America’s special operations forces use a wide variety of vehicles to conduct their covert operations, but few are as dramatic as the Navy’s recent efforts to field a flying submarine that could ferry operators into and out of the battlespace quickly and stealthily.
To date, there has been no evidence to suggest that such a platform is in current operation, but Navy documents from 2010 clearly show that not only was the branch seriously investigating this concept, they concluded that the idea was entirely feasible with the existing technology of the day.
“The submersible aircraft study combines the speed and range of an airborne platform with the stealth of an underwater vehicle by developing a vessel that can both fly and submerge.”-Rick Goddard & Jonathan Eastgate, Submersible Aircraft Concept Design Study
Could Navy SEALs and other special operations units be operating stealthy flying submarines already? It seems unlikely–but based on the Navy’s research, it may only be a matter of time before this concept does come to fruition.
SDVs: The Navy SEALs’ miniature submarines
Today, the U.S. Navy’s premier special operations unit, the SEALs, leverage a number of submersible vehicles for these sorts of clandestine infiltration and exfiltration operations, alongside a variety of other mission sets. These SEAL Delivery Vehicles, or SDVs, have come in at least two operational forms: the Mark 8 SDV, which can transport SEALs, and the Mark 9, which leveraged two Mark 31 or Mark 37 torpedoes to engage surface vessels prior to its retirement. Two new SDVs are already headed for operational service, with the Mark 11 Shallow Water Combat Submersible (SWCS) slated to replace the Mark 8 and the more broadly capable Dry Combat Submersible (DCS) expected to offer even greater capability.
“SDVs aren’t sexy. They are uncomfortable and a lot of work, but they fill a critical and unique capability. The guys that do get to work with the SDVs are a strategic national asset,” one former Navy SEAL told Sandboxx News.
But despite the incredible capability offered by these submersible platforms, they come with some significant setbacks. The Mark 8 SDV and its successor, the Mark 11, are both “wet” submersibles, meaning the occupants are exposed to the water (imagine driving an underwater convertible). This reduces weight and power requirements, but limits how long SEALs can travel in cold water without suffering physically adverse effects. Another significant limitation of the Mark 8 is its operational range; once deployed, the Mark 8 SDV can travel a maximum of just 18 miles with a SEAL team onboard.
While figures for the Mark 11 have not been released, it likely won’t offer a massive increase in range over the Mark 8. The much larger Dry Combat Submersible (DCS) can carry two crew members and eight SEALs, twice that of the Mark 8 or 11 SDVs, and because the crew compartment is “dry,” its operational range isn’t limited by the survivability of its occupants in cold waters. However, its relative size, mobility, and range remain limiting factors in where and how it can be employed.
The Submersible Aircraft Concept Design Study
The idea of using submarines to deploy aircraft has been around as long as submarines and aircraft. The British sank one of their M-Class submarines before World War II attempting to add an aircraft hangar to it, the French carried foldable planes in their Surcouf submarine cruiser in the ’30s, and in the 1950s, the United States developed the AN-1 submarine aircraft carrier design, capable of carrying eight fighters within its hull.
All of these efforts, for one reason or another, found their way to the scrap heap, but the idea of combining the stealthy characteristics of a submersible with an aircraft’s ability to cover ground quickly has remained prevalent to this day.
The challenges associated with the premise, however, are sizeable. An aircraft is inherently low-density, as weight is a constant concern for airborne platforms. Submarines, on the other hand, are incredibly dense–as dense as the water they’re submerged in. Airplane fuselages are likewise designed with weight in mind and could never survive the pressure of being completely submerged in a hundred feet of water, while submarines have incredibly strong hulls that are too heavy for most airborne applications.
A report published by the Navy in 2010 called the “Submersible Aircraft Concept Design Study,” penned by engineers Rick Goddard and Jonathan Eastgate, aimed to determine the feasibility of an aircraft that could easily transition from flight to operating on the surface of the water, and then to operating below the surface with these challenges in mind. This program was covered earlier this year by Brett Tingsley over at The Warzone. The impetus behind the concept, of course, was finding a way to get special operations troops, referred to as Special Forces within the study, into and out of contested areas under a shroud of secrecy. In practice, the premise simply takes the submarine aircraft carrier concept and removes the carrier. Just as seaplanes were intended to eliminate aircraft’s reliance on airstrips, flying submarines could operate secretly in a maritime environment without a submersible aircraft carrier to support them.
It was with this premise in mind that DARPA published a request for designs in 2008, outlining a list of requirements a flying submarine would need to be able to meet in order to be an effective weapon in a near-peer conflict.
