The answer is simple:
Submarines cannot fly, but seaplanes can submerge -- if you build them properly.
That's what the Defense Advanced Research Projects Agency (DARPA) is seeking to develop. A recent Request for Proposal (RFP) from DARPA calls for a submersible aircraft [that] would combine the key capabilities of three different platforms: (1) the speed and range of an aircraft; (2) the loiter capabilities of a boat; and (3) the stealth of a submarine. "By combining the beneficial characteristics the and operating modes of each platform, DARPA hopes to develop a craft that will significantly enhance the United States tactical advantage in coastal insertion missions," according to the RFP.
The irony of the RFP is that the U.S. Navy was developing such a craft some 45 years ago.
The objectives issued by DARPA are for a vehicle that would have an airborne tactical radius of 1,000 nautical miles, a low-level flight radius of 100 nautical miles (which may leverage surface effects), and a submerged tactical radius of 12 nautical miles. The sum of these must be achieved within eight hours. Endurance on the surface has to be 72 hours in sea states up to five between inserting and extracting personnel. The craft's payload objective is eight men and their equipment with a total cargo weight of 2,000 pounds.
DARPA has identified the major challenges to the project as (1) weight, (2) fluid flow regime, (3) structure, (4) lifting surface geometry, and (5) power and energy storage. These factors force the consideration of a seaplane that can submerge as opposed to a "submarine that can fly." The relatively light construction of an aircraft can be submerged to shallow depths, and to even great depths with internal pressurization. But submarine-like vehicles, built to withstand greater depths, are too heavy for consideration.
The U.S. Navy had begun contemplating the merger of aviation and submarine technologies into a single vehicle as early as 1946. By that time several Navy laboratories were looking into the required technologies. When asked by the press in 1946 whether such a vehicle could be produced, Vice Admiral Arthur W. Radford, at the time the Deputy Chief of Naval Operations for Air, replied: "Nothing is impossible."
A decade later, in 1955, studies were being conducted under contract from the Department of Defense by the All American Engineering Company while aviation pioneer John K. (Jack) Northrop was designing such craft. The All American vehicle was to alight on and takeoff from the water on "hydro-skis"; once on the water the craft could be "sealed" and submerge.
Although nothing resulted from these studies, by the early 1960s the U.S. Navy was ready to invest in such a vehicle. A Navy engineer working on the project, Eugene H. Handler, explained, in a 1964 article in the U.S. Naval Institute Proceedings, "there is... a tremendous amount of [Soviet] shipping in the Soviet-dominated Baltic Sea, the essentially land-locked Black Sea, the Sea of Azov, and the truly inland Caspian Sea. These waters are safe from the depredations of conventional surface ships and submarines."
The Navy's Bureau of Naval Weapons -- at the time responsible for aircraft development -- awarding a contract to Convair in 1964 to examine the feasibility of a "submersible flying boat," which was being called the "sub-plane" by those involved with the project. The Convair study determined that such a craft was "feasible, practical and well within the state of the art."
The Bureau of Naval Weapons specified a set of design goals:
air cruise speed 150 -- 225 mph air cruise altitude 1,500 -- 2,500 feetair cruise radius 300 -- 500 n.miles maximum gross takeoff <>
