The TA-4S trainers were not the standard TA-4 with a common cockpit for the student and instructor pilot, but were instead rebuilt by Lockheed with a 28-inch (710 mm) fuselage plug inserted into the front fuselage and a separate bulged cockpit (giving better all round visibility) for the instructor seated behind the student pilot. This arrangement was unique for the RSAF but was not the first by Lockheed (the Lockheed SR-71B and U-2CT/U-2RT/TR-1B/TU-2S trainers also used stepped cockpits). As such converted/rebuilt airframes, these TA-4S trainers were powered by the original Wright J65 turbojet engines as used in the B/C models instead of the Pratt & Whitney J52 used by the purpose-built TA-4E/Fs tandem-seaters from Douglas assembly line; this was the main reason why the RSAF decided to not procure the TA-4E/Fs (if indeed an order was placed) and then having to maintain two different engines to power essentially the same aircraft type (a scenario not unlike having a fleet within a fleet).