WASHINGTON - From the deserts of California to the panhandle of Texas, civilian observers have reported intriguing hints that the Pentagon may be testing one or more super-secret aircraft, including perhaps one with a revolutionary form of propulsion.
They say they have photographed contrails shaped like "doughnuts on a rope," seen delta-shaped aircraft with unusual lights, and heard engines that pulse with enough force to make walls quiver and keep sleepers awake.
There is speculation the craft may be next-generation spy planes, either manned or unmanned, capable of flying several times the speed of sound, matching or exceeding the fastest planes to date.
But the evidence, outlined in recent months in the aviation trade press, is greeted with skepticism by many analysts, including some who follow the shadowy world of the Pentagon's secret programs, or "black budget."
And there has been no consensus among observers on what kind of craft they may be seeing. The recent public evidence contains photographs of a high-altitude contrail over Amarillo, Texas, showing a distinctive pattern that observers have described as like cotton balls or doughnuts strung along a rope.
Steven Douglass, a radio hobbyist who photographed the Amarillo contrail, described the noise of the high-speed craft as a "strange, loud, pulsating roar . . . a deep, pulsating rumble that vibrated the house and made the windows vibrate." According to Aviation Week
magazine, there have been other reports of pulser-like engine noises and the distinctive contrails have been reported over Edwards Air Force Base, near Los Angeles; Portland, Ore.; and Denver.
Experts say witness accounts often are unreliable. Even where there is hard evidence - a photograph of a contrail in the sky or seismic activity linked to unexplained sonic booms - the interpretation is open to question.
Still, the reports in Aviation Week and Space Technology, a respected trade journal, have sparked considerable interest among defense and budget analysts who try to determine what the Pentagon may be up to. "Black" programs, for which budget figures are not disclosed, currently account for an estimated 16 percent of the Pentagon's annual $99.9 billion weapons acquisition program, analysts say.
One source with access to classified budget documents says secret programs under way include work on novel propulsion systems and on "stealthy" reconnaissance craft that can elude radar.
AIR FORCE DENIAL
Air Force officials say none of the recent sightings is linked to any of their programs. But the Air Force's 1993 budget request includes a line item - with no amount given - for a research program called "Senior Citizen." Some analysts suspect the program may harbor money for next-generation spy craft. Secret reconnaissance craft over the years have fallen under the general code name "Senior." The high-flying U-2 was called "Senior Year." The supersonic SR-71 Blackbird spy plane - retired in 1990 - was "Senior Crown."
Analysts said money may also be hidden in other Pentagon accounts such as the Air Force's "Selected Activities" - a $5.5 billion procurement item - or within classified Central Intelligence Agency funds.
William Sweetman, an analyst who has written extensively on advanced aircraft, says there's some evidence the military may be working on a "stealthy U-2," a subsonic plane that can loiter at high altitude and snoop over borders without crossing them. Sweetman also is convinced that a program once code-named "Aurora" continues in some form as successor to the supersonic SR-71, but that "it might well be unmanned."
William Arkin, a former intelligence officer who studies Pentagon programs for Greenpeace, says the military is aggressively pursuing high-speed, unmanned drones for reconnaissance and other uses.
Analysts say the doughnuts-on-a-rope contrail that has been reported by observers is consistent with the possible exhaust pattern of an advanced propulsion design called the "pulse detonation wave engine." The principles of such engines have been described in technical papers at recent aviation meetings. The pulsers use supersonic shock waves - created by successive small detonations within the engine - to trigger fuel combustion. The design is said to be more efficient than jet engines for propelling craft designed to fly at speeds of Mach 3 - three times the speed of sound - or higher.
John Pike, director of the space policy project for the Federation of American Scientists, is skeptical. He says pulse detonation engines likely would have from 100 to 240 detonations a second. Given the probable length of the visible contrail in the Texas photo and the number of visible "puffs" on the rope, Pike estimates the mystery craft would have been traveling an incredible 36 times the speed of sound - 50 percent faster than orbiting spacecraft.
But William Scott, Aviation Week's senior engineering editor, said the magazine's analysis of three sets of contrail photos suggests the mystery craft travels a more plausible three times the speed of sound, comparable to the speed of the SR-71.
Scott, who has heard engine sounds at night from his home near Edwards Air Force Base, says they are unlike any other jet noise. "The pulser is distinctive enough that it wakes people up," he said.
In other sightings, closer to airports, observers have reported unidentified craft that may be close to 200 feet long. Aviation Week, citing anonymous observers near Edwards Air Force Base and elsewhere, has reported the possible existence of a large delta-shaped craft similar in size and shape to the experimental XB-70 bomber of the 1960s.
There have been five reported sightings of the craft, beginning in the summer of 1990. It is described as light-colored, with a red light beneath the nose, amber lights near the wing tips and a white light between the main landing gear doors. The engine noise is described as a low-pitched rumble without pulsing sounds.
The most interesting scientific evidence for possible mystery planes comes from unexplained sonic booms in the Los Angeles area in 1991 and this year. The shock waves were recorded by earthquake sensors. Using known sonic-boom profiles for the space shuttle and the SR-71, seismologist James Mori of the United States Geological Survey estimated the speed of the unknown craft at about three times the speed of sound.
Further analysis by specialists at the Massachusetts Institute of Technology's Lincoln Laboratory determined that in one of the incidents, the shock waves were caused by a plane traveling at about 1.1 times the speed of sound, consistent with speeds reached by conventional military jets.
An Air Force official who asked not to be identified says reports of mysterious delta-shaped aircraft near Los Angeles and elsewhere are not linked to any Air Force activity.
Still, the suspicions about mystery planes linger. The Air Force managed to fly operational F-117 stealth fighter planes at an air base near Tonapah, Nev., for several years in the 1980s without public disclosure.