An update to the "Exotic Propulsion Aircraft" page on the Federation of American Scientists web site that was recently brought to my attention provides a new photograph of a "donuts on a rope" contrail, shot on November 10, 2006. These contrails are interpreted by many people as evidence of secret, high-speed aircraft utilizing Pulse Detonation Wave Engines (PDWE). The "donuts" in the contrails are thought to correspond to the individual detonations that propel the craft.
I would dearly love to believe that the origin of these contrails is a secret aircraft, because I'm fascinated by such things, because I've been waiting to see such a contrail ever since I first read about them in the May 11, 1992, issue of Aviation Week & Space Technology magazine ("New Evidence Bolsters Reports of Secret, High-Speed Aircraft", page 62), and, most particularly, because I observed and photographed two "donuts on a rope" contrails on June 26, 2006.
The photos shown here are of the second contrail, which was the more clearly defined of the two. They were shot from windows on the 25th floor of The University of Texas at Austin Tower using a Canon Digital Rebel camera set at ISO 200 and equipped with a Canon EF 100-400mm f4.5-5.6 image-stabilized lens. The contrails proceeded from east to west. These photos were taken between 8:01 and 8:03 PM CDT.
The final photo, taken at 8:03 PM, shows the origin of the contrail as it neared the western horizon. In order to maximize contrast, the photo's dynamic range has been expanded to its useful limits, a yellow filter applied, and it has been converted from color to monochrome. Unfortunately, there isn't enough detail to identify the aircraft. However, given the specificity of the time and place, FAA flight logs should make an identification possible. (If someone does make that identification, I hope they'll leave a comment with the details.)
For myself, I do not believe PDWE-powered aircraft were responsible for either of the contrails I observed. First, it seems unlikely to me that a secret aircraft would be flown over a large city in broad daylight, especially twice in one afternoon. Second, I heard no remarkable sounds before, or during, my observation of these contrails. Third, and most significantly, the photograph of the origin of the contrail shows that the contrail begins as two separate trails, each one presumably from the jet engines on either side of a commercial airliner. I can only speculate that atmospheric conditions, combined with the air turbulence produced by the aircraft, conspired to merge the two trails into one, and to disturb it at regular intervals, perhaps as a wake vortex periodically intersected the trail.
Of course, I'd much prefer to believe that I'd recorded evidence of a secret aircraft, due to my long-standing fascination with such things (and what photographer wouldn't like to have taken an "important" photograph?), but I believe the evidence points to a prosaic origin in these two cases, and ultimately leaves me in doubt as to the significance of any of the "donuts on a rope" observations.
Disappointing, but interesting, nonetheless.