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Coniglio discusses the operational impact of the latest-generation radar-guided air-to-air missiles (AAMs). AAMs have long been considered as the ideal weapon for fighter aircraft to engage enemy planes, which however, actually took a long time before AAMs established themselves as fully reliable weapons under the unpredictable and often widely different conditions of aerial combat.
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The Operational Impact Of Latest-Generation Radar-Guided AAMs
Air-to-air missiles (AAMs) in their various classes have long been considered as the ideal weapon for fighter aircraft to engage enemy planes. However, it actually took a long time before AAMs established themselves as fully reliable weapons under the unpredictable and often widely different conditions of aerial combat.
The Raytheon AIM-120C AMRAAM, shown here at launch from a F-15C currently dominates the field of BVR air-to-air-missiles in operational and commercial terms.
In particular, serious problems were encountered in developing and perfectioning the more complex longer-range, radar-guided AAM types, which require a high level of integration with the launching platform and its radar. These problems have been largely solved during the last couple of decades, however, and as a result the expectations of AAM performance have become reality. The long sought, medium-range capable AAMs, now commonly classified as BVR (Beyond Visual Range) missiles are today confidently considered as the weapon of choice for all modern fighters, and indeed are actually impacting on both the conceptual design and modes of operation of their intended launching platforms. This perception is further compounded by recent air-to-air combat operations having witnessed an absolute preponderance when not exclusive use of latest-generation BVR AAMs (more precisely, the AIM-120 AMRAAM) rather than short-range dogfight types and guns.
BVRs Old and New
BVR air-to-air missiles have been in service for a long time now, actually since the late '50s. However, earlier BVR types had a rather limited combat value due to their guidance principle of semi-active radar homing (SARH), whereby the launching aircraft had to use its radar to continuously illuminate the single target being engaged and the missile picked the reflected radar signals off the target to guide itself to impact. This principle mandated significant constraints in the changes in flight direction that the launching aircraft could perform without breaking the target lock-on. Also, the use of the radar to further search for other enemy platforms and assess the tactical situation in the combat area was restricted as long as it was required to continuously illuminate the selected target, and the simultaneous engagement of multiple targets was impossible by definition.
The MICA is unique amongst latest-generation Western BVR air-to-air missiles in being available with both a radar (left) and an IR homing head.(Photo: MBDA)
As a consequence of these limitations and constraints, BVR missiles of that age were mainly intended to engage enemy bombers - which also allowed for rather relaxed requirements as regards the missiles' manoeuvrability, particularly in the final phase of the interception trajectory when closing in within lethal distance from the target. For the same reasons the number of AAMs carried was limited, sometime to no more than two rounds.
Rather, the most significant characteristic of modern-generation BVR AAMs is that they all are so-called fire-and-forget types. More precisely, their guidance concept provides a dual mode of operation:
- During the first (and longer) segment of its flight toward the target, the missile steers itself towards the original target's position (as stored in its memory before launch) through inertial navigation algorithms. If appropriate the launching aircraft will, at intervals, update the missile guidance system with the latest information on the target's position via data link;
- When the missile is approaching the estimated position of the target, its guidance system goes autonomous by activating the active radar homing seeker.
Comparison of the respective engagement envelopes of the AIM-7M SPARROW with SARH guidance and the AIM-120 AMRAAM. It should be appreciated that the border line between the command/inertial and the purely inertial sub-phases on the missile's trajectory is variable as a function of the combat conditions. It is thus even possible for either of these two sub-phases to be completely eliminated (i.e., the guidance sequence being purely inertial + active, or command/inertial + active), and by the same token the missile can also be launched in a LOBL (Lock-On Before Launch) mode with no inertial phase at all.
Intuitively enough, the main advantage of this guidance principle (Lock-On After Launch, LOAL) in operational terms is that the fighter's radar only needs to illuminate the target at intervals and for a short time, which opens the way to multiple-target engagement capability. In fact it is now possible, by subsequently illuminating a number of different targets, to update a similar number of missiles that have been launched to independently engage each of them. Moreover, given the very short illumination time required to update the various targets position data, the radar can now spare enough time to perform other modes of operation, in particularly searching for other threats and guaranteeing adequate situation awareness to the pilot even during the missile engagement phase.
