Well, I think it is technically possible to make an aquatic jet. . .
Sure it is, in the sense that a "jet" is a plume of fast-moving fluid of some kind. Modern submarines often use a shrouded-intake pumpjet---it's quieter for one thing.
But of course when we say "jet" we generally mean a bit more than just fluid flowing through a duct...
Mix water with fuel and then combust it?
And in that sense, of course not!
A liquid by definition doesn't compress much, and therefore doesn't expand much, and therefore any chemical reactions you can manage to arrange for in water (which is scanty of free oxygen compared to the air anyway) would be pretty futile, unless you can release so much heat you actually flash the water into gas (or plasma!) that expands anyway despite the pressure at some depth. But such reactions, if we can imagine some way of achieving them (bombard the water with high-energy fission or fusion product particles? Carry pure sodium as "fuel"? Black magic?) would make one noisy, spectacular process and probably be inefficient as hell too. So a "jet" in the sense of actually using the fluid as a part of the source of the energy-producing reaction, which is what an atmospheric turbojet does, is silly in water.
If the desired speed is just 5 knots, I suppose that batteries supplying electricity to the turbojet's starter motor can make the compressors of the jet engine into a sort of pumpjet, if you can bypass the turbine and decouple it mechanically to minimize drag. Now you've got seawater running right through ducts meant for hypersonic, heated airflow; it's hard to believe the same duct and blade shapes can serve well for hot supersonic air and cold salty water flowing at 3 meters per second or so, but maybe a very clever fluid dynamicist can figure that part out. And a good materials science team can figure out how to employ materials that survive and serve well in both environments. And can take the insane thermal transition from the white heat of a jet combustion chamber to the near-freezing temperature of sea water in just a half second or so without bursting, then clear all that salty water out of the entire duct so the turbojet engine can start again without say being thrown centrifugally out of whack by clinging seawater droplets, suffering corrosion, jamming by bits of seaweed and krill and whatnot...
A somewhat less clever but more overall competent fluid dynamicist, or any sort of expert whatsoever, seeing this design, would blanch in horror and (assuming there is some good reason to want a craft with this combination of capabilities!) come back with designs that firmly shut the jet pods to all water flow; probably one would reject the usual modern practice of external jets and instead revert to a more British 1940s/50s approach of embedding the jet engines in the wings or fuselage, so that when their intakes and exhausts are sealed shut they don't leave a hydrodynamic exterior trace.
Then add a separate electrically driven pumpjet, presumably on the tail tip. Or just have a water prop of some kind, folded against the tail cone (or nose cone) like an umbrella when airborne. Or even retracted into the hull and only deployed when submerged.
The drag from the wings and tail are going to be enormous!
Yes it will!
Why in the hell would you bother? It isn’t like you are going to be able to go deep.
That's the multi-billion dollar contract question of course. Whiskey? Tango? Foxtrot!
My best guess is, it's a strategic deterrent concept analogous to a nuclear missile submarine. In peacetime it lurks under the sea when on station, in an unknown location, ready to pop up and fly as a bomber to the target if something goes horribly wrong (or wickedly right) in the world of diplomacy.
And when put that way, the silliness of the concept (at least in a world where submarine-launched ballistic missiles are a viable concept) is pretty plain.
Perhaps someone can come up with some more sensible scenario for a submersible airplane of any kind?
As you say, it's bad enough to submerge an airplane at all, but trying to proof even part of the fuselage against the pressures at any depth would add a huge dead weight to any design, useless except in that case.
So the question of why the heck anyone needs this kind of capability is a rather burning one as it governs the tradeoffs one is willing to make.
Throw enough money at it and might be doable--not like this, having the jets in external pods like that is just stupid in this case. But it looks like more of a stunt than a solution!
Stupid question: disregarding all the other implausible elements of the design, how would you design an ejection system that would function both in air and underwater?
Um, assuming that some need exists to require such a bizarre craft at all, it could be that ejection is just not a priority of design?
The earlier jet bombers had no or only partial provision for crew "ejection" while airborne--some crew if not all were often expected to simply open a hatch and jump out, near-sonic speed be damned. Similarly if you are submerged and have to abandon ship, I suppose down to a certain depth (probably well below anything any half-sane aircraft could dive to without being crushed anyway) you just put on some scuba gear and open the same hatch, and swim for the surface.
Vice versa I suppose if there are fancy ejection pods for near-sonic speed ejection while airborne, these can be designed to also be slowly ejected into water and bear the crew up to the surface, mainly by bobbing up like a cork.
