New Radar-Guided Weapon Under Study
By Barry Miller
Los Angeles—Next-generation Air Force and Navy air-to-air radar-guided missile intended to supplant the Raytheon Sparrow AIM-7F as the medium-range, all-weather armament for new fighters will move into a competitive three-year advanced development late this year.
The weapon is earmarked for four high-performance fighters—USAF’s McDonnell Douglas F-15 and General Dynamics F-16 and the Navy/Grumman F-14 and McDonnell Douglas F-18. A sizable foreign market also is foreseen, particularly among North Atlantic Treaty Organization F-16 customers, but possibly for the Northrop F-18L and F-5 fighters as well, depending on how certain technical features evolve during development.
Five companies are running initial studies of the advanced medium-range air-to-air missile (AW&ST Sept. 20, 1976, p. 9) under contract to what has recently
become a joint Air Force-Navy system program office at USAF’s Armament Development and Test Center at Eglin AFB, Fla. Three of the five—General Dynamics/Pomona, Hughes Aircraft and Northrop—are working with contracts of about $900,000 each; the remaining companies, Raytheon and Ford Aerospace & Communications, are unfunded contractors.
The program office anticipates releasing proposal requests late in the spring for a three-year prototype phase leading to a competitive flyoff between two contractors. An Air Force official indicated the two prototype contractors will be selected late in the summer.
The selection will be narrowed to one contractor for engineering development and a production start on the weapon in the mid-1980s.
The advanced medium-range air-to-air missile (AMRAAM) will incorporate a number of technical advances intended to provide users with a “‘fire-and-forget” capability, at least at short ranges. This will free launching aircraft from the need to illuminate targets through intercept, as they must now do with the semiactively guided Sparrow missile. The fire-and-forget feature is the product of using an active radar seeker, conceptually similar to the terminal seeker in the much larger Hughes Phoenix missile. Squeezing the active seeker into a small missile airframe of the size anticipated, perhaps no larger than the characteristic 5-in.-dia. of a Sidewinder, is a key technological advance. Its feasibility may hinge on the availability of compact, solid-state power sources that are strong enough to yield a 3-5-mi. terminal range.
An active terminal guidance seeker solves the otherwise difficult problem of interfacing the weapon with four radars of different pulse repetition rates in the four aircraft candidates to use missiles. The different pulse rates would complicate accommodation of the weapon if it were semi-actively guided.
The “‘fire-and-forget” performance lessens the chance that a target will become aware that it is under missile guidance illumination and frees the firing aircraft to search for other threats. It also poses the prospect of relieving future aircraft of the need to have or use forward-looking radars for missile attack. The attacking aircraft might be able to acquire targets visually (much as they were required to do with Sparrow in Southeast Asia) and fire the weapons without emitting their own radar energy or might acquire targets based on threat signal emissions using an onboard passive radar homing and warning system.
By having an essentially quiet weapon concept, in which signal emission from the launching platform is minimized or eliminated, the services would minimize the likelihood of the aircraft weapon control system suffering disruption by hostile jamming.
The contractors for the missile are considering a midcourse guidance system sufficiently accurate to bring the weapon to within the approximate volume where the terminal seeker could take control. Most of them are concentrating on strapdown inertial platforms that can be updated through midcourse by radio commands.
The launching aircraft would be able to have several missiles in the air at one time because the weapons will be autonomous or nearly autonomous after launch. This multiple target capability would give the smaller, less-complex new missile much of the same kind of multiple-shot capability that the Phoenix achieves with the F-14 track-while-scan AWG-9 radar.
While the exact size of the new missile is still undecided, studies are focusing on a weapon roughly half the size and weight of the Sparrow AIM-7F but with a comparable range and a few performance features akin to those of Phoenix. The latter would include the active terminal homing and multiple target capability.
“The AMRAAM missile will be a poor man’s Phoenix,” one observer commented. The cost of this weapon, the program office hopes, will be about half that of the Sparrow.
