Something extra: Upgrading the F-101B fire control system

Although the book has yet to be widely released, I have continued adding to and editing the manuscript.  In recent months, I have dug up more information on  the F-101B and modification of its Hughes MG-13 fire control system to counter Soviet developments in electronic countermeasures (ECM) capability to defeat the ability of attacking interceptors to target their weapons on the intruding bomber.  I have a little bit of fact checking to do, so this should be considered provisional at this point but here is what I have.  Enjoy!

Bold Journey: The Interceptor Improvement Program

On 1 November 1961, NORAD sent to ADC a new NORAD Qualitative Requirement (NQR) for a Long Range Airborne Passive Homing (LRAPH) system for implementation. The intent was to provide ADC interceptors with the ability to directly locate and kill Soviet jamming aircraft by providing a broadband receiver, antenna, and display equipment to provide relative azimuth and height to the target.  The LRAPH system would also help address the problem of intercepting low-altitude targets as identified a year previously during the first Sky Shield exercise. A similar system was being considered for the Bomarc B surface-to-air missile. Air Force Systems Command was directed by the Pentagon on 26 April 1962 to begin prototype development of an LRAPH system for all ADC Century-series interceptors: The F-101B, F-102A and F-106A under $475,000 of FY 1962 QRC funding. Implementation of the LRAPH system was to take place in two phases, beginning in January 1963.  The first phase, programmed to take one year to complete, would consist of a redesigned “silent lobing” radar antenna, anti-chaff circuit modifications to the fire control system, and the addition of an infra-red search and track system, or IRST.  The second phase would start about a year later and would add parametric amplifiers and rapid pulse-to-pulse shifting of radar frequency to defeat Soviet sweep jamming systems.  The overall effort would soon become known as the Interceptor Improvement Program.  For the F-101B, the LRAPH system modification was undertaken as Project “Bold Journey.” This final major upgrade to the F-101B weapon system took place beginning in February 1963, but it was to provide a tremendous leap in its operational capability.  Under Project Bold Journey, the Interceptor Improvement Program (IIP) centered on modifications to the Hughes MG-13 to improve the radar’s ability to function successfully in the face of enemy electronic countermeasures and offer improved search and tracking capabilities against low-flying targets.  The modification program coincided with the availability of the new and vastly improved GAR-2B Falcon missile, renamed the AIM-4D as it entered production in February 1963.

The most visible modification associated with the Interceptor Improvement Program was the installation of a Hughes infrared search and track (IRST) system in the forward fuselage, replacing the seldom-used refueling probe.  Infrared sensor technology had first been developed by Germany during World War 2 and saw limited use in detecting Allied night bomber formations and cueing searchlights against them to permit engagement with anti-aircraft guns.  In the late 1940s Hughes had begun to adapt the technology for use in a proposed stellar-inertial navigation system for the Northrop SM-62 Snark intercontinental cruise missile.  By the late 1950s, as a spin-off of the now-cancelled LRI which had resulted in the North American F-108 Rapier, Hughes had developed a new infrared sensor as a component of the extremely advanced AN/ASG-18 fire control system beginning in the spring of 1958.   As developed for the F-108, the initial design provided a field of view of 70 x 140-degrees and an angular resolution of 1 degree.  The prototype Hughes system was designated the X-2, followed by mid-1959 with the X-3.   The Hughes IRST initially operated in 2.5 micron range, using an uncooled lead sulfide (PbS) seeker.  As tested in late 1958 through early 1959, the infrared system was to be capable of detecting a tail-aspect B-47 target from 34.8 nautical miles at 45,000 feet and 10.3 nautical miles from directly ahead.  A Mach 3 bomber or cruise missile could be detected at 76.5 n. m. from any aspect due to aerodynamic heating of the wing leading edges and other structures.  By the beginning of 1959, the system had been described as consisting of a 7-inch diameter silicon “irdome” and a multi-element telescopic lens composed of silicon and sapphire elements, features consistent with the use of a very advanced and sensitive lead selenide (PbSe) array to detect infrared radiation at a range of wavelengths between 3 to 6 microns.  The unit mounted on the F-101B is also 7 inches in diameter.  The IRST used liquid nitrogen to cool the seeker to increase its sensitivity and the unit could be slaved to the radar antenna.  Although the F-108 had been cancelled by the summer of 1959, evaluation of the Hughes ASG-18 fire control system continued, using B-58A 55-0665 as a test bed.  The modification included the installation of two IRST units, identical in appearance to the unit later mounted on the F-101B, on either side of the elongated nose housing the huge new radar.  Modification of the B-58 was completed 2 August 1959 with flight tests conducted from early 1960 through February 1964.

