|Last Updated: Mon Jan 27 11:18:09 UTC 2014|
Advanced Tactical Fighter
[YF-22 and YF-23]
F-22A Raptor [Click for more ...]
Part 1: The Evolutionary Path
The rollout and first flights last year of the YF-22A and YF-23A Advanced Tactical Fighter (ATF) demonstrator aircraft represents another quantum leap in the evolution of air superiority aircraft design.
Unlike earlier designs, the ATF is a careful blend of advanced aerodynamics, propulsion and electronics and involves a degree of system integration never before attempted in a tactical aircraft. The reason for this unprecedented effort is quite clear - the Russians have finally deployed their equivalent to the teen series fighters (see May, June 1990 AA), the Flanker and Fulcrum, and have thus very rapidly closed the technological gap which offered such favourable exchange rates for so long. It is worth noting that the teen series fighters held the high ground for well over a decade which is within itself no mean feat, if we observe the history of fighter development.
It is by turning back to the last two decades of that history that we can fully understand the evolution in tactical thinking that spawned the design concepts now embodied in the ATF. It was in fact about twenty two years ago that a pivotal point in the development of air combat tactics was reached, and a new outlook developed on the issue of air combat performance parameters. The setting for this situation was more than well publicised, as the USAF hammered the Third World infrastructure of North Vietnam with weapons designed to destroy industrialised economies. TAC flew daily raids with fast big and well armed F-105D Thuds and F-4C/D Phantoms. The Thud was initially designed for tactical nuclear strikes and thus sacrificed manoeuvring ability for speed at low level, as such it had no serious competitors in its class. Vietnam saw the Thuds loaded up with 500 lb and 750 lb iron bombs flying low level dive bombing raids on a range of targets, the Thuds carried only an internal 20 mm gatling for self defence.
The Phantom in its C and D incarnations was a minimal modification of the US Navy's established F-4B fleet defence interceptor, the F-4 had a useful bomb carrying ability but significantly served as the USAF's principal air superiority aircraft. It was armed with semi-active radar guided AIM-7D/E missiles, heatseeking AIM-9B and AIM-4D missiles and in some instances, 20 mm gatling centreline gunpods. Their opposition was a mixed bag of second hand MiG-17F, MiG-19S/F-6 and MiG-21F aircraft, armed with 23 mm and 30 mm guns and in some instances, AA-2 Atoll heatseeking missiles. The aircraft were flown by Vietnamese, Russian, Czech, Polish and East German pilots, the latter not publicised for obvious propaganda reasons. The USAF did not fare very well, achieving an air-air kill ratio of 2.18:1 in the period between 1965 and 1968. The reasons were manifold, as inadequacies in missile performance, weapon systems and tactics combined with absurd rules of engagement (ROE) offered every imaginable advantage to the Communists, who did not hesitate to exploit them. What had become very apparent was that the performance characteristics of the aircraft and weapons used were ill matched for the kind of engagements which were entered. The MiGs were light VFR only fighters with simple gyro stabilised gunsights and no fire control worth mentioning, the 17 and 19 being both optimised for climb performance at high subsonic speeds. This gave them good turning ability and respectable subsonic acceleration which combined well to provide the basic elements of good gunfighting air combat performance.
The Phantoms were IFR supersonic interceptors, with air intercept radar, two man crew, beyond visual range (BVR) radar guided missiles and tail aspect only heatseeking missiles, both types optimised for killing high flying bombers. The missiles lacked the manoeuvre performance to successfully kill a nimble and small target unless launch conditions were optimal and the target's manoeuvre options were highly restricted. The Phantom's advantage in transonic acceleration and climb performance was balanced by high wing loading and unspectacular turning performance.
The USAF was clearly unhappy about this situation and measures were sought to improve kill ratios. The demand for a gun and better turn performance spawned the F-4E with a stretched fuselage, improved fuel capacity, chin mounted M61 20 mm gatling gun and manoeuvring slats. The same pressures also led to a major review of air-air tactics which saw the adoption of energy manoeuvrability as a fundamental of the new air combat manoeuvring doctrine. Conceived by a serving USAF fighter pilot, Major John Boyd, energy manoeuvrability revolved about the use of an aircraft's energy state (ie speed/altitude) to gain a positional advantage in a manoeuvring engagement, thus gaining a firing opportunity.
Because the close-in air-air weapons of the period were guns and tail aspect heatseekers, the best firing opportunities resulted from tail aspect shots, this in turn dictated that an aircraft must possess superior sustained turning, acceleration and climb performance to defeat an opponent. While the F-4 had the acceleration and climb performance to kill most current opponents, it was clear that the next generation of fighters had to possess even greater thrust/weight ratios and much better sustained turning performance. The latter was simply not achievable by modifications to existing types which had their origins in the late fifties and hence were aerodynamically highly optimised to strike or interception roles.
The USAF initiated its FX program, while the USN discarded its troubled F-111B bomber turned interceptor in favour of the new VFX. Both the VFX and FX exploited new propulsion technology, discarding afterburning turbojets in favour of afterburning turbofans which offered much better specific fuel consumption in dry thrust and a higher ratio of afterburning thrust to dry thrust. Experience in Vietnam clearly indicated that the endurance/combat radius of the 400 NM class F-4 was inadequate and hence the VFX and FX were designed to a 1000 NM class combat radius. Climb and turn performance dictated low wing loading and good AoA performance this in turn shaping the wing and inlet designs.
First to fly was Grumman's VFX, designated the F-14A, a large twin with swing wings and a pair of TF-30 fans. The F-14A had a large bubble canopy for good visibility during dogfights, a Head Up Display (HUD) gunsight, computer controlled automatic wing sweep and glove vane positioning, a massive AWG-9 pulse Doppler air intercept/fire control radar system capable of tracking multiple targets in ground clutter and an internal M-61 gun. It was bigger, more complex and more expensive than the F-4, but it also offered agility and manoeuvrability without precedent. The first of the teen series fighters had thus made its mark.
