The Advanced Tactical Fighter (2024)

The rollout and first flightslast year of the YF-22A and YF-23A Advanced Tactical Fighter (ATF)demonstrator aircraft represents another quantum leap in the evolutionof air superiority aircraft design.

Unlike earlier designs, the ATF is a careful blend of advancedaerodynamics, propulsion and electronics and involves a degree ofsystemintegration never before attempted in a tactical aircraft. The reasonfor this unprecedented effort is quite clear - the Russians havefinallydeployed their equivalent to the teen series fighters (see May, June1990 AA), the Flanker and Fulcrum, and have thus very rapidly closedthetechnological gap which offered such favourable exchange rates for solong. It is worth noting that the teen series fighters held the highground for well over a decade which is within itself no mean feat, ifweobserve the history of fighter development.

Itis by turning back to the last two decades of that history that we canfully understand the evolution in tactical thinking that spawned thedesign concepts now embodied in the ATF. It was in fact about twentytwoyears ago that a pivotal point in the development of air combat tacticswas reached, and a new outlook developed on the issue of air combatperformance parameters. The setting for this situation was more thanwell publicised, as the USAF hammered the Third World infrastructure ofNorth Vietnam with weapons designed to destroy industrialisedeconomies.TAC flew daily raids with fast big and well armed F-105D Thuds andF-4C/D Phantoms. The Thud was initially designed for tactical nuclearstrikes and thus sacrificed manoeuvring ability for speed at low level,as such it had no serious competitors in its class. Vietnam saw theThuds loaded up with 500 lb and 750 lb iron bombs flying low level divebombing raids on a range of targets, the Thuds carried only an internal20 mm gatling for self defence.

ThePhantom in its C and D incarnations was a minimal modification of theUSNavy's established F-4B fleet defence interceptor, the F-4 had a usefulbomb carrying ability but significantly served as the USAF's principalair superiority aircraft. It was armed with semi-active radar guidedAIM-7D/E missiles, heatseeking AIM-9B and AIM-4D missiles and in someinstances, 20 mm gatling centreline gunpods. Their opposition was amixed 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 Atollheatseeking missiles. The aircraft were flown by Vietnamese, Russian,Czech, Polish and East German pilots, the latter not publicised forobvious propaganda reasons. The USAF did not fare very well, achievingan air-air kill ratio of 2.18:1 in the period between 1965 and 1968.Thereasons were manifold, as inadequacies in missile performance, weaponsystems and tactics combined with absurd rules of engagement (ROE)offered every imaginable advantage to the Communists, who did nothesitate to exploit them. What had become very apparent was that theperformance characteristics of the aircraft and weapons used were illmatched for the kind of engagements which were entered. The MiGs werelight VFR only fighters with simple gyro stabilised gunsights and nofire control worth mentioning, the 17 and 19 being both optimised forclimb performance at high subsonic speeds. This gave them good turningability and respectable subsonic acceleration which combined well toprovide the basic elements of good gunfighting air combat performance.

ThePhantoms were IFR supersonic interceptors, with air intercept radar,twoman crew, beyond visual range (BVR) radar guided missiles and tailaspect only heatseeking missiles, both types optimised for killing highflying bombers. The missiles lacked the manoeuvre performance tosuccessfully kill a nimble and small target unless launch conditionswere optimal and the target's manoeuvre options were highly restricted.The Phantom's advantage in transonic acceleration and climb performancewas balanced by high wing loading and unspectacular turning performance.

TheUSAF was clearly unhappy about this situation and measures were soughtto improve kill ratios. The demand for a gun and better turnperformancespawned the F-4E with a stretched fuselage, improved fuel capacity,chinmounted M61 20 mm gatling gun and manoeuvring slats. The same pressuresalso led to a major review of air-air tactics which saw the adoption ofenergy manoeuvrability as a fundamental of the new air combatmanoeuvring doctrine. Conceived by a serving USAF fighter pilot, MajorJohn Boyd, energy manoeuvrability revolved about the use of anaircraft's energy state (ie speed/altitude) to gain a positionaladvantage in a manoeuvring engagement, thus gaining a firingopportunity.