The Hughes IRST was added to the Voodoo under T.C.T.O. 1F-101B-945 as the first phase of the Interceptor Improvement Program.  In the Century-series interceptors, the IR seeker heads of infrared-guided Falcon missiles could be cued to the target by the IRST, permitting launch and destruction of the target even in the face of intense jamming. Work began at Ogden Air Materiel Area with two prototype aircraft of 4 February 1963.  All 339 F-101B and F-101F aircraft assigned to the 16 squadrons assigned to Air Defense Command received the upgrade, completed at Hill AFB with the last aircraft completed on 11 December 1964. By the end of December Phase I of the ECCM upgrades for the Interceptor Improvement Program for all Century-series interceptors had been completed with the provision of silent lobing radar antennas, anti-chaff circuitry, and the addition of Hughes IRST systems. 

The second phase of modifications under the Interceptor Improvement Program was covered under T.C.T.O. 1F-101B-944.  This second phase included the installation of a hydraulically tunable magnetron, which allowed rapid changes in the waveform of the radar signal and made the MG-13 much less susceptible to both conventional “noise” jamming and more advanced “sweep” jammers that had begun to enter service with Soviet Tu-16P “Badger-J” ECM aircraft in 1962.  Sweep jamming worked by working across a range of frequencies, transmitting at full power to combine the advantages of both spot and barrage jammers.  Success of sweep jammers depended upon performing tuning changes rapidly to keep the enemy radar jammed while preventing reacquisition of the target as the jamming transmitter swept to another frequency.   The key for new Soviet systems such as the “Buket” used on the Tu-16P was the use of a “carcinotron” tube that could manipulate the outgoing signal much faster than the 2-3 minutes it took for a operator to do the same manually with the older SPS-1 spot jamming systems that had to that point been standard equipment on Tu-16 and Tu-95 bombers.  In the case of the “Buket” system, the aircraft carried several automatic transmitters that could simultaneously jam over a wide range of frequencies.  The same basic equipment was under development for an ECM version of the new, supersonic Tu-22 medium bomber that had begun low-rate production in 1960.  It was reasonable to assume that it would only be a matter of time until the similar equipment would be retrofitted to the “Bear.”  By using the new hydraulically tunable magnetron, the interceptor’s radar could play the same game against the jamming bomber by working within a relatively broad frequency range, but change its specific transmitting frequency much more rapidly than the bomber’s ECM equipment and defeat its ability to adjust to the fire control radar and successfully jam it.  Therefore, the hydraulically tunable magnetron gave the upgraded MG-13 tremendously improved ECCM capability. 

The upgraded MG-13 also had an anti-chaff setting, the counter anti-deception jamming or CADJ mode,  that would introduce a bias toward the leading edge of a chaff cloud, closer to the actual target, rather than the radar center or “centroid” of the chaff cloud, permitting easier reacquisition and tracking of the targeted aircraft. Another feature that greatly improved ECCM ability was to incorporate a lobe-on-receive-only (LORO) modification that greatly increased the resistance of the MG-13 weapons control system to deception jamming.  In common with the E-series radars, the MG-13 used conical scan while tracking a target to refine its angular position. Unfortunately, as deception jamming became increasingly sophisticated, conical scan radars had proved relatively easy to defeat, especially with the use of Inverse Gain deception jamming which depended upon the knowledge of the rotation rate of the nutating radar beam.  Adding a LORO feature meant that rotation rate could not be measured during transmission, so the conical scan rate while the antenna was receiving could not be known.  However, the upgraded radar was not as effective against the newer and more sophisticated deception jammers beginning to enter service in the early 1960s.  A deception jammer, or “track breaker” in USAF parlance of the time, worked by gradually introducing a false return signal, “stealing” the range gate or window when accurate signals were expected to return, and then introducing false range and bearing information based on the strength and timing of the spurious return signal.  During practice intercepts, the AN/ALQ-16 unit installed in the Convair B-58A often proved adept at defeating the “Home-On-Jamming” mode of the Voodoo.   Project Bold Journey continued through 1966, with 339 aircraft undergoing modification.  By December 1966 this final phase of ECCM improvements to the F-101B, F-102A, and F-106A had been completed for the Interceptor Improvement Program with the provision of rapid tuning radars and parametric amplifiers for their radar fire control systems. The late-production Block 115 and Block 120 aircraft supplied to the Royal Canadian Air Force under Queen’s Row were not modified under the Interceptor Improvement Program.

