Fighter Wing: A Guided Tour of an Air Force Combat Wing tcml-3
Page 19
When the first test launches of what would become the Aerial Intercept Missile Nine (AIM-9) were conducted in 1953, the missile's snakelike flight path towards the test targets provided the name it would carry for the next half century of service, the Sidewinder. From its first tests against target drones, it was a favorite of the pilots at China Lake, because of its high reliability and deadly accuracy. In addition, it could be rapidly and cheaply retrofitted on older aircraft, so a whole generation of existing fighters could enjoy the benefits of AAMs, without the weight penalty of a massive air-intercept (AI) radar for guidance. Sidewinder was quickly adopted by the Navy and U.S. Marine Corps as the standard short-range AAM of the day.
The effectiveness of the little AIM-9 missile was demonstrated in 1958, when the Eisenhower Administration supplied the Republic of China (ROC)/ Taiwan with Sidewinder AAMs and launchers to equip its F-86 Sabrejets. The ROC Air Force was fighting daily air battles with MiG-17s of the People's Republic of China, over two small islands in the Formosa Straits, Quemoy and Matsu. While the AIM-9s were responsible for shooting down only a small percentage of the MiGs destroyed in the battles (most were still shot down by the.50-caliber machine guns of the ROC F-86s), their impact was immense, and the word quickly spread around the fighter world about the deadly little AAM named after a rattlesnake.
Loral Aeronutronic AIM-9L/M Sidewinder air-to-air missiles loaded onto their launch rails. The seeker is contained in the rounded nose of the missile, and the fins are designed to provide good maneuvering control and minimize drag.
Loral Aeronutronic
Over the next ten years or so, the early models of the AIM-9 (usually the AIM-9B variant) fought in theaters all over the world. In the skies over the Indian subcontinent (the 1965 India-Pakistan War), North Vietnam (1965 to 1973), and the Middle East (the June 1967 Six Day War), the Sidewinder was the most effective AAM in service. It shot down more enemy aircraft than any other AAM of the period, and put the longer-range, heavier, and more costly radar homing AAMs like the AIM-7 Sparrow to shame. So effective was the early AIM-9, that when several fell into Communist hands in the late 1950s and early 1960s, the Soviet Union produced an exact copy, the R-13/AA-2 Atoll, for use on its own fighter aircraft.
For all its successes, the Sidewinder had some significant limitations and shortcomings. Many of these became evident in Vietnam. For instance, the early-model AIM-9B Sidewinder was a relatively short-range (about 2.6 nm./ 4.75 km.) missile, and its seeker could only "acquire" a target and "lock" onto it if the firing aircraft was behind the target (within a 90deg arc centered on the target's line of flight). It also was susceptible to being decoyed by flares, infrared jammers, and even the sun. (If you fired at a target within about 20deg of the sun, the missile would ignore the target and lock onto the sun.) The biggest problem, though, was the pilots' lack of in-depth understanding of the missile's performance "envelope" (aviator jargon for things like, "How fast can it turn, climb, or dive at different altitudes and velocities?"). In addition, the early electronics technology of the day (vacuum tubes) simply lacked the reliability to survive the shock of aircraft carrier landings and the tropical heat and humidity of Southeast Asia. As a result, several efforts were initiated by the U.S. military to improve the Sidewinder and other air-to-air missiles.
In 1968, a U.S. Navy study, the "Ault Report," examined the poor performance of U.S. fighter, radar, and missile systems in Southeast Asia. One of the first results of this study was better training of U.S. pilots, to teach them how to maneuver their aircraft into the "heart" of the missile's lethal envelope, thus maximizing the chances of a kill. The development of Dissimilar Air Combat Training courses (DACT, practice dogfights against fighters with different flying characteristics from your own aircraft, using electronic scoring systems that simulate the performance of real missiles) in the USAF and USN, particularly the Navy's famous Top Gun school, did much to improve the performance of U.S. pilots in combat. As for the Sidewinder, there already was a series of product improvement programs in the works to remake the little AAM.