Because the close-in air-air weapons of the period were guns and tailaspect heatseekers, the best firing opportunities resulted from tailaspect shots, this in turn dictated that an aircraft must possesssuperior sustained turning, acceleration and climb performance todefeatan opponent. While the F-4 had the acceleration and climb performancetokill most current opponents, it was clear that the next generation offighters had to possess even greater thrust/weight ratios and muchbetter sustained turning performance. The latter was simply notachievable by modifications to existing types which had their originsinthe late fifties and hence were aerodynamically highly optimised tostrike or interception roles.

TheUSAF initiated its FX program, while the USN discarded its troubledF-111B bomber turned interceptor in favour of the new VFX. Both the VFXand FX exploited new propulsion technology, discarding afterburningturbojets in favour of afterburning turbofans which offered much betterspecific fuel consumption in dry thrust and a higher ratio ofafterburning thrust to dry thrust. Experience in Vietnam clearlyindicated that the endurance/combat radius of the 400 NM class F-4 wasinadequate and hence the VFX and FX were designed to a 1000 NM classcombat radius. Climb and turn performance dictated low wing loading andgood 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 withswing wings and a pair of TF-30 fans. The F-14A had a large bubblecanopy for good visibility during dogfights, a Head Up Display (HUD)gunsight, computer controlled automatic wing sweep and glove vanepositioning, a massive AWG-9 pulse Doppler air intercept/fire controlradar system capable of tracking multiple targets in ground clutter andan internal M-61 gun. It was bigger, more complex and more expensivethan the F-4, but it also offered agility and manoeuvrability withoutprecedent. The first of the teen series fighters had thus made its mark.

The20,000 lb class TF30-P414 powerplants fitted to the F-14A were a stopgap measure which offered a 1:1 class thrust/weight but not thestunning1.2+ class thrust/weight sought by the Navy, that was to occur with theF-14B which was to be fitted with F401-P-400s, navalised derivatives ofthe emerging Pratt&Whitney FX powerplant. For budgetary reasonsthisnever eventuated, the F-14A having to wait until the nineties for the30,000 lb class F-110-GE-400 powerplant.

TheUSAF's FX subsequently flew in 1972, taking advantage of the 25,000 lbclass F100-PW-100 powerplants and optimised for energy manoeuvrability.Like the F-14 it had a large bubble canopy, HUD, powerful pulse Dopplerfire control radar, low wing loading and internal gun. Because it wassmaller and lighter than the F-14A, it offered a stunning 1.4:1 classthrust/weight ratio and thus set the standard for air combat fighters(see AA Sept/Nov 84).

Bythe mid seventies the F-14A and F-15A entered service and quicklyestablished their superiority over existing aircraft. Cost of ownershiphad however proven to be a major issue as both aircraft cost much moreto buy and even more to maintain, given their more complex powerplantsand avionic systems. Under pressure from legislators, the USAFinitiatedthe Light Weight Fighter (LWF) program to supplement the F-15 with asmaller and much cheaper VFR dogfighter, an aircraft armed with gunsandheatseeking missiles and equipped with a small pulse Doppler radaroptimised for dogfighting alone.

Northrop bid their YF-17, a derivative of their P-530 lightweightfighter, while General Dynamics bid their YF-16, derived from afly-by-wire relaxed static stability technology demonstrator. The YF-17Cobra was a small twin with two 'leaky turbojet' YJ101-GE-100powerplants and a hybrid planform comprised of massive Leading EdgeExtension (LEX) strakes and a moderately swept wing. The YF-16 wasfitted with a single F100 common to the F-15 and employed extensivewing/body blending, also using strakes and a moderately swept wing. TheYF-16 was the first aircraft ever built to be statically unstable andrelied upon a triple redundant analogue computer system to remainflyable.

