I'm doing background work on a story involving a civilization about as advanced as ours materiologically. A significant part of their military operates along the continental shelf zone protecting undersea or coastal bases, as well as submersible carrier battle groups.
Entering the water from the air while under power tends to be fairly destructive to modern aircraft.
Without handwavium-style alloys or configuation changes, I'd like a fighter to be able to fly in the air and underwater as well as survive a controlled transition from air into water.
What is this fighter made of?
A fighter design based upon the rounded, long, and broad wings of a manta ray seem like they would be useful in both media. This helps with some of the basic physics questions, but under what sort of circumstances can something buildable without handwavium materials enter the water (as noted above)?
Often dubbed “missile birds,” gannets can reach speeds of up to 60
miles per hour while careening towards the ocean from heights as
elevated as 100 feet in the air!
If you can figure out how they do it, you have your solution. Or, you can just assume it is possible, since they already do it, and not worry about the science or mechanics.
Nature, and life, so likes to make a mockery of our materialistic inanimate-based view of physics. Animals don't CARE that they are defying our understanding of the rules of classical physics. They do it anyway.
Naughty animals, breaking our laws. How DARE they?
This is vehicle isn't impossible, material composition is the least of your issues. What you have to address is how to ease the transition from air to sea. Slowing down is the first most important aspect. Taking on the transition at full jet speed is likely mechanically impossible let alone likely to kill the pilot.
Tucking in the wings like a pelican is another important aspect, the less surface area making the transition the less drag/resistance/deceleration/G's the aircraft and pilot would suffer.
If you must rely on shapes, I would expect something like the shape of the B2 bomber would be more ideal for this than a manta ray. Inward folding wings would be critical.
Even Pelicans who dive for their food can eventually go blind from the repeated trauma of the dive.
Frank Germano, accredited aerospace engineer, has designed a
revolutionary airship system that incorporates Viktor Schauberger’s
and Nikola Tesla’s pioneering engineering concepts – At the frontal
section of the craft is an air intake plenum (the large opening),
which draws in atmospheric air through a spiral vortex generating
cone. The air is accelerated via a Tesla bladeless disk air pump
system. This accelerated and pressurized air is forced out through an
outer ring of slits located along the side of the craft (think of a
fish’s gills), and the de-oxygenated, pressurized, velocitized air
forms vortices along the outside hull of the airship.
Global Energy Technology’s new airship has the capability to alter its
density (mass as a unit of weight per cubic area) in relationship to
the mass of the surrounding air by the use of collapsible gas bags
that are inside of the shell of the airship into which low pressure,
low density helium is expanded, which causes the overall density of
the aircraft to be lighter-than-air, or by forming a vacuum within the
cells to create vacuum lift to become lighter-than-air.
This exact process and design for can be utilized in Submersibles, as
well (which was, of coarse, Viktor’s original intent!). This creates
some unique possibilities in regards to ocean-going transportation.
Since the craft would now operate under the water’s surface, it would
effectively be mostly immune to weather conditions on the surface.
Speed would be greatly increased over a standard ship’s ocean surface
This ship is centered around the vortex engine as described above and can become lighter than air by filling space inside the craft with helium. Although this is not part of Mr Germano's plan, I assert that per the OPs request this same propulsive mechanism can work underwater, with the ships density increased by filling the helium chamber with water.
One would think a lighter than air craft would not be very fast, because we are used to thinking of zeppelins. An aerodynamic ring shaped balloon around a giant vortex engine might be fast. Also, the leading edge can be sharp which will help when diving into the water - or possibly acting as a ram.
Using current technology, creating a "bubble" of supercavitation around the object will allow you to penetrate the air/sea interface in both directions. The USN has experimented with using different shapes of bullets to allow a gunner on a helicopter to shoot through the water to detonate mines hidden beneath, and the Russian Shkval rocket powered torpedo uses supercavitation to create a "bubble" around itself to allow it to travel through the water at 300kph.
This gives us some parameters. To penetrate the water and create the cavitation bubble, the aircraft or projectile needs a shape capable of pushing the water aside in the correct manner, and also needs to be built solidly enough to prevent buckling or breakup on impact (water being 800x denser than air). A supercavitating bullet provides the basic shape:
When under the water, the vehicle now needs a power plant capable of providing high levels of thrust through the dense fluid media. The Shkval uses a rocket, but this only provides limited range.
Several things need to be accomplished in order for this to work:
A high energy (probably nuclear) power source is needed that can work both in the air and underwater. Using the fluid media to both cool the reactor and provide the thrust may work both in the air (a nuclear jet) and under water (a very high powered water jet).
