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Note: Some answers will refer to not just tanks, as previously I asked about military vehicles in general, then edited it because it was too broad.

So, right now I'm working on a project centred around a speculative sapient race that lives on a planet with no landforms larger than Big Island, Hawaii. In terms of anatomy, they are serpentine (Quite similar to Earth's moray eels, superficially.), except for two five-digit claws, which evolved from bony 'fins' and are used as manipulators.

Their technology is about as advanced and capable as human technology, but it progressed without any form of fire, of course. They achieved metallurgy via electroplating and electroforming, pulling metal ions out of the water to a mandrel on the surface with their bioelectricity. So, what I want to know is what you think their equivalent of tanks would look like.

Basically, I'd like you to help me envision a submarine vehicle that is heavily armoured, has a big gun (Firing self-propelled weapons, of course.), and operates on the front line like a modern tank, but also works underwater, is streamlined, and conforms to all the other requirements for aquatic vehicles. It should be big enough to accommodate a small crew (About 3 or so.) of 3-4 metre long serpentine creatures, and would probably need to have some form of defense on all sides, since the ocean is a 3D environment.

As for the military doctrine, I don't really mind - let's say Leopard-style for now, I can adapt it later to other doctrines.

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    $\begingroup$ Metallurgy could not I think be developed in that way. Developing the ability to smelt material in a gas is non-negotiable from a practical point of view. Electroplating and similar tech won't cut the mustard. The rules of chemistry and physics won't change. Also note that salt water is highly corrosive and more conductive than air - I cannot see them avoiding the need to develop a method to do chemical and industrial processes in sealed gaseous environments. $\endgroup$ – StephenG Mar 31 '18 at 2:59
  • $\begingroup$ you need to define what the purpose of the vehicle is, vehicles are designed for a purpose which massively influence the design, and as it stand this question is way to broad and requires us to come up with an entire technological base of the society as well, that's two big problems with the question. Your best bet is to come up with your own design and ask if it plausible. $\endgroup$ – John Mar 31 '18 at 3:00
  • $\begingroup$ @StevenG - Yes, there is quite a lot of debate as to how underwater metallurgy would be achieved. My very basic understanding of electroplating is that the creatures use their bioelectricity (Sorry, I forgot to mention that in the description) to pull metal ions out of the water to a mandrel on the surface. If that's implausible, then I think, as said, that I'm too far down the line of developing this race to redesign the basis of their technology. Anyway, thanks for contributing. $\endgroup$ – SealBoi Mar 31 '18 at 16:19
  • $\begingroup$ @John - My apologies, I see now that the question was very vague. So, my species and humans, being at a similar level of technological advancement, would have roughly the same warfare requirements. For example, you've got tanks - they're heavily armoured, they've got a big gun, and they work on the front line. Then there's APCs - also armoured, they carry troops. So, what I want to know is how would you create aquatic vehicles that fill these same niches? $\endgroup$ – SealBoi Mar 31 '18 at 16:25
  • $\begingroup$ I suggest picking a single niche, coming up with there entire military complex is still way too broad, you also have the issue that things like guns don't really work underwater, so many niches simply will not exist. Even simple things like their military doctrine will have huge effects, during WW2 germany focused in few super high quality but hard ot make tanks while the russians focused on many lower quality but much cheaper tanks and therefore make a lot more of them. human military practice will only match in broad strokes. $\endgroup$ – John Mar 31 '18 at 22:08
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One first needs to know what the purpose of these military vehicles is in order to describe how they will operate. Looking at modern military equipment, we have machines to transport men and equipment, provide direct and indirect fire, send and receive signals, engineer vehicles to breach, cross or create obstacles and so on (and these are just a few of the various categories of vehicles a modern army uses).

Even then the form of these vehicles is determined by the doctrine of the using military force. Russian tanks look different from Western tanks because Soviet and now Russian ideas on how to use tanks is different from Western ideas. Some of the features considered important on Western tanks, like the ability to depress the main gun @ 10 degrees are not even considered in Russian tanks, since this makes tanks larger, and in any event speed and mass of fire is considered far more important when operating tanks in company and battalion sized manoeuvre units.

enter image description here

The difference in size between an M-1 and a T-90 is largely due to how each side plans to use their tanks

The only real analogues we could get is the vehicles would operate in a 3D environment like submarines, and weapons would need to be self propelled like torpedoes, as water is 800X denser than air, limiting the ability to shoot or otherwise project weapons with guns, speargun like weapons and so on. Some "nations" might prefer small, fast submarines like the Soviet era "Lira" (NATO code name ALPHA) if their doctrine involves raiding or similar strategies. Other "nations" might prefer larger submarines with more on board capacity to carry more torpedoes, or remain on station for longer times if the is important to them.

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Alpha class submarine laid up in dry dock

Other considerations might have to do with crush depths and the ability to operate in the depths of the ocean. If the creatures have a preferred habitat, there will be military advantage to operating well below the normal depth, including vastly increasing the required search volume and taking advantage of thermal layers in the depths to mask sounds and deflect active sonars.

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Bathyscape Trieste II. The crew resides in the tiny sphere under the front of the buoyancy tank. You can see how much different this is from the ALPHA

Operating on the surface might be problematic, you have not told us if the creatures are amphibious or able to operate out of the water. Even if they cannot, they might be interested in the ability of their technology to operate in a different environment. Water is 800X denser than air, so being able to operate out of water provides a huge advantage in terms of speed and range for the same amount of energy. Rockets or shells fired by surfaced submarines will be abele to cover long distances (in their terms), providing the ability to deliver effects with a high degree of speed and even surprise (sensors, torpedoes and mines could be delivered by these means, just as modern navies do on earth).

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SUBROC launch. The torpedo will deliver a multi kiloton nuclear warhead 55 km away

So until you give us an idea of what the creatures intend to do, and how they intend to do it, any answer will be mostly handwaving.

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  • $\begingroup$ First of all, thanks for the informative and helpful answer. So, I think that this idea of different style vehicles for different nations would be great, as the creatures prefer shallower water, and thus most nations are situated on either of two large shallow areas isolated from each other. I imagine these creatures would have similar needs to us in terms of vehicle functions, and would have machines to provide fire on the front line and well behind it, transport troops, do reconnaissance, communicate and so on. They can breathe air, but cannot locomote on land. $\endgroup$ – SealBoi Mar 31 '18 at 16:48
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Submarines. Propelled by electric motors. Moving fast in the liquid really requires streamlines shapes. In addition, a round cross-section minimizes surface area (and thus armor) given the same internal volume.

If you want different types, the fast fighter craft will the long to minimize water resistance, and slower heavily armored vehicles will be more like balls.

The projectile weapons are a lot less effective in the water since unpowered projectile slows down too fast. Torpedoes will totally be a thing, though, along with acoustic weapons (a ping of modern submarine can kill a diver).

Detecting enemy will be important. You can have all sorts of biological senses or technologies: sonar, water vibrations (https://en.wikipedia.org/wiki/Lateral_line), magnetic sensors, even smell.

The only reason to go above the surface would be resources not available underwater. If you are willing to bend the setup a little, maybe they do make metals with fire, and have to go ashore to do that. B/c electricity does not work all that well underwater, and is very dangerous for living things near it.

In fact, I believe for aquatic species, mastery of biology, chemistry and light manipulation is more plausible than metallurgy and electricity. So armor is grown using corals or turtles (fed a diet rich in iron), and computers are quantum ones, not electric. For motors, perhaps they can grow artificial muscles.

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  • $\begingroup$ Well, my rough idea of how they would forge metal is that they use their bioelectricity to pull metal ions from the water and plate a mandrel on the surface. I believe that electroplating and electroforming can produce much more detailed items, which would make it easier for them to create bullet-sized self-propelled projectiles that they could fire from handheld weapons. They could also use their bioelectricity to detect enemies and communicate. I think they would venture above the surface because air is so much less dense than water, meaning missiles could travel much faster. $\endgroup$ – SealBoi Mar 31 '18 at 18:56
  • $\begingroup$ There's this Fluidic computer, I forget the exact terminology but this computing mode uses fluidic interactions of liquids inside valve like logic gates for computation. I've seen it somewhere in this site but I forget. $\endgroup$ – Hendrik Lie Apr 2 '18 at 9:17

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