TheLWF flyoff was won by the YF-16 due in no small measure to itspowerplant commonality with the F-15, and to its smaller size andsomewhat better thrust/weight ratio performance. Subsequently selectedas the standard NATO air superiority fighter, the F-16 is today one ofthe mainstays of Western air power.

Likethe USAF the Navy came under increasing budgetary pressure fromlegislators, who quickly quashed the Navy's plans for a fleet ofsupercarriers equipped with a force of air superiority F-14Bs andstrikeF-14Cs, the latter a strike fighter derivative of the F-14B intended toreplace the underpowered A-6 bomber. The Navy were directed to selectone 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-7and F-4 fleets with a single dual role lightweight fighter bomber. Manyin the US Navy opposed this move, which largely defeated the Navy'sstrategy of acquiring a 1000 NM radius air superiority/strike force.

TheNavy eventually selected the F/A-18A, a substantial redesign of theYF-17 airframe with greater internal fuel capacity, bigger 16,000 lbclass F404 engines and BVR missile capability, absent on the smallerF-16. The F/A-18A was designed to rigid reliability and maintainabilityspecifications, which substantially increased the cost of the aircraft,which by then had acquired a very sophisticated suite of avionicsystems, imposed largely by the need for one airframe to fulfill twodiverse roles. Another side effect of this program was the 'detuning'ofthe F404 powerplant's performance to improve reliability and lifetime.

TheF/A-18A also employed fly-by-wire, but employed a sophisticatedquadruplex digital system where software changes could be employed to'tweak' handling characteristics. The avionic system was built aroundmultiply redundant serial Mil-Std-1553B databusses and employed dualredundant digital computers.

Withthe entry into service of the F/A-18A the US services completed theirreequipping with teen series fighters, eventually building up a forceofseveral thousand aircraft of these four basic types.

Thebasic philosophy of high thrust/weight ratio and sustained turningperformance in transonic dogfights shaped these aircraftaerodynamicallyand hence imposed fundamental constraints to the other performancecharacteristics of this family of aircraft.

Evolution did not stand still however, and the early eighties saw thedeployment of one of the most significant air combat weapons of itstime- the all aspect heatseeking Lima model of the established AIM-9Sidewinder missile. The AIM-9L did not require a tail chase position tolock on to a tailpipe, it was quite happy to lock on from any angleincluding 12 o'clock ie head-on. Soon after deployment the AIM-9Lprovedthat much of the air combat tactics textbook had been obsoleted, inthatinstantaneous turning performance became far more important thansustained turning performance. The ability to point the nose at anopponent quickly and loose off a missile became far more important thanthe ability to follow through multiple turning manoeuvres to acquire atail aspect gun/heatseeker firing position. While a tail aspectpositiondid improve the kill probability of the missile by reducing thetarget'sevasive manoeuvre options, the AIM-9L's all aspect performance wasstillsuperb, as learned the hard way by the Argentine and Syrian air forcesin 1982.

Clearly the day of the classical dogfight was almost over, in that thefirst aircraft to acquire its opponent would be first to fire and mostlikely to win the engagement. Only if the first firing opportunity wereto fail and the combatants were to pass each other and then engage in aturning dogfight, was there a major requirement for sustained turningperformance. Yet again, however, the all aspect capability of theAIM-9Lwould convert fleeting nose on target opportunities into real firingopportunities.

Thefirst aircraft to take advantage of this situation was the NavyF/A-18A,which had its flight control software tweaked up to optimiseinstantaneous turn performance, this combined with its sophisticatedHUDand fire control radar dispelled any doubts of the aircraft'slethality,given earlier criticism of its low (ie 1.1:1 !) combat thrust/weightratio. Acceleration and climb performance, and low energy bleed inmanoeuvring at transonic speeds were the key parameters in this classofengagement.

