In this world FTL travel is accomplished by entering an alternate dimension, let's call Aqua. Aqua is a very simple, albeit strange dimension. A great expanse of water as far as civilization has mapped stretches out, dotted with islands, and an ocean floor. Not a planet, just a strange expanse of water.

Different locations along an X-Y plane in Aqua correspond to specific locations along an X-Y-Z space in the main (normal) dimension. The center-of-mass of the object exiting Aqua has to be roughly (only roughly) sea level, and likewise an object entering Aqua always enters roughly sea level. This allows trips of thousands of light years in the main dimension to be reduced to simple months of travel in Aqua.

Gates can be opened up between the main dimension and Aqua, and the gates are designed to hold the water and atmosphere back, creating a barrier for the water. Gates can only be made sufficiently removed from gravity wells, thus being limited to space.

How Aqua works, as how the jumps to and from aqua work, and the general nature of Aqua are all outside the scope of this question, unless they'd affect the design requested at the end of this post. Consider it all handwavium.

What is within the scope of this question is what the vessels would be constructed like: this is a question about design.

Points of consideration:

  • Movement in Aqua: Floating like boats? Submarines? Like aircraft?
  • Vessels need to be designed for the lack-of-atmosphere in space to the pressures of Aqua (similar to Earth's)
  • Vessels need to be designed to be able to accelerate/travel in both space and Aqua.
  • Trade transports should be constructed as economical as possible.
  • Space military vessels need to be able to protect themselves or be protected in a sensible fashion both in space (obviously), and in Aqua. Military vessels might be carriers, transports, or vessels meant for direct combat.

...What would be a sensible design for these vessels?

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    $\begingroup$ @JDługosz Instead of adding another 600 pages onto my to-read list, would you mind touching on what the linked book says on the topic? $\endgroup$ – Ranger Aug 15 '16 at 18:48
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    $\begingroup$ The space ship is made from a submarine. $\endgroup$ – JDługosz Aug 15 '16 at 18:50
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    $\begingroup$ Yeah, that Star Trek movie pretty much made me angry, when they put the Enterprise under the water. In that case, I knew the engineering for water vs. space is so divergent, that it wouldn't make any sense for them to build for that/plan for it, ever, even with the magic that is Scotty. Because, also, designing the ship to also come in and out the atmosphere seems a little weird to me, when there are shuttles (though not as maddening as ocean/space). $\endgroup$ – Erin Thursby Aug 15 '16 at 19:34
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    $\begingroup$ In your case, because water travel gets you to FTL, there is a motivation to engineer such a tough conversion. It would be quite expensive. $\endgroup$ – Erin Thursby Aug 15 '16 at 19:38
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    $\begingroup$ One major factor in design of vessels is energy usage. We don't know the energy requirements of the process as a whole, but how important is the efficiency of the design in Aqua. The more hydrodynamic the shape needs to be, the more it will affect the design of the vessel. $\endgroup$ – Cort Ammon Aug 15 '16 at 20:12

Take a look at how we move cargo around on earth - we don't have amphibious 18-wheelers running cargo across the ocean to stores on land, we have a network of ports, trainyards, and distribution centers. I'd imagine something similar for Aqua - the gates would be like ports, where spacecraft in normal space arrive and transfer their cargo/passengers through the gate. On the aqua-side of the port, they'd be loaded into planes or freighters for long-haul trips. If the gate is underwater, they'll be loaded into a submarine or aquatic elevator to reach the gate from a surface terminal.

However, it does make some sense to move spacecraft through Aqua. If you have a port that's underdeveloped, or a gate that has just opened and needs ships to explore the far side, you might have a fleet of Aqua-side Spacecraft carriers, designed to haul complete, functioning ships through Aqua, in much the same way there are car-carriers and Ferries on earth.

Special thanks to Erin Thursby for reminding me that infrastructure needs something to build it

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    $\begingroup$ So that you don't have to lose your ship on the other side, why not make them like aircraft carriers--except they have your spaceship inside? The Aqua Trawlers wouldn't move far from the gate and they wouldn't be good at space travel at all--they'd be very very slow and have like, little itty jets which would go away when they travel in the Aqua, but they use to maneuver (but not really travel) in space. Have the jets break down a bunch. $\endgroup$ – Erin Thursby Aug 15 '16 at 19:42
  • $\begingroup$ You're opening comment is needlessly condescending and adds nothing to your answer. Can I ask why you thought it worth including? $\endgroup$ – Ranger Aug 15 '16 at 19:45
  • $\begingroup$ @ErinThursby If your aim is to keep your ship that's an option. It makes sense to have some spacecraft-carriers to move ships to underdeveloped ports/newly opened gates that need exploration ships. But for established systems and commerce, you'd be passing the cargo and not the ship. FedEx doesn't load delivery trucks onto their planes, they load cargo onto the planes, and then load it into trucks. $\endgroup$ – UIDAlexD Aug 15 '16 at 19:46
  • $\begingroup$ @NexTerren My apologies, Nex. I didn't mean to be rude. $\endgroup$ – UIDAlexD Aug 15 '16 at 19:46
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    $\begingroup$ @UIDAlexD would think that they would have many different kinds of transport--shipping containers that get transferred on to space type vehicles, cruise ship type vessels, which may or may not have room for spaceships inside them. Sure, what you're saying for cargo makes sense, but this is a story, so of course I'm picturing our intrepid heroes doing more than dropping off cargo, but actually going through the Aqua. Of course, this may be pure world-building, but I'm thinking about it from a story perspective. $\endgroup$ – Erin Thursby Aug 15 '16 at 20:03

