In the early days of Daylight Hihg altitude bombing, during WWII, contrails helped german to spot and trace B17 & 24 formation. Therefore, intensive researches were made to find a way to consistently reduce those "here we are" advertising signs.. .Finally, a man was able to design a kit to be fitted on the exhausts of the A/C. At this stage germans engineers had devellopped efficient radars and needed no more to have the incoming raid to be reported visually. The brillant system was given up.. .Amazingly, since a couple of years, NASA started researches on the same subject, as it is thought that airliners contrails create an artificial "cloud coverage" with potential effects on environment. They are using the WWII works to devellop "contrail reducing kit/engine" for tomorrow's civilian jet ..... .I can't remember the name of this WWII era engineer, but at least, his work maybe usefull.