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 deadly 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.
Hughes Missile Systems
Vietnam was a wake-up call for the fighter community. They didn’t have the right weapons for the job; and that stung them. Then it took several more years, and more proposed Sparrow variants, for the truth to finally hit home. They needed a new BVR missile. The argument for a radically new missile was simple. If an enemy fighter force with an all-aspect, IR missile faced off against a U.S. fighter force using Sparrow, the U.S. force might barely break even in the critical kill/loss ratio that separates victory from defeat.
Thus came a specification for a new kind of BVR missile: It would have the same fire-and-forget capabilities as Sidewinder, but much greater reliability and speed; it cou
ld be carried on much smaller fighters than the Sparrow; and it would throw away the concept of “maximum range” for a more useful and deadly measuring stick—the no-escape zone. No escape means that any target aircraft inside the new missile’s performance “envelope” would be unable to get away, no matter how hard and fast it punched the afterburner or how violently it maneuvered. Because the AIM-7 series had neither the brains nor the energy for such sophisticated maneuvering, it was relatively easy for a skilled pilot to evade, especially with the warning that even a primitive RWR provided.
Five different manufacturers vied for the opportunity to build the Advanced Medium Range Air-to-Air Missile, or AMRAAM. In 1979, the competition was whittled down to just two contractors: Raytheon Corporation and Hughes Missile Systems. After two years of development and competition, Hughes won the biggest AAM contract of the century in 1981. The contract was for twenty-four developmental missiles with options for production of an additional 924, and plans for up to 24,000. The missile, designated as the AIM-120, would take almost a decade to bring into service.
Hughes brought strong credentials and a wealth of experience to the problem of developing AMRAAM. They were builders of the long-serving AIM-4 Falcon series of AAM, and the most powerful AAM in history, the mighty AIM-54 Phoenix. Phoenix, which came into service in 1974 on the Navy F-14 Tomcat fighter, was the first true “fire-and-forget” radar-homing AAM, and has been the airborne shield for the fleet for over two decades. Known as “the buffalo” by the fleet aircrews for its impressive size and weight, Phoenix has a range of up to 100 nm./182.9 km. and the ability to engage multiple targets with multiple missiles at the same time. One of the key objectives of the AMRAAM program was to give pilots of single-seat fighters like the F-15 and F-16 the same kind of firepower and tactical capabilities as the F-14 Tomcat, with its two-man crew and powerful AWG-9 radar /fire control system. It would be a technical challenge to pack so much performance into a much smaller airframe.
Unfortunately, the AMRAAM program ran into terrible technical problems. For years, AMRAAM development and testing failed to go smoothly, mostly because everything in the AIM-120 was generations ahead of the best technology on the Sparrow. The advanced electronics, structures, and rocket motor were difficult to design, qualify, and produce. The real hang-up, though, was the software. The AIM-120 is driven by microprocessors running hundreds of thousands of lines of computer code—more than any AAM in history. After each line of code is written, it has to be validated through rigorous testing. Any faults or problems have to be isolated and fixed, and then the process begins again. This cycle continues until the code is ready to be loaded on tape cassettes for delivery to units equipped with AMRAAM. If this sounds frustrating, try to remember the last time a commercial software program “bombed” on your computer. You probably lost an hour or two of work, rebooted, and drove on, muttering a curse on the programming “geeks” who left the bug in the code. But in a system like an AAM, the software has to be perfect. If it is not, you’ve just thrown $300,000 of the taxpayers’ money into the toilet, and potentially put an aircraft and crew at risk. This was the problem the AMRAAM program faced as the 1980s wore on. Schedules slipped, and the project ran over budget. Hostile members of Congress repeatedly tried to kill the program; and several critical General Accounting Office reports raised doubts that the program could ever “get well.” Finally, Congress threw down the gauntlet, mandating a series of successful live-fire tests before full production of the missile would be authorized. Things started to look grim.
Then, some good things began happening. Fully validated software tapes began to arrive at test sites, and missiles began to fly straight and true against their drone targets. To some of the missile’s critics, it appeared that a miracle had happened. In fact, AMRAAM had followed the normal path of a system controlled by computers and software. It is a hallmark of software-driven systems that they are virtually useless until a valid version of the software is available. But when the day comes that a technician plugs in the final release version of the software, it usually works exactly as promised. Like the Army’s Patriot SAM and Navy’s Aegis Combat System, AIM-120 came of age when its software was finally ready. The final validation of AMRAAM came at the White Sands Missile Range when an F-15C ripple-fired four test AIM-120s at four jammer-equipped QF-100 target drones, maneuvering aggressively and kicking out flares and chaff decoys. Dubbed the “World War III Shot” by test directors, it resulted in all four drones going down in flames. All of the Congressionally mandated tests were passed.