The first of these programs produced versions of the missile for the USAF (the AIM-9E) and the USN/USMC (the AIM-9D). Both versions, fielded in the mid-1960s, featured improved seekers that were cooled (thermoelectrically in the case of the AIM-9E, gas cooled for the AIM-9D). The — E models were converted from earlier AIM-9 B-model Sidewinders, and provided better low-altitude performance than the — B model. In addition, the — D had a more powerful rocket motor for greater range (up to 11 nm./20.11 km.) and an improved warhead. Later, the USAF further modified the — E model Sidewinder to the AIM-9J configuration, with improved aerodynamic surfaces and control for greater maneuverability and range (about 9nm./ 16.46 km.). This second generation gradually expanded the launch envelope, range, and performance of the various Sidewinder versions, so that a pilot could launch from anywhere aft of an enemy aircraft's wings (the rear 180deg hemisphere), from a fairly high off-angle (from the target aircraft's centerline), and from maximum and minimum ranges. These versions of the Sidewinder helped the USN and USAF fighter forces decimate the North Vietnamese MiGs in 1972, and they were the backbone of the Free World's short-range AAM inventory for the rest of the 1970s. A third generation, the AIM-9L, saw service in the 1980s.
The final, and currently deployed, third-generation version of the Sidewinder is the AIM-9M. Like the earlier AIM-9L variant, the "Mike" as it is called, is used by the fighter forces of the USN, USMC, and USAF on virtually every combat aircraft with an air-to-air capability. The Air Force normally deploys the "Mike" on fighters such as the F-15 Eagle and F-16 Fighting Falcon. Its dominant feature is the tubular airframe, 5 in./ 12.7 cm. in diameter, to which the forward (guidance) and aft cruciform fins are attached. At the front of the missile is a tapered nose section with a hemispherical seeker window at the tip. The 5 in./12.7 cm. dimension has been one of the little missile's great virtues, as well as its biggest vice. On the plus side, it has meant that the basic missile and interface has remained relatively unchanged for over forty years. This has allowed aircraft designers to find a variety of inventive ways of adding Sidewinder to the weapons suite of fighters. In the case of the F-16 and F-18, the primary AIM-9 launchers were placed on the wingtips. The down side is that packing improvements into a 5 in./12.7 cm. tube can be difficult. For example, Israeli and Soviet/Russian AAM designers long ago abandoned the small diameter airframe, so they could pack larger motors and warheads inside.
It's what is inside that airframe tube that counts, and the Sidewinder does as much with the limited space available as any missile in the world. The current — M version is some 113 in./287 cm. long, with a forward canard (BSU-32/B) wingspan of 15 in./38.1 cm., and a rear stabilizer (Mk 1) wingspan of 24.8 in./63 cm. Weighing in at 194 lb./88.2 kg., it was first produced in fiscal year 1981. At the front of the missile is the WGU-4A/B Guidance Control Section (GCS). Inside the GCS is the seeker, which is the ultimate in single-element infrared sensitivity. Composed of an indium-antimonide (InSb) detector element, cooled by an open-cycle Joule-Thompson cryostat, it is mounted on a gimbaled "head" behind a magnesium-fluoride (MgF, a fragile material, but selectively transparent to infrared radiation in the seeker's particular wavelengths) seeker dome/window. The seeker element feeds into a signal processor, which generates the commands for the missile's four guidance fins, which are mounted on the side of the seeker-guidance section. The real beauty of the current system is that it scans in two different wavelengths or "colors." This means that it is looking at both short- and middle-wavelength (infrared) light as well as the long-wavelength (ultraviolet) spectrum. It is a deadly combination.
Just forward of the rocket motor is the WDU-17 Annular Blast Fragmentation (ABF) warhead section. Previously, there had been a great deal of criticism over the relatively puny size of the Sidewinder's warhead. Thus, when development of the third-generation AIM-9 began, the designers decided to enhance the destructive power of the 25 lb./11.36 kg. warhead. The previou
s versions had provided a mixed bag of weapons effects. The solution was a new kind of proximity fuse that would detect when a target aircraft got into lethal range and detonate in such a way that the force (and fragments) of the warhead would impact directly into the target aircraft. Composed of a ring of four pairs of laser-emitting diodes (somewhat like the IR emitter/detectors on your TV/VCR remote controls) and laser detectors, the DSU-15/B Active Optical Target Detector uses the laser detector ring as a way of determining when a target aircraft is within range. If the missile should miss the target (a rare occasion due to the accuracy of the guidance system), the warhead is designed to detonate and spew its fragmentation pattern at the target aircraft. This is a particularly effective kill mechanism, since the second- and third-generation Soviet fighters that the AIM-9L/M was designed to attack had no self-sealing fuel tanks or fuel bladders. In fact, Soviet designs like the MiG-23/27 Flogger and the MiG-25 Foxbat usually had only a thin skin of aircraft-grade aluminum or stainless steel between their fuel supply and the open sky. This meant that if so much as a single hot fragment penetrated a fuel tank, the Soviet aircraft was probably going to be headed down in a ball of flames.
