Fighter Wing: A Guided Tour of an Air Force Combat Wing tcml-3
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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 could 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, an
tenna, 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 aircraft, 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.
Just aft of the seeker guidance section is the AMRAAM's armament section, which contains the warhead and target-detection device. The warhead is an ABF-type warhead built by Chamberlain Manufacturing which weighs in at a hefty 50.6 lb./23 kg., using a ring of contact plus/laser proximity fuses, just like the AIM-9M. While not as powerful as the big warhead on the AIM- 7M, it can down virtually any aircraft in the world today.
Just behind the armament package, and taking up fully half the AIM- 120's length, is the single-grain, ducted rocket motor built by Hercules. It is a fine compromise between a fast-burning, high-impulse motor and one which burns with lower thrust for a longer time. What makes this possible is the small size and low aerodynamic drag of the AIM-120 airframe. The missile rapidly accelerates to about Mach 4 (plus the speed of the launch aircraft), and can sustain this with an intelligent autopilot designed to conserve the vital "smash" energy that creates the no-escape zone. The result is a missile with the ability to virtually guarantee a kill against an approaching head-on target out to something like 40 nm./73.2 km. In a "tail chase" engagement, which requires the missile to overtake the target, this range drops to probably around 12 nm./21.9 km. These numbers should be considered approximate, because DoD is very sensitive about the precise no-escape range at various points of the AMRAAM envelope. At the rear of the missile are the maneuvering fins. Hughes found that rear-mounted maneuvering fins enhance the ability to turn rapidly during the terminal endgame.
So how do you launch an AMRAAM missile shot? If you are flying an F-16C, you select an air-to-air mode for the radar such as BORE (Boresight — i.e., where the radar is sighted down the centerline of the aircraft) or TWS, DOGFIGHT (where the radar is in a mode useful for close-in dogfighting). Then you thumb the missile selection switch on the control stick for AIM- 120, and select either SLAVE or BORE to program the missile radar to accept commands from the F-16's onboard APG-68 radar. The SLAVE option locks the missile seeker onto whatever target the aircraft's radar is currently tracking, while BORE simply points the aircraft's radar straight ahead along your line of flight — the first target it sees will be locked. Once a radar contact is established, the onboard weapons computer establishes a fire control solution, including elapsed time from missile launch until the AMRAAM's radar goes active. At this point, the F-16's Heads-Up Display will begin to give you steering cues to bring the aircraft into range to fire. Once the HUD gives you an IN RANGE indicator, you press the weapons release ("pickle" switch) on the control stick. At this point, the missile is launched and will accept updates from the radar (if you have selected a FIRE AND UPDATE mode) until either you break radar contact from maneuvering or the missile hits the target. At this point, you are ready to either select another target or evade. Total time for the engagement? Well, on my first try in the F-16 simulator at Lockheed's Fort Worth Plant, I was able to do it in about eight seconds. It is that simple, just like playing the computer game Falcon.
So, what is the future for AMRAAM? For starters, there are exports. Great Britain, Norway, Sweden, and Germany have already become customers for the AIM-120. Additional nations will certainly be added to this list. New versions of the missile are on the drawing board, getting ready for test and production. The most important of these will be the AIM-12 °C, designed for internal carriage on the Lockheed F-22A stealth fighter, which will enter service early in the 21st century. This new version of the AIM-120, with smaller control surfaces and a much smaller stowage profile, will give the F-22 lethal air-to-air firepower without compromising its ultra-smooth low-observable profile. AMRAAM is really a flying computer with a big bang attached. With continuous software improvements, it will be a corner-stone of the U.S. AAM arsenal well into the middle of the 21st century.
FUTURE DEVELOPMENTS: THE AIM-9X
Once upon a time in the 1980s, there was a master plan for future U.S. AAM development. This plan included the introduction of AMRAAM, as well as the replacement of both the AIM-9L/M Sidewinder and the AIM-54 Phoenix. Unfortunately, with Congressional restrictions and budget cuts, the end of the Cold War, and some badly managed programs, this master plan fell apart before it could be implemented. The AIM-54 replacement, known as the Advanced Air-to-Air Missile (AAAM), was stillborn when the requirement died with the Soviet Union in the late 1980s. But the most painful loss for fighter crews was the Sidewinder replacement.
Originally, the AIM-9's successor was to be a European-built system known as the AIM-132 Advanced Short Range Air-to-Air Missile (ASRAAM), built by a consortium of British Aerospace and Bodenseewerk Geratechnik (BTG) of Germany. Under a multi-national Memorandum of Understanding (MOU) signed in 1981 by the United States and a number of NATO nations, all agreed to adopt AMRAAM and ASRAAM as their standard AAMs. Unfortunately, the United States and Germany dropped out of the program. While the AIM-132 has continued development, and will go into service with the Royal Air Force in the late 90s, the result was disarray in Western AAM procurement.
Today the next generation of American short-range AAMs is being conceived in the halls of the Pentagon and the engineering design shops of Hughes and Raytheon. The missile is tentatively called the AIM-9X, and if it goes into production, it should put the United States back into the game of short-range dogfighting in the 21st century. In January 1995, Hughes Missile Systems and Raytheon Corporation won a competition to develop separate proposals for the new model of Sidewinder. Final selection of a prime contractor will happen in 1996, with service introduction sometime in the early years of the 21st century. While the exact configurations that the two contract teams will submit to the AIM-9X JPO are proprietary,
there are probably many common features. These include:
• Seeker—The seeker will probably be a staring (constantly viewing the target) IIR array with many detector-array elements, each one sensitive enough to track a target at all aspects. It will be backed up by an advanced signal processor, designed to actually look for the signature of a particular aircraft configuration (such as a Mirage 2000 or a MiG- 29), providing it with a basic NCTR function. Also, it will be capable of tracking targets from a high "off-boresight" mode (the ability to lock up a target well off the launch aircraft's centerline — maybe more than 60deg — and then fly directly off the launch rail to a hit).
• Helmet-Mounted Sight (HMS)—The Navy and Air Force have finally accepted the inevitability of the HMS as the visual sighting system for future manned combat aircraft. The big advance planned for the U.S. HMS will be that HUD symbology will be superimposed on the sight glass, directly in front of the user's right eye. Studies indicate that this will provide a two-to-four-second improvement in overall reaction time to launch an AIM-9X, and will also make AMRAAM shots more rapid and accurate.
• Warhead—The current generation of ABF warheads, while quite adequate for killing a MiG-23 Flogger or MiG-25 Foxbat, may not perform as well against newer Russian and Western designs. These blast-fragmentation warheads were designed to perforate the target's fuel tanks, igniting catastrophic fires on any plane not equipped with self-sealing fuel tanks and fire suppression systems. Plans are afoot to design warheads that specifically target other aircraft systems like the engines or the crew. This will keep the AIM-9X a highly lethal contender in the endless contest between the warhead engineers and the "vulnerability engineers" who design aircraft protective systems for use well into the middle of the 21st century.