|Last Updated: Mon Jan 27 11:18:09 UTC 2014
of Fighter Capability
Parts 1 - 3
First published in Australian Aviation
October, November, December 1999
© 1999 - 2005 Carlo Kopp
The change in the wider regional threat environment, and concerns about airframe fatigue life, have raised the prospect of an early replacement of the RAAF F/A-18A Hornet. Understandably, this has created much debate on the subject of possible candidate aircraft. To place the subject matter in perspective, it is therefore well worth exploring some of the basic issues.
The starting point for any such discussion must be the mission which the aircraft is to perform, because optimisations of airframes, propulsion and sensor packages of one mission can strongly influence an aircraft's suitability for another. As discussed in earlier analyses on the Hornet replacement, any RAAF replacement fighter must have the ability to successfully defeat an numerically superior opponent flying the Su-27 Flanker, or evolved derivatives thereof. If this aircraft is to in the near term escort, and in the longer term also replace the F-111, it must be able to at least match the F-111's 1000 NMI class combat radius with a robust payload of weapons. Both of these factors indicate that range will be a vital issue, as vital as air combat performance and strike capability.
A modern combat aircraft is essentially a high performance sensor and weapons platform, and to gauge the measure of this importance, it is worth noting that 40-60% of the cost of a modern fighter is in its onboard avionic package, and embedded software. A good indicator of this trend is that Boeing are building the F/A-18E for about the same cost that they are building the latest F-15E derivatives, despite these being a bigger and better performing airframe. The cost of the radar, InfraRed Search & Track (IRS&T), ESM/RWR (radar homing/warning receivers), ECM (jammers), Missile Approach Warning System (MAWS), thermal imaging targeting system(s) (FLIR/Laser), IFF/communications, mission computers and stores management computers, databussing, cockpit displays, Helmet Mounted Sight and software required to make it all work together rivals the cost of the basic airframe, powerplant(s), fuel, electrical and other systems. So much so that an incremental increase in airframe size does not have the effect on aircraft price which it may have had two decades ago. The increasing level of miniaturisation and declining cost of high performance embedded computers have translated into vasty smarter and more capable missiles, which in turn require vastly more capable offensive systems to exploit them, and defensive systems to evade them.
One of the side effects of this revolution in weapons and sensors is stealth, which evolved as a specific response to increasing missile and radar capability. Stealth impacts airframe design through shaping and materials, and sensor design through the need to minimise electromagnetic emissions in all bands, leading to low probability of intercept techniques in active sensors, and higher sensitivity in passive sensors. Experience in the US suggests that a stealthy airframe has a flyaway cost about 25-35% greater than an equivalent conventional airframe, indeed the inflated cost figures we see in the media reflect more than anything US accounting practices which lump basic R&D and manufacturing/support expenditure into the type specific program cost.
We are seeing a strong in air superiority toward Beyond Visual Range (BVR) combat, which demands highly capable and comprehensive sensor suites, fusion of data from multiple sensors, and the ability to evade or defeat the opponent's sensors.
This is the context in which a modern fighter must be assessed. How well does the sensor package fit the mission, how well does the airframe fit the mission, how well is the package blended together, how well can the airframe and sensor package support the weapons carried.
A modern fighter radar built to support air superiority missions must have dogfighting modes to engage close targets at high relative angular rates, as well as providing high detection range performance against small targets at long ranges, looking down into clutter, and having the ability to track, identify (Non-Cooperative Target Recognition or NCTR) and target multiple bogies. The recent availability of stealth treatments for existing aircraft, as seen with the USN F/A-18C/E/F or Russian absorbent coating work applied to the Flanker, indicates that detection range performance will be an important issue in coming decades, since reduced-RCS (Radar Cross Section) fighters may have signatures a factor of ten or more below existing in service aircraft. This in turn roughly halves the detection range of such aircraft in comparison with non-treated airframes.
While InfraRed sensors such as IRS&T/FLIR/Laser may provide good capability at WVR ranges, and under suitable weather conditions, even at BVR ranges, they cannot compare with the true all weather capability of radar at any range. Therefore defeating radar provides a tremendous tactical payoff.
All other things being equal, the easiest way to increase radar detection range performance is to use a bigger antenna, which not only improves range, but also angular resolution at a distance, vital for BVR combat. This is a strong argument for aircraft with wide forward fuselages, which usually leads to a bigger airframe. An example here would be Hughes APG-70 series of fighter radars, which share a very high degree of commonality between types at a module level. The "bigger" radars have bigger antennas and more powerful transmitters.
Another technology which is beginning to emerge in fighter radars is the electronically steered phased array, and in particular the active array. A conventional fixed slotted array antenna, or a passive phase shifter based phased array, as used in the B-1B or Russian N-011 Ph/Zhuk Ph, both use a conventional Travelling Wave Tube transmitter feeding into the antenna. Active arrays however are quite different, with each tiny slot in the antenna having its own electronically controlled transistor receiver/transmitter/phase-shifter module. An active array can achieve potentially better receiver sensitivity by placing the first receiver stage right behind the antenna slot, as well as vastly improving reliability since the loss of any module in several hundred costs a fraction of a percent of performance.
The less obvious advantage of the active array (or phased array) antenna is its potential for stealthiness (Low Probability of Intercept or LPI). Whereas a mechanically steered antenna exhibits repetitive scan behaviour, easily identified by a hostile warning receiver, a phased array can be cleverly programmed to do a pseudo-random scan pattern, which means that pulse trains are no longer detected periodically by an opponent. If pseudo-random scanning is combined with spread spectrum modulation techniques on individual radar pulses (ie direct spreading and frequency hopping), a significant reduction in detected power per bandwidth can be achieved. Most established radar warning receivers and ESM systems are totally deaf to such radars.
The recent advent of anti-radiation variants of established radar guided BVR AAMs suggests that those who choose to fly with a conventional non-LPI radar will be exposed to yet another form of BVR AAM attack.
