We have all seen commercial jets flying high overhead with streaking white contrails behind them. When seeing a 747, it is possible to see four of these contrails leaving the aircraft before they mix together in the atmosphere and gradually disappear.

Typically, the contrails of these and other conventional jets create a relatively smooth, unbroken line in the sky. On days when the high atmosphere is very still and cold, contrails can remain in the sky many minutes after the aircraft has left the area.

Serrated Contrail

In the early 1990s, a new kind of contrail was discovered. These contrails were also white, and followed a relatively straight line, but they were also more jagged than the usually smooth, more common contrails. And they were also apparently created by aircraft traveling very fast and at very high altitude.

But it was the characteristics of the contrail itself that baffled viewers at first. The contrails were described by some eyewitnesses as "donuts on a rope," or "beads on a string," because they appeared to be white strings with beads spaced at short, precise intervals. They were unlike any other contrail seen before.

Adding to the mystery, no one had definitively identified the aircraft that made these bizarre contrails. Some experts suggested that these contrails could not be made by conventional aircraft because the "structure" of the contrails meant another kind of engine was responsible for making them. And no previously known aircraft was believed capable of creating such a contrail.

The Aurora

During that time, in 1992 and 1993, the alleged Aurora hypersonic aircraft was capturing the attention of the news media, after gaining strong interest among followers of military aircraft. Mainstream newspapers and magazines across the country, and even around the world, devoted column space to the Aurora as the new star in the U.S. Air Force's stable of top-secret aircraft. It was speculated that this superfast spyplane probably utilized one of two possible high-performance engine types. One was called the pulse detonation wave engine (PDWE), and the other was called a pulsejet engine.

These contrails, as depicted in the photos shown, suggest the use of a pulse detonation wave engine (PDWE). This is an engine system in which a short detonation tube is located beneath the fuel layer of a scramjet engine. During the engine's operation, a sustained series of "detonations" occurs. The resulting wave of energy diffracts at the exit point of the tube and decays into a blast wave. The exiting shock is transmitted through the fuel layer, which it compresses and heats to ignition temperature. A rapid burning occurs at that location.

Strong, unsteady shocks are a powerful mechanism for stimulating the mixing and combustion in a scramjet combustor. Theoretically, this leads to the design of shorter, lighter engines, with higher performance at high flight speeds. The detonation tubes also significantly contribute to the engine thrust, and they apparently have minimal operating penalties. These "detonations" produce an engine signature with very low, loud, booming tones, and a serrated contrail indicative of the violent activity within the engine.

The PDWE and pulsejet engine concepts are believed to be the closest available technology that could create one of these unique contrails. It is widely speculated that these are also the most likely engines which could power an Aurora aircraft. If the Aurora does use a PDWE or a pulsejet, the contrail left behind by the Aurora should look very much like the photos shown here.

More accounts of these contrails can be found below:

Wyoming, September 2000 Several images and a sighting report of a "donuts" contrail over Wyoming in September 2000.

Montana 1997 "Donuts" Photo An AAP reader shares a sighting account and analysis of a contrail image released in June 2000. Is this the first look at the aircraft that makes the "donuts" signature?