Analytical Estimation of Infrared Signature of Converging and Converging-Diverging Nozzles of Jet Engine

Jet engine hot parts (e.g., jet nozzle) are a crucial source of aircraft’s infrared (IR) signature from the rearview, in 1.9-2.9 μm and 3-5 μm bands. The exhaust nozzle design used in a jet aircraft affects its performance and IR signature (which is also affected just by performance) from the engine layout. For supersonic aircraft (typically for M _∞ > 1.5), a converging-diverging (C-D) nozzle is preferred over a convergent nozzle for optimum performance. The diverging section of the C-D nozzle has a full range of visibility from the rearview; hence, it was not considered a prudent choice for low IR observability. This theoretical study compares the IR signature of the C-D nozzle with that of the convergent nozzle from the rearview in 1.9-2.9 μm and 3-5 μm bands for the same thrust. It is found counterintuitively that the IR signature of the engine with a convergent nozzle is higher than that of an engine with a C-D nozzle for the same thrust, in spite of the lower visibility of engine hot parts mentioned earlier. For an engine with a C-D nozzle, directional IR radiance from the boresight is reduced by ~25% and ~12% (relative to the engine with a convergent nozzle) in 1.9-2.9 μm and 3-5 μm bands, respectively.