The 20,000 lb class TF30-P414 powerplants fitted to the F-14A were a stop gap measure which offered a 1:1 class thrust/weight but not the stunning 1.2+ class thrust/weight sought by the Navy, that was to occur with the F-14B which was to be fitted with F401-P-400s, navalised derivatives of the emerging Pratt&Whitney FX powerplant. For budgetary reasons this never eventuated, the F-14A having to wait until the nineties for the 30,000 lb class F-110-GE-400 powerplant.
The USAF's FX subsequently flew in 1972, taking advantage of the 25,000 lb class F100-PW-100 powerplants and optimised for energy manoeuvrability. Like the F-14 it had a large bubble canopy, HUD, powerful pulse Doppler fire control radar, low wing loading and internal gun. Because it was smaller and lighter than the F-14A, it offered a stunning 1.4:1 class thrust/weight ratio and thus set the standard for air combat fighters (see AA Sept/Nov 84).
By the mid seventies the F-14A and F-15A entered service and quickly established their superiority over existing aircraft. Cost of ownership had however proven to be a major issue as both aircraft cost much more to buy and even more to maintain, given their more complex powerplants and avionic systems. Under pressure from legislators, the USAF initiated the Light Weight Fighter (LWF) program to supplement the F-15 with a smaller and much cheaper VFR dogfighter, an aircraft armed with guns and heatseeking missiles and equipped with a small pulse Doppler radar optimised for dogfighting alone.
Northrop bid their YF-17, a derivative of their P-530 lightweight fighter, while General Dynamics bid their YF-16, derived from a fly-by-wire relaxed static stability technology demonstrator. The YF-17 Cobra was a small twin with two 'leaky turbojet' YJ101-GE-100 powerplants and a hybrid planform comprised of massive Leading Edge Extension (LEX) strakes and a moderately swept wing. The YF-16 was fitted with a single F100 common to the F-15 and employed extensive wing/body blending, also using strakes and a moderately swept wing. The YF-16 was the first aircraft ever built to be statically unstable and relied upon a triple redundant analogue computer system to remain flyable.
The LWF flyoff was won by the YF-16 due in no small measure to its powerplant commonality with the F-15, and to its smaller size and somewhat better thrust/weight ratio performance. Subsequently selected as the standard NATO air superiority fighter, the F-16 is today one of the mainstays of Western air power.
Like the USAF the Navy came under increasing budgetary pressure from legislators, who quickly quashed the Navy's plans for a fleet of supercarriers equipped with a force of air superiority F-14Bs and strike F-14Cs, the latter a strike fighter derivative of the F-14B intended to replace the underpowered A-6 bomber. The Navy were directed to select one of the two USAF LWF contenders under the Naval Air Combat Fighter (NACF) program, the intention being being to replace the ageing A-4, A-7 and F-4 fleets with a single dual role lightweight fighter bomber. Many in the US Navy opposed this move, which largely defeated the Navy's strategy of acquiring a 1000 NM radius air superiority/strike force.
The Navy eventually selected the F/A-18A, a substantial redesign of the YF-17 airframe with greater internal fuel capacity, bigger 16,000 lb class F404 engines and BVR missile capability, absent on the smaller F-16. The F/A-18A was designed to rigid reliability and maintainability specifications, which substantially increased the cost of the aircraft, which by then had acquired a very sophisticated suite of avionic systems, imposed largely by the need for one airframe to fulfill two diverse roles. Another side effect of this program was the 'detuning' of the F404 powerplant's performance to improve reliability and lifetime.
The F/A-18A also employed fly-by-wire, but employed a sophisticated quadruplex digital system where software changes could be employed to 'tweak' handling characteristics. The avionic system was built around multiply redundant serial Mil-Std-1553B databusses and employed dual redundant digital computers.
With the entry into service of the F/A-18A the US services completed their reequipping with teen series fighters, eventually building up a force of several thousand aircraft of these four basic types.
The basic philosophy of high thrust/weight ratio and sustained turning performance in transonic dogfights shaped these aircraft aerodynamically and hence imposed fundamental constraints to the other performance characteristics of this family of aircraft.
Evolution did not stand still however, and the early eighties saw the deployment of one of the most significant air combat weapons of its time - the all aspect heatseeking Lima model of the established AIM-9 Sidewinder missile. The AIM-9L did not require a tail chase position to lock on to a tailpipe, it was quite happy to lock on from any angle including 12 o'clock ie head-on. Soon after deployment the AIM-9L proved that much of the air combat tactics textbook had been obsoleted, in that instantaneous turning performance became far more important than sustained turning performance. The ability to point the nose at an opponent quickly and loose off a missile became far more important than the ability to follow through multiple turning manoeuvres to acquire a tail aspect gun/heatseeker firing position. While a tail aspect position did improve the kill probability of the missile by reducing the target's evasive manoeuvre options, the AIM-9L's all aspect performance was still superb, as learned the hard way by the Argentine and Syrian air forces in 1982.
Clearly the day of the classical dogfight was almost over, in that the first aircraft to acquire its opponent would be first to fire and most likely to win the engagement. Only if the first firing opportunity were to fail and the combatants were to pass each other and then engage in a turning dogfight, was there a major requirement for sustained turning performance. Yet again, however, the all aspect capability of the AIM-9L would convert fleeting nose on target opportunities into real firing opportunities.
The first aircraft to take advantage of this situation was the Navy F/A-18A, which had its flight control software tweaked up to optimise instantaneous turn performance, this combined with its sophisticated HUD and fire control radar dispelled any doubts of the aircraft's lethality, given earlier criticism of its low (ie 1.1:1 !) combat thrust/weight ratio. Acceleration and climb performance, and low energy bleed in manoeuvring at transonic speeds were the key parameters in this class of engagement.
