement to avoid visual detection unless you're quite close to the surface. As well, due to its motion, it should be relatively soundless and could carry quite a few people depending on its length.

This can be accomplished by a series of treads to pull the body along on the outside as well as pull the sand through the middle. They wouldn't need to be all encompassing as the natural flow through the middle should keep the sand moving just by the outer treads. The crew/personal (and possible even controls quarters would likely be best handled as an American football-shaped cavity that would be attached to the outer shell that pulls it along. This shape would allow the sand to flow around it while still being attached to the outer treads.

In all honesty, you could probably just take a submarine body and strap some treads to the outer shell to pull it along while providing a tube-like shape to move through the sand. The greatest disadvantage with either of these designs is that turning at any sort of speed is pretty much impossible and the only way any sort of turning is accomplished is through different tread speeds. Of course, you could also stop completely and just have one side of the treads moving forward and the other in reverse, but it's still going to be a slow endeavor.

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  • $\begingroup$ How would you pull the sand through the body and at the same time transport humans in the body of the mechanical sandworm? At the moment I can't imagine the design of this reverse-drill $\endgroup$ – Secespitus Jan 17 '17 at 20:35
  • $\begingroup$ @Secespitus Much like a submarine: In very cramped quarters. Basically, you'd need to have football (American) shaped cavities to allow for sand to flow around these sections. Due to this, you'd displace a bit more sand, but still likely not enough to be visible on a live Google maps-like satellite imagery. $\endgroup$ – rangerike1363 Jan 17 '17 at 20:43
  • $\begingroup$ Ah, okay. Thanks for the clarification. Interesting idea. $\endgroup$ – Secespitus Jan 17 '17 at 20:59
  • $\begingroup$ @Secespitus - I edited my answer for clarity due to your questions so I hope that helps. $\endgroup$ – rangerike1363 Jan 17 '17 at 21:06
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    $\begingroup$ the main problem with that is that sand is extremely abbrasive. It's not like water where the main problem is keeping it out. $\endgroup$ – jwenting Jan 18 '17 at 14:47
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Sandstorms

Earth/Dune-type planets are nothing like Mars, our sandstorms are massive. One can use publicly available satellite imaging to plan the travel accordingly, so if there are only visible spectrum range satellites (or if the storm can block other wavelength), it will be very easy hide from them under the storm, moving along with it.

This traveling mode probably would allow the vessel to move half-"submerged" in sand while staying unnoticed, and if storms are occuring often enough, the vessel can submerge completely and wait for few days..

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I am not sure that a digging machine should unavoidably leave a trail. Perhaps it is possible to avoid recognizable trail by digging deep enough. Or your planet could have giant wormlike animals, who make trails indistinguishable from the digging machines.

But a mechanical sandworm has to overcome some technical issues:

  1. It has to endure the pressure of the sand, and thus have a heavy hull.

  2. Since the displacement of millions of tons of sand is quite energy-consuming. (The drag is extremely high there.) the machine would require a serious power plant (probably nuclear) to move freely. But the waste heat should go somewhere.

  3. They have to have some emergency systems, preferably smaller diggers as escape pods, to dig out the crew, if the digging system fails, and they get stuck.

  4. It has to protect its moving parts from sand penetrating the machinery, and scour or block it from inside.

  5. If the speed would be small, they would need some simple life support to survive while isolated from the atmosphere for extended period of time.

  6. Navigation, sensor and communication systems probably won't be too effective from under 250m of sand. Therefore they would have to rely on compass and inertial navigation. I am not sure, that even sonars will work. So they would probably prefer pre-mapped routes to avoid collisions with rock and stone-surface hills.

It seems to me, that such machine is possible, but will be monstrous and slow.

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  • $\begingroup$ This is why my initial answer involved pulling sand through the middle of the machine as the displacement would only be equivalent to the motion machinery and the travel compartments. You're also forgetting that due to its weight, sand shifts rapidly at the surface and if the displacement is small enough any trail would be rapidly covered/blown away. $\endgroup$ – rangerike1363 Jan 18 '17 at 15:04
  • $\begingroup$ Possibly you misunderstood me, I have said, that IT IS probably possible, to have trailless sanddigger, while John said, that it is not possible. But I mentioned great worms masking the trails, for the case if John could prove somehow, that it is indeed impossible. $\endgroup$ – b.Lorenz Jan 18 '17 at 15:15
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If there are no prebuilt tunnels, then your characters must dig the tunnel as they go, so you are talking about a Tunnel Boring Machine. The technology exists, especially when you think about using it to dig through sand. The main problems TBM machines have experienced in the past have to do with digging through rock.

Nor would such a machine necessarily leave a trail, if it is filling in the hole behind itself as it digs in front. If it is near the surface, yes, it will probably disturb things enough to leave a visible track. But if it goes deeper and fills in behind, there should be no question of leaving an observable trail.

