One down, one to go - or, at least, that would appear to be the case, following the US Department of Defense's recent decision on its Hughes AIM-9 Sidewinder replacement, which ended the battle for the air-to-air missile's succession.
The next decision to be made is over which company will provide the next-generation AIM-120 advanced medium-range air-to-air missile (AMRAAM).
Many industry observers remain perplexed, however, by the choice of the winner of the AIM-9X design contest. The Hughes Evolved Sidewinder is not a revolutionary design, providing only an incremental improvement in capability over late-model AIM-9s. By comparison, the "fat-body" Raytheon air-to-air missile (AAM) submission and the British Aerospace advanced short-range air-to-air missile (ASRAAM) variant were technically more radical in approach.
Although the US Air Force is wedded to the concept of "domination", as evinced by its description of the Lockheed Martin/Boeing F-22 Raptor as its "air-dominance fighter", it would appear outwardly to be satisfied with what it terms "equivalence" when it comes to within-visual-range (WVR) AAMs.
While there is a gap in projected combat performance between the F-22 and other fighters in development, including Russia's faltering fifth-generation projects, the same cannot be said of the Sidewinder successor.
Operational philosophy
Russian missile-design house Vympel is continuing to upgrade its R-73 (AA-11 Archer), as well as a follow-on weapon. Israel's Rafael Python 4 is also being offered in the export market. The performances of these missiles, in terms of off-boresight engagement capabilities and kinematic performance, are arguably close to those of the winner of the AIM-9X competition.
On operational philosophy, some argue that the emphasis, even down to a WVR target of around 6km (3nm), is placed on using the AMRAAM as the weapon of choice for all pre-merge engagements. This would leave the AIM-9X as the choice for very-close-in combat only - known as the "fur-ball".
While this requires a high-off-boresight and high-agility capability, the kinematic engagement envelope of the missile need not compare with that of either an ASRAAM or an R-73. Future projects, however, may involve even more-capable technology.
One programme which has recently begun to emerge is the USAF's dual-range missile (DRM) project, being run by the Wright Laboratory Armament Directorate at Eglin AFB in Florida. This project would appear to unite technologies which have previously been under study independently for several years.
DRM Integrated Product Team leader Jeff Jones describes the programme as a "technical initiative-developing advanced technologies within the context of a fictitious system". If these technologies were to prove successful, however, then at least elements of the fiction would almost certainly become fact.
A DRM-class missile is projected as being available between 2010 and 2015, potentially providing a successor (or successor technologies) to the Evolved Sidewinder and the AIM-120. The DRM project is intended to design a weapon capable of addressing both the WVR and beyond-visual-range (BVR) engagement regimes - arenas which have traditionally remained separate, with good reason.
The design demands of weapons optimised for the WVR and BVR battles have, until now, been contradictory. For the aerodynamic requirement, drivers for the BVR weapon were for a relatively clean airframe, to minimise drag and optimise range - WVR weapons designs have bristled with destabilising and control surfaces for maximum manoeuvrability - irrespective of the drag penalty.
There are precedents for a DRM-type approach. Matra BAe Dynamics' Mica offers both active-radar and imaging-infra-red (IIR) variants, intended to address BVR/WVR engagements, while its ASRAAM, despite being an IIR WVR missile, has a range envelope which extends into the BVR arena.
Both designs (particularly that of the Mica), however, represent compromises in trying to address conflicting needs. The Mica needs two seeker designs, while both it and, to a lesser extent, the ASRAAM, have been criticised over their manoeuvrability in minimum-range engagements. Jones maintains that "-the DRM is not a compromise weapons concept".
The attractions of a single missile meeting both WVR and BVR needs are considerable. On the ground, support and logistics are considerably simplified, while aircraft-weapons load-out is almost certainly improved.
Sqn Ldr Robin Birtwistle, the UK Ministry of Defence's desk officer for AAM operational requirements, also sees the advantages of what he terms as a single "multi-role" AAM. Birtwistle, like his US counterparts, does not foresee the emergence of a DRM-type missile before 2010. He also suggests that, for the medium term, "-modularity should be sought to allow missiles to grow in capability, perhaps to include multi-spectral seekers".
Van of the revolution
If the Evolved Sidewinder enshrines a conservative developmental approach, then the DRM is in the van of the revolution. Every area of guided-weapons technology is being pushed to the edge of the envelope, and then beyond.