At the rear part of the airframe is the rocket motor. Over the years, the USAF and USN have differed over what they want from the propulsion system of the Sidewinder. In fact, this has been the basic philosophical difference between USAF and USN since the first of the improved Sidewinders began to roll off the lines in the 1960s. The Mk 36 rocket motor in the AIM- 9M favors the USAF point of view. With the Mk 36, an M-model Sidewinder can theoretically fly out to a range of up to 11 nm./20.1 km. with a maximum flight time of one minute.
What does all of this mean when it comes to real-world combat? Well, consider the performance of AIM-9L/M-series AAMs in U.S. service over a ten-year period from 1981 to 1991. In that time, some twenty-two missiles were fired, with sixteen guiding to hits, resulting in some thirteen "kills." During the same period, foreign clients have scored an even better record, with two kills going to Saudi pilots, twenty-five to Royal Navy Sea Harrier pilots in the Falklands, sixteen to Pakistani aircrew, and probably several dozen more to the Israelis. This run of success may never be duplicated by any future model of AAM.
For all the high technology and old-fashioned ingenuity that have gone into making Sidewinder so successful, it is still among the easiest of missiles to use. When the pilot of an F-16C wants to launch an AIM-9M at a target, all that is required is to select AAM from the stores control panel. At this point, the seeker in the nose of the missile begins to look for a target in front of the fighter. If the radar is already locked onto a target, the seeker head can be slaved to the radar, and the seeker will lock onto the desired target. The pilot is informed of the lock-on through an audio tone in his/her headset. When the tone becomes a solid "growl," the missile is ready to launch. At this point, all the pilot has to do is squeeze the trigger gently, and the missile is on the way. The pilot of the F-16 is now free to fire another missile, seek another target, or just "get the hell out of Dodge City," should that be necessary.
AIM-120 AMRAAM
The pilots call it the "Slammer," and it is the fastest, smartest, most deadly AAM in the world today. It works so well that an F-15 pilot compared shooting down enemy aircraft with the AIM-120 to "clubbing baby seals, one after the other… whomp… whomp… WHOMP!" It is a telling statement, even more telling when you consider that the AIM-120 Advanced Medium Range Air-to-Air Missile (AMRAAM) program was nearly stillborn because of development problems and Congressional opposition. Its long and painful gestation, particularly in software and production engineering, came close to killing it repeatedly in the 1980s. Yet just four years into its service life, the initial model, the AIM-120A, is the most feared missile in the history of air warfare. In spite of that, AMRAAM would never have been needed if its predecessor, the AIM- 7 Sparrow III, had not been such a terrible disappointment.
The AIM-7 Sparrow was born as the Sperry XAAM-N-2 Sparrow I out of a 1946 Navy program called Project Hot Shot. Hot Shot sought to find an airborne solution to the kinds of jet and kamikaze aircraft encountered at the end of World War II. While it went into production in 1951, the first Sparrow AAM did not intercept a test target until 1953 at Inyokern in California, and finally went into USN fleet service in 1956. That first radar homing AAM utilized a "beam riding" radar guidance system that was really only capable of hitting large, bomber-sized targets flying straight and level. Realizing the limitations of the Sparrow I, in the late 1950s, the Navy began a program to improve the missile into a weapon with greater tactical capability. Out of this effort came the AIM-7C Sparrow III, produced by Raytheon in Massachusetts. This new version retained the basic airframe and propulsion package, but used a new guidance scheme known as "semi-active" homing, in which the radar of the firing aircraft "illuminates" a target aircraft with its radar, and the missile seeker homes in on the reflected radar energy. This puts the burden of the intercept problem on the aircraft's radar, allowing the missile to be smaller, lighter, and supposedly simpler. If it were only that easy!