A new philosophy for air combat tactics was thus developed by the USAF, who envisaged long range medium to high altitude penetration of hostile airspace by supersonic cruise capable fighters with all aspect fire and forget missile armament. A key element in the new strategy was the AIM-120 Amraam missile (AA Sept 86), an active radar guided intelligent fire and forget BVR missile, designed to replace the established semi-active radar guided AIM-7. Coupled with a suitable fire control radar the AIM-120 allows a single fighter to salvo up to eight rounds at eight separate inbound targets within the acquisition geometry of the fire control radar.
Manoeuvring at sustained supersonic speeds, the new look air superiority fighter could outmanoeuvre SAMs and most AAMs, while always retaining an energy advantage over the subsonic/transonic speed range optimised teen series turning dogfighter. The aerodynamic and propulsion design compromises which supported sustained turning performance at high subsonic and transonic speeds seldom improved the ability of the aircraft to sustain high energy manoeuvres at Mach 1.5 class speeds, where much of the wing was enveloped in the shock cone produced by the nose of the aircraft.
The philosophy of first-look first-shoot reflected in the need for superior sensor capability and low observability ie Stealth. The former and latter requirements result in the need for a frequency/spatially agile radar and high performance InfraRed Search & Track (IRST), while the latter requirement imposes the need for all of the tricks of the Stealth trade, ie shaping, skinning and detailing.
The first aircraft to fly which embodied some of the new technology was the ill-fated F-16XL. A radical redesign of the F-16A, the XL was a supersonic cruise demonstrator with a cranked arrow delta wing optimised for that flight regime. The aircraft was, on the strength of published reports, a major technical success, with two demonstrators eventually flying. The highly swept inboard wing section of this aircraft produced substantial vortex lift at supersonic speeds, while also improving instantaneous turn rate and extending the 9G manoeuvre envelope well above Mach 1. An additional benefit of the new configuration was a substantial increase in internal fuel capacity, providing a 120% improvement in combat radius performance.
Early Lockheed ATF Concept (1988).
The F-16XL suffered the fate of many pioneering aircraft before their time, its F-16E dual role strike fighter derivative lost out in a flyoff against MDC's bigger and more capable F-15E Strike Eagle, thus ending all prospects for its eventual production. Many observers attributed its demise to a political strategy played by the USAF, to prevent an older generation airframe derivative from being used by legislators as an excuse to kill off or postpone the ATF program. Equipped with Amraam, higher thrust engines and new radar, the F-16XL could cover a large part of the role envisaged for the ATF at substantially lower unit and program costs. As an older generation airframe however its infrared and radar signatures are substantial and this would greatly reduce its effectiveness (although trivia of this nature hardly ever bother astute decisionmakers such as politicians...).
The ATF program had its origins in numerous USAF air combat studies carried out in the late seventies and early eighties, when intelligence information revealed the Soviets' early flight testing of the Fulcrum and Flanker. From the observed geometry of the airframes it was clear that both types would have the vortex lift performance to challenge the teen series aircraft in turning dogfights, by the same token both Soviet fighters would be handicapped by their geometry in both supersonic manoeuvre and low observability performance.
The ATF was to be the successor to the F-15, a long range air superiority fighter with the performance to kill any other tactical aircraft and the operating radius to threaten targets deep inside the USSR while flying from bases in Western Europe. This was to be achieved by the use of a highly integrated airframe/systems/propulsion design exploiting advanced aerodynamics, engines and stealth technology, the latter to delay an opponent's initial firing opportunity for as long as possible, and thus capitalise on the large Radar Cross section (RCS) of the Fulcrum and Flanker.
Subsequent to studies, an RFP was issued in July 1986, and two contractor teams, Northrop/McDonnell-Douglas and Lockheed/Boeing/General Dynamics were selected in October 1986 for the initial 50 month demonstration/validation phase flyoff. The rollout of the prototypes was initially scheduled for mid 1989, but ongoing slippages have delayed this until the middle of last year.
Part 2 provides a technical comparison of the YF-22A and YF-23A prototypes
MDC F-4 Phantom II. The F-4 was the mainstay of the USN's and USAF's tactical fighter force in the SouthEast Asian conflict. The Phantom offered superb acceleration and climb performance for its day, while carrying an impressive air-air payload of 4 heatseeking AIM-9D and 4 semi-active radar AIM-7E missiles. In engagements with the smaller and nimbler NVAF MiGs the F-4 was hampered by poor missile performance and reliability, inadequate radar lookdown performance and the absence of a gun for close in engagements.
Grumman F-14A Tomcat. Grumman's large F-14 fighter was the first of the teen series aircraft to fly and deploy. It was equipped with a pair of 20,000 lb class TF-30 afterburning fans and a computer controlled variable geometry wing to provide superb turning and acceleration performance, while its massive pulse Doppler AWG-9 fire control radar and Head Up display allowed the targeting of 100 NM class AIM-54 Phoenix missiles, AIM-7 Sparrow, AIM-9 Sidewinder missiles and an internal M-61A1 20 mm gun. It has remained a formidable dogfighter to this day.
MDC F-15A Eagle. The F-15A was designed, like the F-14A, for high thrust/weight ratio to provide superlative acceleration, climb and turn performance. Like the F-14, it is equipped with a high power long range lookdown/shootdown pulse Doppler radar. Armed with a mix of AIM-7 and AIM-9 missiles and an internal M-61 gun, the F-15 has repeatedly demonstrated its capability in the Middle East. The most recent subtype, the dual role strike fighter F-15E, is structurally strengthened for 9G manoeuvres.
