Although the earlier F.2 variant performed its role well, continuing safety fears regarding the use of HTP on a day-to-day basis prompted the RAF to investigate alternatives. Based on an old NAA proposal ( the NA-349 ), the F.3 replaced the Stentor rocket motor with a third J-79 engine, fed by a pair of intakes located above and behind the original ones. The new engine was located between and above the original engines, creating a dorsal hump which also blended in to the new intakes. The high engine location left sufficient space in what remained of the original weapons bay for sufficient additional fuel tankage to allow the aircraft to retain a reasonable range with internal fuel only, though it was rare to see the F.3 without external tanks fitted as the third engine made the F.3 quite a thirsty bird.
The location of the new engine meant that the original single all-moving fin could not be retained in its original position, as there was no space above the engine for the required actuators and support structure, and servicing access to the engine would also have been compromised ( hanger height was also a factor ). Instead the decision was made to revert to the layout proposed by NAA for the original A3J, with twin outward canted fins located near the edges of the rear fuselage. After the initial test flights the wingtips were drooped to counter lateral instability at high speed. The canopies were also altered to a single clamshell design to provide better visibility for both pilot and WSO, and a 30mm Aden cannon was added under the forward fuselage, though this last feature involved deleting the IRST system.
Whilst the third engine conferred excellent performance the F.3 was also capable of carrying out extended patrols at low speed, and the prototypes demonstrated the ability to loiter with two engines shut down to save fuel ( in-flight engine start up took less than 2 minutes under combat conditions ). Unfortunately the cost of the extensive conversion meant that only 34 were completed before production switched to the F.4
Built from an Airfix kit, dorsal hump is actually a Tornado lower front fuselage, new intakes cut down from Tornado ones, lots of filler, fins are tailplanes from another Vigilante, canopy and cockpit from a Tornado, tailcone is an old Mirage nosecone.