For mud-beating missions, the current generation of radars provide focussed Synthetic Aperture Radar (SAR) and Ground Moving Target Indicator (GMTI) modes, which provide very high resolution groundmapping and the ability to detect, identify and track vehicles on the ground. The state of the art in this technology is an ability to map with 3-1 ft resolution from tens of NMIs, while concurrently operating a GMTI mode and overlaying the two images in real time (Lockheed/Norden APG-76). This technology also has an inherent ability to identify target types, and differentiate between wheeled vehicles, tracked vehicles, rotating radar antennas and hovering helicopters, if enough computing power is available. Since it is radar, cloudbase and humidity are no longer an issue in precision bombing, should (cheap) GPS guided bombs and glide weapons be used.
Electronic Warfare Systems
Traditionally the EW suite on a fighter comprised a warning receiver of some type and a package of defensive jammers, complemented by a chaff and flare dispenser. This was generally adequate for dealing with older generation SAMs, AAMs and AAA. Only aircraft specialised for the SEAD mission, such as the F-4G Weasel and Tornado ECR carried more sophisticated precision homing and rangefinding equipment used to hunt down hostile emitters.
The growing sophistication of the air-air and surface-air threat, typified by highly jam resistant AAM and SAM seekers, has brought about major changes in this area.
The trend today is to use a true ESM (Electronic Support Measures) receiver on a fighter, rather than the established and relatively simple Crystal Video Receiver (CVR) or scanning superhet (SSH) based RWRs. The ESM uses typically a channelised receiver, which is significantly more sensitive than older CVR, and has higher probability of intercept to the SSH, and thus if designed properly can not only detect threats at much greater ranges, but also has the potential to detect lower power emissions from stealthy radars. Moreover, we are seeing the use of interferometric techniques for the precision angular measurement of threat position, and passive rangefinding techniques to provide accurate surface emitter location. This means that the modern ESM fitted to a fighter is as much a defensive aid, as an offensive targeting tool to support BVR AAM launches, and attacks on surface based radar systems.
While the adoption of a stealth airframe defeats most missile threats at long ranges, at very close ranges (ie inside eyeball range), a stealth aircraft is almost as exposed as a conventional aircraft, and thus defensive electronic countermeasures (DECM) and expendables will continue to be used, on both stealthy and conventional airframes. A package for a stealth aircraft can however be simpler, since it need only deal with terminal threats.
A contemporary DECM suite is substantially more complex than its predecessor of one or two decades ago. While the basic model of a set-on receiver and jammer is retained, the jamming techniques generators are vastly more sophisticated. Moreover, we are seeing a trend toward the use of highly integrated defensive suites, where a central controller box is used to coordinate jamming with the release of expendables, to achieve maximum effect.
Expendable decoys have evolved considerably in recent years. Radar threats have evolved to the point where the dropping of chaff is seldom effective, and this has led to the development of the expendable repeater decoy, which dangles on a parachute and rebroadcasts the impinging radar emissions from a threat, to seduce the missile away. More recently, electronic counter-countermeasures (ECCM) features have evolved to defeat such decoys by discriminating by velocity. The response to this ECCM has been the adoption of the towed decoy, which follows the aircraft at the end of a long cable. Simpler towed decoys contain a ram air driven repeater package, smarter decoys have an optical fibre embedded in the cable and provide for much more sophisticated seduction and general jamming techniques, using a jamming techniques generator carried on the aircraft itself.
Other than towed decoys, we are also seeing lightweight gliding and powered decoys (eg Northrop-Grumman MALDS), which can by carried in large numbers and emulate the radar signatures, emissions, and flight profile of a full sized fighter or bomber. A strike package can therefore launch multiple MALDS and use these to seduce hostile air defences.
We are also seeing a trend to fit conventional aircraft with Missile Approach Warning Systems (MAWS), either aft or fore and aft. MAWS are either radar or optical, with radar based systems providing angle/range/velocity and optical systems (IR/UV) typically only angle information on an inbound threat. A MAWS is typically integrated with the rest of the defensive suite, to enable the best possible application of jamming and expendables to defeat the missile seeker.
Another increasingly common defensive aid is the Laser Warning Receiver (LWR), designed to detect the rangefinding component of the Flanker/Fulcrum IRS&T/Laser fire control package. It is most often integrated as an adjunct to the RWR/ESM package.
IR jammers designed to defeat heatseeking missiles do not appear to be common for fighter applications, although they have seen wide deployment on "slow movers" like helos and transports.
It is evident that the contemporary EW suite on a state of the art fighter is vastly more capable than established technology, both as a defensive aid and an offensive package. The stealth centred trend in modern air warfare, where anything you emit can and will be used against you, suggests that we will see a growing focus on long range passive detection of threats for defensive and offensive purposes.
Traditionally, airborne optical sensors were split between specialised thermal imagers, usually equipped with laser rangefinders, for mud-bashing missions, and specialised stabilised TV telescopes for target identification, and IRS&T for passive target acquisition. The division was clear cut, and the equipment design focussed on a single mission.
Examples of the air-ground optimised thermal imagers abound, either as embedded systems (F-117A IRADS, A-6 TRAM) or self contained pods (Pave Tack, Lantirn, TIALD, Litening). TV telescopes were used widely on the USAF F-4 and USN F-14 as BVR visual identification sensors, while the IRS&T has seen a major revival with the Russians fitting an IRS&T/laser rangefinder as standard equipment on the MiG-29 and Su-27 series.
We are now seeing a trend toward merging the functions of the TV telescope, IRS&T and forward looking navigation thermal imager into a single device. The technological development behind this is the Indium Antimonide single chip Focal Plane Array camera, similar in design to the single chip CCD cameras which are so commonly used now. Such a device can be operated as a passive IRS&T to search for airborne targets over a wide field of view, or to zoom in on a specific target for BVR visual identification and missile targeting. In the air-mud scenario, such a device can provide the pilot with HUD or steered helmet visor projected thermal imagery (EF2000, F-16 CAS demonstrator) of the terrain he is penetrating, and surface targets he is to attack.