Dubbed the 'Electric Jet', GD's F-16 was the first tactical aircraft to employed relaxed static stability and fly-by-wire control. Initially acquired as a low cost VFR dogfighter armed with an internal M-61 gun and heatseeking AIM-9 missiles, the later F-16C is a truly multirole tactical aircraft, with wide angle holographic HUD, and provision for Lantirn terrain following radar/FLIR pods and the AIM-120 Amraam BVR missile.
The F-16XL was a supercruise technology demonstrator derived from the basic F-16 airframe/powerplant. The cranked arrow delta wing allowed the aircraft to cruise supersonically on dry thrust, improved the manoeuvre envelope substantially while providing enough additional internal fuel capacity to increase the combat radius by 120%. Sadly it never entered production, losing to the F-15E in a competitive flyoff for the Dual Role Fighter program. As a teen series airframe lacking stealth capability, it cannot compete with the newer ATF aircraft.
Using a hybrid planform wing and digital fly-by-wire, the MDC F/A-18A was the last of the teen series fighters. It was the first tactical aircraft to employ a fully digital Mil-Std-1553B bussed avionic system under the control of redundant digital computers. A multirole derivative of the YF-17 airframe, the later F/A-18 is a fully capable all weather strike fighter which retains excellent air superiority performance.
The Su-27 Flanker is the most capable aircraft in the Russian inventory and is expected to be a hot seller in the Third World market (Above is the aircraft in Chinese livery with R-77 Adder radar guided missile and R-73 IR missile below). Aerodynamically it reflects the design philosophy of the teen series aircraft, employing vortex lift, high thrust/weight ratio and fly-by-wire control. Equipped with a large pulse Doppler radar, internal 30 mm gun, BVR and heatseeking missiles, it is a formidable opponent. Recently tested on the V-MF's new CVAN, the Flanker has the combat radius and performance to contest any teen series aircraft.
Part 2: YF-22A and YF-23A - A Technical Comparison
At the time of writing the Northrop/MDC YF-23A and Lockheed/B/GD YF-22A had both completed their respective demonstration/validation flight test programs . While the USAF have not revealed much about the internals and performance of the aircraft, their airframe geometry and known powerplant parameters reveal much of their design philosophy and performance. Both aircraft reflect their prime contractors' respective philosophies of stealth aircraft design as much as they reflect their common mission profile.
Principal airframe/propulsion design objectives were sustained supersonic cruise on dry thrust, high energy manoeuvrability, superior combat radius to the F-15 with all weapons and fuel carried internally and low signatures.
Both ATF prototypes are approximately 10% larger than the F-15 and both carry approximately twice the internal fuel of an F-15C, while both have about 50% more wing area at about 30% greater combat weight. As such both aircraft clearly illustrate the long range air superiority mission which was originally envisaged for the aircraft, penetrating deep into Soviet airspace to destroy air defence aircraft and to disrupt Soviet offensive air operations. The decline of the Soviet empire during the last 18 months has understandably led to many US politicians calling for the scrapping of the ATF program, in view of the 'diminished threat'. This myopic posture needless to say wholly disregards the fact, that the USSR itself is quite unstable and could well slip back into hardline Stalinism, and also ignores the reality that the USSR will sell the Flanker and Fulcrum to any party who can pay for it. It is likely that that these capable teen series class aircraft will become as common in the Third World as the ubiquitous Fishbed. The mere perception that a capability matching that of the frontline Western aircraft is present will be destabilising - the instance of Iraq with its Fulcrums and Fencers is a case study, their tactical and technical incompetence clearly underscoring this sad phenomenon.
The reality is that capabilities are a good measure of intent, it is unrealistic at the least to assume that any nation will expend vast sums of money to acquire specific weapons systems without seeing how that expenditure will further its interests. Long range air superiority aircraft such as the Flanker serve a clearly defined role, offensive strategic air war.
How the ATF performs this role is best judged by a closer look at the design philosophy of the airframe, propulsion and weapon system.
The ATF airframes represent another quantum leap in air superiority airframe design, as great as that represented by the teen series fighters. Two new and key capabilities were integrated in the ATF program, low observability (ie stealth) and supersonic cruise.
The objective of low observables is to reduce the performance of hostile radar and infrared surveillance, tracking and guidance systems. Existing airframes perform poorly in this respect, and thus only a new airframe design can address the problem.
Supersonic cruise serves several purposes, providing for fast and deep penetration into hostile airspace, while offering the supersonic cruise fighter a major energy advantage over subsonic/transonic dogfighters which it can both outmanoeuvre and outlast in a supersonic engagement. The high corner speed of such aircraft also provides a major manoeuvring advantage when evading SAMs at altitude, enhancing survivability on deep penetration missions. Supercruise required major advances in propulsion technology and nontrivial concessions in airframe design.
Low observability in the ATF designs is achieved by a range of measures, how these are applied clearly illustrates the heritage of the respective designs.
The Lockheed/B/GD YF-22 employs planform shaping and faceting with blended facet boundaries, the latter a necessary concession to high performance aerodynamics. This is apparent in the shape of the nose, the fuselage sides about the inlets and engines, and the upper forward fuselage. Lockheed/B/GD used serrated edges extensively, as with the F-117A, to control the returns from panel boundaries, this is very visible on the undercarriage and weapon bay doors.
The planform results in a multiple lobe design, as the boundaries of the major surfaces are not parallel with respect to each other. Planform return lobe structure is defined by the radiation pattern lobes resulting from surface wave reflections which occur at the leading and trailing edges of the airframe's major surfaces. The objective of lobing is to concentrate this unavoidable radar return into specific directions so as to minimise frontal/aft/beam aspect return and maximise scintillation in the direction of the lobe. Scintillation is a measure of how rapidly the size of the return varies with angle, the greater this variation, the more difficult a target is to track. The lower the number of lobes and the narrower the lobes, the lower the probability of detecting any return.
