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Strange contrails, described as being like "Donuts-on-a-rope", have been sighted many times, and are commonly associated with hypersonic propulsion systems and the Aurora spyplane and Brilliant Buzzard aircraft.
How these air contrails are produced is still a speculatory matter. Scientists and researchers alike say that they are the product of Pulse Detonation Wave Engines or experimental Pulsejet Propulsion systems. However, under closer examination and testing, results show that an aircraft travelling at such high velocities would be travelling at such a speed as to the "donuts" being too far apart to be a result of the suspected propulsion systems.