DARPA’s Concept of Operations (CONOP) for flying submarines
The Navy’s 2010 study largely leveraged DARPA’s outlined requirements, or Concept of Operations, for a submersible aircraft aimed at transporting special operators. The first was obvious: that the flying submarine could be deployed from existing Navy or auxiliary platforms. The vehicle had to be able to land and take off unassisted from the surface of the water, with an in-flight range of 400 miles or more. It needed to be able to transit at least 12 miles once submerged, and loiter for up to 72 hours while hiding from detection.
Perhaps most importantly of all, it also had to be able to traverse those same 12 miles beneath the waves and 400 in the sky on the way back, as well.
In keeping with DARPA’s own efforts, the U.S. Navy Special Warfare Center and Office of Naval Research used the same requirements in their study. However, in order to emphasize real-world applications, they further incorporated the requirements that the vehicle must be fully submerged during its 72-hour loiter time, and must be able to complete the entirety of the mission without refueling.
The Navy came up with two flying submarine designs
The study determined that a flying-wing/blended-body design was best suited for managing the rigors of both flight and operating beneath the surface of the ocean. So, the Navy set about creating two potential flying submarine designs that took similar, but slightly different approaches to solving the litany of challenges before them.
The first design, dubbed Variant 1, had a 92-foot wingspan and a total weight of 37,000 pounds. It was 36 feet long, could carry a 750-pound payload, and cruise at speeds of 200 miles per hour while airborne. The second design, logically called Variant 2, was similar, but boasted a larger 109-foot wingspan, a shorter overall length of 34 feet, and the same speed and payload capabilities, despite a slightly higher overall weight of 39,000 pounds.
Both designs leverage multiple watertight compartments, with one keeping the two-person crew separate from a single personnel compartment capable of carrying six special operators, and the other using two smaller personnel compartments that could each hold three. In both designs, the wings would carry fuel in a membrane that would allow any unoccupied space in the wings to be flooded with seawater while submerged. Likewise, the personnel compartments were intended to be free-flooding when the vehicle was underwater.
Two turbofan motors would propel the vehicle while in flight and on the surface of the water, which would be sealed using torpedo-style doors while submerged to protect them from seawater. Submerged propulsion would be managed by a drop-down azimuthing pod with electric motor, capable of maintaining a speed of 6 knots.
Both variants of the flying submarine concept were to operate at depths of around 30 meters (98.4 feet) and be able to take off and land on specially designed inflatable floats.
While the numbers associated with the Variant 1 and Variant 2 designs were very similar, they differed far more on the arrangement of their respective crew compartments. In both arrangements, the quarters were quite cramped, so the team compared the internal volume of their cockpit to the long-duration flying Rutan Voyager, the first aircraft to fly around the world without stopping, and the Apollo capsule that brought astronauts home from the moon.
The Navy’s flying submarine offered 180 cubic feet of space for two pilots during potential missions of up to 84 hours in duration, which was less than the the 216 cubic feet allowed by the Apollo capsule for a longer 144 hour occupied duration and a third occupant, and much more than the 35 cubic feet offered by the Voyager for a 216 hour flight. Based on these comparisons, 180 cubic feet seemed feasible.
In Variant 1, the personnel compartment for special operators was placed forward of the cockpit, as these compartments would flood during submerged operation, placing the pressurized cabin as close to the vehicle’s center of gravity as possible.
In this configuration, the vehicle’s batteries are stored on either side of the personnel compartment, in line with the turbofan engines.
Operators would deploy through a large hatch on top of the vehicle near the equipment locker. Both turbofan engines would be placed far to the rear and as close to the centerline of the vehicle as the cabin would permit, though the study points out that analysis should be done regarding performance during a single-engine failure. If the engines were mounted too far apart and one were to fail, it could place the flying submarine in an unrecoverable flat spin — a problem that cost the Navy as many as 40 F-14 Tomcats.
This arrangement resulted in an exterior appearance that was more triangular in shape than the alternative, explaining its slightly longer dimensions.
Variant 2, on the other hand, sought an even lower profile along the vehicle’s centerline, accomplished by distributing equipment and personnel across a wider area. As discussed in earlier sections, the personnel compartment carrying special operators would be split in two, placing three operators on each side of the pressurized cabin in reclined chairs to maximize headroom while minimizing height.
This design places the turbofan engines, ballast tanks, air pressure system, and some other components in the same places as Variant 1.
The batteries in Variant 2 are placed ahead of the pressurized cabin and between the personnel compartments. Operators would again exit through hatches on top of the vehicle, but would have to swim to its rear to grab their bags, scooters, and other mission-specific equipment.
The result is a shorter, wider exterior design with a slightly lower centerline. This arrangement proved heavier, due to its added width and associated fairing.