Where a conventional radar is carried for the air superiority mission, there is much to be said for silently hunting for targets with an IRS&T, and in any BVR scenario the ability to ID the bogie before shooting is helpful to say the least. With an NCTR capable LPI radar, the need for the IRS&T is lesser, and open to some debate.
The thermal imager/designator pod has a future as long as Laser Guided Bombs continue to be used. The current trend is for all weather capable GPS guided bombs to be used against fixed targets, relegating the LGB to the role of a niche Battlefield Air Interdiction/Close Air Support weapon for use against moving targets. This may or may not persist with the evolution of SAR/GMTI and Millimetric Wave Imaging seekers for GPS guided bombs and glide weapons. Until the latter mature, the LGB will still be around.
An issue with GPS guided bombs will be Rules of Engagement mandating strict visual identification of targets prior to attack. Given the CNN factor, and the Law Of Armed Conflict (LOAC), considerable pressure will exist in many circumstances to visually ID targets. Whether this significantly prolongs the life of the laser and television guided bombs remains to be seen.
Cockpits and Computers
The cockpit is another area which has seen significant evolution in recent years. From the "steam gauge" analogue cockpits of the sixties, with a scope for radar and FLIR, we have seen the increasing proliferation of glass displays. In recent years the thermionic Cathode Ray Tube (CRT) has been supplanted by the flat panel Liquid Crystal Display (LCD), which eats much less power and volume, and provides a more stable picture with better image registration.
A late teen series fighter, or current Russian fighter, will have three or more glass colour displays, providing the pilot with separate screens for radar, navigation, FLIR, systems and weapons status and EW activity. The drawback of this arrangement is that the pilot is presented with a deluge of information, which he/she must integrate in his/her head, not an easy task by any means in the heat of battle.
This in turn has resulted in a trend toward performing the integration in software, and presenting the pilot with a combined display. An example would be a colour terrain map with the location of the target, planned ingress and egress routes, surface based defences circled by detection and engagement radii, airborne threats and associated wedge shaped radar and missile detection and engagement envelopes, and the position and status of friendly aircraft. The air to air equivalent would be "decluttered" by removing the surface map.
This technology significantly reduces workload and speeds up pilot response times, since the task of sorting threats and targets and developing the "big" tactical picture is performed by software. The pilot can concentrate on flying the aircraft, making tactical decisions and attacking his targets. In the first-shot-is-the-killing-shot game, response time is everything from a pilot's perspective, and taking the load off the pilot will make a huge difference.
The weapon system is in modern aircraft mostly controlled by switches on the throttle and stick (EF2000 adds voice input for everything but weapon launch), which means that a pilot can operate his weapon system heads up at any time in air-air combat, and much of the time in mud beating combat.
The Helmet Mounted Sight (HMS) has now become a must item for fighters, with 4th generation (4G) WVR AAMs. We have seen this technology evolve from simple mechanical "ring and bead" sights, through simple optical reticles, to sophisticated visor projection schemes which present the pilot with optically collimated missile boresight reticles, threat status data and aircraft and weapon system mode indicators. Top of the line HMS include the ability to display FLIR/IRS&T imagery (EF2000) and may include integrated stereoscopic image intensifiers (EF2000), or FPA thermal imagers embedded in the helmet. A key issue in HMS equipped helmets will continue to be weight, since in air-air close in combat every extra gram of helmet mass translates into neck strain for its wearer. At 9 G a 2 kg helmet weighs effectively 18 kg.
Automation in the offensive and defensive systems is paralleled by increasing automation in flight management functions, and the now common ability to poke a cartridge into a socket when the pilot climbs into the aircraft, to completely preprogram the mission flight plan and prebriefed threat environment into the aircraft.
Software controlled stores management using "smart" Mil-Std-1760 digital weapon stations is now a defacto standard for any state of the art fighter. Whether we look at current build older airframes, like the F-15I/S, or newer airframes, like the F/A-18E or EF2000, smart digital weapon stations are the norm and provide unparalleled flexibility in integrating new weapon types, since all that is needed is the addition of more software, and appropriate clearance testing.
The baseline for onboard computers has also moved up significantly. Whereas eighties generation aircraft were hamstrung by a US DoD directive to use the Mil-Std-1780 16-bit (defacto PDP-11) architecture, the latest generation of aircraft commonly exploits the militarised variants of the latest commercial processor chips. The Rafale for instance uses a SPARC architecture RISC processor. We can expect to see this trend broaden, with militarised variants of the MIPS R-series, DEC Alpha and Intel i960 chips proliferating further.
The future clearly lies with highly integrated cockpit/HMS environments and weapon systems, for very good reasons, and that what we see today in the F/A-18E/F, Eurofighter and F-22A will be the benchmark for 21st century fighters and bombers. The F/A-18A Hornet used to be referred to as the "Tron Machine", yet it today compares in capability to modern systems like an IBM PC-XT to a Pentium Pro. It is evolution in action, to paraphrase Larry Niven.
In the second part of this feature we will explore propulsion and airframe issues in modern fighter design, and relate these to capability and survivability of combat aircraft in the evolving air combat environment.
The afterburning low bypass ratio turbofan is now the standard powerplant for a modern fighter, providing typically dry Specific Fuel Consumption of the order of 0.7-0.8 lb/lb/hr, and full afterburning SFC of the order of 2 lb/lb/hr. The hot end failures, frequent compressor stalls and durability problems of first and second generation afterburning fans are now much less common problems. Current engines in this class are highly durable, typically employ a "smart" digital engine controller and allow the pilot generally carefree engine handling in most or all regimes of flight. Typically, extra performance can be extracted at an expense in TBO, or vice versa.
