In June 1943, Hawker fitted a Typhoon with four steel "Mark I" rocket rails under each wing. Trials at the Aeroplane and Armament Experimental Establishment (A & AEE) and Air Fighting Development Unit (AFDU) showed that the combination of the RP-3 rocket and the stable, high-speed platform of the Typhoon was promising. Carrying the eight rails and rockets, it was found that the top speed was reduced by , with no adverse handling effects. As a result, the Mk I rails and RP-3s were first fitted to production aircraft of 181 Squadron in October 1943. At first attempts were made to arm Typhoons with either bombs or rockets depending on requirements but it was soon decided that squadrons would specialise. By D-Day, the 2nd TAF was able to field 11 RP ("Rockphoon") Typhoon squadrons and seven "Bombphoon" squadrons.