unknown aircraft

If you think it broke the sound barrier then yes F117 was sub sonic. But speed is relative and with that sudden zoom no signs of a zoom in then it's hard too tell I mean just because it looks fast does not mean it is fast.

From blacklist:

is it possible that contrails created by engine mechanism to hide engine's heat ?

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The ideal liquid to cool the jet exhaust is water, which was formerly used to increase thrust/engine power. But to use it to cool your jet pipe needs a a very different set up and more water, because you would use it for longer than the time to take off, than seems reasonable to me.

a Contrail too hide your aircraft would be a poor move it's like trying too hide the fact your zipper is down with a huge neon sign that reads "LOOK AT ME!". there have been chemical methods too hide a contrail but these have the conspiracy paranoids and environmentalists banging on the door. In the B2 they installed a Lidar (Light Detection And Ranging) system that can detect when a trail is forming so the pilot can make adjustments too his flying too prevent it from giving himself away.