B-2 Bombers over Arkansas (1 Viewer)

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As Terry pointed out, contrails act like a giant arrow in the sky, pointing directly to the plane. This is a serious problem for stealth and spy planes, which might otherwise go undetected. From a military perspective, even a short non-persistent contrail is something that should be avoided. The simplest method of contrail avoidance, and one that has been used since WWII, is to not fly in regions of air that support contrail formation. You can do this by flying low enough that it's too warm, or by making smaller modifications in altitude to avoid flight levels with high humidity.
Unfortunately high altitude humidity is hard to measure. So the U2 spy plane designers resorted to a rather low tech solution, a small rear-view mirror that allowed the pilot to see if he was making a contrail. Later planes used automated contrail detection techniques such as LIDAR.
Contrails form due to moisture in the aircraft exhaust. So an obvious way of preventing contrails would be to remove the moisture. This is not a trivial thing. Extracting the moisture is inevitably going to severely degrade the performance of the engine.
Contrails also form because the exhaust cools quickly enough so that the water vapor condenses before it's been too diluted by the surrounding air to do so. So you could prevent contrails either by making the exhaust hotter so it takes longer to cool, or by mixing it with the ambient air quicker. Once again there's no easy way of doing this without degrading the performance of the engine.
Contrails also generally require condensation nuclei. Often this comes from soot and sulfur and other byproducts in the engine exhaust such as carbon particulates. However it would not prevent contrail formation, as it's very unlikely we could get it down to zero, and there's other condensation nuclei in the air anyway. The use of ultra-low sulfur fuel can provide a reduction in the initial density of the contrail.
Another technique that has been suggested and tried over the years seems initially to be counter-intuitive. It is to add many times MORE condensation nuclei to the exhaust. The large number of condensation nuclei, create a lot more ice crystals, but they are very small, and if the size can be kept under half a micron then they will be sub-visible. It's not a guaranteed fix though, as the contrail can still continue to grow visible if conditions are humid enough. Presumably by the time the trail become visible, the plane will be long gone.
In 1962 the US Air Force developed a contrail suppression technique which resembles chemtrails. It consists of a chlorosulphuric acid spray, powered by compressed nitrogen, that is mounted near the engines. This is sprayed into the exhaust, the heat breaks it down into hydrogen chloride and sulphur trioxide, which acts as the nuclei. As you might guess, the acid was very corrosive, and you needed a lot of it – as much as 3% of the weight of the fuel, it is not environmentally friendly (spraying tons of corrosive chemicals into the air) and the additional complicated machinery to the aircraft is prohibitive. So this approach was only investigated by the military.
One of the more interesting and recent contrail suppression technologies is that the ice crystals in the contrail can be broken down by zapping them with either electromagnetic radiation like microwaves or ultrasonic sound waves. The idea is that the radiation or sound energy will melt or break up the ice crystals into sub-visible sizes, or delay cooling past the crucial mixing point. This has a great advantage over the chemical methods in that it's very clean. No additional pollution is added beyond the extra energy required from the engines. The method is combined with automated contrail detectors, so it's only activated when contrails would form. This greatly reduces the energy needed.
 

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