In 1918 the Anglo-American company Cosmos bought Straker-Squire (also known as Brazil Straker), a car and bus manufacturing firm which had branched out into aircraft engine repair and manufacture. This began by first reconditioning, then substantially redesigning and re-manufacturing Curtiss OX-5 engines. Due to the quality of this work, they were the only company permitted to manufacture Rolls-Royce aircraft engines under licence, building Hawk and Falcon engines, major components for the Eagle engine and also 600 Renault 80hp 8Ca engines. Over 1,500 engines were produced in total. The company was one of the first to be brought under Admiralty control, and Fedden and his draughtsman Leonard Butler designed two engines during the war; the Mercury and the smaller Jupiter.
The Bristol Jupiter engine designed by Roy Fedden whilst at Cosmos
In 1919 the range consisted of the 450 hp Jupiter, the 300 hp Mercury and the smaller 100 hp Lucifer. On 9 April 1919 a Bristol Scout F fitted with a Mercury engine set two British records at Farnborough achieving the time to 10,000 ft and 20,000 ft records.
Despite selling off the engineering side of Brazil Straker, which continued to successfully build cars, Cosmos Engineering also produced a small number of Fedden-designed cars. The Cosmos 10.5 of 1919, featured an air cooled 3cyl radial layout 994cc engine of 16 hp and was significantly innovative for its time. Only a limited number were made, and it was followed up by the larger 10.5 CAR which never got beyond prototype stage.