Convair Submersible Nuclear Ramjet
Ok, this one is a bit of a weird one so you'll have to bear with me.
Information on this is fairly sparse, with most of what we know coming from two presentations devoted to design work for atomic powered aircraft being done for the US Navy in the early 1960s. The second report describes a pencil-shaped vehicle, approximately 24.7 feet in span and one hundred and forty feet in length. No real data on this second proposal (which is what I have drawn) exists, but it would likely be somewhat similar to the first proposal: while submerged, the vehicle would have most of its internal volume filled by seawater in massive ballast tanks. I While underwater or running on the surface, propulsion would come from using the nuclear reactor as a water ramjet at a low power setting, using the steam directed rearward as a waterjet. Submerged speed for the first proposal was estimated at a whopping 100 knots maximum. The later proposal was given a potential top speed of Mach 4 and carried at least 24 nuclear weapons in its payload bay.
The vehicle was designed to lurk for extended periods of time underwater, either cruising about or remaining still and silent. Because of the duration of missions, a crew of nine would be housed in a lead-lined compartment forward. If a mission launch command was received, the internal tanks would be pressurized and the water expelled through the nuclear reactor to create superheated steam that, if run at full power, would create enough thrust to launch the craft vertically out of the water like an SLBM. The engine would continue to function somewhat like a rocket motor as the vehicle climbed, pitched over, and accelerated to a sufficient speed that the reactor could transition from water to air as the reaction mass to power the ramjet. The vehicle would fly at high speed and low altitude over enemy territory, using terrain-following radar to stay below hostile radar. At the end of the mission, the crew would select an area of open water and throttle the ramjet down. As the craft slowed, it would enter a climb inducing an aerodynamic stall. As it stalled, airbrakes near the nose would extend to stabilize the vehicle as it fell tail-first towards the sea. The engine would transition back to rocket power for the "suicide burn", safely lowering the craft into the water. Once in the water, the internal tanks would again flood to submerge the vessel and increase the protection for the crew.
Not much information exists on this proposal, so I've had to assume that the Aerospace Projects Review drawings (based on the originals) are accurate and take some artistic liberties of my own.