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 < 30,000 lb submerged speed 5 -- 10 knotssubmerged depth 25 -- 75 feet submerged range 40 -- 50 n.miles submerged endurance 4 -- 10 hourspayload 500 -- 1,500 lb takeoff and land in State 2 seasSeveral firms responded to a Navy request and a contract was awarded to Convair to develop the craft. The flying boat, which would alight and takeoff using retractable hydro-skis, would be propelled by three engines -- two turbojets and one turbofan, the former for use in takeoff and the latter for long-endurance cruise flight. Among the more difficult challenges of the design was the necessity of removing air from the engines and the partially full fuel tank to reduce buoyancy for submerging. Convair engineers proposed opening the bottom of the fuel tank to the sea, using a rubber diaphragm to separate the fluids and using the engines to hold the displaced fuel.
To submerge, the pilot would cut off fuel to the engines, spin them with their starter motors for a moment or two to cool the metal, close butterfly valves at each end of the nacelles, and open the sea valve at the bottom of the fuel tank. As the seaplane submerged, water would rise up into the fuel tank beneath the rubber membrane, pushing the fuel up into the engine nacelles. Upon surfacing, the fuel would flow back down into the tank. The only impact on the engines would be a cloud of soot when the engines were started.
When the engines were started their thrust would raise the plane up onto its skis, enabling the hull, wings, and tail surfaces to drain. The transition time from surfacing to takeoff was estimated to be two or three minutes, including extending the wings, which would fold or retract for submergence. Only the cockpit and avionics systems were to be enclosed in pressure-resistant structures. The rest of the aircraft would be "free-flooding." In an emergency the crew capsule would be ejected from the aircraft to descend by parachute when in flight, or released and float to the surface when underwater. In either situation the buoyant, enclosed capsule would serve as a life raft.
The craft would have a two-man crew and could carry mines, torpedoes or, under certain conditions, agents to be landed or taken off enemy territory.
The Navy Department approved development of the craft, with models subsequently being tested in towing tanks and wind tunnels. The results were most promising. But in 1966 Senator Allen Ellender, of the Senate's Committee on Armed Services, savagely attacked the project. His ridicule and sarcasm forced the Navy to cancel a project that held promise for a highly interesting "submarine." Although the utility of the craft was questioned, from a design viewpoint it was both challenging and highly innovative.
DARPA would do well to check the Navy's historical records as it embarks on the development of a flying submarine -- -oops, I mean submerging seaplane.
-- Norman Polmar