How to take a flying submarine to war
According to DARPA’s Concept of Operations and the conclusions drawn by the Navy’s study, the following is an approximation of a flying submarine could be leveraged in covert operations.
The vehicle would be deployed by any surface vessel large enough to carry it and equipped to place it in the water. Two vehicle operators (pilots) would enter the pressurized crew compartment and begin their pre-flight checks as the Navy SEALs or other special operations divers donned their equipment and stored the rest in the vehicle’s gear locker.
Once the flying submarine was in the water atop two specialized inflatable floats that eliminated the suction effect a hull can have on the surface, its two turbofan engines would propel the vehicle to 100 knots, at which point, the unusual aircraft and all on board would take to the sky.
Once airborne, a lightweight fly-by-wire control system fed through a flight control computer would manage the aircraft’s twin-flap control surfaces that allowed roll-independent yaw control, thus eliminating the need for a vertical tail. The vehicle would remain airborne until it was approximately 12 nautical miles from shore, which is the conventional limit of territorial waters. In other words, even if someone were to spot the aircraft flying along its route, it would be seen flying over international waters.
Using the same inflatable floats, the flying submarine would land on the surface of the water. If necessary, the vehicle could then traverse further on the surface of the ocean, turning through modulating the output of the turbofan engines independently. It would then deflate its floats and fill its ballast tanks, submerging the vehicle and flooding all compartments except the pressurized cabin.
The SEALs inside, donning wetsuits and completely submerged in water, would draw breath from the same high-pressure air system that keeps the cabin pressurized and refills the vehicle’s floats. Based on industry capabilities in 2010, the study posits that storing enough air to meet all operational requirements internally would be entirely feasible, so there would be no need for any means of refilling the air tanks.
The vehicle would then deploy its underwater propulsion pod, using an electric motor to propel it forward and a combination of the motor’s orientation and the vehicle’s flight control surfaces for maneuvering.
“It is envisaged that the pilot will be assisted by a flight control system acting between the manual yoke inputs and the final flap deflections and that this system should be able to deal with the smaller deflections needed for underwater maneuvering.”-Rick Goddard & Jonathan Eastgate, Submersible Aircraft Concept Design Study
The flying submarine, now more submarine than airplane, would approach the shore fully submerged, opening its hatches and deploying the combat divers while deep enough to avoid detection. Once out of the vehicle, the divers would grab their gear from the equipment locker and travel on using submersible scooters or by swimming to shore.
The special operators would then have 72 hours to conduct clandestine operations in enemy territory while the vehicle’s two pilots remained onboard and fully submerged. Once the special operations team completed their mission, they would return to the water, swimming back to the vessel and reentering it while submerged, reconnecting to its onboard air supply. At which point, the pilots would drive the vehicle back into international waters before surfacing, inflating the floats, and taking off once again to fly back to the ship that deployed it or to a different designated recovery point.
What happened to the Navy’s flying submarine concept?
After testing a scale model, the Navy’s report concluded that “feasible vehicle concepts can be generated using current technology and materials” to design and build a real special operations flying submarine, but as far as the public is aware, that’s the end of the story.
To be perfectly clear, it’s difficult to say if this study resulted in further development of a flying submarine or if it was quietly shelved alongside other exotic but seemingly feasible programs.
It’s important to remember that the United States was not particularly concerned with Great Power Competition or the possibility of near-peer conflict in 2010. In fact, this study came two years after a different effort to deploy low-observable drones from the vertical launch tubes of Ohio-class ballistic missile submarines was canceled for budgetary reasons and one year before the F-22 Raptor program was canceled after just 186 out of 750 intended fighters were delivered.
To be frank, America’s defense apparatus was funding combat operations in multiple theaters against opponents with no notable air or sea power… so efforts to field stealthy or otherwise highly advanced platforms in either environment were simply not the priority.
It’s possible, however, that development continued on the flying submarine concept at a low volume and scale, and to be honest, the United States may even have some such platforms in service today. After all, the F-117 Nighthawk first took to the skies in 1981, but wasn’t revealed to the public until 1988. Today, America has successfully flown a technology demonstrator for its Next Generation Air Dominance program, assembled three B-21 Raiders, and regularly operates the highly secretive RQ-170, all without most of us getting a good look at any of them. It stands to reason that there are other programs we remain entirely in the dark about.
The Navy may have concluded that it was feasible to build a flying submarine, but that doesn’t mean the Navy felt it was practical. The truth is, if there is such a vehicle in service today, we likely won’t know for years to come… but if there isn’t, the Navy’s engineers had an interesting bit of advice for future endeavors.
According to them, the secret is approaching the problem with the intention of building a submersible aircraft, rather than a flying submarine. Apparently, planes do better underwater than submarines do above it.