While the depiction of an Aurora aircraft seems consistent with the confines of present technological imagination, other SR-71 follow-on prognosticators suggest more alien craft. Aviation Week & Space Technology has proposed a current black aircraft that is something akin to a 1960s Sci-Fi concept. They depict an elongated diamond shaped vehicle (one hesitates to call it an aircraft) similar to a "flattened football." The airframe's dimensions might be 110 x 69 feet. Due to intense heat, the vehicle would have a heat-streaked appearance similar to that of the space shuttle. Contrary to intuition, the aft body would appear distinctly more pockmarked than the fore sections, as if the most intense heat was experienced in this region.
This vehicle would have a dual propulsion system. Jet engines buried in the fuselage would propel the vehicle to supersonic speeds, when a novel external burning mechanism would take control as the fundamental propulsion method:<1>
"In the high Mach regime, misted fuel is ejected from the fuselage midsection -- the 'break point' of the elongated diamond -- across the aft surface tiles, into the area between the fuselage and a shock wave attacked to this break. In essence, the sloping, converging aft fuselage sections form the inside of a 'nozzle' and the shock boundary constitutes the outer surface, creating an expanding exhaust effect, much like that on a conventional rocket. The fuel is ignited by surface heating -- or other means -- creating combustion that accelerates the aircraft up to the Mach 6-8 regime."
Eliminating the human life-support requirements at Mach 8, this flaming, hypersonic pumpkin seed would be unmanned and capable of on-board self- control. This vehicle would be as destructive as it is unusual. A payload of 120- odd nuclear weapons would be dispensed as the flaming stone skipped across enemy skies.<2>
The technical and trade press literature includes a number of references to exotic propulsion concepts that may find application in advanced military aircraft. These include pulsed detonation engines, external combustion engines, and waveriding aerodynamics.
Pulsed detonation engines, also referred to as pulsed detonation wave engines:<3>
"... use a shock wave created in a detonation -- an explosion that propagates supersonically -- to compress a fuel-oxidizer mixture prior to combustion, similar to supersonic inlets that make use of external and internal shock wave for pressurization."
Although early experimental work was conducted on such propulsion concepts in the 1940s,<4> a recent review noted:<5>
"... there has been no previously reported use of PDE devices in any past or recent flight vehicle."
"External propulsion," like the "flaming pumpkin seed" mentioned previously, is another hypersonic propulsion technique currently being actively explored. During the 1950s and 1960s research began on exotic external-combustion propulsion systems. An aircraft would achieve hypersonic flight by pumping fuel from its midsection into a cone of air bounded by its shock wave.<6> Interest in this technique continues.<7>
"NASA said yesterday it wants to modify one of its three newly acquired SR-71A Blackbirds to prove the concept of burning hydrogen fuel outside an engine's exhaust nozzles to improve overall flight efficiency....explore a key propulsion concept for the X-30 National Aerospace Plane known as external burning....Engineers want to inject hydrogen fuel into the air stream under the NASP's engines and ignite it to increase pressure near the nozzles and reduce drag....and fly at speeds up to Mach 3."
Another exotic propulsion technique is "waveriding," in which a vehicle's shock wave remains attached to the leading edge of the aircraft's body in hypersonic flight. This makes it appear that the aircraft is riding its own shockwave. It has been reported that at least one aerospace corporation has developed and is marketing a concept for an unmanned hypersonic vehicle that is designed to operate at speeds around Mach 10 or higher.<8>
External Combustion Pulse Detonation Engine Aircraft<9>
The previous budget analysis pertaining to Aurora is also applicable to the Exotic Propulsion Aircraft. However, while the $1.5 billion appropriated for the Special Update Program is consistent with an effort to develop and test a single high-speed high-altitude aircraft, it is far from clear that this amount would support more than one such effort. It may also be questioned whether decision-makers would choose to carry more than one competing propulsion concept to the prototype flight stage of development.
Thus while budgetary considerations render plausible the existence of test prototypes of either Aurora or a more advanced Exotic Propulsion Aircraft, the simultaneous existence of both is much less plausible. Budget and financial data do not discriminate between the relative plausibility of these two classes of vehicles.
Three classes of observations have been reported to suggest the existence of an Exotic Propulsion Aircraft, possibly using a pulse detonation engine. These observations include distinctive exhaust contrails, unusual engine noises, and intercepted radio transmissions. Various reports suggest that this aircraft's "slow-frequency (about 1 Hz.) pulsing sound" was accompanied by a thick, segmented smoke trail or contrail. Lighting patterns indicated the aircraft is approximately 100 feet long.<10>
Observations of distinctive exhaust contrails that have been associated in some reports with pulsed detonation engines date back to 1989:<11>
"Described as "cotton balls strung on heavy yarn" by D.C. Card, a mechanical engineer who saw it forming at high altitude over Denver in 1989, this distinctive contrail matches exhaust patterns that could be expected from a pulse detonation engine. At its high altitude, the unknown dark-grey aircraft was barely visible in late- afternoon sunlight."