With the problems of testing behind, the first production missiles began to be delivered in late 1988, becoming operational in 1991 when 52 AIM- 120As deployed with the F-15Cs 58th Tactical Fighter Squadron (TFS) of the 33rd Tactical Fighter Wing (TFW) to the Persian Gulf, in time for the end of Desert Storm. As it turned out, the missile did not get a chance to shoot at anything before the end of hostilities, but did acquire plenty of “captive carry” flight time, which is critical to “wringing out” the problems of any new airborne weapons system. The new missile’s chance for combat finally came on the morning of December 27th, 1992, when a USAF F-16C assigned to the 33rd TFS of the 363rd TFW, patrolling a no-fly zone in Iraq, shot down an Iraqi Air Force MiG-25 Foxbat with a single, front aspect, “in-your-face” AIM-120A shot. This also was the first USAF kill for the F-16. Three weeks later, on January 17th, 1993, the AMRAAM/“Viper” combination scored again, when a 50th TFW F-16C escorting an F-4G “Wild Weasel” encountered an Iraqi MiG-23 Flogger in one of the no-fly zones. After sparring with the MiG for several minutes, it launched a single AIM-120A from the outer edge of the missile’s no-escape zone. The missile guided true, downing the MiG as it tried to escape. Later, as the missile was rapidly acquiring the nickname of Slammer from the aircrews, the AIM-120A/F-16C combination scored again over Bosnia. A single AMRAAM scored a kill against a Serbian attack aircraft, this time hugging the ground, dodging through mountainous terrain (three other kills in this engagement went to AIM-9M Sidewinders). The Slammer had silenced its critics, downing three enemy aircraft with three shots—a perfect combat record. No other missile in history, even the legendary AIM-9 Sidewinder or AGM-84 Harpoon, did so well during its combat introduction. This amazing performance deserves a closer look.
If you walk up to an AMRAAM at the factory, the first thing you notice is that it looks a lot like the old AIM-7 Sparrow: a pointed nose cone on a cylindrical airframe with two sets of cruciform guidance/stabilization fins. On the surface, nothing special. As you look closer, subtle differences begin to appear. The AIM-120 is considerably smaller than the Sparrow, based on a 7 in./17.8 cm.-diameter airframe tube, as opposed to the 8 in./20.3 cm. barrel section on an AIM-7. It is shorter, measuring 12 feet/3.7 meters, with a center (stabilizing) fin span of 21 in./53.3 cm., and a rear (guidance) fin span of 25 in./63.5 cm. And it weighs in at a modest 335 lb./152 kg., compared to the hefty 500 lb./227.3 kg. of the AIM-7. This weight difference makes it possible to mount the AIM-120 on launch rails designed for the smaller AIM-9 Sidewinder. In fact, F-16s often carry two AIM-120s on the wing-tip missile launchers. The smooth integration of the F-16 and the AIM-120 makes the missile a favorite among Viper drivers, who claim that they can now shoot anything the larger F-15 can.
At the front of the missile is the seeker section with its electronics, antenna, and batteries. Under the nose radome is the gimbaled radar antenna of the missile. Unlike Sparrow, AMRAAM does not depend on the AI radar of the launching aircraft to illuminate the target to provide guidance for the missile. Instead, the Hughes engineers have built a complete AI radar system into the nose of the AIM-120. The missile can hit a fast-moving airborne target all by itself. All the radar of the launching aircraft has to do is send the missile the three-dimensional position, course, heading, and speed of the target. The missile then flies out to a point where it switches on its own radar. If the target is anywhere inside the radar “cone” of the AMRAAM’s seeker, it locks up the enemy ai
rcraft, interrogates it with IFF to make sure that it is not a “friendly,” then initiates the endgame and streaks in to the kill.
Because it does things that were previously limited to missiles over three times its size and weight, the seeker of the AIM-120 is where the magic happens. The radar antenna of the seeker (produced by Microwave Associates) looks just like a miniature of the dish on the APG-63 and functions in exactly the same way. Just aft of the gimbaled mounting for the radar is the seeker/guidance electronics package. Here are mounted the circuit cards for the Watkins-Johnson signal processor, transmitter, receiver, the digital autopilot, and the battery array. All of this is contained in a series of modules about 24 in./61 cm. long and about 6 in./15.2 cm. in diameter—a marvel of packaging and miniaturization. At the rear of this package is the Northrop strapdown Inertial Reference Unit (IRU), which is the heart of the guidance system. It contains three small gyros (one each for the roll, pitch, and yaw axes), and senses the movement of the missile along its flight path. This allows the AMRAAM’s guidance electronics to calculate any deviations from the programmed flight path and generate course corrections.
Since all of the AIM-120’s electronics are microprocessor-controlled modules, they are easily upgraded by adding new software (uploaded through the aircraft data bus, or inserted on new Programmable Read Only Memory chips). In addition, as the circuits resulting from Pave Pillar and other programs come on-line in the 1990s and beyond, it will be possible to keep the missile up-to-date, including software upgrades rapidly produced during wartime. There are already plans to replace the mechanical gyros in the AMRAAM’s IRU with much more accurate ring-laser gyros. There are even studies to evaluate fitting AIM-120 with a GPS receiver, to enhance its navigational accuracy.