Anew philosophy for air combat tactics was thus developed by the USAF,who envisaged long range medium to high altitude penetration of hostileairspace by supersonic cruise capable fighters with all aspect fire andforget missile armament. A key element in the new strategy was theAIM-120 Amraam missile (AA Sept 86), an active radar guided intelligentfire and forget BVR missile, designed to replace the establishedsemi-active radar guided AIM-7. Coupled with a suitable fire controlradar the AIM-120 allows a single fighter to salvo up to eight roundsateight separate inbound targets within the acquisition geometry of thefire control radar.

Manoeuvring at sustained supersonic speeds, the new look airsuperiorityfighter could outmanoeuvre SAMs and most AAMs, while always retaininganenergy advantage over the subsonic/transonic speed range optimised teenseries turning dogfighter. The aerodynamic and propulsion designcompromises which supported sustained turning performance at highsubsonic and transonic speeds seldom improved the ability of theaircraft to sustain high energy manoeuvres at Mach 1.5 class speeds,where much of the wing was enveloped in the shock cone produced by thenose of the aircraft.

Thephilosophy of first-look first-shoot reflected in the need for superiorsensor capability and low observability ie Stealth. The former andlatter requirements result in the need for a frequency/spatially agileradar and high performance InfraRed Search & Track (IRST), whilethelatter requirement imposes the need for all of the tricks of theStealthtrade, ie shaping, skinning and detailing.

Thefirst aircraft to fly which embodied some of the new technology was theill-fated F-16XL. A radical redesign of the F-16A, the XL was asupersonic cruise demonstrator with a cranked arrow delta wingoptimisedfor that flight regime. The aircraft was, on the strength of publishedreports, a major technical success, with two demonstrators eventuallyflying. The highly swept inboard wing section of this aircraft producedsubstantial vortex lift at supersonic speeds, while also improvinginstantaneous turn rate and extending the 9G manoeuvre envelope wellabove Mach 1. An additional benefit of the new configuration was asubstantial increase in internal fuel capacity, providing a 120%improvement in combat radius performance.

EarlyLockheed ATF Concept (1988).

TheF-16XL suffered the fate of many pioneering aircraft before their time,its F-16E dual role strike fighter derivative lost out in a flyoffa*gainst MDC's bigger and more capable F-15E Strike Eagle, thus endingall prospects for its eventual production. Many observers attributeditsdemise to a political strategy played by the USAF, to prevent an oldergeneration airframe derivative from being used by legislators as anexcuse to kill off or postpone the ATF program. Equipped with Amraam,higher thrust engines and new radar, the F-16XL could cover a largepartof the role envisaged for the ATF at substantially lower unit andprogram costs. As an older generation airframe however its infrared andradar signatures are substantial and this would greatly reduce itseffectiveness (although trivia of this nature hardly ever bother astutedecisionmakers such as politicians...).

TheATF program had its origins in numerous USAF air combat studies carriedout in the late seventies and early eighties, when intelligenceinformation revealed the Soviets' early flight testing of the Fulcrumand Flanker. From the observed geometry of the airframes it was clearthat both types would have the vortex lift performance to challenge theteen series aircraft in turning dogfights, by the same token bothSovietfighters would be handicapped by their geometry in both supersonicmanoeuvre and low observability performance.

TheATF was to be the successor to the F-15, a long range air superiorityfighter with the performance to kill any other tactical aircraft andtheoperating radius to threaten targets deep inside the USSR while flyingfrom bases in Western Europe. This was to be achieved by the use of ahighly integrated airframe/systems/propulsion design exploitingadvancedaerodynamics, engines and stealth technology, the latter to delay anopponent's initial firing opportunity for as long as possible, and thuscapitalise on the large Radar Cross section (RCS) of the Fulcrum andFlanker.

Subsequent to studies, an RFP was issued in July 1986, and twocontractor teams, Northrop/McDonnell-Douglas andLockheed/Boeing/GeneralDynamics were selected in October 1986 for the initial 50 monthdemonstration/validation phase flyoff. The rollout of the prototypeswasinitially scheduled for mid 1989, but ongoing slippages have delayedthis until the middle of last year.

