Two weeks ago, The War Zone
published a brief story on a picture of a curious-looking black object, described as a “tow target,” but with a shape reminiscent of an SR-71 Blackbird spy plane, taken at Kirtland Air Force Base in New Mexico in 1980. Thanks to information readers sent to us, especially from Grigory Omelchenko, we have now been able to identify a contemporary program to develop a high-speed towed aerial target to support the testing of infrared and electronic countermeasures, or IRCM and ECM, respectively. That target, known as TDU-X, is very similar in size and shape to the object seen in the picture from Kirtland.
The caption for the 1980 picture of the object at Kirtland, which is available through the U.S. National Archives and Records Administration, simply says “Technicians conduct a bench test on a diagnostic tow target.” There is very limited information readily available on the TDU-X, as well. It is worth noting that the U.S. military’s Aeronautical and Support Equipment Type Designation System (ASETDS) uses “TDU” as the general category code for towed targets. An “X” suffix of some kind is often used within the U.S. military to denote still-experimental systems that have not yet received some form for formal nomenclature, such as the Air Force’s T-X trainer, which subsequently became the T-7A Red Hawk, and the Navy’s FFG(X) frigates, now known as the Constellation class.
The mystery tow target photographed at Kirtland Air Force Base in 1980.
“The TDU-X tow target is a large center of gravity towed vehicle that has the payload capacity to carry IRCM/ECM devices for airborne testing,” according to a 1976 report from Georgia Institute of Technology’s Engineering Experiment Station (EES). “The target has subsystems to support the IRCM/ECM devices. These subsystems are infrared/radar signatures, scoring, command receiver, telemetry and beacons. Those subsystems that radiate or receive a signal have unique antenna pattern requirements.”
Between 1975 and 1976, personnel at the EES had conducted tests regarding the “radiation properties of several antenna systems” on the TDU-X, as well as BQM-34 Firebee and BOMARC target drones, the latter of which had been converted from retired surface-to-air missiles. That work was done under contract to what was then known as the Air Force Armament Laboratory (AFATL) at Eglin Air Force Base in Florida. AFATL has since evolved into the Munitions Directorate of the Air Force Research Laboratory (AFRL) and remains at Eglin.
The EES tested emissions from five antenna systems on the TDU-X. These included multiple L and X-band antennas, treated as a single system, along with a G-band beacon, all used to help track the target from air and the ground, according to the report. There was also an antenna used to send telemetry data to nodes on the ground and another one to receive instructions from offboard platforms. Lastly, there was an antenna linked to a Digital Doppler (DIGIDOPS) system used to help score the performance of the countermeasures onboard.
Georgia Institute of Technology
A drawing of the TDU-X from the 1976 EES report showing the locations of various antennas.
Georgia Institute of Technology
A complete description of the functionality of the TDU-X’s antenna’s from the 1976 EES report.
“Computer techniques were utilized to model the TDU-X tow target and to calculate antenna radiation pattern coverage for the antenna systems,” the report added. “A scale model of the target was fabricated and the computed patterns were verified by actual measurements.”
There is a drawing of the TDU-X in the EES report, but it’s unclear if it depicts the scale model used in the testing or the actual system that this subscale test article was meant to represent. The general design, with its elongated fuselage and two short wings with nacelle-like pods on their tips, each of which has two outward-canted fins attached to it, is virtually identical to a subscale wind tunnel model that the Air Force tested in 1971.
Those wind tunnel tests were conducted at the Arnold Engineering Development Center (AEDC) at what was then Arnold Air Force Station in Tennessee on behalf of AFATL. This model “tow target” was tested in five configurations, with different nose cones and wing-tip fin arrangements, at simulated speeds of between Mach 0.5 and Mach 0.9. What is today known as the Arnold Engineering Development Complex, also abbreviated AEDC, at Arnold Air Force Base, remains one of the Air Force’s premier wind tunnel facilities. A report on those tests does not offer any additional information on the development of these towed targets, when the project first began, and what the requirements might have been for any final design at that time.
A wind tunnel model of a tow target design that the Air Force tested in 1971. This planform shows a clear relationship to the TDU-X and the tow target photographed at Kirtland in 1980.
A picture of the model actually in a wind tunnel at the Arnold Engineering Development Center in 1971.
Different fin configurations on the wind tunnel model.
An “inlet” nose configuration, rather than a full nose cone, on the wind tunnel model.
The report from AEDC says that the wind tunnel model was 1/6th scale and has a diagram that shows its dimensions. It had an overall length of just over 31 inches, with the solid nose cone fitted, and a wingspan between, not counting the wingtip fins, of just over eight and a half inches. As such, the full-size tow target it was meant to reflect would have been around 15 and a half feet long and have a wingspan of around 4.3 feet. The War Zone had previously estimated that the tow targeted photographed at Kirtland in 1980 was around 12 feet in length, so 15 feet would make sense.
The basic dimensions of the tow target model that underwent wind tunnel testing in 1971.
It’s not clear how long the TDU-X program lasted or whether it led to the actual adoption of this towed target. At least some actual flight testing did occur. A picture, seen below, of an Air Force F-4D Phantom II fighter jet carrying one of these targets on its centerline pylon, in a 1975 document offers an overview of Air Force Systems Command (AFSC), to which AFATL was assigned at the time. The caption simply says “F-4 Phantom aircraft carries a TDU-X tow target on a test of electronic counter measure devices in a controlled environment,” but does not say where it was taken. The aircraft is seen flying over a beach, which could indicate it was shot over the Florida coast near Eglin. To this day, the Air Force and other U.S. military services make use of expansive ranges over the Gulf of Mexico off Florida for tests and training.