Of the four aircraft candidates for the new missiles, the F-16 appears to offer the main size constraint. The aircraft is not now equipped nor intended to control Sparrow launches. If it were equipped with Sparrow, the fighter would suffer a noticeable performance degradation amounting to a 10-15% penalty in aerodynamicperformance and time to accelerate.
A radar-guided missile would be important armament for the F-16 in a European environment where weather so frequently precludes use of visual weapons.
European Advances
Entrants in the missile competition are attempting to tap advances made in air-to-air missile technology by West European companies. Marconi Space and Defense Systems, Ltd., is Ford Aerospace’s radar guidance subcontractor for the effort, and the British EMI, Ltd., is one of several companies Ford is considering as its fuzing contractor. Marconi made the monopulse seeker for the United Kingdom Skyflash modification to the Sparrow AIM-7E, which has demonstrated what is widely regarded as a remarkable success in tryouts at the Pacific Missile Test Center. Skyflash has scored 11 hits in 12 firings,
six of which were contact hits, demonstrating the reduced miss distance capability of the monopulse seeker. The latter type of seeker provides look down, shootdown performance against low-flying targets, and competitors on the new missile are expected to include monopulse features in their active seekers.
Northrop has a license agreement with Engins Matra under which it will obtain aerodynamic and control data relating to the French company’s Magic 550 missile for incorporation in the new U.S. missile. The French missile, Northrop believes, has greater maneuverability and presents less drag than comparably sized missiles like Sidewinder. It also has an autopilot. The Magic achieves its maneuverability with an extra group of four fixed control surfaces ahead of the movable control canards in the forward portion of the missile. The combination of the two smooths flight at high angles of attack. The weapon has a spin free tail, with each of four tail surfaces hinged on a bearing, which avoids imparting any rolling moment to the missile.
Northrop is teamed in its effort with Motorola, whose active solid-state seeker work on what is called a fast acquisition search and track (Fast) seeker has led to a $7.2-million tri-service development contract. Under this contract, Motorola is building five active radar seekers, packaged in both 8-in.-dia. and the Sidewinder 5-in. dia. versions. Some of the service interest in the Motorola seeker is as a possible replacement for the Sidewinder infrared seeker.
As applied to the new missiles, the Motorola sensor is expected to reduce the wandering of the missile’s aim point, which occurs at short distances from the target as the phase front returns from its radar signals combine. Reducing the aimpoint error or miss distance would permit use of smaller warheads, hence result in a smaller missile. The seeker uses an Impatt diode power source. There is a possibility the fuzing function could be handled by the seeker instead of requiring a separate unit.
Northrop’s design approach also would incorporate a strapdown inertial platform developed by Northrop’s Precision Products Dept. and now in test at the Naval Weapons Center. The sensor developed for possible application in the Chaparral air defense missile system meets the Sidewinder 5-in. envelope objective. The three-gyro, three-accelerometer package with small digital computer resolves sensor outputs into a proper coordinate system. The platform can accept command inputs over a one-way radio link.
The new missile is expected to be entirely digital, using a reprogrammable processor for both midcourse and terminal guidance calculations. These processors are smaller, more flexible and more reliable than analog equivalents previously used in small tactical missiles. They also have important cost implications: equipment can be modified more readily during the missile’s operational lifetime, and changes can be made during engineering development by altering software.
A digital guidance seeker offers an additional bonus in its ability to implement increasingly sophisticated guidance laws. With range, range-rate, angle and angle-rate data available, the seeker’s computer could obtain target acceleration data from the derivatives of the rate information, provided the signals are clean.Since the missile computer knows its own acceleration, based on the platform’s information, it could determine exactly what the target is doing.
During current studies, the five contractors have to perform tradeoff studies and prepare cost and performance estimates. They must conduct guidance hardware experiments and deliver seekers to Holloman AFB, N. M., by May for testing. The contractors are now working to a draft joint services operational requirement prepared after the studies got under way originally as a lightweight radar missile with backing from the Defense Advanced Research Projects Agency.