Aircraft engine's infrared lock-on range due to back pressure penalty from choked convergent nozzle

The study of lock-on envelop of an aircraft is of fundamental interest for determining its susceptibility to infrared (IR) guided threats. The performance of IR signature suppression system for aircraft engine is affected by the penalties associated with its installation. This paper gives the qualitative and quantitative assessment on the IR lock-on range (RLORLO) due to increase in the engine back-pressure penalty, for several operating conditions. As a case study, the effect of reducing the exit area of a choked convergent nozzle of a turbojet engine on its IR signature is analyzed. Reduction in exit area of choked convergent nozzle below the design point value results in optical blockage of hot parts and therefore, solid angle subtended by the hot parts reduces. But the effect of back-pressure penalty is to increase the IR signature from the rear aspect and in the net, the IR signature and RLORLO for the same thrust level increases. Polar plots in 2-D of RLORLO for different view angles with reduction in choked convergent nozzle exit area in horizontal and vertical planes are examined for 3–5 μm band.