In more accurate terms, modern BVR missiles should be described as "partial fire-and-forget", in that the launching aircraft can only "forget" its missile(s), and thus fully recover its freedom of action and manoeuvre only after the last update information has been sent to the last missile in a salvo engaging a number of targets. Under certain circumstances (e.g., short engagement range, engagement of nonmanoeuvering targets), however, it is possible to reduce the duration of the first (inertial + update) phase of the trajectory or even completely eliminate it with a shift to a Lock-On Before Launch (LOBL) mode, or/and to perform this phase under inertial guidance only with no updates. These modified guidance principles result in a true fire-and-forget capability.
The Russian Vympel R-77 features a unique aerodynamic configuration combining vestigial cruciform wings with lattice tail control surfaces. These surfaces consist of a metal frame containing a blade-like grid assembly which combines a greater area, and thus control authority, with reduced weight and size. As a result, these surfaces require less powerful actuators than conventional fins, and have a lower RCS. Furthermore, the flow separation which occurs at high angles of attack enhances the missile's turning ability (maximum turn rate is given at 150/sec.). The development of this apparently simple control concept, which is referred to by the Russians as 'gas dynamic declination devices', took several years of theoretical studies and wind tunnel testing.
Beyond these distinctions, it is undeniable that modern BVR AAMs offer a vastly better operational flexibility and combat potential than their predecessors, including e.g. the possibility of mid-course update being provided from a different platform than the launching aircraft.
BVR Guidance Considerations
As briefly mentioned above, most current-generation BVR missiles use inertial guidance + mid-course update for the first part of their fly-out, followed by active homing for the terminal phase via a miniature fully autonomous radar. A partially different approach had originally been adopted for the Israeli DERBY, whereby there was no possibility of sending updating messages to the missile in flight; this was not the result of a technical shortcoming, but rather reflected a deliberate choice in favour of an uncompromising "fire-and-forget" approach as well as a way to simplify integration with different fighter types. More recently, however, indications have emerged to that effect that a mid-course update function has been added, perhaps as a result of customer's request.
However, by no means this is the only way to satisfactory guide BVR AAMs, while removing undue burden and limitations on the fighter's radar system. Indeed, infra-red active homing guidance has been for a long time the most common way to achieve a practical, and even more radical, fire-and-forget mode of operation for such kind of weapons. Until recently, however, IR guidance has been considered, particularly in the West, suitable only for short-range engagements, and in fact it has been the standard guidance principle for nearly all dogfight AAMs since the '50s. Latest new-generation IR seekers boast a much expanded practical engagement range, while the introduction of dual-frequency range systems (featured most notably by the IR-guided version of the MICA) allows better target definition and reduces vulnerability to flares and other IR countermeasures. The combination of an IR homing seeker with inertial + mid-course update via data link, similar to the solution as adopted for radar-guided weapons, offers an alternate way to achieve BVR engagement with minimum warning to the enemy (particularly in the last and most critical phase of the interception sequence), by way of performing the final attack run with a totally passive sensor instead of an active radar seeker. By the same token, the possible simultaneous use of two BVR AAMs with radar and IR seeker, respectively would make it much more difficult for the target to deploy appropriate self-defence countermeasures. That not only because of different countermeasures being required at the same time, but also because the possible warning given to the target by the short-spaced energy "bursts" from the launching aircraft's illuminating radar would most probably be related to a single radar-homing BVR, with no specific indication of an IR-guided missile being also on its way.
It is very significant to note that this very attack procedure, i.e. the simultaneous launch of two similar missiles fitted with radar and IR seeker, respectively has been for a long time (up to and including the R-27 family) the standard operating procedure of Soviet/Russian interceptors, long before the introduction of TWS-capable radars combined with new-generation BVR AAMs. Indeed, the Russians are apparently striving to maintain a dual-mode guidance approach even through their current economic difficulties, as Vympel is studying (at least) two additional variants of their R-77 fitted with different guidance seeker heads, to supplement the basic model featuring the nowadays conventional active radar/inertial + mid-course update approach. One of the alternative guidance system is based on a latest-generation IR seeker head (which is exactly in accordance to the above mentioned tradition), while the other uses an innovative guidance system based on passive radio frequency. Such seekers are typically adopted for air-to-surface antiradar missiles, but had never been previously used for AAMs.
