This Day in McDonnell Voodoo History: February 4th

In 1963, a major mid-life upgrade of the F-101B began under Project Bold Journey.  In the first phase, an advanced infrared system was installed, replacing the refueling probe in the nose.  The addition of the new IRST allowed the F-101B to detect the heat signatures of approaching bombers and cruise missiles, allowing targeting of improved AIM-4D Falcon missiles even in the face of intense jamming.   Similar equipment was also retrofitted onto Convair F-102A and F-106 interceptors.

Hughes Aircraft Company had begun work on infrared sensor systems in the early 1950s while developing a stellar-inertial guidance system for the Northrop SM-62 Snark intercontinental cruise missile.  This, in turn, had been based on German research conducted during World War 2.  By 1958, Hughes was testing infrared search-and-track systems but due to the inherent limitations of their uncooled lead sulfide detector arrays, they lacked the desired range and were vulnerable to countermeasures such as flares.  Since lead sulfide seekers could only detect the relatively short-wavelength infrared radiation produced by hot engine parts, they were limited to rear-quarter use.  

Photo credit: USAF

Hughes pioneered research on other compounds and optical elements that would be effective in the 3-6 micron range, where the seeker could detect the heat signature of the relatively cooler exhaust plume and hot wing leading edges and other structures of supersonic targets, significantly increasing detection range regardless of target aspect.  For the new Hughes system, the new detector array used lead selenide, coupled with optical elements composed of exotic materials such as sapphire.  

Originally developed for the canceled North American F-108 Rapier interceptor,  the technology was transferred to existing Century-series interceptors.  By 1962, the Soviets were introducing new, upgraded electronic warfare equipment such as the "Buket" system for a dedicated ECM escort version of the Tu-16 Badger medium-range bomber.  NORAD exercises against Strategic Air Command bombers also highlighted the vulnerability of its interceptor force to low level penetration as the radar returns of the huge bombers were often lost within the even larger radar return from the ground.  The Hughes IRST system allowed the F-101B Voodoo and other interceptors to target bombers, even in the face of intense jamming as well as increasing the probability of detecting low-altitude intruders.  In the case of high-speed, high-altitude bomber or cruise missile targets, the sensitivity of the Hughes IRST often allowed a greater detection range than the MG-13 radar system of the F-101B.

IRST technology fell by the wayside as the next generation of fighter-interceptors introduced solid-state radars using traveling wave tubes (TWT) and master oscillator power amplifies (MOPA) to allow the radar to sense the different Doppler shift of low-altitude targets and enable true "look-down, shoot-down" capability.  However, advanced developments of this technology are becoming more common to augment advanced radar capabilities and provide effective sensors against stealthy or small, low-signature targets such as missiles.  That technology all traces back to the Hughes systems installed in the F-101B and its stable mates in the early 1960s.