Recently, I have been researching supercruise and its impact on IR stealth. Some of you may have read this
particularly pages 35-40, where Stillion argued that supercruising greatly compromise IR stealth and therefore supersonic speed should not be attempted. Specifically, table 4 claims thats supercruising at M1.6 increase the IR detection range by a factor of 2.9. If this is true and we assume the subsonic IR detection range to be 50km (PIRATE IRST claimed range), then a supercruising J-36 at M1.6 would be detected at 145km. This would imply that the supercruise capability of the J-36 are only relevant to rapidly moving from point A to B and are not well suited to air combat due to the IR stealth penalty. It would also imply that subsonic drones or even the B-21 are the ideal platform for future air combat, which is Stillion’s main point in the report.
View attachment 144869
Actually, Stillion made several mistakes that significantly overestimated the IR stealth penalty. First, he incorrectly claims that the Mach cone is visible in IR and increase the effective area of a supersonic object. In fact, Mach cones are invisible and the front projection area of supercruising aircrafts are the same as subsonic aircrafts. As a result, his calculations overestimate the IR emission of supercruising aircrafts by a factor of around 10.
Second, he assumes the shock cone temperature applies uniformly to the visible forward projected surface area. This is not true as the temperature decreases moving away from the tip of the nose. So the average aircraft is cooler than his calculated numbers, although the specific degree depends on the design.
Third, he failed to account for IR reflections from the sun and earth. An IRST detects the combined the IR radiation from the thermal radiation from the object itself as well as reflected light. How signifiant are the reflection from the earth? According to this paper (
), quite a lot. In fact the earth irradiance overshadows the IR emission at between 20 to 40 C. This roughly corresponds to supercruising at M1.5.
View attachment 144864
Additionally, there are a number of strategies aircraft designers can use to suppress IR signatures. These include reducing surface emissivity, concentrating emissions in atmospheric absorption bands and directional emission to reflect IR radiations away from threat sensors (the last strategy is particularly viable for stealth aircrafts because the same geometry good for radar stealth is also good for IR stealth).
Interestingly enough, most of the paper I came across while researching this topic from Chinese authors, and they have unique tastes in making their graphical illustrations on how their research may be applied. Some examples below:
View attachment 144866
View attachment 144867
View attachment 144868
In conclusion, we should dispell the myth that supercruise is not compatible with IR stealth. The signature increase was overstated and there are many strategies to manage signature, with very active research coming out of China.
References:
