As the recent Kosovo conflict proved, there is still a need for air-to-air missiles
Stewart Penney/LONDON
Despite the concentration on the performance of air-to-ground weapons during recent conflicts, the need for air-to-air missiles has not diminished. Last year's Kosovo conflict may have highlighted the use of precision-guided strike weapons, but six Yugoslav air force aircraft were destroyed in the air during the conflict.
The persistent air threat means that nations continue to seek air-to-air weapons which, in conjunction with the sensors on-board and data available to a fighter from elsewhere, will guarantee an acceptable exchange ratio and air supremacy. Although most recent engagements have used beyond visual range weapons, rules of engagement and the potential for a long-range engagement to become a close-in dogfight means the short-range missile remains a key part of the armoury. At the same time, rocket motor developments and lower drag configurations mean missile maximum range is improved, while electronic safety circuits, shorter reaction times and improved manoeuvrability mean the minimum range has been shrunk.
Europe and the USA are developing three new short-range missiles: the IRIS-T, built by a six-nation consortium led by Germany's BGT; the Anglo-French Matra BAe Dynamics (MBD) ASRAAM and Raytheon's AIM-9X development of the venerable Sidewinder. These weapons are designed for the generation of fighters soon to enter service, such as the Boeing F/A-18E/F Super Hornet, Eurofighter and Lockheed Martin/Boeing F-22 Raptor, as well as widely used types such as the Lockheed Martin F-16, earlier F/A-18s and the Panavia Tornado. The weapons are a response to the fielding, and export, of missiles such as Russia's Vympel R-73 (AA-11 Archer) and Israel's Rafael Python 4.
Greater sensitivity
ASRAAM - developed for the UK Royal Air Force and Royal Navy and ordered by Australia - and the AIM-9X - developed for the US forces - share a common Raytheon developed imaging infrared (I²R) seeker with a 180° field of view. I²R has greater sensitivity and, unlike earlier seekers, can distinguish between the target and countermeasures such as flares, and even between different parts of the aircraft allowing aim point selection when coupled with suitable signal processing. Aim point selection allows the missile to target a specific point on the aircraft rather than heading for the hottest spot, usually the jet pipe. Many aircraft have limped home with damaged exhausts and a tail peppered with shrapnel. The technique also allows for different types to be attacked in different locations. "Type A" may be best defeated by a hit close to the intake, whereas "Type B" will most likely be destroyed by a strike at the wing root. In short, missile miss distances have moved from an area around the aircraft to one on the aircraft, says an industry source.
The seeker is based around a 128 x 128 element staring focal plane array. The 16,384 elements give greater sensitivity than earlier generation weapons, says Bill West, senior manager air-to-air programmes business development at Raytheon. As well as allowing the missile to differentiate between the target and countermeasures, the seeker reassesses the view hundreds of times a second, he adds, describing the ASRAAM/AIM-9X seeker as the only fifth-generation system in production.
West says the combination of the focal plane array and commercial off-the-shelf (COTS) processors and other subsystems allows Raytheon "to do smart things with the data" such as complex image processing. He says: "It was a good decision by the customers [the US Department of Defense and UK Ministry of Defence] to select focal plane arrays as they are the way ahead and proof the weapons against the future". As processors develop, the manufacturer will be able to further increase the complexity and volume of software in the seeker that will give scope to defeat any new countermeasures developed to overcome the AIM-9X or ASRAAM.
Raytheon is on its fourth evolution of the AIM-9X processor, if the demonstration-validation phase unit is counted. Now a Motorola PowerPC chip is used. "We take advantage of the commercial world," says West, adding that the 1,000 processors acquired each year for the AIM-9X is "a minimum dent" in the supply which would not be the case if a chip unique to the missile had been developed. Raytheon is also introducing the PowerPC into AMRAAM and the SM-2 naval surface-to-air missile, benefiting from economies of scale and commonality.
MBD has also taken advantage of the continual improvement in processors and is using upgraded units in the ASRAAM which give the seeker head an improved field of view and the missile greater manoeuvrability. The Anglo-French company suggests that the increased memory available with its new processor will also allow a more rapid response to future customer requirements, partly because software-led improvements can be introduced without requiring major hardware updates.
Off-boresight capability
A further advantage of the AIM-9X/ASRAAM seeker's focal plane design is the ability to give the missile a high off-boresight capability without the need to fit the missile with a drag-inducing, large diameter hemispherical dome. A high off-boresight seeker has a larger field of view and can lock on to a target that is not directly ahead, without the pilot needing to manoeuvre the aircraft to allow the missile to "see" the target. This is combined with helmet-mounted sights which allow the pilot to look to the side of the aircraft and acquire a target. This means dogfights will cease to be won by the first pilot to get on the opponent's tail.
A Raytheon source says the AIM-9X's seeker is better matched to the motor, which is the same design used in the AIM-9L/M. The weapon's detection distance was previously well within the missile's maximum range. "Now with the sensor, motor and reduced drag, full advantage can be taken of the missile's kinematics."
IRIS-T, meanwhile, uses a two-colour seeker based around a 128 x 2 array which uses a scanning mirror to build a larger picture of its ±90° field of view. Matra BAe Dynamics uses a similar system on the IR-guided version of its medium-range Mica missile. Mica is also available with a radar seeker. Another difference between IRIS-T and AIM-9X is that the former uses a Ku-band radar as the fuze, whereas Raytheon's missile has a laser sensor.