The current crop of conventional low bypass fans falls squarely into two categories, the "small" category, typified by the evolved 22,000 lb (A/B) class GE F404 used by the Hornet family, and the new 20,000+ (A/B) class EJ200 to be used in the EF2000. The "large" engine category is dominated by 30,000 lb (A/B) class variants of the P&W F100 and GE F110. It is expected that growth variants of these engines will deliver 35,000 lb (A/B) in post 2000 airframes.
There is a clear and established trend for growth variants of engine types to be fitted to airframes in service, or late build variants of established airframes. In this sense assessing the performance of any established airframe type must be done in the context of what likely powerplant it is to use in the post 2005 timeframe. An F-15 or F-16 variant built in 2005 is likely to use a 35,000 lb class engine, as compared to the 25,000 lb and 29,000 lb engines used in fielded airframes.
Conventional exhaust nozzles for the larger engines have seen development in two separate directions. One is typified by recent US testing of a reduced RCS nozzle, employing absorbent materials and flat petals with a serrated trailing edge to break up the characteristic all aspect RCS of circular tailpipe edge. The other direction in development is 2-D and 3-D thrust vectoring (TVC), intended to improve sustained turning performance, especially in regimes where aerodynamic controls begin to lose effectiveness. The Russians are clearly leading the pack here, with 2-D TVC nozzles to be fitted to the Indian Su-30MKI, and a 3-D nozzle providing pitch/yaw control under development. Whilst there is still some debate under way as to the merits of high sustained turn rate performance in the age of 4th generation 50G+ WVR missiles and HMS, it is clearly one way of giving a very large fighter turn rate performance competitive with much smaller and lighter airframes. In the very long term, 3-D thrust vectoring offers the potential for smaller tail surfaces, or none at all, the latter removing the RCS penalty of tail surfaces altogether.
The most radical propulsion development in recent years has been the P&W F119 supercruising turbofan for the F-22. Conventional fighter turbofans do not cope well with sustained dry supercruise, since the higher inlet temperatures in turn raise the whole temperature profile across the engine, with unhealthy consequences for the turbine stages. Should a conventional fan be run for more than several minutes or tens of minutes in this regime, it will be spitting turbine blades through the rear fuselage. The F-22 requiring sustained supercruise as part of its basic mission needed a quantum leap in engine capability, and the F119 won the flyoff in 1990/91 against GE's F120. Critical design features are cooling of both counter-rotating turbine stages, and the use of titanium in the six stage compressor. The combustors are convection and film cooled. The result of this effort is a powerplant delivering twice the dry supersonic thrust of the F-15's F100-PW-200, and 1.5 times the afterburning thrust, with the same engine durability, all in about the same size as the F100/F110.
Unlike avionic systems, which have seen at least two generations of evolution since the RAAF selected the Hornet, airframes have evolved at a more sedate pace. We have seen much wider use of composite materials for structural components, especially load bearing skins, and more common use of exotic materials such as titanium and lithium aluminium alloys. All have improved structural stiffness and strength with a cost penalty, since all of these materials are more expensive to produce and much more difficult to fabricate than the traditional Aluminium alloys. An important plus is that the fatigue behaviour, especially of composites, is superior to that of Aluminium thus providing better airframe durability. Another useful attribute of composites is their potential to be laminated with radar absorbent materials, thus contributing to stealthiness. At a system level, the use of lighter and stronger materials translates into less weight in structures for a given volume and thus more space for fuel and systems.
In terms of air superiority fighter airframe configuration, two forms are dominant at this time. The first is the twin tail straked arrangement, typified by the Flanker, the F-15, the F/A-18E and the F-22A. The second is the combined delta-canard, typified by the EF2000, the Rafale, the stillborn Lavi and the Gripen. In both instances the airframe is designed to provide the best possible high Angle of Attack (AoA) turning performance and controllability.
One big issue in fighter airframes has always been the "big fighter vs small fighter" argument, and it is clear that this will also be a core issue in the RAAF's selection of a Hornet, and later F-111 replacement. The other big issue in fighter airframe design is that of what is it primary airframe optimisation.
Fighter airframes are usually exceptionally well suited to one task, reasonably good at a range of other tasks, and marginal for some tasks. In the days of single purpose highly specialised airframes, this was never an issue, since a designer built an aircraft for a particular mission and that was all it ever did. Advances in avionics and shrinking budgets spawned the idea of the multirole fighter, which by virtue of tacking on extra bits of avionic equipment could perform a wide range of tasks, and if we are to believe proponents of the model, equally well. Certainly there are good economic and strategic/doctrinal arguments for multirole fighters, all deriving from the idea that you never have idle assets. The flip side of the argument is overcommitment of assets in combat, since you thought you saved money by buying half the equivalent number of multirole assets, and suddenly find you don't have enough airframes to fly counter-air and strike at the same time.
In practice, the multirole model has met with varying degrees of success over the last two decades. An example of a failure would be the MiG-23 Flogger, which never had the air superiority performance to hold its ground, just like the stillborn naval F-111B. More successful were the F-16 and F/A-18, both born as lightweight transonic dogfighters. Their limitation as bombers lay primarily in limited payload radius performance, low level ride quality for deep penetration, and initially with the F-16, limited tools for precision weapon delivery and defence penetration. The most successful example is without doubt the F-15E/I/S which has proven to be almost as good a bomber as the F-111, and improves on the counter-air lethality of the F-15C.
Much of the ballooning cost of modern fighters is a direct result of packing them full of expensive avionic systems to provide a multirole capability, and the often small purchase price differentials between big and small fighters are a direct result of this effect. Indeed the most significant cost difference between large and small fighters today is the extra cost of maintaining a bigger, twin engined airframe and the associated fuel, hydraulic, bleed air and other accessory systems.
The now classical air superiority aerodynamic performance model is based on the idea of superior energy manoeuvrability, a concept created by the USAF's John Boyd. In this model, a fighter gains a manoeuvre advantage to fire its weapons by outclimbing, outaccelerating, outturning and outlasting its opponent in a manoeuvring engagement. With the to BVR combat and high off-boresight 4th generation WVR AAMs supported by HMS, optimising an airframe today for the close in high AoA subsonic/transonic turn and burn will not yield the return it may have in the days of the AIM-7F Sparrow and AIM-9H/J Sidewinder.