When the Sparrow system was conceived just after World War II, the electronic technologies that make guided missiles effective and reliable just weren't there. Early airborne radar/missile designers had to make do with vacuum tubes, early analog computers, and complex, bulky logic circuit boards. Thus Sparrow has spent its long service life hamstrung by primitive technology. For example, keeping the target illuminated throughout the flight of the missile required the launching aircraft to remain in a tactically disadvantageous position — flying straight and level instead of maneuvering aggressively. This became particularly evident in Vietnam, when unrealistic ROE were politically determined at the Presidential level. The ROE prohibited use of the Sparrow at medium/Beyond-Visual-Range (BVR) distances where it was capable of destroying an enemy target with little risk to the launching aircraft. (BVR meant greater than 20nm./36km. Pilots prefer to think in terms of a missile's "no escape zone," an ever-changing teardrop-shaped volume of space with dimensions that are classified.) This forced crews of the heavy F-4 Phantoms that used the Sparrow to close to visual range with the more agile North Vietnamese MiGs, making it nearly impossible to maneuver the big fighter's radar onto the nimble enemy interceptors.
An F-16C launches a Hughes AIM-120 Advanced Medium Range Air-to-Air Missile (AMRAAM) during testing. The fighter also carrier AIM-9 Sidewinder missiles on the wingtips.
Hughes Missile Systems
And then the firing sequence was a nightmare. The AIM-7E2 version of the Sparrow III, used throughout the Vietnam War, had over ninety electrical, pyrotechnic, and pneumatic functions that had to work perfectly in the proper order, and took over three seconds just to get out of the launch well and on its way to the target. If that was not bad enough, the AWG-10 radar system on the U.S. Navy F-4J was roughly comparable in parts count and design complexity to the Surveyor-series of unmanned moon probes launched in 1966. And the lunar probe only had to function in the relatively benign vacuum environment of the moon for a month or two. The AWG-10 had to function after being slammed around repeatedly by catapult takeoffs and carrier landings in tropical conditions. As a result, the Sparrow III, as well as the radar systems of the various models of F-4s, had severe reliability problems. The Project Hot Shot engineers had never anticipated the possibility that AIM-7 missiles and their associated black boxes might be catapulted off aircraft carriers and do arrested landings three times a day in the steaming heat of Southeast Asia for weeks on end. In short, nobody had anticipated the nature of air warfare in the real world, and the Sparrow AAM was one of the victims of that lack of vision.
In short, it would be nice to say that the radar-guided Sparrow has been as successful as its heat-seeking cousin, the AIM-9 Sidewinder. But it would be a lie. The AIM-7 has been a disappointment, despite tens of billions of dollars spent on it and its fire control radars. When it is used properly, and the breaks go its way, it can be the most deadl
y of AAMs. But its designers promised a "silver bullet," and it never delivered, proving that no matter how much money you throw at a program, basic design limitations cannot be overcome. Some of the technologies the AIM-7 was based on were just fundamentally flawed. Nevertheless, the Sparrow has served for five decades, and continues to soldier on, periodically improved and updated. It became a primary weapon of the F-15 Eagle, and is carried on most other U.S. and NATO fighters capable of air-to-air operations (such as the U.S. F-14 Tomcat and F-18 Hornet). Slowly and painfully, shortcomings and problems were overcome, at a cost of billions of taxpayer dollars. Finally, some forty-five years after it was conceived, the Sparrow III got its day in the sun during Operation Desert Storm. The good news was that the final major version of the missile, the AIM-7M, shot down more Iraqi aircraft (twenty-four) than all other weapons combined, and that it was over four times as effective as it was in Vietnam. (In Vietnam, the AIM-7 had a success rate of about 9 %, while in Desert Storm, depending how you interpret the data, it was about 36 %.) The bad news was that almost half the AIM-7s launched failed to function properly, and only about one Sparrow in three actually hit and killed anything. Out of seventy-one AIM-7Ms fired in Desert Storm, only twenty-six hit their targets, for twenty-three kills. It was as good a performance as the Sparrow ever gave, and it stank. Luckily, there was already a replacement on the way.
Two Hughes Missile Systems technicians move an AIM-120A Advanced Medium Range Air-to-Air Missile from the assembly line to the shipping area.