By the middle of 1992, it was reported that:<12>
"Donuts-on-a-rope contrails produced by an unknown high-speed, high-altitude aircraft have been reported throughout the U.S. and Europe, suggesting that the classified "pulser" is no longer confined to a test range... In late January, a similar contrail -- described as a "coiled spring" -- was seen over Scotland behind a very fast aircraft flying east to west. The distinctive contrails have been spotted during daylight hours over Portland, Ore.; Washington Dulles International Airport, Va.; Denver, Colo.; and Edwards AFB, Calif."
In addition, "observers" have claimed that they have heard various distinctive noises emanating from the California sky.<13>
"Observers...tell of a swift, high-altitude light that accompanies the pulsing noise....the light moved from horizon to horizon -- well over 100 miles -- in under a minute."
One observer described the noise as a:<14>
"... very, very low rumble, like air rushing through a big tube."
Another individual said it sounded as if:<15>
"... the sky itself is tearing."
While the engines are supposedly reminiscent of heavy-lift rockets, California's seasoned sky-gazers say the sound is definitely not caused by rockets.
A link between the sightings of the donut-on-a-rope contrail and the reports of distinctive sounds was established in a sighting on 23 March 1992 near Amarillo, Texas. Steven Douglas took a series of pictures of the contrail, describing the engine noise as a:<16>
"... strange, loud pulsating roar... unique... a deep pulsating rumble that vibrated the house and made the windows shake... similar to rocket engine noise, but deeper, with evenly timed pulses."
In addition to providing the first photographs of the distinctive contrail previously reported by many, the significance of this sighting was enhanced by Douglas's reports of intercepts of radio transmissions:
"Air-to-air communications... were between an AWACS aircraft with the call sign "Dragnet 51" from Tinker AFB, Okla., and two unknown aircraft using the call signs "Darkstar November" and "Darkstar Mike." Messages consisted of phonetically transmitted alphanumerics. It is not known whether this radio traffic had any association with the "pulser" that had just flown over Amarillo."
Reports from Scotland suggest that this vehicle has been tested on a world-wide basis:<17>
"People living nearby reported strange ear-splitting noises and mysterious smoke-rings in the sky early this year."
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.
<1> Scott, William, "Scientists' and Engineers' Dreams Taking to Skies as 'Black' Aircraft," Aviation Week & Space Technology, 24 December 1990, page 44.
<3> Scott, William, "Renewed Interest in Pulsed Engines May Be Linked to Black Aircraft," Aviation Week & Space Technology, 28 October 1991, pages 68- 68.
<4> Eidelman, Shmuel, et al, "Review of Propulsion Applications and Numerical Simulations of the Pulsed Detonation Engine Concept," Journal of Propulsion, vol. 7, no. 6, November-December 1991, pages 857-865.
<5> Eidelman, Shmuel, et al, "Pulsed Detonation Engine Experimental and Theoretical Review," AIAA/SAE/ASME/ASEE 28th Joint Propulsion Conference, Nashville, TN, 6-8 July 1992, paper AIAA-92-3168.
<6> Pope, Gregory, "America's New Secret Aircraft," Popular Mechanics, December 1992, page 35.
<7> "NASA plans to test spaceplane external burning concept on SR-71A," Aerospace Daily, 5 March 1992, p.371.
<8> "Secret Advanced Vehicles Demonstrate Technologies For Future Military Use," Aviation Week & Space Technology, 1 October 1990, page 21.
<9> Scott, William, "Scientists' and Engineers' Dreams Taking to Skies as 'Black' Aircraft," Aviation Week & Space Technology, 24 December 1990, page 44.
<10> ibid., page 20.
"Multiple Sightings of Secret Aircraft Hint at New Propulsion, Airframe Designs," Aviation Week & Space Technology, 1 October 1990, page 22.
Pope, Gregory, "America's New Secret Aircraft," Popular Mechanics, December 1991, page 35.
<11> Scott, William, "Renewed Interest in Pulsed Engines May Be Linked to Black Aircraft," Aviation Week & Space Technology, 28 October 1991, pages 68- 6
<12> "New Pulser Sightings," Aviation Week & Space Technology, 6 July 1992, page 13.
<13> Pope, Gregory, "America's New Secret Aircraft," Popular Mechanics, December 1991, page 35.
<14> "Possible Black Aircraft Seen Flying In Formation With F-117s, KC-13s," Aviation Week & Space Technology, 9 March 1992, page 66.
<15> Pope, Gregory, "America's New Secret Aircraft," Popular Mechanics, December 1991, page 35.
<16> Scott, William, "New Evidence Bolsters Reports of Secret, High-Speed Aircraft," Aviation Week & Space Technology, 11 May 1992, pages 62-63.
<17> Campbell, Christy, "Secret US Spy Plane is Kintyre's Dark Visitor," Sunday Telegraph, 26 July 1992, page 1.
<18> Eidelman, Shmuel, et al, "Pulsed Detonation Engine Experimental and Theoretical Review," AIAA/SAE/ASME/ASEE 28th Joint Propulsion Conference, Nashville, TN, 6-8 July 1992, paper AIAA-92-3168.
<19> Scott, William, "New Evidence Bolsters Reports of Secret, High-Speed Aircraft," Aviation Week & Space Technology, 11 May 1992, pages 62-63.
<20> "Blast from the Past," Aviation Week & Space Technology, 25 November 1991, page 23.