The contemporary approach is to stay out of WVR AAM engagement envelopes if possible, and instead of flying small subsonic/transonic circles around an opponent at close quarters, the trend is to fly supersonic and pick off the opponent with BVR AAMs. Unless your fighter has good sustained supersonic manoeuvre ability and persistence you do not have the option of disengaging, as you will be shot in the back with a BVR AAM. Without good supersonic performance you will be committed to fight it out at close quarters unless you can kill the opponent with a pre-merge BVR missile shot. An airframe built for this style of air combat must have the ability to fly high G supersonic manoeuvres with minimal energy bleed, high installed dry thrust for supersonic persistence, and a hefty load of gas to maintain the pace. Compared with transonic teen series fighters, the need for high thrust/weight ratio and low wing loading is much greater as these are critical performance parameters for such high energy manoeuvres.
Low RCS and infrared signature can be a tremendous advantage in this style of air combat, since it can dramatically shrink an opponent's BVR engagement envelopment, while the best possible radar, IRS&T and ESM detection and tracking performance are clear assets in this model.
Clearly flying high energy supersonic manoeuvres will require
both wing design optimisation, and plenty of gas to burn. If the
supersonic drag characteristics of the wing are not well matched to
model, more thrust will be required in turn limiting persistence,
especially if reheat is required to sustain such manoeuvres. Big
fighters like the F-15, the evolved Su-27 and the F-22A (and the
stillborn and smaller F-16E/XL) have a major advantage in this style of
air combat, since the eyeball range argument for small airframes
The pure air defence mission is today firmly shifting away from the "rolling out of a HAS, lighting the burners and blasting out on a maximum speed/maximum RoC intercept" mission, to the forward Combat Air Patrol mission, under the watchful eye of the AEW&C platform. In the Australian context, the latter aspect of the role is by far the more important, given our geography. In this model, persistance with a hefty load of BVR AAMs, and radar/IRS&T/ESM performance are decisive measures of success. So yet again, gas is the decisive parameter for success. If the bad guys are defenceless bombers then the demand for supersonic manoeuvre performance is lesser than for air superiority sorties, but if they are nasty late model Flankers capable of BVR shots, then the requirements of this air superiority model apply yet again. Given the toward multirole fighters over the last decades, most engagements will be against aircraft which can shoot back. The pure air defence fighter (Tornado ADV, Foxhound, F-106) is now a historical artifact.
In the surface and maritime strike missions, the decisive measure of performance, sensor capability being equal, is payload/range performance. The more stores you can carry further, the better. A typical land strike sortie for a conventional fighter or bomber would involve an efficient cruise climb to the boundary of the opponent's IADS, upon which the penetrating aircraft would descend to low level, and using either automatic terrain following radar or eyeball Mk.1, hug the ground to avoid area defence SAMs and hide from fighters in clutter. A laser guided bomb is then tossed at the target and the aircraft then does its best to quietly sneak out in the same manner it got in. This is the now classical Hi-Lo-Lo-Hi mission profile.
Flying such sorties at transonic or supersonic speed below 1000 ft AGL demands a high wing loading on deep penetration sorties, to reduce crew fatigue, and improve the stability of the airframe as a bombing platform. At low level the aircraft is exposed to point defence SAMs, AAA and with the deployment of the pulse Doppler radar equipped Fulcrum, Flanker and teen series fighters, BVR AAM attack.
With the advent of GPS guided glide munitions, and powered standoff munitions, in many instances the need for low level penetration will decline, moreso if fighter escorts are available to keep the opponent's interceptors at bay. However, it is clear that the single unescorted penetration mission is today the exclusive domain of the stealth aircraft. The sanctuary of low altitude is largely gone with universal availability of pulse Doppler capable fighter radars, AEW&C radars, SAM engagement radars and SAM/AAM seekers. The only instance where low level penetration still offers a useful advantage is where a pure SAM/AAA surface threat exists, and terrain allows a masked approach until weapon release. While much of the Asia-Pacific today still fits this model, the ongoing proliferation of AEW&C and the Flanker suggest that its days are clearly numbered.
The US approach to this environment has been to adapt the F-22A, originally defined for an air superiority mission alone, during its development phase to drop internally carried GPS guided bombs on a Hi(subsonic)-Hi(supercruise)-Hi(subsonic) mission profile exploiting its stealth, supercruise and ESM to bypass hostile defences and attack its targets under all weather conditions, using its APG-77 radar to precisely map the aimpoint programmed into the nav-attack system. This is a direct application of the F-117A/B-2A penetration model, and is planned for the JSF should it go into production. It is envisaged that stealthy strike will be used to break the opponent's air defence system, upon which the reduced threat will allow the carriage of additional external weapons.
In the absence of stealth, and the presence of a modern fighter and SAM threat, the only manner in which targets can be attacked with low loss rates is either by strike packaging, using SEAD aircraft to take down the SAM/AAA threat and fighter escorts to keep fighters away, or by shooting $0.5-1.5 million cruise missiles from outside hostile defences. Both are much more expensive than flying in individual penetrators dropping $20,000 GPS guided bombs, especially in a sustained air war scenario, indeed the longer the war lasts, the cheaper the stealth model becomes, both in terms of airframe losses and munitions expended.
Whatever air-air or air-ground mission we explore it is quite clear that range/persistence is of critical importance, moreso given the developing style of BVR air-air combat. At this point is worth making some comparisons between small and large fighters, to drive home some important points.
The first area to explore is that of achievable combat radius. Typical modern turbofans exhibit a slight improvement in SFC with altitude, but do experience a loss in dry thrust with altitude, so much so that at cruise altitudes the achievable dry thrust is about 35-45% of that at sea level. The typical cruise regime for jets is a constant Mach number cruise climb, at altitudes between 20,000 and 40,000 ft subject to the type.