The operational implications of this guidance system equipping a long-range AAM become obvious when looking at the exceedingly "cooperative" targets (in terms of powerful electromagnetic emissions because of their peculiar mission) represented by AEWS&C and JSTARS-type aircraft. It is pretty evident that a passive radar homing (and thus totally "silent") guidance seeker for the R-77, particularly if combined with an integral rocket-ramjet engine to increase the missile's engagement range up to perhaps 150km, would pose a very significant threat to these aircraft, which are an absolutely indispensable asset for network-centric warfare.
The Rafael DERBY was originally designed with no mid-course update capability, but this function appears being now available.
Data Link Spreading
A significant improvement to further enhance the flexibility and effectiveness of the whole interception procedure, in particular with the most recent generation of BVR AAMs, it expected to come through the use of sensors, distributed in a number of aerial platforms other than the launching aircraft, to acquire the targets and contribute toward guiding the missiles. These platforms could for example be another fighter with a tactical command role (as envisaged for the Su-30 and MiG-31 as well as the JAS-39D GRIPEN two-seaters), or dedicated AEW&C aircraft. The tactical command role could also be expanded to cover missile guidance through data link update, thus allowing the launching fighters to remain "silent" and manoeuver irrespective of any constraint due to target illumination requirements.
The MIDS (Multi-function Information Distribution System), which is being introduced in more and more aircraft, also offers other advantages, such as overall tactical situation awareness being shared by all the components of a force operating in that specific operational area.
Obviously enough, such a widespread reliance on data link as a force multiplier increases the vulnerability to jamming, which however "robust" the system is always a possibility. This risk is further compounded by the relatively long flight time of BVR missiles, which increases the chances of the enemy discovering the threat and having the time to activate effective countermeasures.
The Terminal Manoeuvrability/ Energy Equation
The new generation of BVR missiles, represented by the American AIM-120 AMRAAM, the Russian R-77, the future European METEOR and the lighter MICA, the Israeli DERBY and its South African R-DARTER clone all offer an engagement range which, while depending on various external conditions would, in the best cases and against a head-on target, reach between 50-100km. This key performance element has been accompanied by an important change in the typical targets to be engaged, whereby large and low-manoeuvrability bombers have given way to fighters or fighter/bombers including the latest extremely agile generation. In addition, even small-size/low-cost combat aircraft are nowadays equipped with sophisticated EW suites capable, among other active and passive defensive functions, to warn the pilot of a missile threat so that he can react by implementing appropriate countermeasures (this function of often completely automatic) and performing adequate evasive manoeuvres.
The longer engagement range and the shift in priority targets to highly manoeuvrable types have made it very critical for AAMs to possess, including in the final phase of their trajectory to the target, a very high manoeuvre potential (in the order of tens of "g") to expand the "no escape" zone around the target. To address this requirement, the traditional mix of aerodynamic control surfaces and single-stage rocket motors of former generation BVRs is giving way to new and in some case totally innovative solutions.
The MICA, which is characterised by a dual role being intended both for short-range and long-range engagements, boasts Thrust Vector Control (TVC) with four vanes deflecting the rocket motor exhaust flow, hence giving extreme manoeuvre capability immediately after launch. The missile also features four large tail fins for aerodynamic control also after engine burn-out. To maintain adequate terminal manoeuvre energy, the MICA rocket motor is located in the middle of the body with some equipment being positioned to the rear around the exhaust, which of course requires adequate insulation. This design choice significantly reduces the forward shift of the centre of gravity due to the propellant burning out, which is unavoidable in traditional missile configurations with the engine occupying the whole centre-rear part of the airframe. As a consequence, the strong stability increase and consequent manoeuvreability decrease due to the centre of gravity being shifted too forward in the terminal phase of the trajectory is completely eliminated.