The Northrop/MDC YF-23 employs planform shaping with extensive blending, the latter technique used to advantage with the large B-2A. Blending has the major strength of not compromising high speed aerodynamics, the blended airframe offering very low drag by avoiding vortices which may be produced by a faceted geometry. In addition to RCS reduction through shaping, the YF-23 also employs carefully shaped exhausts to conceal the engine hot end, yet another technique developed during the B-2A program [Editor's Note 2005: the exhaust troughs used air cooled inserts resulting in the lowest IR signature of any fighter ever built].
The unusual 'diamond' planform of the YF-23 is a 2 major lobe design, as all major edges fall into groups of two parallels.
The result of the low observables techniques employed with these aircraft is a major reduction in aircraft detectability by radar, and in the YF-23, also detectability by Infra-Red Search & Track (IRS&T) systems. This will radically shrink the usable envelope of hostile radar guided weapons and in the instance of the YF-23, also heatseeking weapons.
Lockheed/B/GD chose a somewhat conservative hybrid planform airframe layout, reminiscent of the F-15 and F/A-18, with closely spaced engines, long inlet tunnels, outward canted vertical tails and rudimentary strakes over the inlet boxes to promote vortex lift over the outboard wing sections at high AoA. The characteristics of this general layout are well understood, the forward sloped inlets providing good airflow characteristics at high AoA and the conventional tail providing good controllability under such conditions, apparently earlier attempts at using a V-tail did not yield the desired results. The close spacing of the engines reduces inertia in the roll axis, but may penalise survivability. Weapon bays are located on the sides of the inlet boxes and a single central bay is located beneath the centresection, all located well aft of the inlet to preclude ingestion problems. Typically AIM-9s fit in the inlet bays and AIM-120s in the split central bay.
The single piece canopy cockpit is well elevated to maximise the pilot's situational awareness.
Northrop/MDC chose a far more radical airframe layout, driven by the objectives of stealthiness and supercruise. The extensively blended fuselage has rudimentary chines which smoothly blend into the wing leading edge, the blending allowing good area ruling and low supersonic drag. The low wing aspect ratio is used to optimise supercruise performance. The ventral trapezoidal inlets feed the engines via stealthy S-bends, and the rear boattail and submerged dorsal exhausts were specifically aimed at low drag and infrared signature. The YF-23 employs an unconventional V-tail with a planform consistent with the airframe lobing strategy. The large centresection area will provide substantial body lift at high AoA thus improving turn performance, a technique used in the F-14 and Flanker. While the widely spaced engines result in some roll rate penalty, they are sufficiently separated to avoid fratricide in the event of turbine breakup. Two tandem weapon bays are employed, the aft bay is reported to be very large and contains pairs of staggered AIM-120s, the forward bay carrying AIM-9s.
The YF-23 employs a two piece canopy, the cockpit is like it's competitor's well elevated for good visibility.
The exhausts of the two aircraft differ radically. Lockheed/B/GD had chosen a layout aimed at maximising lower speed manoeuvrability via the use of thrust vectoring, even though this was not a mandatory USAF requirement. Two dimensional thrust vectoring nozzles provide vectoring to enhance response in pitch. Northrop/MDC on the other hand rated stealth and drag so important, that they employed a serrated planform beavertail with B-2-like submerged ventral exhaust troughs. This approach reduces both depressed tail aspect infrared emissions and tail aspect radar cross-section, but precludes any vectoring.
Both prototypes are reported to employ relaxed static stability, with multiply redundant digital fly-by-wire control systems.
The navalised ATF derivative planned to replace the Grumman F-14 as the USN's principal air superiority fighter has yet to materialise. Lockheed/B/GD have proposed a variable geometry wing derivative of the TAC design, in order to accommodate the Navy carrier recovery an launch requirements, ie low speed on approach and high lift at low speed on catapult launch. At the time of writing no information was available on the Northrop/MDC proposal.
The unique and new supersonic cruise mission profile of the ATF has had a major impact upon the powerplants to be used for the aircraft. The higher combat weight of the aircraft in comparison with the F-15 imposed a need for greater installed afterburning thrust, in the 35,000 lb class per engine, to maintain the preferred 1.4:1 class combat thrust/weight ratio, while the supercruise profile imposed the need for high dry thrust particularly within the supersonic part of the envelope. The latter requirement was particularly painful, as it forced a move to higher temperatures within the engine, particularly the turbine.
The two bidders for the ATF powerplant are Pratt & Whitney and General Electric with their YF119 and YF120 designs respectively. The P&W YF119 is the lower risk of the two designs, an advanced low bypass ratio turbofan. The GE YF120 is more radical, as it is a variable cycle engine capable of adjusting its bypass ratio to the optimum for a given flight regime.
GE's involvement with variable cycle engines dates back a decade, with a major technology demonstration program built around a substantially redesigned YJ101 (former YF-17) powerplant. This was followed by work on an F404 derivative, this providing the foundation for GE's variable cycle technology. The core of the YF120 was derived from work done during the government sponsored ATEGG (Advanced Technology Engine Gas Generator) and JTDE (Joint Technology Demonstrator Engine) programs. Subsequently early development XF120 engines underwent testing at the USAF Systems Commands AEDC facility. Ground test prototype YF120s have been under test since late 1989.
Internal details of the YF120 are, not surprisingly, classified. The engine is known to be a two shaft design with a minimum number of rotating stages, a fan which has been speculated to be a single stage design and a compressor using integrated bladed rotors. In common with earlier GE VCEs, the YF120 uses VABI (Variable Area Bypass Injector) technology to alter engine bypass ratio. The YF120 is reported to use aerodynamically actuated VABIs, in which respect it differs from earlier designs which used mechanical actuation. Typical VABI technology used in earlier GE designs saw the use of sliding sleeves which would reduce the cross section at the fan exit entry to the bypass duct, and at the tailpipe exit from the bypass duct.