Part2 provides a technical comparison of the YF-22A and YF-23A prototypes

MDC F-4 Phantom II. The F-4 was themainstay of the USN's and USAF's tactical fighter force in theSouthEastAsian conflict. The Phantom offered superb acceleration and climbperformance for its day, while carrying an impressive air-air payloadof4 heatseeking AIM-9D and 4 semi-active radar AIM-7E missiles. Inengagements with the smaller and nimbler NVAF MiGs the F-4 was hamperedby poor missile performance and reliability, inadequate radar lookdownperformance and the absence of a gun for close in engagements.

Grumman F-14A Tomcat. Grumman's large F-14 fighter was the first of theteen series aircraft to fly and deploy. It was equipped with a pair of20,000 lb class TF-30 afterburning fans and a computer controlledvariable geometry wing to provide superb turning and accelerationperformance, while its massive pulse Doppler AWG-9 fire control radarand Head Up display allowed the targeting of 100 NM class AIM-54Phoenixmissiles, AIM-7 Sparrow, AIM-9 Sidewinder missiles and an internalM-61A1 20 mm gun. It has remained a formidable dogfighter to this day.

MDCF-15A Eagle. The F-15A was designed, like the F-14A, for highthrust/weight ratio to provide superlative acceleration, climb and turnperformance. Like the F-14, it is equipped with a high power long rangelookdown/shootdown pulse Doppler radar. Armed with a mix of AIM-7 andAIM-9 missiles and an internal M-61 gun, the F-15 has repeatedlydemonstrated its capability in the Middle East. The most recentsubtype,the dual role strike fighter F-15E, is structurally strengthened for 9Gmanoeuvres.

Dubbed the 'Electric Jet', GD's F-16 was the first tactical aircraft toemployed relaxed static stability and fly-by-wire control. Initiallyacquired as a low cost VFR dogfighter armed with an internal M-61 gunand heatseeking AIM-9 missiles, the later F-16C is a truly multiroletactical aircraft, with wide angle holographic HUD, and provision forLantirn terrain following radar/FLIR pods and the AIM-120 Amraam BVRmissile.

TheF-16XL was a supercruise technology demonstrator derived from the basicF-16 airframe/powerplant. The cranked arrow delta wing allowed theaircraft to cruise supersonically on dry thrust, improved the manoeuvreenvelope substantially while providing enough additional internal fuelcapacity to increase the combat radius by 120%. Sadly it never enteredproduction, losing to the F-15E in a competitive flyoff for the DualRole Fighter program. As a teen series airframe lacking stealthcapability, it cannot compete with the newer ATF aircraft.

Usinga hybrid planform wing and digital fly-by-wire, the MDC F/A-18A was thelast of the teen series fighters. It was the first tactical aircraft toemploy a fully digital Mil-Std-1553B bussed avionic system under thecontrol of redundant digital computers. A multirole derivative of theYF-17 airframe, the later F/A-18 is a fully capable all weather strikefighter which retains excellent air superiority performance.

TheSu-27 Flanker is the most capable aircraft in the Russian inventory andis expected to be a hot seller in the Third World market (Above is theaircraft in Chinese livery with R-77 Adder radar guided missile andR-73IR missile below). Aerodynamically it reflects the design philosophy ofthe teen series aircraft, employing vortex lift, high thrust/weightratio and fly-by-wire control. Equipped with a large pulse Dopplerradar, internal 30 mm gun, BVR and heatseeking missiles, it is aformidable opponent. Recently tested on the V-MF's new CVAN, theFlankerhas the combat radius and performance to contest any teen seriesaircraft.

At the time of writing theNorthrop/MDC YF-23A and Lockheed/B/GD YF-22A had both completed theirrespective demonstration/validation flight test programs . While theUSAF have not revealed much about the internals and performance of theaircraft, their airframe geometry and known powerplant parametersrevealmuch of their design philosophy and performance. Both aircraft reflecttheir prime contractors' respective philosophies of stealth aircraftdesign as much as they reflect their common mission profile.

