Finally, in 1998, another aircraft spotter videotaped two unusual contrails in quick succession. One of the sights appeared to be a fireball, while the other was described as "doughnuts on a rope." However, this latter bit of evidence is particularly shaky, as this sort of contrail is not all that rare with passenger airliners either.

Although these observations are intriguing, they are also difficult to reconcile with one another. While many observers agree on the unusual sounds created by these vehicles, a range of descriptions are provided as to the nature of these sounds. The pulsating tone emanating from these sightings has been taken as an indication of the use of some form of pulse detonation engine. Some observers report a characteristic frequency as high as 60 Hertz, while others suggest a frequency as low as 1 Hertz.
But a technical analysis of pulse detonation engines suggests that engines operating at the thrust levels associated with military aircraft would operate a between 100 and 200 Hertz (pulses per second).<18> While doppler shifting may reconcile this value with the reported 50-60 Hertz pulsation, it is more difficult to reconcile this with the reports of a 1 Hertz pulsation.
It is also difficult to reconcile a pulse rate of 100-200 Hertz with the observed donut-on-a-rope contrails.<19> The association of these contrails with a pulse detonation engine would seem to be predicated on the observation that each "donut" is a product of a single pulse detonation. Based on published photographs, the "donuts" appear to be approximately 100 meters apart. Assuming a detonation pulse rate of 100 Hertz, this would imply a velocity of 10 kilometers per second, or 36,000 kilometers per hour (roughly Mach 36), one- and-one-half times orbital velocity. While it is asserted that the Exotic Propulsion Aircraft is a high-speed vehicle, this is at least four times faster than the speeds normally associated with this aircraft.
In addition, a closer examination of the published photographs reveals a significant irregularity in the spacing between the donuts on the rope. This would seem to be inconsistent with the normal functioning of a pulse detonation engine.
While various assumptions might be brought forward to reconcile these observations with theory, the evidence is not obviously consistent with the presence of a pulse detonation engine.
And the unusually loud, rumbling sonic booms reported along the California coast actually have a precedent in a deep black program well prior to the 1970s era THAP. According to an historical Lockheed document, during test flights at high altitude, the Lockheed A-12 "Oxcart" (an SR-71 predecessor) would make an "ominous rumble on the ground.<20> Thus, while the reports of unusual auditory signatures may be indicative of the existence of some type of advanced air vehicle, they do not appear to constitute conclusive evidence of the existence of an Exotic Propulsion Aircraft.