This arrangement allows the engine to smoothly optimise its bypass ratio to the flight regime. For maximum afterburning thrust on takeoff or efficient subsonic long range cruise, a high bypass ratio is set. For supersonic cruise a turbojet is approximated, with very low or zero bypass ratio. Turbojets are considered optimal for supersonic flight as their dry thrust drops far more slowly than that of a fan with increasing vehicle airspeed. The ATF flight profiles are sufficiently unconventional to create major difficulties for a fixed bypass ratio engine designer attempting to reconcile the diverse demands of lower speed operation and supersonic cruise.
This must have been the case with P&W, who have bid a fixed bypass ratio turbofan derived from the ATEGG/JTDE programs and the company's existing F100 family of fans.
Published reports indicate the GE engine has demonstrated better supercruise performance than its conventional rival and it is very likely that GE's gamble with a more radical technology will yield the desired payoff. The gain in overall engine performance in comparison with existing teen series fighter powerplants is clearly illustrated by a Lockheed/B/GD flight envelope chart for the YF-22 which shows a military thrust envelope for the YF-22 as greater at all airspeeds and altitudes than the afterburning envelope of the F-15C. In the thrust/drag limited low altitude regime the YF-22 dry envelope is 7% greater than that of the F-15C, given the similar configuration of both airframes and greater wetted area of the YF-22 this suggests dry thrust in excess of 25,000 lb per engine.
Avionics, Cockpit and Weapon System
Avionics is an area where the ATF will offer a radical improvement over existing systems. From the outset avionics were a key aspect of the ATF program. Initial studies were aimed at a distributed architecture designated Pave Pillar, the objective of which was to employ physically separate common computing modules for the aircraft's vital systems. This would provide superior tolerance to battle damage and internal systems failures, while reducing the requirement for unique spares modules. A high level of integration was also sought in the comm/nav systems and electronic warfare systems, under the USAF Icnia (Integrated Communications, Navigation and Identification Avionics) and Inews (Integrated Electronic Warfare System) technology development programs respectively.
A major system level requirement was supportability in the field and very high reliability, the latter a must in view of the complexity of the aircraft. The scale of effort in this area is reflected by a requirement for a combat turnaround of 15 minutes (cf 35 minutes for F-15), a requirement for 9 support personnel/airframe and 6.8 C-141 loads of support equipment, in comparison with the existing 17 for a 24 aircraft TAC squadron.
The YF-23 avionic system is built around a core integrated system using Unisys 32 bit GPPE (General Purpose Processing Element) modules. The original 3 CPU 1750 architecture mission computer arrangement was discarded as the support hardware requirements were excessive, and the computational power inferior.
Signal processing is done with a single dedicated processor, sliced between two large physically separated 75 card racks, with redundant functional modules spread between the racks to enhance survivability.
The YF-22 avionic system is built around Hughes CIP (Common Integrated Processor) modules. Lockheed/B/GD have apparently opted for unconventional liquid cooling of the processor modules to reduce hardware operating temperatures.
Weapon system software is to be implemented in US DoD standard ADA language, it is not clear whether the production code will be to current ADA or revised ADA 9X standard.
The INEWS electronic warfare systems are being developed by two contractor teams, TRW/Westinghouse/Tracor/Perkin Elmer for the YF-23 and Sanders/GE/Motorola/HRB for the YF-22.
The sensor suite will be dominated by an active phased array radar. The radar will employ electronic antenna scan exploiting over 1,000 transmit/receive/phase-shifter elements, each of which is a wholly self contained module. This arrangement results in a highly robust design which doesn't require mechanical pointing, as main lobe shape and direction are controlled electronically, and which gracefully degrades in performance as modules fail. The use of electronic beam shaping/pointing provides major advantages as this class of radar may timeshare its antenna between modes, optimise lobe shapes to modes, tolerate violent manoeuvring and also selectively direct nulls at troublesome jammers as a potent ECCM. Both contenders would employ a Westinghouse/TI radar design, initially flown in 1989 and derived from the URR (Ultra Reliable Radar) program.
This radar is the most radical step in fighter air intercept radar design since the first pulse Doppler sets were introduced in the early seventies, and offers diverse upgrade paths through software changes in the beam control subsystem and the signal processing subsystem.
The radar is to later be supplemented by an EOSS (Electro-Optical Sensor Suite) which is essentially an advanced IRS&T set. Both contenders are to employ a Martin Marietta/GE system using focal plane array (FPA - see earlier TE) technology. The advantage in a FPA design is higher sensitivity and the absence of moving parts, scanning being accomplished electronically. At the time of writing it was unclear as to whether a cheaper mid-infrared PtSi or InSb design would be adopted, or whether a long-infrared HgCdTe design would be employed. While the latter can detect airframe skin friction, it is more demanding in cooling and signal processing. The difficult requirement is to detect and track targets against an IR background at low level, at altitude the background environment is easier to deal with. At the time of writing the EOSS was deferred as it was considered too immature for a low risk production design.
The cockpits of both the YF-22 and YF-23 will be conventional 'glass' arrangements, although Lockheed/B/GD have opted for LCD technology in preference to CRT displays. The YF-22 uses no less than 6 LCDs, typically providing 512x512 pixel resolution with 4,096 colours. An advanced HUD will be employed, as will the USAF's new G-suit technology currently being introduced on the F-15. Both contenders are reported to use conventional control layouts, the sidestick controller not being used.
Flight testing of both avionic suites has taken place on dedicated testbed aircraft, Boeing using a 757 and Northrop/MDC a well reworked BAC-111.