The classical Breguet equation tends to lose some accuracy when jet range performance is considered, primarily since some of the basic assumptions do not hold very well for low aspect ratio wings and a drag environment where parasitic drag dominates over lift induced drag. The critical factors for range in fast jets are the fuel fraction (a measure of fuel capacity against weight) and the aircraft's drag. A typical rule of thumb for jets is that 75% of the total drag is parasitic drag and only about 25% is lift induced drag. It follows therefore that a range advantage is held by the aircraft which has the higher fuel fraction and lower parasitic drag, factors such as SFC and cruise Mach number being constant.
Let us consider a generic small fighter, and a generic large fighter, each with weight, fuel loads, and installed thrust produced by averaging the values across each class (ie no possible vendor bias here, 3 current large fighters and 4 current small fighters, Western and Russian types inclusive). We end up with a large fighter with an empty weight of 32,600 lb, an internal fuel load of 23,500 lb, 40,000 lb of total dry SL thrust and 62,000 lb of reheated SL thrust, with a wing area of 706 square feet and fuel fraction of about 42%. Doing the same for the small fighter, we end up with an empty weight of 22,700 lb, an internal fuel load of 10,600 lb, 24,120 lb of total dry SL thrust and 36,700 lb of reheated SL thrust, with a wing area of 416 square feet and a fuel fraction of about 32%. Interestingly, for the statistically inclined, the variance on these parameters is not very big.
Assuming that the aircraft have a very similar lift to drag ratio and cruise at the same Mach number and the same SFC, then the ratio of relative range performance is given by the ratio of the natural logarithms of the ratios of total weight to empty weight, excluding stores. Plugging in these numbers yields a result which suggests that the large fighter will have about 40% better range. In practice the unrefuelled clean range advantage of a larger fighter varies between 10% and 50%, and should be roughly halved for combat radius.
The drag term in the denominator of the range equation is critical when assessing the relative range/radius performance of fast jets, particularly due to the dominance of parasitic drag sources. While the fuel fraction of both small and large fighters can be significantly improved with external tanks, moreso with the smaller fighter, the price to be paid is additional parasitic drag, which offsets to some degree the improvement in fuel fraction. Drag also bites with externally carried weapons. In practice therefore care must be exercised since idiosyncrasies of particular designs may introduce significant drag at cruise speeds and thus impair combat radius performance. A good example and one which has caused much embarrassment to its users is the F/A-18A/C. Development F/A-18s with a better fuel fraction did indeed outrange the early F-16, but production aircraft with a much draggier pylon design fell short by a solid margin.
Clearly the superlative range of the Flanker is a direct consequence of its blended airframe geometry and large fuel fraction, which means that it need not carry draggy external tanks. Whatever parasitic drag it suffers through stores alone is very modest, moreso since the largest of these are mostly carried semi-conformally. While the USAF have not released combat radius and range numbers for the F-22A, with its large internal fuel fraction and drag free internally carried weapons we can expect it to outperform both the F-15E and Flanker for combat radius.
Conformal fuel tanks (CFT) have become a very popular measure to improve the fuel fraction of a fighter without much of the drag penalties of external drop tanks, indeed a well designed CFT can contribute to area ruling and actually reduce the transonic and supersonic wave drag. CFTs are available for the F-15C/D, permanently fitted to the F-15E/I/S and under development for the F-16C Block 60 and EF2000.
The other basic aerodynamic parameter of interest is agility, basic measures of which are the combat thrust to weight ratio, and combat wing loading, both defined for an aircraft weight with a given stores load and 50% of internal fuel. Higher thrust to weight ratio translates into better climb rates, acceleration and given similar wing design, sustained turn rates. Wing loading directly affects climb rate and turning performance.
We now calculate the combat thrust/weight ratios, dry and reheated, and wing loadings for the generic large and small fighter aircraft, assuming 50% total internal fuel load and 2,000 lb of weapons, ie 4x BVR and 4x WVR AAMs. For the large fighter, we get thrust/weight ratios of 0.86 and 1.33 dry and reheated respectively, with a wing loading of 65.5 lb/ft^2. For the small fighter we get slightly worse numbers of 0.8 dry, 1.22 reheated and 72.2 lb/ft^2, about 10% below and above, respectively, the large fighter, but hardly decisively inferior. We here use values at sea level, but since the thrust loss factor with altitude will be similar for both types, the performance ratio between the types at altitude will not vary significantly from sea level.
Now this calculation assumes that both fighters are operating at their respective radius limits on internal fuel, in a clean air superiority configuration. The small fighter will under such conditions achieve typically about 70-85% of the combat radius of the large fighter.
Now let us assume that we wish to fly the small fighter out to a combat radius equivalent to that of the big fighter. This means that we load it up with extra drop tanks, possibly also scab on conformal tanks (F-16, EF2000), accepting a solid drag penalty, and carry a total fuel load at takeoff identical to the internal fuel load of the big fighter (ie better fuel fraction for the smaller fighter). Now this is probably a little optimistic, but still reasonably close to reality.
Recalculating the combat weight, we get a revised thrust weight ratio of 0.7 dry, 1.06 in reheat, and a wing loading of 82.3 lb/ft^2. These numbers are interesting, if we compare them to the performance figures for the big fighter at this radius. The small fighter has 20% lower dry and reheated thrust weight ratios, and a 26% higher wing loading. Whatever performance gain we might pick up by giving the small fighter a lesser weapon load and perkier engines, we lose on the additional fuel load. The small fighter is well behind the big fighter in agility, since the ratio of remaining internal fuel weight to total weight is much higher. A small fighter with superior agility at 50% internal fuel, compared to a large fighter, will almost certainly fall behind the large fighter in such a scenario.