I think you just need to do a little reading on TBM's.

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    $\begingroup$ Your link states, that you need slurry to dig through sand with a "slurry-separation plant, usually outside of the tunnel". Is it plausible to fill the hole behind, if you normally need to get water to the TBM? And I could'nt find anything about the speed of such a TBM. How long would the journey probably take? $\endgroup$ – Secespitus Jan 17 '17 at 21:31
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    $\begingroup$ You may need a little hand-waving to deal with some of the technical details. I'd omit the slurry (most of its need is because the material has to be expelled and not put back into the tunnel), and go with scoop-like drills digging out the sand in front, blowing to the back with fans, through conduits that go around the habitable space and reinforce the vehicle. In the back, you have to press the sand back into the tunnel, and you use that pressing to also propel the vehicle forward. $\endgroup$ – user11864 Jan 17 '17 at 21:49
  • $\begingroup$ Big Bertha moves at 10m per day (newatlas.com/worlds-largest-drilling-machine-bertha/28311). I'd theorize a much smaller radius and with the fact that you don't have to line the tunnel, you should be able to move faster. But I doubt that current technology could produce anything that would move at more than 10 to 20 miles per hour, if that. Dissipation of heat and exhaust would also be problems. I'd go with a Hydrogen combustion engine. Its only byproduct is water, which you can use to push the sand back into place at the back, and put your exhaust heat into it as well. $\endgroup$ – user11864 Jan 17 '17 at 21:54
  • $\begingroup$ @AgapwIesu You should add the comments to the answer. $\endgroup$ – T.J.L. Jan 18 '17 at 0:30
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Why set aside the technological issues related to the sustainability of the cities?
They actually solve your problem, quite nicely.


No need to go under the sand or use fancy tech:

Infrastructure is already in place

Major cities will already be connected, so your heroes simply must stow away on an existing transport system rather than spending the money to create their own.

  • Boats. In a desert world, cities will likely be abundant where there's water - similarly to the Egyptian Nile River, which supports ~the bulk of the population there. If the main transport system between cities is canals - and it may very well be - your characters can hide in or on boats. This has the added bonus of relatively quick travel compared to walking - and boats are harder to stop and search.
  • Roads. Some areas can't have sand that thick - like coastlines, watersheds, mountain ranges, and river deltas. These will be great candidates for road building, and, similarly to the above suggestion, your characters can hide in vehicles with relative ease.
  • Flight. If planes and helicopters exist, they must be able to cross great distances. There is always the possibility to use one.
  • Literally any existing transportation. If only visible-light detection is being used - with a handwave to identify the targets on sight - nearly any means of getting around will work. Disguises with prosthetics, closed vehicles in search-free zones, and secret compartments will all do the trick. Just like in real life, your characters can take advantage of what already exists! No need to bore expensive tunnels.
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Realistically, you would use the same approach opposing sides used during Cold War: if you know orbits and time of satellites passing your location, you can plan accordingly, and hide only when satellite is overhead. Then you can dig yourself down, and you won't need to go too deep.

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Chunnels

The Channel Tunnel is a construct that stretches beneath the sea, from the United Kingdom to the European continent. In a desert world, as-the-crow-flies travel will be the most efficient (dunes aren't exactly great for travel) - and tunnels through the rigid / firm sandstone will likely hold, with the benefit of being easier to bore than tunnels in firm oceanic crust.

Some estimates put the length of the tunnel at 50 kilometers, meeting the mark exactly.
This is no difficult feat engineeringwise, and the tunnels will probably already exist - as direct routes between cities - for ease of access.


Of course, all of this hiding business is pointless. See my other answer "Infrastructure is Already in Place" for a better solution.

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My answer may sound like a joke, but let's go anyways:

Travelling trough sand is incredibly hard. After all, it's all glass broken in very, very little pieces. Unless there is a very good reason to dig trough it all the way (other than "avoiding orbit detection"), I would recommend the cheapest, easiest way to go:

Give your crew 30 sand-bycicles (and a camo suit).

Because it's awesome.

The trails left behind the bikes will be almost invisible from orbit, if not totally covered by small winds in minutes. And because they are very small (and sand-colored), the members of the crew would be nearly unnoticeable on such a large surface, on the contrary of a ship that would remove tons of sand and leave a giant trail visible from space.

50 miles a day by bike may be a lot if the crew is not trained however. On a flat surface, riding a bike 45 miles is about the same in energy requirement as running 5 miles or so. Sand dunes being made of... well, sand, I guess it would take some training to ride the same distance within a day, so it's not a trek for everybody.

Also I assume the purpose of the ship is not just for the crew, but for good and supplies too. Each bike can have dual bags on each wheel, up to 30 liters capacity by bag. That's already 60 to 120 liters of gear by person (depending of their respective weight and training). That can go up to 3600 liters of goods for the whole crew. And I'm not even talking about bycicle trailers...

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