Lightweight shielding. Unless you intend to cook the on board systems and possible crew, the reactor will need to be shielded. Conventional shielding will weight many tons, degrading performance.
High strength construction. Hitting the water at sufficient speed to create the cavitation bubble without collapsing the vehicle is a must. Once again, weight is the enemy of performance.
Crew protection. Slamming into the water is going to create high "g" forces, so the crew and all systems and electronics will need to be cushioned somehow.
Retractable control surfaces. The wings will need to be folded in before hitting the water, and underwater control surfaces will need to be deployed. When surfacing, the wings need to snap out quickly in order to achieve flight once clear of the water and while the jet engine is building thrust in the air.
So you can see this will be a very complex and expensive piece of kit. Someone will need to be working out the calculus to see if it makes sense to divert resources to building these , or making more regular submarines and aircraft instead.
As an alternative, a long range underwater torpedo might be created. When it reaches the end of its programmed underwater journey, it fires a rocket engine to make the last leg at high speed, with the possibility of it surfacing and converting into a missile to confuse the defence, which needs to worry about both underwater and missile attacks.
Your aliens should build light and with a very high structural resistence. So I would go with some sort of composite material (like carbon fiber, kevlar or maybe something like the spider silk), which should give the structural resistence they need.
So, assuming that they solved the propulsion problem, probably using two propulsion methods to have the best of the two worlds, the next problem is the transition. As you noted a good form is something like the manta ray. Another good form can be the one of the B2 spirit, but more compact. An important point is that it should not have any external payload.
All that anyway don't completly solve the problem, since you still have the transition between air and water. The only solution here is to be slow, ideally they should hover before going from air to water while when the fighter go from water to air, since it has some lift problems so it should go as fast as possible.
Something to consider: The weak link in current high speed fighters is not the materials, it's the pilot. They get squishy when subjected to very high G-Forces. That's what is going to happen when transitioning from one medium to the other without changing the vehicles shape. There are ways you can cope though.
If your craft must be fixed wing, consider a slower, graceful approach. Have the craft approach at an extremely shallow vector, almost parallel to the surface, while shedding a lot of speed. Once it gets to a given speed, then it can dive. There are a lot of aircraft out there that land on the surface of a body of water. The downside is that those shapes may be ungainly in air to air combat.
What I am picturing is a long, slender fuselage with a V shaped bottom, like a cigarette boat. The wings are going to be backward swept and centered along the far rear of the fuselage, with smaller control fins farther forward, kind of like This Beechcraft. The difference will be that the wing should be mounted high rather than low, so that it can clear the surface of the water faster when trying to generate enough lift to get in the air
Jet propulsion should be fine for the air, but you may have to find something different for underwater propulsion. Normal jet engines are going to get swamped and stop working underwater.
I do know for certain that if you try to keep any sort of fixed wing configuration, a high angle of attack going into the water is going to result in your plane being ripped apart or your pilot being smeared in a thin layer on the inside of the craft
Take a look at the folding wings concept as mentioned earlier. The first thing I think of is a tube-like body into which the wings can fold completely. The propulsion can utilize the same propeller that is used in flight as in water. The key might be a changeable pitch propeller and an engine that supports lower RPMs under water. The body in more detail should be fairly long and needle-shaped to reduce drag in water as in the air. This way, the HOW is more important than the WHAT in the construction and should this way be completely possible with today's materials and constructional knowledge.
Dropping a streamlined "pipe" into water will create the least resistance possible and probably lessen the impact when transitioning between mediums enough to not have to slow down too much. Naturally, the angle of entry is paramount for not wrecking the craft. Exiting the water and returning to flight
is probably a bit more difficult. The craft should be able to accelerate to a great enough velocity for the wings to fold out and the engine spin up to speed to create adequate lift. Another solution is to have a fuselage that is shaped in such a way that the craft can drive across the surface to an adequate velocity where the wings can give enough lift. Of course, I thought about rockets, but that seems wasteful for repeated takeoffs and landings and adds another level of complexity.
Weaponry in the air will probably be similar to our weapons; Machineguns, missiles etc. The machinegun barrels can be covered with hatches if needed when submerged and the missiles can be in internal compartments of the body of the vehicle. However, underwater the scenario changes. Here the increased resistance will render bullet-type weapons useless beyond a few short meters. Torpedoes are heavy and big and would probably not be efficient. Even small torpedoes would have limited range and efficiency due to their size.