The first thing that jumps to mind is that if you have the tech to create a barrier that would hold back the water pressure of a huge expanse of water and still have it be penetrated by the site spaceship then what is stopping you from putting one of these "field generators" or whatever it is in your spherical ship and thus removing the problem of water tight and airtight. The field of would simply prevent the water from touching the ship.

Another thought that I had was maybe in this water world you find an element after visiting the first time that is one hundred percent hydrophobic and/or repels hydrogen molecules for some strange reason again simply making the vessel unaffected by the water and only affected by the pressure, at which point you take a submarine design make it out of this element and slap some thrust engines on it. (crude I know)

My final thought is make a sphere that has one entrance make it of something that doesn't rust and make it large/strong enough to withstand the pressure and instead focus on the propulsion tech and up with a gravity drive or something that can do two thing one : alter the gravity of the object that holds the drive (reducing the pressure on the sphere ) and two: work in almost any "atmosphere"


The real key to moving underwater is the static pressure increases at a very rapid rate as you go deeper into the water. On Earth the increase is one bar for every 10 m of depth. It is a bit unclear how the pressure increases with "depth" in your universe, but using this assumption, submersible spacecraft would have to be immensely strong in order to reach greater depths (assuming there is anything interesting or useful "at depth" in your universe).

In terms of how this would affect spacecraft, you are essentially building nuclear submarines for use in this "ocean". A warship would probably resemble a nuclear attack submarine like the USS Virginia or the Indian INS Arihant, being a compact cylinder with hydrodynamic ends and some sort of propulsion system. The requirement for a "sail" might not be there, since it is unclear to me if the submarine/spaceship ever "surfaces" in the alternate universe. Cargo ships would be much larger, resembling "Boomers" like the USS Ohio class or Russian Typhoon class boats.

Spacecraft, OTOH, are not constrained to resist pressure from external forces, and can be built much more lightly. Deep space craft generally only have to contain an interior pressure of one bar against the external vacuum, and even atmospheric craft like a shuttle are not subject to crushing pressure like a submarine. In a realistic setting, they also need massive radiating surfaces to reject waste heat, and since they would be subject to the tyranny of the rocket equation, every gram counts. They will be built as lightly as possible in order to maximize deltaV and minimize the use of fuel and reaction mass. The only exceptions would be an ORION nuclear else drive or something that is propelled by an external energy source like a laser beam.

The best way to contrast the two might be to think of an airship and a submarine. The airship is analogous to the spaceship; lightly built to operate in the atmosphere, while the submarine is strongly built to resist the pressure of the water. Both vehicles use buoyancy to remain "afloat" in their medium, and both superficially resemble each other for aerodynamic and hydrodynamic reasons, but neither could operate in the medium of the other.

The best possible solution would be to have some sort of trans shipment point at the interface between the two universes, and have separate ships for each set of conditions.



It seems to me that craft in your universe will simply fly, as this is quicker than sailing and places less stress on the craft. Its possible some more bulky items might be transported by bulk freighter, but assuming that your spacecraft travel using some sort of energy drive rather than reactive mass, and have energy shields to protect themselves from space debris and high speed particles, and are capable of atmospheric flight - it's simpler to just fly. As all of these are pretty common sci-fi tropes and clichés, and it's harder to explain reaction craft and energy shields unless you go properly hard-scifi, this is the route I would go down. The only exception for this could conceivably be if your craft are not capable of flight, in which case you're probably looking at bulk cargo being transported and more important items and passengers being transported "locally" to aqua by air.

To give you some idea, the Lockheed SR-71 Blackbird holds the official Air Speed Record for a manned airbreathing jet aircraft with a speed of 3,530 km/h (2,193mph) set in the 70's.[source] The current surface water-speed record is 511 km/h (318mph) also set in the 70's Source. Underwater, U.S. Navy and GE are jointly developing the Underwater Express, an undersea transport capable of controllable speeds up to 100 knots (185 km/h, 115mph) through supercavitation, and underwater weapons systems taking advantage of the supercavitation concept are already in existence, with a rumoured top speed of around 800 knots (1481 km/h, 920 mph) for the German "Barracuda" anti-torpedo missile.[Source]

Essentially, water is hard to move through - much harder than air to the point where underwater, the fastest objects actually are more like missiles flying through their own little pocket of air than torpedo's [source]. You're going to have real trouble getting and maintaining anywhere close to what you might imagine to be space-travel worthy speeds under- or over-water.


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