The Cox-Klemin XS was a 1920s American experimental scout biplane, the first aircraft to be both launched and recovered from a submarine.
Based on a design by the US Bureau of Aeronautics for a simple single-seat scout seaplane that could be disassembled and assembled quickly. Instead of building the aircraft itself the Bureau of Aeronautics contracted the Cox-Klemin Aircraft Corporation to build six aircraft designated XS-1. The aircraft were powered by a 60 hp Lawrance L-4 radial engine. One aircraft was re-engined in 1923 with a Kinner engine and re-designated XS-2.
As part of a series of studies conducted by the United States Navy after World War I into the possibility of submarine borne observation and scouting aircraft, the submarine S-1 became the experimental platform for this project late in 1923. The XS-1, XS-2 and the Martin MS-1 were used for the trials mounted in a cylindrical pod behind the conning tower. After surfacing the aircraft could be rolled out and assembled, it was then launched ballasting the sub until the deck was awash. The first full cycle of surfacing, assembly, launching, retrieving, disassembly, and submergence took place on 28 July 1926, on the Thames River at New London using the XS-2.
After further trials during 1926 all the experimental aircraft were scrapped.