Soviet Wild Weasels: Part Three (Missiles)

Kh-28 missile being loaded on Vietnamese Sukhoi fighter bombers

I briefly touched upon the subject of anti-radiation (or anti-radar) missiles in the previous blog post on the specialized aircraft designs that were developed in Russia as counterparts to the American Wild Weasels. Despite the introduction of nuclear weapons in Soviet suppression of enemy air defense (SEAD) doctrine in 1959, there were misgivings within the Soviet air forces on the effectiveness of nuclear weapons against targets like early warning radars and air defense sites. The first dedicated anti-radar missile (ARM) was the Kh-28 (NATO code name AS-9 Kyle), work on which began in 1963. Initially the Kh-28 was designed to be used by the proposed Yakovlev Yak-28N I mentioned in the previous blog post. Work on the Kh-28 by the Raduga design bureau proceeded in tandem with the much larger Kh-22 anti-shipping missile- in fact, in many aspects the Kh-28 is a scaled-down version of the larger Kh-22 that was carried only by bomber aircraft.

Like its larger sibling, the Kh-28 used a liquid propellant rocket engine as the technology of the day meant that solid rocket motors, while being easier to handle and employ, didn't have the range that was needed for the Kh-28, which was on the order of 75 miles, considerably more than any American ARM design at the time. Weighing approximately 1,500 lbs, the Kh-28 was not a light missile. When the Yak-28N was canceled in 1972, when the Kh-28 went into service, it was to be carried by the Sukhoi Su-17 Fitter fighter bombers, a single missile on the centerline station with an associated guidance pod carried on the right inboard wing pylon. The targeting system for the Kh-28 was tuned specifically to counter the American Nike-Hercules SAM radars and as well as the British Thunderbird and Bloodhound SAM radar systems. The first production versions of the Sukhoi Su-24 Fencer strike bomber also had provisions to carry the Kh-28 on one wing pylon and the associated guidance pod on the opposite wing pylon. Besides the liquid fuel handling issues, the main drawback of the Kh-28 was that it had to be preset to a given enemy radar frequency on the ground before the mission. Despite this, the Kh-28 remained in production until 1983 and was exported to several client states. 

Kh-25MP on a Sukhoi Su-17 Fitter

When the Soviet air forces introduced the Mikoyan MiG-27 Flogger strike derivative of the MiG-23 fighter, in th early 1970s, the Kh-28 was simply too large for the MiG-27 to carry. As a result, a smaller ARM had to be developed for use by the Flogger. The Kh-27 was developed by the Zvezda design bureau as a variant of the Kh-23 air-to-surface missile which had the NATO code name AS-7 Kerry. The Kh-23 was roughly in the same class as the American AGM-12 Bullpup missile. The Kh-27 homed in on enemy radar emissions like the Kh-28, but it featured a new solid rocket motor and an onboard autopilot that allowed it to be fired outside the radar range of its target. Being smaller in size, though, the Kh-27 had a much shorter range of approximately 30 miles, just a bit longer than its American contemporary, the AGM-45 Shrike. A MiG-27 carrying the Kh-27 needed a guidance and targeting pod to be carried as well. The Kh-27 entered service in 1977, but was limited in its deployment as it was rapidly superseded in front line service by the improved Kh-25. The numbering seems odd that a lower number is more advanced than the Kh-27, but the Kh-25 (NATO code name AS-10 Karen) is actually a family of missiles with different warheads and seeker heads that was developed from the Kh-23 family of missiles. In the Kh-25 family, the specific ARM variant is designated Kh-25MP which uses the same seeker head as the Kh-27. Both the MiG-27 Flogger and the Sukhoi Su-17 could carry and employ the Kh-25MP. Being a modular family, the Kh-25 family of missiles' closest counterpart in the West would be the AGM-65 Maverick, but having a longer range than the Maverick. The ARM version of the Kh-25, the Kh-25MP, has its own NATO code name, AS-12 Kegler.