Principal airframe/propulsion design objectives were sustainedsupersonic cruise on dry thrust, high energy manoeuvrability, superiorcombat radius to the F-15 with all weapons and fuel carried internallyand low signatures.

BothATF prototypes are approximately 10% larger than the F-15 and bothcarryapproximately twice the internal fuel of an F-15C, while both haveabout50% more wing area at about 30% greater combat weight. As such bothaircraft clearly illustrate the long range air superiority missionwhichwas originally envisaged for the aircraft, penetrating deep into Sovietairspace to destroy air defence aircraft and to disrupt Sovietoffensiveair operations. The decline of the Soviet empire during the last 18months has understandably led to many US politicians calling for thescrapping of the ATF program, in view of the 'diminished threat'. Thismyopic posture needless to say wholly disregards the fact, that theUSSR itself is quite unstable and could well slip back into hardlineStalinism, and also ignores the reality that the USSR will sell theFlanker and Fulcrum to any party who can pay for it. It is likely thatthat these capable teen series class aircraft will become as common inthe Third World as the ubiquitous Fishbed. The mere perception that acapability matching that of the frontline Western aircraft is presentwill be destabilising - the instance of Iraq with its Fulcrums andFencers is a case study, their tactical and technical incompetenceclearly underscoring this sad phenomenon.

Thereality is that capabilities are a good measure of intent, it isunrealistic at the least to assume that any nation will expend vastsumsof money to acquire specific weapons systems without seeing how thatexpenditure will further its interests. Long range air superiorityaircraft such as the Flanker serve a clearly defined role, offensivestrategic air war.

Howthe ATF performs this role is best judged by a closer look at thedesignphilosophy of the airframe, propulsion and weapon system.

Airframe Design

The ATF airframes represent another quantum leap in air superiorityairframe design, as great as that represented by the teen seriesfighters. Two new and key capabilities were integrated in the ATFprogram, low observability (ie stealth) and supersonic cruise.

Theobjective of low observables is to reduce the performance of hostileradar and infrared surveillance, tracking and guidance systems.Existingairframes perform poorly in this respect, and thus only a new airframedesign can address the problem.

Supersonic cruise serves several purposes, providing for fast and deeppenetration into hostile airspace, while offering the supersonic cruisefighter a major energy advantage over subsonic/transonic dogfighterswhich it can both outmanoeuvre and outlast in a supersonic engagement.The high corner speed of such aircraft also provides a majormanoeuvringadvantage when evading SAMs at altitude, enhancing survivability ondeeppenetration missions. Supercruise required major advances in propulsiontechnology and nontrivial concessions in airframe design.

Lowobservability in the ATF designs is achieved by a range of measures,howthese are applied clearly illustrates the heritage of the respectivedesigns.

TheLockheed/B/GD YF-22 employs planform shaping and faceting with blendedfacet boundaries, the latter a necessary concession to high performanceaerodynamics. This is apparent in the shape of the nose, the fuselagesides about the inlets and engines, and the upper forward fuselage.Lockheed/B/GD used serrated edges extensively, as with the F-117A, tocontrol the returns from panel boundaries, this is very visible on theundercarriage and weapon bay doors.

Theplanform results in a multiple lobe design, as the boundaries of themajor surfaces are not parallel with respect to each other. Planformreturn lobe structure is defined by the radiation pattern lobesresulting from surface wave reflections which occur at the leading andtrailing edges of the airframe's major surfaces. The objective oflobingis to concentrate this unavoidable radar return into specificdirectionsso as to minimise frontal/aft/beam aspect return and maximisescintillation in the direction of the lobe. Scintillation is a measureof how rapidly the size of the return varies with angle, the greaterthis variation, the more difficult a target is to track. The lower thenumber of lobes and the narrower the lobes, the lower the probabilityofdetecting any return.