BGT IRIS-T marketing manager Gerhard Dussler says the 128 x 2 element array produces a 128 x 128 size picture 80 times a second and, like its competitors, uses aim point selection for greater accuracy and to defeat IR countermeasures. He claims the array allows a smaller radome that eliminates dome-heating problems. Dome heating caused by a missile's high speed is an issue for a sensor which relies on detecting differences in temperature. A high temperature dome will appear as background radiation, often obliterating the image of a target at a distance.
Dussler says the two rows of 128 elements are offset, creating a so-called "staggered linear array" so that data does not "fall through the gaps" created when the rows are aligned.
Defeating I²R guided weapons
Directed IR countermeasures (DIRCM) are becoming the standard means of defeating I²R guided weapons. BGT says that scanning arrays are less susceptible than staring arrays as the former receive the DIRCM energy for fractions of a second whereas the latter receive it continuously. DIRCM manufacturers are integrating lasers into their systems to defeat scanning arrays, so missile manufacturers will need to find a way to defeat such countermeasures.
IRIS-T is a collaborative programme led by BGT with industrial participation from Canada, Greece, Italy, Norway and Sweden. Dussler says a six nation programme has the advantage of proving a large "home" market and means the missile will be integrated with a range of fighters when it enters service - including the F/A-18A/B Hornet, Eurofighter, F-16, McDonnell Douglas F-4 Phantom, Tornado and Saab/BAE JAS39 Gripen.
Although ASRAAM and the AIM-9X share a common sensor, their configurations diverge at the back end of the missile which manoeuvres the weapon. ASRAAM relies on fins at the rear of the fuselage and body lift while the AIM-9X, like the Russian R-73 and IRIS-T, has a combination of tail fins and thrust vectoring, also know as jet vane control. Raytheon's West says the combination allows the missile to "actively fly off the launcher" and perform square turns immediately after launch, improving reaction time and reducing the minimum engagement zone. The AIM-9X has a significantly different aerodynamic shape to earlier Sidewinders, which relied on moving canards for control, with smaller fins allowing it to be carried internally by the F-22 Raptor and the Joint Strike Fighter. The fin span has been reduced by 120mm (4.8in) to 445mm while the canard span has been clipped by 280mm to 355mm. The resultant drag reduction also improves the missile's range. The US forces consider thrust vectoring crucial for a dogfight missile. As well as its AIM-9X solution, Hughes (which subsequently merged with Raytheon) also offered a version of ASRAAM equipped with jet vane control.
BGT's Dussler says that, although the thrust vectoring control is only of use when the motor is burning, it provides a turning radius which is "nearly independent" of firing altitude, as available lift declines with the reduced atmospheric pressure at high altitudes. But once the dual pulse motor has expired, the missile relies on the lift from relatively-large fixed wings along the weapon's centrebody. He adds that not relying on body lift also means the missile does not bleed energy when manoeuvring.
No-escape zone
The combination of new seekers and improved kinematic performance allow manufacturers to claim significant improvements to the new generation missiles' no-escape zones. While exact figures are classified, BGT, MBD and Raytheon all suggest their missiles restore the balance of power over the Russian R-73/AA-11, which in turn had a better engagement profile than earlier generations of Sidewinder.
The future for AIM-9X includes a Defense Acquisition Board (DAB) review set for September, which should lead to a low-rate initial production go-ahead in November. Testing continues until 2002. Once the on-going engineering and manufacturing development phase is completed, follow-on testing and evaluation will integrate the AIM-9X with other US platforms including the F-16, F/A-18E/F and F-22. Testing has involved the Boeing F-15 Eagle and F/A-18C/D Hornet. West says a pre-planned product improvement (P³I) cycle has been contemplated. P³I has been the basis for the continuing upgrading of Raytheon's medium-range AIM-120 AMRAAM and allows the integration of new technologies on a planned basis. US research centres have "road-maps" extending into the middle of the next decade which include new materials, fuels, motors, actuators, seekers, fuzes and warheads. Initial P3I is likely to focus on replacement of the components held over from earlier Sidewinders, including the single-stage motor, fuze and warhead.
While AIM-9Xdevelopment continues, the US Air National Guard and US Air Force Reserve - which will receive the missile after the USAF, US Navy and US Marine Corps - are evaluating Israel's Python 4 as an interim replacement for older Sidewinders. The missile, which uses a two-colour IR seeker and moving canards for control, is undergoing separation and other tests.
Rafael has an agreement with Lockheed Martin that allows the latter to build the missile in the USA. A similar agreement covers the Popeye air-to-surface weapon, which is built in Alabama by Rafael/Lockheed Martin joint venture PGSUS as the AGM-142 Have Nap. The pair also has an agreement covering Python 4 developments variously dubbed Python 4I or Python 5. Lockheed Martin Electronics & Missiles will fund the upgrade work - including a new motor and perhaps seeker - in return for Rafael technology.
IRIS-T is moving towards separation trials on the fighters of the six nations in the programme. The move from separation to firing trials is set for the end of next year or early 2002 with production planned to start in 2003.
ASRAAM's service entry is also behind schedule. It should have been fielded in 1998, but will not be handed over to the initial customer, the UK's Royal Air Force, until towards the end of this year.
The relentless requirement for air superiority, and the number of nations which continue to develop indigenous air-to-air missiles, means that service entry will not be the end of the story as BGT, Matra BAe Dynamics and Raytheon each strive to stay ahead of the competition.
Source: Flight International