Basic physics cannot be escaped in this game. If you want to fight at longer ranges you need a bigger airframe, and there is little else you can do about it. Inflight refuelling is clearly a must for small fighters, but this can complicate things tactically since a tanker is a high value asset and will probably require its own fighter escorts if it is to refuel small fighters with minimal external fuel load, since the last refuelling must be done much closer to the contested area.
No amount of well spoken argument can change the fundamental physics involved, and lay readers should give this some careful thought. In summary, all other capabilities being equal, a small fighter can contest a big fighter successfully only at shorter unrefuelled ranges, and the initiative will thus be in the hands of the user of the big fighter.
The final part of this series will explore low observables issues in modern fighters.
In this final part of this feature we will explore low observables issues in modern fighter design, and attempt to relate these to capability and survivability in combat aircraft.
As discussed in earlier analyses, stealth provides a decisive advantage in a BVR oriented air combat environment, as well as allowing for unescorted deep penetration into well defended airspace for strike missions. The question however will arise as to much much stealth is really required for a given level of threat. Much has been published about the reduced RCS of more recent conventional fighters. This needs to be more carefully explored, since stealth is new and excluding the few initiated, most observers will have little intuitive insight into the basic issues.
An important distinction must be made here between "true" stealth aircraft with an all aspect RCS below -30 dBSM (0.001 m^2 / square metres) and reduced RCS aircraft, with typically head on RCS values between 0 and -10 dBSM (1 - 0.1 m^2). The former can be detected by large early warning radars and SAM acquisition radars inside 20 NMI or less, by big fighter AI radars at about 10 NMI, and locked on by missile seekers at 2-3 NMI. This contrasts starkly with the detection range performance against reduced RCS conventional aircraft, which can be detected by large early warning radars and SAM acquisition radars at 60 - 100 NMI, big fighter AI radars at 40-70 NMI, and locked on by missile seekers at 5 - 8 NMI.
Therefore, a reduced RCS fighter may be competitive against a fighter/AAM threat, or little point defence SAM system. It will not be particularly competitive against a modern area defence SAM system like the SA-10 or SA-12, designed to engage cruise missiles and standoff missiles like the SRAM, which have RCS values of about -10 dBSM (0.1 m^2). This is moreso the case since reduced RCS fighters usually retain very large beam and tail aspect RCS, which means that their ability to fly the air-ground penetration mission differs only marginally from conventional fighters.
One important point to make here is that there is no necessary relationship between aircraft size and RCS, ie a big fighter which is sensibly shaped even without specific RCS reduction measures may have a much lower RCS than a significantly smaller aircraft which is less sensibly shaped. The assertion that airframe size differences in fighters amount to significant RCS differences is simply not true. This may seem counterintuitive, but reflects the physics of radar scattering, which are dominated by shaping.
The popular media assertion that "stealth is in the paint and washes off in the rain" is beyond absurd, and its popularity indicates how little stealth is understood outside the radar and stealth community.
If we look at the major RCS contributors to any airframe, viewed head on, we will find a major RCS contribution from the aircraft's basic shape, and what are termed "flare spot" contributions from smaller design features on the airframe. The total RCS is the sum of the shaped related RCS and the flare spot RCS values. Looking at the basic shape of the aircraft, from a head on perspective the biggest RCS contributors will be the leading edges of the wings, tailplane or canards, vertical tail(s), the inlets and the nose radar bay and cockpit. Of particular interest will be any airframe features which form acute angles or dihedral or trihedral corners, since these form excellent broadband wide angle radar reflectors, producing an equivalent RCS far greater than their geometrical size for the upper microwave bands of interest (SAM/AAM guidance bands).
With an established airframe, there is little that can be done to reduce the RCS contribution of the leading edges, other than apply a radar absorbing coating which can help reduce but not eliminate the signature. If the option of a systematic RCS reduction redesign such as that applied to the F/A-18E/F is available, then leading and trailing edges, and panel boundaries can be aligned to scatter energy away to the sides. Straight edges and panel boundaries, angled away from the normal, are highly desirable here.
The nose radar bay can be filled with broadband absorber material behind the antenna, and if a phased array is used, it can be tilted up slightly to bounce the return upward. A "tuned" or bandpass radome, transmissive in the AI radar's band alone, can significantly reduce nose area RCS to out of band threats like early warning radars, AEW&C and SAM guidance radars.
Cockpit canopies and windshields can be laminated or coated with conductive materials to make them radar opaque and hide the highly reflective cockpit interior.
Inlets are a major problem area, since the inlet entry edges are nice reflectors and the inlet tunnels behave like blanked off waveguides, which guide impinging energy to the fan face(s), where it is modulated with an engine signature, and then guide it back out through the inlet to be nicely radiated out again, over a wide angular range. Inlet treatments are quite difficult, since absorbent linings must be used in the inlet tunnel, and inlet edges must be treated with absorbent materials, and also possibly geometrically realigned to scatter away from the boresight. A clear giveaway of poor inlet RCS performance are rectangular inlet leading edges which are aligned at right angles to the aircraft's boresight.
The other major issue with reduced RCS and conventional aircraft is the issue of external stores. If the aircraft has an RCS of -10 dBSM (0.1 m^2), but is carrying a package of drop tanks and weapons with an RCS of 3 dBSM (2 m^2), then the stores will clearly compromise the aircraft. While conformal or semiconformal carriage are helpful, there is no substitute for internally carried stores. This is all the more important for the strike mission, since the aircraft must contend with large high performance early warning and SAM radars.
Having addressed major RCS contributors, we need to look at the smaller flare spot contributors. These can have RCS values up to 0.5 m^2, particularly if they resonate at a particular wavelength. Good examples of flare spots are the various little air scoops which adorn an airframe, RWR/ESM and nav/comm antennas, semi-conformal weapon stations, unfortunately aligned panels, air vent grilles, gaps between control surfaces and the airframe, and optical sensor domes such as MAWS, FLIR and IRS&T. Should we be aiming for significant stealth performance, then we even need to eliminate rivet heads, drain holes and gaps between panels.