Another way to maintain a better manoeuvre potential in the terminal phase is to adopt an engine with a longer burning time. Velocity over the trajectory is thus more regular without any undesirable reduction in the terminal phase (it should be remembered that the effectiveness of aerodynamic control surfaces is proportional to the square of the velocity). To achieve the desired high terminal velocity the rocket motor can also have a dual-stage burn rate, typically providing a short-duration higher thrust for acceleration after launch and then a lower thrust for the longer cruise flight to the target, with a higher average velocity in the final phase of the trajectory. Unfortunately, however, this solution is not easy to match with the ever increasing requirement for longer range and hence longer flight time/burn time, as traditional rocket motors are more suitable to a very rapid acceleration being followed, after propellent burn-out, by a corrected ballistic trajectory with rapid velocity decay. The best way to satisfy both of the contradictory requirements for longer range and high terminal velocity/manoeuvre energy is arguably replacing the rocket with a different engine, namely the ramjet.
The METEOR is expected to be the first operational AAM to introduce a very promising, but not so easy to develop compound engine configuration, a rocket/ramjet or ducted ramjet. The initial impulse is provided by a rocket motor occupying the would-be ramjet combustion chamber, which accelerates the missile to the high speed needed to ignite the ramjet. The latter then takes over as a long-endurance sustainer engine, with the additional advantage of the possibility of varying the flow and thus the thrust (hence, a throttleable engine) during the trajectory to optimally suit a specific flight profile. The final advantage of such a very efficient engine configuration is a significantly longer engagement range and high speed, hence high energy manoeuvrability, particularly in the critical terminal phase near the target thanks to the fact that the engine is still producing thrust.
A computer-generated image of the Kentron Dynamics R-DARTER missile being launched from a Brazilian Air Force's F-5E fighter (F-5BR upgrade).
The Russians certainly boast both the widest theoretical background and largest practical experience in rocket/ramjet-powered missiles, and indeed in the recent past Vympel did show several models of a derivative version of the R-77. The exact status of this programme (R-77M) is rather uncertain, however, due to the company facing difficulties in finding domestic or external sources of funds to complete the development phase.
Platform Impact
BVR AAMs have traditionally been associated with medium- to large-size fighters, as these types boasted the relatively heavy and physically large radars, necessary to use radar-guided missiles of the former generation. The smaller radars, which typically equipped lightweight and medium-size fighters, could not provide adequate multi-mode operating capability, and in particular were unable to work in the track-while-scan (TWS) mode, which is necessary to track multiple targets while also simultaneously performing other functions. Furthermore, earlier BVR AAMs were much bigger and heavier (more then double the weight) than their short-range counterparts. As a result, fighters below a certain size were typically restricted to carry nothing more the two/four short range AAMs, with the consequent operational limitations.
Integration of the METEOR to the EURO-FIGHTER started in early October 2003 following 'fit and form' trials at BAE Systems. The missile's configuration has since been radically changed, with the removal of the four mid-section wing surfaces and the shift to a lifting body concept with smaller tail fins. The METEOR will still use conventional bank-to-turn flight in the mid-course phase, shifting to almost instantaneous skid-to-turn for the terminal phase. Test firings of the ramjet propulsion system are scheduled to start in the second half of 2004. Captive flight tests will begin in mid-2005, with the first airborne firing scheduled from a GRIPEN in mid-2006. Development is planned to be completed by 2010.
In recent years, however, a new generation of very small and compact fighter radars has been introduced to offer multi-mode operations, up to and including the engagement of multiple aerial targets with TWS procedures and while maintaining the pilot alerted about the tactical situation confronting him. This has led to a substantial levelling of the combat performance of small and large fighters, at least in terms of their practical engagement range - which is now effectively a function of the actual combat range of the BVR missile types being carried, rather than the detection/tracking range of the respective radars.
It is intuitive enough that while a larger fighter aircraft will typically carry a radar capable of longer-range target search and acquisition, however this sensor, while providing better tactical opportunities due to the larger volume swept at longer range, still cannot allow for a target to be engaged and a range, that exceeds the maximum figure the missile is capable of. That is the reason which justifies the upgrading of older fighter types such as the diminutive F-5E or the MiG-21 to carry modern BVR AAMs (the Israeli DERBY and the Russian R-77, respectively), and which makes such upgraded aircraft still credible combat types in the BVR arena.
