Submersible Aircraft

DARPA's Submersible Aircraft was put on hold in July 2009 by the Congressional Budget Office. In 2008 DARPA solicited [DARPA-BAA-09-06] innovative research proposals on the topic of a Submersible Aircraft. In particular, DARPA is interested in a feasibility study and experiments to prove out the possibility of making an aircraft that can maneuver underwater. The proposal needs to outline a conceptual design along with identifying the major technological limitations that need to be overcome in order to maneuver an aircraft underwater. In addition to the conceptual design studies, performers need to outline experiments or computational models that will be used to demonstrate that the major technological limitations can be overcome.

DARPA was soliciting innovative research proposals on the topic of a submersible aircraft. In particular, DARPA is interested in a feasibility study and experiments that would provide proof of concept for the realization of an aircraft that can maneuver underwater. The proposal needs to outline a conceptual design along with identifying the major technological limitations that need to be overcome in order to maneuver an aircraft underwater. In addition to the conceptual design studies, performers need to outline experiments and/or computational models that will be used to demonstrate that the major technological limitations can be overcome.

In order to assure that the U.S. maintains its tactical advantage for future coastal insertion missions, DARPA is interested in exploring radical new technologies that can provide a game changing DoD capability for inserting small teams, clandestinely, along coastal locations. One such technology is a submersible aircraft. A submersible aircraft 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 and the operating modes of each platform, DARPA hopes to develop a craft that will significantly enhance the United States' tactical advantage in costal insertion missions.

Prior attempts to demonstrate a vehicle with the maneuverability of both a submersible and an aircraft have primarily explored approaches that would endow flight capability to platforms that were largely optimized for underwater operation. Unfortunately these prior attempts have been unsuccessful largely because the design requirements for a submersible and an aircraft are diametrically opposed. DARPA believes that, in order to overcome these diverging requirements, it is essential to properly formulate the problem statement. The Submersible Aircraft BAA was specifically designed to solicit innovative concept designs coupled with feasibility experiments and/or computational models that provide initial proof of concept for the technologies that would ultimately enable a submersible aircraft platform.

The Submersible Aircraft program is designed to explore the possibility of developing a single platform that is capable of both flying through the air and submerging below the water. The difficulty with developing such a platform arises from the diametrically opposed requirements that exist for an airplane and a submarine. While the primary goal for airplane designers is to try and minimize weight, a submarine must be extremely heavy in order to submerge underwater. In addition, the flow conditions for a submarine and an airplane are different, due to the order of magnitude difference in the densities of air and water. Platform velocities vary considerably as well and there are also significant differences in the design constraints that arise from the two radically different loading modes that need to be supported by the structural systems.

An airplane's structure acts as a pressure vessel and as such the required skin thickness is relatively narrow, while a submarine is required to withstand enormous crushing loads and consequently the required skin thickness is an order of magnitude larger. Structural systems represent one third of the total platform weight for both submersible and aircraft platforms. However, the weight of a submarine's structure precludes the possibility of flight. The geometry requirements for lifting surfaces capable of operating in air and water have different design drivers. The lifting surfaces on a submarine are small appendages that are placed in such a manner that they are always submerged even when the submarine is surfaced. In contrast, the wings on an amphibious aircraft are placed high on the structure to minimize any potential contact with the water. Finally the power plants of submarines and aircraft have radically different densities. Aircraft engines are relatively light weight because they operate in an environment were there is plenty of available oxidizer. Submarine power plants on the other hand must rely on a snorkel or be air independent and therefore must either carry their oxidizer or use batteries or nuclear power, neither of which is particularly light weight.

The platform should be capable of transporting 8 operators, as well as all of their equipment, with a total cargo weight of 2000 pounds. There are three range objectives set for the platform that correspond to the anticipated three modes of operation: 1) airborne; 2) surface; and 3) subsurface. The minimal required airborne tactical radius of the platform is 1000 nautical miles (nm). The minimum surface tactical radius (defined as flight near the surface of the water which may or may not leverage the ground effect) is 100 nautical miles. The minimum subsurface tactical range is 12 nautical miles. Note that the ranges quoted are the tactical (i.e. one-way) ranges. The platform would need to be able to transit into theater, insert and extract personnel without refueling and this would require the total operational range to be 1000 nm airborne, 200 nm surface, 24 nm subsurface. The extraction is considered complete once the surface transit is finished. At that point in the mission the submersible airplane could meet up with additional air or sea support assets and refuel.

Given the list of diverging requirements and design considerations, the difficulties involved in developing a submersible airplane are clear. It is difficult to find a common solution space when the driving variables for each problem are diametrically opposed. Thus, it is extremely important to properly formulate the requirements for this design concept, because these design requirements will ultimately determine whether a solution space actually exists.