The ATF will be armed primarily with the AIM-120 AMRAAM ARH BVR missile, supplemented by a short range all aspect heatseeker, the AIM-9M at this time. A design requirement is the carriage of four Amraams, these must be ejected from internal bays at launch. An internal gun will be employed, although it appears that the gun is absent on all prototypes.
It is clear from published accounts that the ATF is an enormous step forward in aerodynamic and low observables performance in comparison with the teen series fighters and their Soviet counterparts. Both competitors have repeatedly supercruised on dry thrust with speeds of 1.58 Mach reported for both airframes with YF-120 powerplants. In addition the YF-23 attained 1.8 Mach in afterburner and reports indicate that the final maximum speed figures have been classified by the USAF.
Both airframes offer 1.4:1 class combat thrust/weight ratio performance and combat wing loadings well below 60 lb/sqft, therefore the energy manoeuvrability performance will equal if not exceed that of the F-15. Controllability at high AoA has been reported as excellent for both types, in the absence of hard data it is therefore difficult to estimate whether Lockheed/B/GD's claimed advantage in manoeuvrability will be decisive.
Tactical radius and cruise speed are also critical parameters for the mission, in both areas the ATF is well ahead of the teen series fighters. Again in the absence of hard figures it is difficult to establish whether Northrop/MDC's greater speed and radius performance are a decisive advantage. Certainly the ATF's 25,000 lb class fuel capacity must offer a major gain in radius in comparison with the 13,000 lb class F-15, how much more will depend on the flight profile. Reports suggest the YF-22 consumes 30% less fuel in supercruise than an F-15 in afterburner, suggesting an SFC of about 1.5 lb/lbt/hr which is about twice the dry SFC of an F100-PW-100. Therefore on a purely supercruise mission profile the additional fuel may not offer a gain in radius, however a mixed subsonic/supercruise profile almost certainly would, the gain inversely proportional to the ratio of time spent in supercruise vs subsonic cruise. Both airframes are designed for boom refuelling.
The combined effects of the airframe and powerplant designs will see a toward supersonic engagements, where current generation aircraft optimised for transonic/subsonic manoeuvring with afterburner cannot measure up. The current generation fighter will suffer shortfalls in persistence due increased fuel flow and sustained manoeuvring performance due aerodynamics optimised for turning at lower speeds.
Low observables performance is an area where Northrop/MDC will almost certainly win out over Lockheed/B/GD, due to the effort expended on the design of the rear fuselage exhaust area and due to the use of blending and lobing techniques which offer far lower numbers of airframe discontinuities. Any discontinuity promotes surface wave scattering, therefore the smoother the design the lesser the scattered return. As the RCS figures are classified, it is not clear how great a performance margin exists. Tail aspect radar and infrared performance must be superior in the Northrop/MDC design simply as a result of the geometry used.
Lockheed/B/GD compromised low observables performance to achieve greater agility, whereas Northrop/MDC focussed on stealth, speed and radius performance [Editor's Note 2005: more recent disclosures indicate that the YF-23 was capable of achieving all specified agility points without thrust vectoring, unlike the YF-22]. The USAF's decision will clearly illustrate which of these parameters are considered of greater value in the projected strategic air war of the future.
The Perspective View
To the Australian observer the ATF underscores the revolution under way in tactical air warfare, with stealthiness, radius and agility growing significantly against the existing generation of aircraft. The ATF will be substantially more expensive than smaller multirole fighters such as the F/A-18A, but also offers vastly superior performance in the long range air superiority mission.
[Editor's Note 1997: since the time we published this item in 1991, the outcome predicted in this article has indeed come to pass, with the PRC and India about to deploy large numbers of advanced Flankers, and even Malaysia deploying the potent MiG-29SE with the deadly Archer missile. In hindsight, we correctly anticipated current events, the early replacement of the Hornet now a distinct possibility]
[Editor's Note 2005: since the updated version of this article went online in 1997, Malaysia has opted for the Su-30MKM, Indonesia a mix of Su-27SK and Su-30MK, and China's planned numbers of Su-27SK, Su-30MKK and Su-30MK2 apt to reach 380 or more]
In the current regional air defence environment the F/A-18A has no serious rival. This could however change with a regional acquisition of the Fulcrum and Flanker. While the Fulcrum could be readily tackled in BVR and visual engagements, the larger Flanker would present a serious problem particularly in extended range BVR engagements due to its superior radius and radar performance. Well flown Flankers could present a serious problem for the RAAF as they have greater persistence, superb manoeuvring performance and a larger envelope for firing radar guided missiles.
As the Gulf war demonstrated, modern radar guided missiles are far more lethal than their Vietnam era predecessors and the initial pre-merge phase of an engagement has thus become far more dangerous. Closing fighters now have the option of a head-on BVR missile shot, a situation where radar and RCS performance are critical. Evading an inbound missile can severely disadvantage the defending fighter in terms of geometry and energy state, this in turn penalises it once the merge occurs and a turning dogfight is initiated. A Flanker with its powerful radar and BVR missiles has thus a major advantage over an F/A-18A, which can only employ its manoeuvrability and weapon system to an advantage once a turning engagement has been entered. In a close in turning fight it has the advantage of smaller size and better dogfighting radar modes, but will suffer an energy disadvantage if the Flanker is flying at a lower fuel state. Similarly the Flanker will have an advantage in persistance, given fuel state.
The ATF with its low frontal RCS has a distinct advantage over any current opponent in any such engagement, allowing it to ruin its opponent's entry into the engagement, and then apply its supersonic agility and persistence to force the engagement on its terms. The reduced RCS and in the YF-23 IR signature, will also reduce the usable radius of its opponent's weapons, while allowing the ATF to disengage more readily, itself not suffering any penalties in missile guidance effectiveness.
