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The original was posted on /r/characterrant by /u/ApartRuin5962 on 2023-09-10 17:38:56.

The Justice League Javelin, the Black Adam Hawk Cruiser, and the Subnautica Seamoth are all aircraft and/or spacecraft which are perfectly-functional submarines. This has bothered me for a long time, but when we had a real-world fatal accident this June from a company trying to use next-gen aerospace materials for a deep sea submarine I think it should bother you too.

  1. The required structure is fundamentally different for containing 1 atmosphere of pressure with the lightest-possible mass vs. keeping out dozens or hundreds of atmospheres of pressure (with mass almost a non-issue). Let’s say you have a hatch rated for 1 atmosphere of pressure in either direction: it’s fine in space, but underwater it will implode at a depth of just 33 feet. A good submarine is an insanely heavy airplane, and a good airplane is an insanely flimsy submarine.
  2. Steel is the standard submarine material, aluminum is preferred for airplanes. Carbon fiber has been used on some aircraft and spacecraft but it caused the catastrophic loss of the Titan. Titanium has been used on both, but the thickness is insanely different (1/10 of an inch for the hull of the ISS, 2 in for the crew sphere on DSV Alvin) and there are reports of its fatigue and corrosion in seawater requiring a ton of costly maintenance on Russia’s Alfa class.
  3. There’s a reason why the Navy uses different aircraft than the Air Force: just a little saltwater spray tends to ruin the delicate and complex machinery of a modern jet. Immersing a jet in saltwater for hours or even weeks is going to fuse your ailerons into a solid hunk of rust like the Antikythera Mechanism. The History Channel is gonna have a documentary about how your landing gear is probably a mechanical astrolab made by ancient aliens.
  4. In an emergency it’s nice to have an aircraft or spacecraft which can make a water landing and float. A damaged submarine aircraft might accidentally land with its ballast tanks open and dive right to the bottom.
  5. Cabin volume and mass are at a premium on aircraft. For dive control you need either ballast tanks or dive weights, leaving very little room for crew and payload.
  6. The wings of an aircraft are way too goddamned big to work as good submarine control surfaces.
  7. There’s no propulsion system which could plausibly work in air, space, and underwater. Even a rocket engine is going to have a hard time forming a proper exhaust cone with the much higher-pressure fluid around it. Water is tough to move through: it stops bullets in a few feet, which is why most underwater vehicles use propellers or impellers to use that heavy fluid for reaction mass. I seriously doubt that a propeller could be manufactured which would work as a perfectly-good screw underwater, and even then you’d need one hell of a gearbox to switch from the insanely high RPMs of an airplane propeller to the insanely high torque of a submarine propeller.
  8. The two classes of vehicles need to be resistant to totally different kinds of damage. A spacecraft is going to run into micrometeorites and flecks of paint traveling at hypersonic speeds and needs a “whipple shield” to disperse all that kinetic energy. A submarine is a massive vehicle which will likely bump into other massive objects and needs to survive high-momentum collisions which probably generate enough force to crush the entire ISS like a tin can.

In summary, I like submarines and spaceplanes but I’m tired of sci-fi artists telling us that one vehicle magically does both. It would be infinitely cooler to see a spaceplane deploy a purpose-built submarine, a submarine aircraft carrier launch a spaceplane, or have a hangar and landing pad rise out of the water to accept aircraft before being sealed up and lowered back down to the underwater base. I understand “rule of cool” but I think we could make more sense and have more cool vehicles your setting if we split the spaceplane and submarine roles into two distinct vehicles.