Paging Mr. Tom Swift: Your Country Needs a Swimming Airplane
Tim Eisele pointed out that the Defense Advanced Research Projects Agency is shopping for a submersible aircraft. Or, at least, they want to buy a feasibility study. Quoting:
DARPA is soliciting 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.
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. [...]
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 is 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.
PROGRAM OVERVIEW
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.
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.
DARPA has identified five major concept design objectives that must be met in order to prove the tactical significance of a submersible aircraft platform as well as five major technical challenges that must be overcome in order to enable the development and demonstration of a submersible aircraft. The major concept design objectives include the following: 1) range; 2) loiter; 3) payload; 4) depth; and 5) speed. The major technical challenges involve the following design parameters: 1) weight; 2) fluid flow regime; 3) structure; 4) lifting surface geometry; and 5) power and energy storage. The major concept design objectives and technical challenges are described in greater detail in Section 1.2.1 Program Metrics. DARPA believes that the risks posed by these concept design objectives and technical challenges can be dramatically mitigated by properly posing the requirements for this platform. The design concept being evaluated here is for a submersible aircraft, not a flying submarine. It is expected that the platform will spend the bulk of its time in the air and will only spend short periods of time submerged. While it is hard to envision a propulsion system that could ever get a craft with the weight of a submarine airborne, it may be possible to submerge an extremely buoyant platform like an aircraft if the operating depths can be minimized. The goal is to reduce the vulnerability of an insertion mission by submerging the transport platform. The fact that the platform need only submerge to shallow depths to significantly reduce the vulnerability of the mission, allows for some flexibility in overall design constraints. The benefits of operating at shallow depths are a dramatic reduction in the crushing loads applied to the structure, along with the ability to supply air to the platform and power plant via a snorkel. DARPA has analyzed the design space and relevant technologies and believes that by focusing the design on submerging an aircraft at shallow depths for a short period of time, a tractable solution to the submersible aircraft challenge problem might be possible.
The development and demonstration of an innovative approach to a submersible aircraft that overcomes the five design objectives and technical challenge areas listed above would be a revolutionary advance in the DoD’s ability to transport operators to coastal locations. [...]
While it may be possible to design a notional submersible aircraft, if the design does not enable tactically significant range, loiter time, payload, depth, and speed it will have little relevance to any DoD mission. Thus, DARPA has identified the following concept design objectives in order to be able to discriminate platforms that would be considered tactically significant.
Range 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.
Loiter The platform should be capable of loitering in a sea-state five, in theater between inserting and extracting personnel for up to 3 days (72 hours). The craft does not need to be submerged during loitering operations; it can operate at the surface.
Payload The platform should be capable of transporting 8 operators, as well as all of their equipment, with a total cargo weight of 2000 pounds.
Depth The operating depth of the platform will be constrained by balancing the need to reduce depth in order to minimize structural loads and snorkel complexity with the need to increase depth in order to minimize any potential signatures that could be generated by perturbing the free surface. The effect that the submerged platform will have on the free surface is exponentially proportional to the depth, therefore the platform should be able to operate at a relatively shallow depth and only have the snorkel affect the free surface.
Speed The speed of the platform in each mode of operation must allow the system to complete a tactical transit (1000 nm airborne,100 nm surface ,12 nm sub-surface) trip in less than 8 hours. This 8 hour time must include any time required by the platform to reconfigure between modes of operation.
The technothriller practically writes itself, doesn't it?
DARPA is soliciting 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.
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. [...]
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 is 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.
PROGRAM OVERVIEW
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.
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.
DARPA has identified five major concept design objectives that must be met in order to prove the tactical significance of a submersible aircraft platform as well as five major technical challenges that must be overcome in order to enable the development and demonstration of a submersible aircraft. The major concept design objectives include the following: 1) range; 2) loiter; 3) payload; 4) depth; and 5) speed. The major technical challenges involve the following design parameters: 1) weight; 2) fluid flow regime; 3) structure; 4) lifting surface geometry; and 5) power and energy storage. The major concept design objectives and technical challenges are described in greater detail in Section 1.2.1 Program Metrics. DARPA believes that the risks posed by these concept design objectives and technical challenges can be dramatically mitigated by properly posing the requirements for this platform. The design concept being evaluated here is for a submersible aircraft, not a flying submarine. It is expected that the platform will spend the bulk of its time in the air and will only spend short periods of time submerged. While it is hard to envision a propulsion system that could ever get a craft with the weight of a submarine airborne, it may be possible to submerge an extremely buoyant platform like an aircraft if the operating depths can be minimized. The goal is to reduce the vulnerability of an insertion mission by submerging the transport platform. The fact that the platform need only submerge to shallow depths to significantly reduce the vulnerability of the mission, allows for some flexibility in overall design constraints. The benefits of operating at shallow depths are a dramatic reduction in the crushing loads applied to the structure, along with the ability to supply air to the platform and power plant via a snorkel. DARPA has analyzed the design space and relevant technologies and believes that by focusing the design on submerging an aircraft at shallow depths for a short period of time, a tractable solution to the submersible aircraft challenge problem might be possible.
The development and demonstration of an innovative approach to a submersible aircraft that overcomes the five design objectives and technical challenge areas listed above would be a revolutionary advance in the DoD’s ability to transport operators to coastal locations. [...]
While it may be possible to design a notional submersible aircraft, if the design does not enable tactically significant range, loiter time, payload, depth, and speed it will have little relevance to any DoD mission. Thus, DARPA has identified the following concept design objectives in order to be able to discriminate platforms that would be considered tactically significant.
Range 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.
Loiter The platform should be capable of loitering in a sea-state five, in theater between inserting and extracting personnel for up to 3 days (72 hours). The craft does not need to be submerged during loitering operations; it can operate at the surface.
Payload The platform should be capable of transporting 8 operators, as well as all of their equipment, with a total cargo weight of 2000 pounds.
Depth The operating depth of the platform will be constrained by balancing the need to reduce depth in order to minimize structural loads and snorkel complexity with the need to increase depth in order to minimize any potential signatures that could be generated by perturbing the free surface. The effect that the submerged platform will have on the free surface is exponentially proportional to the depth, therefore the platform should be able to operate at a relatively shallow depth and only have the snorkel affect the free surface.
Speed The speed of the platform in each mode of operation must allow the system to complete a tactical transit (1000 nm airborne,100 nm surface ,12 nm sub-surface) trip in less than 8 hours. This 8 hour time must include any time required by the platform to reconfigure between modes of operation.
The technothriller practically writes itself, doesn't it?