Camm's response to this rejection was to further develop the design, during which a retractable undercarriage was introduced and the unsatisfactory Goshawk engine was replaced by a new Rolls-Royce design, initially designated as the "PV-12", which went on to become famous as the Merlin engine. In August 1934, a one-tenth scale model of the design was produced and dispatched to the National Physical Laboratory at Teddington. A series of wind tunnel tests confirmed the aerodynamic qualities of the aircraft were in order, and in September 1934, Camm again approached the Air Ministry. This time, the Ministry's response was favourable, and a prototype of the "Interceptor Monoplane" was promptly ordered.