When it became apparent that the conversion to F.3 standard was costing nearly as much as a complete new build aircraft, the RAF asked for alternatives which would give similar performance but an affordable cost. After a 'paper based' competition, the F.4 configuration proposed by BAe Warton was selected. The F.4 modifications included replacing the original 17,000lb thrust J79 engines with RR Spey 205 engines delivering 25,000lb thrust each in reheat, giving similar total thrust to the F.3 but with significantly lower weight, cost, and fuel consumption. The combined dry thrust of over 25,000lbs also conferred the ability to cruise at supersonic speeds without the use of afterburner.
Apart from the revisions to the tail to accommodate the larger exhausts, the Spey installation also required larger intakes and revised internal ducting to cater for the greater mass airflow. The F.4 also included many of the features developed for the F.3, including the drooped wingtips, and revised cockpit, though the Aden cannon was moved further aft and offset to starboard to avoid shooting off the IRST ?, with ammunition stored in the lower electronics bay in space made available thanks to advances in electronic miniaturisation reducing the volume occupied by the avionics fit. A proposal to fit two cannon on the sides of the forward fuselage were abandoned due to concerns over engine ingestion of gun gases. The F.4 also received small leading edge root extensions.
The major increase in the combat capability of the F.4 however came with a new weapon system. The advent of the digital AN/APG-71 radar in the late 1980s finally provided the Vigilante with the AIM-54 Phoenix capability previously impossible due to there being insufficient space in the airframe for the AN/AWG-9. Two Phoenix could be carried in tandem under the fuselage centreline ( conversion to/from Skyflash/AMRAAM configuration took around 1 1/2 hours ), and four of the wing pylons could also carry the big missile.
Built from the Airfix kit, nose from a Phantom, tailcone is actually an F-15 nose, exhausts also from an F-15 ( couldn't find any Spey ones ). Intakes widened by fitting std ones a bit further out and filling the gap with putty, canopy and cockpit from a Tornado. Cannon is cut down from an Airfix Hawk item.