While flare spots have much lower unit signatures than major airframe components, there are usually a great many of them and their effects are additive. Moreover, they often produce their RCS over a wide angular range, and thus will provide a stable rather than scintillating radar return. Therefore any airframe RCS reduction effort, if serious, will almost certainly need to deal with the most troublesome minor flare spots on the airframe.
The wide range of values associated with reduced RCS aircraft reflects essentially the amount of effort expended on RCS reduction. A comprehensive redesign like that performed on the F/A-18E/F will yield a much lower RCS than a minimal effort to make the canopy opaque, fit absorber around the radar antenna, and put absorber in key areas around the inlet. In any event it is worth stressing here that a reduced RCS aircraft still has a 100 times or greater RCS than a true stealth aircraft, even if it is 10-100 times smaller than that of an untreated conventional aircraft.
Any assertions by some manufacturers that reduced RCS aircraft provide a significant improvement in survivability against an area defence SAM threat are a simple marketing ploy, designed to be swallowed by stealth-illiterates. At best, a reduced RCS fighter when clean and carrying semiconformal AAMs, will have increased survivability against a fighter/AAM threat or short range point defence SAM threat.
Infrared signature is another aspect of aircraft observables which deserves scrutiny, since the Flanker and Fulcrum both carry respectable IRS&T sets, and by 2020 we can expect this to be a standard fit on most fighters. Such equipment can typically detect a fighter tailpipe on dry thrust at altitude, from tens of miles away, and a large afterburning plume signature out to distances of 100 NMI or more. As a result, this places a big premium on dry engine thrust performance, since a fighter which requires generous use of afterburning thrust to maintain speed and agility will beacon its position and likely intent from afar. Again, this reinforces the argument for big fighters vs small fighters.
So the argument of "how much stealth is really necessary" is a sensitive one, since even small perturbations in threat capability, like the acquisition of fighter IRS&T and modern area defence SAMs can render any minimal incremental stealthing measures quite impotent. Since the Flanker and SA-10 are now deployed in the wider region, and the SA-12 may deploy soon, a reduced RCS fighter offers only a marginal survivability advantage over a conventional fighter when dealing with this level of threat.
The last decade has seen some significant developments in fighter technology, which suggest that many of the established measures of acceptable capability are now marginally relevant. If we project ourselves 25 years ahead, it is clear that given current trends in technology, a competitive fighter will need to have an advanced passive sensor suite, a low probability of intercept radar, a good measure of stealthiness, sufficient dry thrust/weight ratio to perform sustained supersonic manoeuvring, and sufficient persistence to sustain supersonic combat at a useful radius.
How well the currently available fighters fit this paradigm, whether in baseline or evolved variants, is a very interesting question. The smaller fighters will be penalised by the problem of agility/signatures vs range, reduced RCS fighters will be penalised by external stores at any range, and all will be penalised by the costs associated with extremely capable, but complex and expensive sensor packages.
Half or more of the cost of a modern fighter is in the sensor/avionics/software/weapons package, and as time progresses, this is likely to become the dominant cost factor in the game, if current trends are followed. The cost penalties between airframe sizes will become secondary to the cost penalties between sensor/avionics/weapons packages. Given the small size of such packages today, even a small airframe can fit a highly competitive package.
The ADF fighter replacement problem decomposes into two problems, that of finding an airframe with the range/agility and signatures performance to be competitive, and that of finding a suitable sensor/avionics/weapons package to match. Since some packages are firmly wedded to their respective airframes, the puzzle becomes all the more complex to solve properly.
A key strategic issue for the RAAF will be the degree of autonomous counter-air and strike capability inherent in a given package, since the RAAF will never have the resources to field large numbers of highly capable information gathering assets like AWACS, Rivet Joint, JSTARS and imaging/radar/Elint satellites. This suggests that the RAAF will have to lean in the direction of more capable fighter avionic packages, increasing the baseline cost regardless of airframe size.
The great hope of many in the Australian defence establishment, the yet to fly JSF, is to achieve low cost by minimising the capability of its onboard sensor packages, and relying on the JTIDS and other datalinks to carry targeting data to it from AWACS, Rivet Joint, JSTARS, UAVs and satellites. As a adjunct fighter to designed to operate as part of a massive US force package, this is a workable model, where the JSF supplements the highly autonomous F-22. As a primary fighter built to operate with maximum autonomy, this is a marginal strategy, akin to the idea of the F-16A which had to go through extensive sensor and weapons upgrades to be genuinely useful outside of its original day VFR dogfighter mission.
The F-22A can clearly provide the required capability, but will do so at some cost premium which is yet to be determined. Therefore, it will be exposed to political attack from vendors of other airframes, dutifully doing their best to sell their own products. Since it is also being subjected to systematic political and media attack in the US, this will increase the political cost to be borne by an Australian government which might like to acquire it.
The remaining, conventional aircraft in the marketplace will fall short in the areas of stealth performance, and with the smaller types, effective combat radius performance.
Given that nearly all current build fighters fall into the USD 45-70M unit flyaway cost bracket, whatever choices the RAAF makes will be expensive. And at this time there is little certainty on post 2000 production volumes and pricing for most types, given the volatility in the international arms market. We can expect that whatever choice is made, between 10 and 20 billion dollars will be required for this program. With political expectations of stable defence expenditure at odds with a regional arms race, and competing demands for resources by the other services, throwing away capability to keep down costs will be a tempting political choice for our national leadership. It will not however change the wider regional situation, nor will it reduce the risks that go with such a strategic environment.
Maintenance of a credible conventional deterrent capability hinges on the ability to inflict unacceptable attrition upon an opponent's military and infrastructure assets. In a wider regional arms race situation, this will inevitably be costly, as the capabilities of other players grow. However, unless Australia chooses to pursue other, less politically acceptable means of strategic deterrence, the cost of capable long range air power is one which we as a nation must simply accept.
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