"Advances in power technology - such as hose in the new German U31, employing a silent fuel-cell system in conjunction with air-independent propulsion -are making non-nuclear submarines more combat effective. Adding some non-nuclear submarines to the U.S. Navy would offer new flexibility in submarine production and operations. Given the mission requirements laid out in "Sea Power 21" and in the Quadrennial Defense Review Report, non-nuclear submarines bring nearly the same unique intelligence-gathering capability as nuclear submarines, especially against less capable adversaries."