TheNorthrop/MDC YF-23 employs planform shaping with extensive blending,thelatter technique used to advantage with the large B-2A. Blending hasthemajor strength of not compromising high speed aerodynamics, the blendedairframe offering very low drag by avoiding vortices which may beproduced by a faceted geometry. In addition to RCS reduction throughshaping, the YF-23 also employs carefully shaped exhausts to concealtheengine hot end, yet another technique developed during the B-2A program[Editor's Note 2005: the exhaust troughs used air cooled insertsresulting in the lowest IR signature of any fighter ever built].

Theunusual 'diamond' planform of the YF-23 is a 2 major lobe design, asallmajor edges fall into groups of two parallels.

Theresult of the low observables techniques employed with these aircraftisa 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 guidedweapons and in the instance of the YF-23, also heatseeking weapons.

Lockheed/B/GD chose a somewhat conservative hybrid planform airframelayout, reminiscent of the F-15 and F/A-18, with closely spacedengines,long inlet tunnels, outward canted vertical tails and rudimentarystrakes over the inlet boxes to promote vortex lift over the outboardwing sections at high AoA. The characteristics of this general layoutare well understood, the forward sloped inlets providing good airflowcharacteristics at high AoA and the conventional tail providing goodcontrollability under such conditions, apparently earlier attempts atusing a V-tail did not yield the desired results. The close spacing ofthe engines reduces inertia in the roll axis, but may penalisesurvivability. Weapon bays are located on the sides of the inlet boxesand a single central bay is located beneath the centresection, alllocated well aft of the inlet to preclude ingestion problems. TypicallyAIM-9s fit in the inlet bays and AIM-120s in the split central bay.

Thesingle piece canopy co*ckpit is well elevated to maximise the pilot'ssituational awareness.

Northrop/MDC chose a far more radical airframe layout, driven by theobjectives of stealthiness and supercruise. The extensively blendedfuselage has rudimentary chines which smoothly blend into the wingleading edge, the blending allowing good area ruling and low supersonicdrag. The low wing aspect ratio is used to optimise supercruiseperformance. The ventral trapezoidal inlets feed the engines viastealthy S-bends, and the rear boattail and submerged dorsal exhaustswere specifically aimed at low drag and infrared signature. The YF-23employs an unconventional V-tail with a planform consistent with theairframe lobing strategy. The large centresection area will providesubstantial body lift at high AoA thus improving turn performance, atechnique used in the F-14 and Flanker. While the widely spaced enginesresult in some roll rate penalty, they are sufficiently separated toavoid fratricide in the event of turbine breakup. Two tandem weaponbaysare employed, the aft bay is reported to be very large and containspairs of staggered AIM-120s, the forward bay carrying AIM-9s.

TheYF-23 employs a two piece canopy, the co*ckpit is like it's competitor'swell elevated for good visibility.

Theexhausts of the two aircraft differ radically. Lockheed/B/GD had chosena layout aimed at maximising lower speed manoeuvrability via the use ofthrust vectoring, even though this was not a mandatory USAFrequirement.Two dimensional thrust vectoring nozzles provide vectoring to enhanceresponse in pitch. Northrop/MDC on the other hand rated stealth anddragso important, that they employed a serrated planform beavertail withB-2-like submerged ventral exhaust troughs. This approach reduces bothdepressed tail aspect infrared emissions and tail aspect radarcross-section, but precludes any vectoring.

Bothprototypes are reported to employ relaxed static stability, withmultiply redundant digital fly-by-wire control systems.

Thenavalised ATF derivative planned to replace the Grumman F-14 as theUSN's principal air superiority fighter has yet to materialise.Lockheed/B/GD have proposed a variable geometry wing derivative of theTAC design, in order to accommodate the Navy carrier recovery an launchrequirements, ie low speed on approach and high lift at low speed oncatapult launch. At the time of writing no information was available onthe Northrop/MDC proposal.