As modified under Project Bold Journey, the upgraded Hughes MG-13 IIP radar operated in I-band in a frequency agile mode between 8500 MHz and 9250 MHz. The pulse repetition frequency (PRF) was 910 Hz for a 0.5 microsecond pulse width, for a range displays of 6, 20, and 40 nautical miles in SP (short pulse) mode and 416 Hz for a 1.0 microsecond pulse width to allow a range of up to 200 miles in LP (long pulse) mode, with 40, 80, and 200 nautical mile range scales. The radar operated in a pulse repetition jitter mode with 70 microseconds jitter and used LORO processing for angle tracking, able to operate at four separate LORO frequencies of 89 Hz, 122 Hz, 145 Hz, and 185 Hz.  At peak power, the MG-13 IIP unit transmitted at 250 kilowatts and as with the original unit and earlier Hughes radars used an RK6249 magnetron tube.  The normal scope display for the radar intercept officer (RIO) was a B-scope, displaying range on the y-axis and azimuth on the x-axis. The antenna could be steered with a joystick on the RIO’s right instrument panel, moving the stick side-to-side to move the antenna in azimuth and using a thumbwheel on the inboard side at the top of the stick to adjust elevation.  With the “Action” switch depressed, the range gate marker was adjusted and placed over the target blip by fore-aft movement of the stick. Once centered on the blip with the range marker lined up, release of the action switch enabled the MG-13 unit to automatically track the target. A-scope display could also be selected to plot receiver signal amplitude on a horizontal range trace. Although the A-scope did not display azimuth, the antenna could be steered to find the highest amplitude signal.  The A-scope display was useful, however, in finding and identifying the true target contact in the midst of jamming.  The 25-inch diameter parabolic antenna could be set to automatically scan in 4 bar, 2 bar, and 1 bar modes to the center or to either side of the flight path, along with a boresight mode.  The antenna limits for elevation were 35 degrees up and 15 degrees down.

The new sensor capabilities of the F-101B were paired with a new version of the Falcon missile, the AIM-4D.  Developed as the GAR-2B by mating the infrared seeker head from the F-106’s GAR-4A missile to the body of the GAR-2A, the new weapon was far more sensitive, less vulnerable to glare or countermeasures, and for the first time in an infrared-homing missile offered at least a theoretical all-aspect capability in both pursuit and lead collision modes.  The AIM-4D was distinguished by a white moisture-proof sleeve covering the forward half of the missile making it much more reliable than the earlier GAR-2A.   The missile armed after motor burnout at 1.4 seconds after launch.  Operating in the 3.8 to 5.4 micron range, the improved IR seeker was sensitive to temperatures ranging from about 200 to 750 degrees Centigrade, able to home on to relatively cooler aircraft engines and exhausts than the earlier GAR-2A.  The seeker had a field-of-view of 6.5 degrees and a gimbal limit of 48 degrees.  The AIM-4D seeker could be slaved to either the fire control radar or the IRST system.  Compared to the GAR-2A, the AIM-4D had a much shorter minimum range, as low as 2,000 feet, a similar engagement envelope out to 90 degrees target aspect angle, and the ability for forward hemisphere engagement.  Just forward of 90 degrees abeam of the target, maximum engagement range ranged from 17,000 feet to 10,000 feet from directly head-on to the target.  The Falcon was now no longer a strictly “tail-chase” weapon and gave the Voodoo a front hemisphere engagement option besides the Genie rocket, now carrying the new designation AIR-2.  The AIM-4D entered service in July 1963 with both the F-101B and F-102A, quickly replacing the older GAR-2A (now AIM-4C) in United States Air Force service while the unmodified Canadian aircraft continued with the GAR-2A. 

During the mid-1960s timeframe of Project Bold Journey, the F-102A was undergoing similar upgrades in sensors and missiles while a retractable IRST was coupled with the more advanced weapons of the F-106 to provide the same tactical advantages for American interceptors.  The Hughes IRST was reportedly sensitive enough to detect a Soviet Tu-95 “Bear” bomber from the front hemisphere due to the massive heat signature produced by its NK-12 turboprop engines and it was not unusual to detect targets on the IRST before achieving a lock with the radar.  Although the IRST in F-101B maintenance manuals is referred to generically as the “IR Receiver and Closed Cycle Cooling System”, the same basic system is referred to as the “90-C IR-search-track set” for the F-102A and the AN/AAS-15 for the Vought F8U-2NE Crusader serving with the U.S. Navy.  The downside of the Hughes IRST was that, at least for its first year of service, it was not at all reliable and spare components were in very short supply, often forcing squadron-level maintainers to use field-expedient solutions to keep the systems functioning.