Were the RAAF confronted with the Flanker, it would have little option other than to consider a two tier force employing the ATF as the long range air superiority element. This in turn however raises questions about whether our political leadership would be prepared to acquire such aircraft, even in limited numbers, given the expense and perceived specialised role. Numbers are a major issue in this context, a very small number of top tier aircraft may not yield the desired effect but will incur the fixed overheads resulting from supporting the type. A large number would be costly, and this would result in interservice political problems.
Hopefully this question will not need to be considered during the projected lifetime of the F/A-18A force, allowing the RAAF to look at cheaper second tier follow-on fighters employing the technology advances currently seen in the ATF.
The USAF at the time of writing envisaged about 500 ATFs to replace the frontline elements of the F-15C force, a reduced requirement against the original 750 airframes, with an additional 450 Navy airframes. The size of the production run would have a major impact on unit costs, given the substantial R&D overhead. Political debate on the usefulness of the aircraft has been heated, as many US politicians consider it to be a specialised asset targeted at defeating Soviet air power in a NATO theatre conflict. While this is clearly not the case, laymen of such calibre seldom allow facts to interfere with their righteous crusades.
The reality is that both the Fulcrum and Flanker if flown by competent pilots and applied appropriately, could successfully contest teen series fighters. The sheer incompetence of the Iraqi air force in the Gulf should not colour perceptions of the worth of the Fulcrum and Flanker. They are serious players and the high production rate of the Fulcrum reflects its status as one of the USSR's hottest exports, almost certainly supplanting the Fishbed as the Third World's premier tactical aircraft.
With shrinking budgets the US will be stretched to meet its commitments and this will reflect in lesser numbers of tactical aircraft available for bushfire conflicts such as the Gulf campaign. Both TAC and the Navy will require a new air superiority fighter by the turn of the century, simply due to airframe fatigue. Both the F-14 and F-15 are in the process of production windup and shutdown.
The question of course remains, will sanity win out ? The ATFs are both quite clearly good implementations of this class of aircraft, unlike the stillborn A-12 which was killed off earlier this year as it could not meet its design specification. US observers repeatedly commented early this year that the A-12 was in a more secure position politically than the ATF as the Navy's A-6E force is now block obsolescent and almost out of life. The F-14 and F-15 have at least a decade of useful life left in them.
Alternatives proposed to the ATF vary from F-15s reengined with ATF powerplants to the revival of the F-16XL, although the latter would require a major redesign to provide some measure of stealthiness.
The most reasonable outcome would be low rate production of the ATF to be later supplemented by a smaller fighter, in the same fashion as the F-15/F-16 programs developed. Whether this eventually occurs remains to be seen, and thus the ultimate fate of the superb ATF contenders is unclear.
Table 1. Performance Comparison - YF-22A, YF-23A, F-15C, F/A-18A
The ill fated A-12A Avenger II was to be a stealthy interdictor replacing the A-6E and F-111 family. The US Navy envisaged the use of the long range ATA in conjunction with the navalised ATF to provide a 1000 NM + power projection capability. The ATA was cancelled earlier this year, as it had become severely overweight and could not meet design performance requirements with a pair of 12,000 lb class F404 engines. The expense of a major redesign with 18,000 lb class F110 engines was substantial and the US DoD killed the program, leaving the Navy with a fleet of obsolescent A-6Es. A short term fix is the adoption of an enlarged strike derivative of the F/A-18, supplemented by strike capable F-14Ds. In the longer term, an AX strike aircraft is envisaged, but no major funding has been allocated at this time.
The YF-22 was optimised for agility with some resulting loss in stealthiness. The general layout is similar to the F-15 and F/A-18, but much larger. RCS reduction is achieved largely through planform shaping and faceting, resulting in a multiple lobe design. Thrust vectoring is employed to improve pitch response.
The YF-23 was optimised for speed, range and stealth at some expense in agility, compared to its rival. The general layout is unique and exploits much of the design technique developed in the B-2A ATB program. RCS is reduced through careful planform shaping and blending, with a unique low drag tail which conceals dorsal exhausts in troughs to reduce both RCS and IR emissions (USAF).
The ATF is designed for a 1:1 class dry thrust/weight ratio and supersonic dry cruise. This provides it with a major energy advantage over a teen series (or teenski series) opponent, which it can outmanoeuvre and outlast in a supersonic engagement (USAF).
The ATF will carry its missiles internally to minimise RCS. Both the AIM-120 Amraam and AIM-9 are ejected from their bays at launch, so that the increase in RCS due open bays is transient and thus cannot allow tracking. The missiles will be supplemented by an internal gun (USAF/Lockheed/Boeing).
The ATF has been designed for a minimal frontal RCS to provide a major advantage in the high noon shootout pre-merge phase of an engagement. A conventional opponent cannot shoot until a lock is acquired, and thus is likely to get hit in the face with an Amraam fired by the closing ATF before he can acquire the ATF. Once a turning engagement is entered, the high dry thrust/weight ratio of the ATF will confer a major energy advantage. A measure of this is a Lockheed report which indicates the YF-22 dry envelope is greater than the reheated envelope of the F-15C!
The ATF is designed to be long legged, with 25,000 lb class internal fuel capacity [Editor's Note 2005: actual production F/A-22A vehicle capacity is closer to 21,000 lb] supplemented by inflight refuelling. This provides it with phenomenal range in subsonic cruise and excellent persistence in supercruise. In strategic air warfare terms, the ATF can penetrate deep into hostile airspace to defeat defending fighter aircraft and disrupt any attempts at offensive air operations, the ultimate application of Lanchester's laws (USAF).
|Artwork, graphic design, layout and text © 2004 - 2014 Carlo Kopp; Text © 2004 - 2014 Peter Goon; All rights reserved. Recommended browsers. Contact webmaster. Site navigation hints. Current hot topics.|