In parallel with these radar advances, AAMs such as the MICA, the DERBY and the R-DARTER are now available that are capable of BVR engagements at ranges of tens of kilometres, yet weigh slightly more than the typical value for short-range IR-guided missiles (some 90-100kg) and can be carried on the standard launching rails for these latter missiles. The implications for lightweight fighters are clear.
Coming to larger models, latest-generation BVR AAMs offer not only much more flexibility of use, but also the potential for significant changes in the tactical combat concepts and thus related training and organisation of air operations. The same advantages of reduced weight and dimension which enhance the combat capability of lightweight fighters also result in medium/heavy planes being capable to carry a higher number of weapons for enhanced combat persistence, or contribute towards reducing the aerodynamic drag penalties and combat weight when fewer weapons are carried.
A further operational advantage of modern BVR AAMs is their less specialised nature when compared to earlier types, i.e. their remarkably wider-spectrum engagement capability also at very short range. All modern AAM types have been designed to offer the capability to be launched, still with a very high probability of success, at a target Within Visual Range (WVR), directly in the true fire-and-forget mode with no cruise phase, with the seeker head being immediately active. Possibly the best example of such a comprehensive short-to-medium range engagement capability is given by the MICA, its TVC control allowing extreme manoeuvrability immediately after launch - an essential feature to expand the no-escape zone at very short range. The MICA combines this short-range capability with a maximum range not that different from other BVR types.
The only significant limitation of the lightest models amongst the latest generation BVR types seems to be the warhead's weight, about half that of the bigger weapons, which in some cases could have an adverse limiting effect on the damage being inflicted onto the target. However, the effectiveness of fuze and the specific characteristics of the warhead (e.g., directed rather than omni-directional detonation) can do much to overcome these limitations and enhance the effectiveness of a small explosive charge.
The reduced dimension of the new BVR AAMs are also likely to bring about additional advantages in terms of encouraging internal carriage in both manned and unmanned (i.e. UCAV) future combat platforms. The need to enhance the low signature characteristics of combat aircraft, together with the too often neglected aerodynamics penalties of external weapons carriage are expected to provide a powerful rationale for internal carriage being seriously investigated.
Given these premises, the ratio of BVR versus WVR AAMs carried in the future by fighters will probably tend to go in favour of the former types in view of their better intrinsic versatility. Only the extremely agile WVR AAMs, capable of high off-boresight engagement at very short-range and designed for optimal use with an Helmet Mounted Sight, would still maintain their value. A threat to the predominance of radar-guided BVR AAMs could however come from the widespread introduction of low radar signature characteristics and further enhanced ECM, which are becoming a standard built-in feature of all combat aircraft. Both these elements could in fact significantly reduce the effective range of engagement and the probability of success for radar-guided weapons, thus effectively reversing the recently acquired edge of medium-range AAMs which has made BVR engagement the preferred tactical procedure.
It is thus certainly possible that, oddly enough, short-range dogfight would again become a nearly unavoidable occurrence in most aerial engagements. At which point, the circle will be closed again with short-range, extremely agile IR-guided AAMs and even the gun coming back in fashion as leading assets.
The Missile Life Issue
The recent air campaigns in the Balkans and the long endurance patrols over Iraq to enforce the "No Flight Zones" have highlighted a specific problem with air-launched guided weapons in general and AAMs in particular: namely, the expected useful life of such weapons when actually carried during repeated operational missions, which however do not lead to the missiles being fired in anger. Repeated TO/ landing cycles and long patrol flights, frequently at medium or even low altitude in relatively turbulent air and with evasive manoeuvres when tracked by enemy radars, have been found to cause a significant reduction in the MTBF (Mean Time Between Failure) and thus the inspection/repair/overhaul intervals. This problem needs to be energetically addressed particularly for the latest generation of guided weapons, because it has a not insignificant impact on the operational availability of an adequate number of weapons and on their life-cycle cost. As a rule, current requirements specify an initial flying time between overhaul in the range of 500 flight hours, to be expanded or reduced according to the peculiarities of the missions being flown.
Copyright Moench Verlagsgesellschaft mbH 2003