settings the exit area was too big for the closed engine nozzle giving over-expansion. Free-floating doors were added to the ejector allowing secondary air to control the primary jet expansion. Reasons for C-D nozzle under-expansion and examples For complete expansion to ambient pressure, and hence maximum nozzle thrust or efficiency, the required area ratio increases with flight Mach number. If the divergence is too short giving too small an exit area the exhaust will not expand to ambient pressure in the nozzle and there will be lost thrust potential With increasing Mach number there may come a point where the nozzle exit area is as big as the engine nacelle diameter or aircraft afterbody diameter. Beyond this point the nozzle diameter becomes the biggest diameter and starts to incur increasing drag. Nozzles are thus limited to the installation size and the loss in thrust incurred is a trade off with other considerations such as lower drag, less weight. Examples are the F-16 at Mach2.0 and the XB-70 at Mach3.0. Another consideration may relate to the required nozzle cooling flow. The divergent flaps or petals have to be isolated from the afterburner flame temperature, which may be of the order of , by a layer of cooling air. A longer divergence means more area to be cooled. The thrust loss from incomplete expansion is traded against the benefits of less cooling flow. This applied to the TF-30 nozzle in the F-14A where the ideal area ratio at Mach2.4 was limited to a lower value. What is adding a divergent section worth in real terms? A divergent section gives added exhaust velocity and hence thrust at supersonic flight speeds. The effect of adding a divergent section was demonstrated with Pratt & Whitney's first C-D nozzle. The convergent nozzle was replaced with a C-D nozzle on the same engine J57 in the same aircraft F-101. The increased thrust from the C-D nozzle ( at sea-level take-off) on this engine raised the speed from Mach1.6 to almost 2.0 enabling the Air Force to set a world's speed record of which was just below Mach2 for the temperature on that day. The true worth of the C-D nozzle was not realised on the F-101 as the intake was not modified for the higher speeds attainable. Another example was the replacement of a convergent with a C-D nozzle on the YF-106/P&W J75 when it would not