The DRM designers have threatened to tear up the hand-book on radar-antenna design. For BVR weapons, either semi-active radar or, more recently, active-radar flat-plate antennas have set the standard. The antenna has been mounted behind an ogival (S-shaped) nosecone, in part determining the diameter of the missile. (The acquisition range of a radar seeker is directly proportional to the diameter of the antenna.)
The Wright Laboratories project is looking at conformal-array seekers (CAS), mounted externally on the missile airframe, described by Jones as having both active and passive capabilities. The ring-like conformal arrays would provide very high off-boresight acquisition coupled with extremely high-angle tracking rates. Both of these have been limited on previous imaging and IIR seeker designs.
Missiles such as the ASRAAM, Python 4 and R-73M have off-boresight capability of around 100¹. The near-term target for DRM technology development, within the next five years, is to have a design capable of 150¹ off-boresight cueing. The mid-term target, some 15 years hence, is for a spherical cueing capability.
Multi-spectral community
Jones remains reluctant to expand on the underlying technology behind the CAS, beyond saying that it emerged from small-business research conducted some three years go. The CAS uses electronically steerable arrays to provide both lateral- and potentially forward-hemisphere radar coverage.
As now funded, the CAS project is in the exploratory development phase, with $2 million allotted to cover fiscal years 1996-8. The aim is to build a "breadboard" seeker for ground tests.
As well as providing a greatly expanded off-boresight track capability, the CAS potentially circumvents one of the major headaches of the multi-spectral community: how to package two seekers within a single nose section.
While stand-off air-to-surface-weapon designers now have both the volumetric and financial latitude to use dual-mode seekers, the problems for the air-to-air fraternity remain greater, even if it is accepted that a dual-mode approach is correct.
One senior source within the European missile-design community suggests that, heretical question though it might be, one basic issue to address in considering dual mode seekers is why they are needed in the first place.
Dual-mode technology has support partly on the basis that an IIR seeker will produce a smaller miss distance than a radio-frequency (RF) seeker. This, suggests one official, misses the point. "Lethality is the concern, not the miss distance," he says.
This is not to say that a dual-mode seeker is the wrong path to pursue; rather it is to stress the need to place technology firmly within the parameters of operational aims.
Using a CAS as the RF element of a dual-mode missile would potentially free the nose-section volume, traditionally associated with radar antennas, to house another seeker. It might also potentially get round one of the main cost drivers associated with multi-spectral-seeker design for AAMs.
The conventional approach to an IIR/RF seeker has been to attempt either to marry the RF antenna with a small IIR array, using a shared aperture, or to use a dedicated IIR array asymmetrically mounted on the forward section of the missile, using a separate aperture. There are concerns associated with these design approaches, however.
The IIR window size may limit the acquisition field of view in the case of an asymmetric configuration, while both approaches raise the issue of how to manage aperture heating. In a WVR engagement, heating of the dome is not an overriding concern, except at the edge of the engagement envelope. For medium-range BVR engagements, dome heating becomes an issue in that the IIR array is at risk of being blinded by the ambient temperature of the aperture because of aerodynamic heating.
Removing the need to place an RF array at the front of a BVR missile would potentially allow for an IIR seeker to be substituted in the same space. The need to deal with dome heating could be obviated by using a pop-off aerodynamic cover, as favoured by air-to-surface-missile designers, with the IIR aperture exposed only in the final stages of the engagement.
The passive RF element of the CAS would allow for a missile to be used against emitting ground-based air-defence radars as an anti-radiation missile, a far-term (25-year-plus) target for the DRM developers. It could also be optimised to home on the emissions of target air-intercept radars.
The other key technology area to be addressed in the near term is at the blunt end of the missile - that of propulsion. The Air Superiority Missile Technology (ASMT) project is intended to demonstrate a hybrid tailfin/reaction-jet flight-control system using an AIM-120 airframe. The success of this kind of propulsion approach is crucial to the entire aim of the DRM initiative, since it is through this that the missile's "super manoeuvrability" for the close-in air battle will be derived. The near-term goal for the DRM is a minimum engagement distance of 450m (1,500ft), with a mid-term goal of 300m.
The mid- and near-term targets for the missile no-escape zone are, respectively, double and quadruple that of the AIM-9X in the WVR environment.
