Rolls-Royce had a design proposal, the RB.169, for the aircraft at the time of Concorde's initial design[91] but "to develop a brand-new engine for Concorde would have been prohibitively expensive"[92] so an existing engine, already flying in the supersonic BAC TSR-2 strike bomber prototype, was chosen. It was the BSEL Olympus Mk 320 turbojet, a development of the Bristol engine first used for the subsonic Avro Vulcan bomber.