US Navy Eyes Technology to Cloak Subs

This article first appeared in Aviation Week & Space Technology.

The U.S. Navy is investing in research that could make its submarines even stealthier with cloaking technology that seems to come straight out of a Tom Clancy thriller or a Star Trek movie script.

Developed by New York-based Weidlinger Associates with U.S. Navy Small Business Innovation Research (SBIR) funding, the technology involves the carving up and altering of aluminum to give it "elastic properties" -- a form of what the company calls "metal water," says Jeffrey Cipolla, a Weidlinger senior associate and principal investigator.

Defense analysts say the technology could be a real game-changer for undersea warfare, a lethal hide-and-seek battle in the depths. Any type of system that makes it easier to hide and more difficult to seek would provide a real advantage.

Submarine warfare is all about acoustics -- and while quieter submarines have long enjoyed the upper hand, detection technologies are now on the advance.

The new technology being developed, the company says, will allow the Navy to deploy underwater systems with reduced radiated noise signatures, smaller active sonar signatures and less vulnerability to multistatic surveillance systems.

The underwater acoustic-evading technology for submarines and unmanned undersea vehicles, or UUVs, now in SBIR Phase II development, creates a coating that features broadband passive waveguides, which redirect acoustic energy around an object, "rendering it nearly undetectable" to active Sonar, the company says.

Current acoustic coating technology falls into two categories: "decouplers," intended to create a large mismatch between hull and water and thereby suppress radiated noise; and "absorbers," which are designed to absorb incident sonar pings.

Cloaking technology encompasses a range of functionally graded materials that combine both effects, the company says, but act by redirecting rather than absorbing incident energy. Ideal cloak coatings, in theory, annihilate both radiated noise and sonar strength.

The coatings also decouple structural vibrations from the water, the company says, reducing passive sonar signature.

The cloak "suppresses all scatters and reflected waves," Cipolla said on June 4 during a presentation at the Navy's annual 2012 Opportunity Forum.

One of the keys to the technology, Cipolla says, is the development of metamaterials. A metamaterial can be made of metal or another solid, but is formed into a microscopic engineered structure (such as a grid or a lattice).

Weidlinger's technology involves creating aluminum with a hexagonal cell structure, forming it into a cylindrical cloak for the submarine hull.

The approach uses the new theory of "transformation acoustics" to derive material layers of different densities and elastic properties, but otherwise conventional mechanical engineering structural, material and analytic methods are employed. As an added benefit, the technology decouples the hull vibrations from the surrounding water, suppressing radiated noise.

"New metamaterials promise to improve radically the ability of structures to evade sonar detection," the company says, adding the physics of these "acoustic-mapped metamaterials" (AMM) is novel: by careful design of their elastic properties, a coating made of AMM guides waves around and away from a cloaked internal region, rendering the object inside undetectable by active sonar."

Like all technologies under development, the company cautions, "AMM will require considerable effort before it becomes a practical reality. AMM theory has only recently been cast in a usable form."

Previous theories based on extensions of electromagnetics are not "physically realizable," the company contends. "Consequently, we propose to conduct extensive numerical experimentation and development to minimize risk. The AMM theory will be deployed to a well-verified finite element code."

Using that kind of development, Weidlinger says, will enable the company to test the AMM theory on submarine-like shapes. The company also plans to conduct finite element studies of AMM microstructures "to ensure that the proposed materials exhibit the desired, complicated and unique elastic properties."

The unit-cell analyses will guide the design of producible composites, the company says. Weidlinger adds that it is consulting with "established manufacturers of naval composite materials, to ensure the AMM microstructures can be manufactured at full scale and low cost."

There are some caveats, however. The exact system will be platform-dependent and the resulting cloak is a heavy garment for the sub -- and the Navy better get used to that idea, Cipolla says, if it wants to pursue this technology.

"Any cloak technology worth its salt is going to be heavy," he says, adding that there will be no "spray-on coating."

The software the company has developed for the technology has reached technology readiness level (TRL)-5, he says. The material maturity is about a TRL-4 and the overall concept is at a TRL-3, Cipolla says.

Credit: U.S. Navy

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