 An exhaust plume contributes a significant infrared signature.The F-117 reduces IR signature with a non-circular tail pipe (a slit shape) to minimize the exhaust cross-section and maximize the mixing of hot exhaust with cool ambient air.The F-117 lacks afterburners, because the hot exhaust would increase the infrared signature, and breaking the sound barrier would produce an obvious sonic boom, as well as surface heating of the aircraft skin which also increases the infrared footprint.