Kh-58 on display in front of a Sukhoi Su-24 Fencer

Despite the tactical flexibility afforded by the more advanced Kh-27/Kh-25MP missile, there was still a need for a missile with the long range that the older generation Kh-28 possessed. The Raduga design bureau was asked again to develop a successor to the Kh-28 with the same range and performance but having more advanced features and more tactical flexibility. This missile became the Kh-58 (NATO code name AS-11 Kilter) which featured a solid rocket motor to dispense with the handling of corrosive fuels. It was a different design than what the Raduga OKB had been used to, most of its work on cruise missile type weapons, so the Kh-58 was its first design to have cruciform fins. Carried on a specialized pylon adapter that swings the missile downward and away from the launch aircraft before rocket ignition, the Kh-58 was introduced in 1982 just before the Kh-28 ended production. Designed from the outset to counter the radar systems of the Hawk and Patriot SAM missiles, the Kh-58 is a big missile weighing 1,400 lbs with a range of 75 miles. Its carrier aircraft include the MiG-25BM Wild Weasel variant of the Foxbat, the Su-22 Fitter, and the Su-24 Fencer. While the necessary equipment was internally carried on the MiG-25BM (mentioned in the previous blog post), both the Su-22 and the Su-24 needed an external guidance pod. On the Sukhoi Su-24 Fencer this is the Fantasmagoria pod that functions much like the HTS (HARM Targeting System) used on the Lockheed Martin F-16CJ variant of the Falcon. An improved version of the Kh-58 then followed that was designated Kh-58U and it possessed a remarkable range of 155 miles. The Kh-58's closest analog in the West is the Anglo French AS37 Martel missile, but with a much more impressive range than the Martel. 

Kh-31 test round on the intake pylon of an Su-27 Flanker

When the United States introduced the AGM-88 HARM (High-speed Anti-Radiation Missile), it was clear that American ARMS were gaining in parity with Soviet designs. The HARM had a speed well in excess of Mach 2 and a range greater than 60 miles. The response from the other side of the Iron Curtain came from the Zvezda design bureau which developed the Kh-31 (NATO code name AS-17 Krypton). The Kh-31 is a scaled-down derivative of the much larger P-270 Moskit ship-launched anti-ship missile (NATO code name SS-N-22 Sunburn). The Moskit was unique at the time in using a ramjet to sustain the rocket engine for long ranges at high speeds. the Kh-31 uses a similar ramjet system that acts as a solid rocket engine at ignition, then switches over to a ramjet once it reaches its high cruise speed of Mach 3.6! In fact, the design team for both missiles was the same. Weighing the same as the Kh-58 missile, the Kh-31 was much faster, more advanced and highly accurate. The Kh-31 entered service in 1988 with an anti-shipping variant becoming operational a year later that targeted the advanced AEGIS radar systems of US Navy surface combatants. Unlike previous Russian ARM designs, the Kh-31 is much more flexible and can be carried by nearly all of the tactical aircraft in Russian service from the MiG-29 Fulcrum, the Sukhoi Su-24 Fencer, and the entire Su-27/Su-30 Flanker family. China has a license built version of the Kh-31 that can be carried by its Su-30 Flankers and its Xian JH-7 strike fighters. The first versions delivered were designated KR-1 and were optimized against Taiwanese radar frequencies, but more recently a version of the Kh-31 with more Chinese-specific avionics has entered service as the YJ-91.

Interestingly, when the US Navy canceled its AQM-127 SLAT (Supersonic Low Altitude Target) in 1991, the remaining funds from the program were used in 1996 to purchase a small number of Kh-31s from Russia for use as MA-31 target drones. The missiles were delivered to Boeing lacking the warheads and military avionics and Boeing (back then the work was being done by McDonnell Douglas before it merged with Boeing) added the necessary equipment to convert it into a supersonic target drone. While a follow-on buy was planned, the MA-31 was really on an interim solution as it didn't meet the requirements fully that were laid down for the SLAT program. In 2000 Orbital Sciences was given the definitive contract for the GQM-163 Coyote target missile.

Sources: Soviet/Russian Aircraft Weapons Since World War II by Yefim Gordon, Midland Publishing 2005.  

Wild Weasel Fighter Attack: The Story of the Suppression of Enemy Air Defences by Thomas Withington. Pen and Sword Aviation, 2008