Traditionally, WVR weapons have been designed to use either aerodynamic control or thrust vectoring, or a mix of both, to achieve the required manoeuvrability. Using aerodynamic control for high agility has meant, as in the case of the R-73 or Python 4, that absolute range is sacrificed because of the drag penalty of the numerous control surfaces. Thrust-vector control, using guide vanes entering the motor plume, also wastes energy.
High AoA control
The ASMT will have rear-mounted reaction jets, delivering up to 2.7kN (600lb) of thrust each, for missile manoeuvring. In a WVR engagement, the reaction jets, coupled with a flight-control system which is capable of flying the missile at extremely high angle-of-attack (AoA) flight regimes, would be used to engage a manoeuvring target which is within the acquisition "sphere".
Controlling a missile at extremely high AoA was originally explored at Wright Laboratories under the Alternate Control Technology project. This reached the stage of examining wind-tunnel test models of tail-surface-only designs. One solution examined to the issue of maintaining control while the missile was manoeuvred at high AoAs and relatively slow speed, was the use of cruciform nose-mounted control surfaces. At high AoAs, the tendency is for an aerodynamic body to depart radically from controlled flight, not ideal for the successful conclusion of an air-to-air engagement. Flip-out control surfaces could also figure on a DRM design.
For a BVR engagement, the reaction jets would not be used in the initial stages, the missile following a conventional fly-out flight profile, to conserve motor energy. Only in the end game would the reaction jets be used, to provide the necessary capability to successfully engage a 9g-manoeuvring target.
Conformal radar arrays, coupled with reaction-jet control, are intended to provide the DRM with another key advance over the AIM-9X - the ability to engage a target in the rear hemisphere of the launch aircraft.
US missile designers are now looking to develop a weapon which is capable of engaging targets to the rear of the launch aircraft, which has long been a design ambition of Russian manufacturer Vympel, with a variant of the R-73. Whereas Vympel has taken the relatively simple expedient of pointing both the missile and the launch pylon to the rear, the aim of the DRM project is to use the missile's "super-manoeuvrability" from a forward launch station to engage targets in the rear hemisphere.
The near-term target is for an engagement capability in the rear hemisphere of 25km, with the 25-year aim to be able to engage targets out to 60km. The CAS would also allow the missile to track the target at very high off-boresight angles during what could effectively be viewed as the mid-course phase of the engagement.
As with their Russian counterparts, the interest in a rear-hemisphere engagement capability is predicated on the ability to locate the target. For the DRM, the approach would appear to be similar to that of Russia: equip the launch aircraft with a rear-facing radar.
So far, the USAF has not identified a platform to be fitted with radars providing coverage in the rear hemisphere, although it remains a stated ambition to fit side-panel antennas to the F-22. It is only a small step to fitting additional antenna arrays to the rear of the aircraft.
Improving lethality
Another area of research which will feed into the DRM initiative is the Programmable Integrated Ordnance Suite, a collaborative US/UK Defence Research Agency project. One of the problems with the present generation of warheads is that the vast majority of the explosive energy is wasted. It is estimated that up to 75% of this energy goes in the wrong direction. One way of improving lethality would be to try to ensure that more than 25% of the warhead's energy is expended in the general direction of the target.
The warhead event is effectively inseparable from that of the performance of the seeker. In terms of lateral warheads, the time constants are extremely demanding, given the small time frames required for a successful detonation.
One approach being examined is to shift away from a lateral explosion to that of a forward-facing detonation. This would ideally place the target at the centre of the lethal radius of the warhead explosion.
Warhead effectiveness is also coupled to that of seeker effectiveness. One promised advantage of a dual-mode, active-radar/IIR would be that the latter seeker would provide for improved air-point selection. IIR technology promises to give the missile designer the ability to provide the AAM with the ability to recognise and categorise the type of aircraft it is attacking, and to select the most vulnerable point.
Given the highly ambitious nature of the DRM project, it is unlikely that, in the near term, many of the required technologies will be considered deployable. It is conceivable, however, that some, such as the reaction-jet control, could be deemed mature enough for consideration for inclusion on existing designs.
Since an AMRAAM airframe is being used as the test vehicle, it is reasonable to speculate that a future derivative of the AIM-120 could have such a manoeuvre-propulsion design, if the USAF wanted to extend its service life and expand its engagement envelope further beyond improvements now planned. The same could also be said for the CAS.
What the DRM project indisputably does is to set out the technology agenda for the generation of AAMs beyond those now in the development stage, such as the AIM-9X. If successful, not only will the DRM determine the agenda, it may also provide the solution.
Source: Flight International