A US Air Force F-4D Phantom II fighter jet with a TDU-X tow target on its centerline pylon.
It’s also not entirely clear how it was meant to be employed in practice, but if it was at all similar to other towed targets, it would have used some system to reel it out behind an aircraft carrying it in flight. Radars and infrared seekers, as well as infrared cameras, could then be pointed at it from other aircraft, as well as from positions down below, to test the efficacy of any IRCM/ECM payloads it might be carrying.
A tow target trails behind and below a US Navy QF-4B Phantom II. The high-tensile wire that connects the target to the aircraft is not clearly visible. This shows a typical way of employing such systems.
It’s not clear if the TDU-X was designed to be used destroyed as part of any such testing. The description of the tow target photographed at Kirtland as a “diagnostic” system could indicate that it, as well as the earlier TDU-X, may have been simply intended to gauge the performance of radars, seekers, and other sensors against the payloads it was carrying in non-destructive tests.
In addition, in a way, as described, the TDU-X sounds very much like a predecessor to modern towed electronic warfare decoys that are employed on various kinds of combat aircraft used by the Air Force, as well as the U.S. Navy and Marine Corps, along with foreign military forces.
The TDU-X is also clearly related to the object seen in the 1980 picture from Kirtland, though the latter notably has what appear to be ram-air turbine blades on the fronts of its wing-tip pods. There is no indication from any of the available material on the TDU-X design that it included, or was expected to eventually feature, any similar turbine arrangement. It is worth noting that many towed target systems are attached to aircraft via pods that have ram-air turbines to power the reel system that extends and retracts them. These fans could be linked to power generation systems to run other internal systems, including electronics, without a direct connection to the aircraft, as well. Some podded electronic warfare systems use ram-air turbines for this exact reason.
A close-up of what appear to be ram-air turbine blades on one of the wingtip pods on the tow target photographed at Kirtland in 1980.
A US Navy QF-4B with a different kind of tow target system on its centerline pylon. Ram-air turbine blades are clearly seen on the portion of the system that remains attached to the aircraft.
The shape of the fuselage of the black “tow target” at Kirtland is also different in some respects from what we’ve seen of the TDU-X design. At least one side of the forward fuselage is notably flat, with a bulge closer to the wings, rather than the uniformly cylindrical shape of the TDU-X’s fuselage.
A close-up of the Kirtland tow target’s fuselage showing the flat portion of one side and a bulge closer to the wings.
Beyond that, the Kirtland tow target has the logo of the Air Force Weapons Laboratory (AFWL), rather than AFATL. AFWL has since morphed into the Directed Energy Weapons Directorate of AFRL and remains at this base.
In the 1970s and 1980s, AFWL was also involved in directed energy work, particular lasers, making good use of its proximity to the Sandia Optical Range, also in New Mexico, which was subsequently renamed the Starfire Range. With this in mind, it’s interesting to note that a written response from the Pentagon to a question from the Senate Appropriations Committee regarding the Fiscal Year 1983 defense budget request mentions that the “first in-flight propagation of destructive laser energy to a tow target” was among the “specific accomplishments to date” for the NKC-135A Airborne Laser Laboratory (ALL) aircraft.
A written response from the Pentagon to a question from Senate Appropriations Committee regarding the Airborne Laser Laboratory program in relation to its Fiscal Year 1983 budget request.
The Air Force conducted various tests involving the NKC-135A ALL aircraft, which was fitted with a 10.6-micrometer carbon dioxide laser, between 1975 and 1984. That testing also including shooting down air-to-air missiles, including AIM-9 Sidewinders, and target drones. It’s certainly possible that Kirtland also used later TDU-X variants, or more substantial derivatives of that design, to support its airborne laser work.
The NKC-135A Airborne Laser Laboratory aircraft. The turret, and associated hump, on top of the fuselage, housed components of the carbon dioxide laser system.
What might have happened to this black tow target and how many more of them AFWL might have had remain unclear. The example in the 1980 picture has has “7702-4” painted on the side, which could indicate it was the fourth example in a batch of them. It’s also not clear exactly what programs, beyond the possible connection to the ALL testing, they might have supported and how.
It is also worth mentioning that two separate people claimed this thing was being developed as a towed decoy for the B-1A bomber before it was canceled. It is possible that an offshoot of this towed target program did have that concept in mind, but we cannot find any evidence that anything came of it. A towed electronic warfare and even infrared-enabled decoy for the B-1A would have been quite useful for ensuring its survivability.
The B-1A was envisioned to have one of the most advanced electronic warfare suites ever created. Was a towed decoy part of this program? We don’t know, but we intend to find out.
Such a decoy would have to of been capable of being stable at high speed and the small towed decoys we know today were just not possible during the 1970s when the B-1A was being developed. It’s also worth noting that the B-1A’s born-again successor, the B-1B, was one of the first aircraft in the USAF’s stable to received modern towed decoys. So maybe there is something to these claims, we just need to find documentation of it.
The War Zone has reached out to AFRL’s History Office to see if any additional information is available. In the meantime, the available information about the TDU-X has already provided interesting insights into other work the Air Force was doing that is clearly related to the mystery object photographed at Kirtland in 1980. At the very least, the article we see in the image at Kirtland was a derivative of what we now understand the TDU-X was, and the potential modular nature of that program could mean the article in question was itself and TDU-X of some sort.
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