Propulsion

The unique and new supersonic cruise mission profile of the ATF has hada major impact upon the powerplants to be used for the aircraft. Thehigher combat weight of the aircraft in comparison with the F-15imposeda need for greater installed afterburning thrust, in the 35,000 lbclassper engine, to maintain the preferred 1.4:1 class combat thrust/weightratio, while the supercruise profile imposed the need for high drythrust particularly within the supersonic part of the envelope. Thelatter requirement was particularly painful, as it forced a move tohigher temperatures within the engine, particularly the turbine.

Thetwo bidders for the ATF powerplant are Pratt & Whitney and GeneralElectric with their YF119 and YF120 designs respectively. The P&WYF119 is the lower risk of the two designs, an advanced low bypassratioturbofan. The GE YF120 is more radical, as it is a variable cycleenginecapable of adjusting its bypass ratio to the optimum for a given flightregime.

GE'sinvolvement with variable cycle engines dates back a decade, with amajor technology demonstration program built around a substantiallyredesigned YJ101 (former YF-17) powerplant. This was followed by workonan F404 derivative, this providing the foundation for GE's variablecycle technology. The core of the YF120 was derived from work doneduring the government sponsored ATEGG (Advanced Technology Engine GasGenerator) and JTDE (Joint Technology Demonstrator Engine) programs.Subsequently early development XF120 engines underwent testing at theUSAF Systems Commands AEDC facility. Ground test prototype YF120s havebeen under test since late 1989.

Internal details of the YF120 are, not surprisingly, classified. Theengine is known to be a two shaft design with a minimum number ofrotating stages, a fan which has been speculated to be a single stagedesign and a compressor using integrated bladed rotors. In common withearlier GE VCEs, the YF120 uses VABI (Variable Area Bypass Injector)technology to alter engine bypass ratio. The YF120 is reported to useaerodynamically actuated VABIs, in which respect it differs fromearlierdesigns which used mechanical actuation. Typical VABI technology usedinearlier GE designs saw the use of sliding sleeves which would reducethecross section at the fan exit entry to the bypass duct, and at thetailpipe exit from the bypass duct.

Thisarrangement allows the engine to smoothly optimise its bypass ratio tothe flight regime. For maximum afterburning thrust on takeoff orefficient subsonic long range cruise, a high bypass ratio is set. Forsupersonic cruise a turbojet is approximated, with very low or zerobypass ratio. Turbojets are considered optimal for supersonic flight astheir dry thrust drops far more slowly than that of a fan withincreasing vehicle airspeed. The ATF flight profiles are sufficientlyunconventional to create major difficulties for a fixed bypass ratioengine designer attempting to reconcile the diverse demands of lowerspeed operation and supersonic cruise.

Thismust have been the case with P&W, who have bid a fixed bypass ratioturbofan derived from the ATEGG/JTDE programs and the company'sexistingF100 family of fans.

Published reports indicate the GE engine has demonstrated bettersupercruise performance than its conventional rival and it is verylikely that GE's gamble with a more radical technology will yield thedesired payoff. The gain in overall engine performance in comparisonwith existing teen series fighter powerplants is clearly illustrated bya Lockheed/B/GD flight envelope chart for the YF-22 which shows amilitary thrust envelope for the YF-22 as greater at all airspeeds andaltitudes than the afterburning envelope of the F-15C. In thethrust/drag limited low altitude regime the YF-22 dry envelope is 7%greater than that of the F-15C, given the similar configuration of bothairframes and greater wetted area of the YF-22 this suggests dry thrustin excess of 25,000 lb per engine.

Avionics, co*ckpit andWeapon System

Avionics is an area where the ATF will offer a radical improvement overexisting systems. From the outset avionics were a key aspect of the ATFprogram. Initial studies were aimed at a distributed architecturedesignated Pave Pillar, the objective of which was to employ physicallyseparate common computing modules for the aircraft's vital systems.Thiswould provide superior tolerance to battle damage and internal systemsfailures, while reducing the requirement for unique spares modules. Ahigh level of integration was also sought in the comm/nav systems andelectronic warfare systems, under the USAF Icnia (IntegratedCommunications, Navigation and Identification Avionics) and Inews(Integrated Electronic Warfare System) technology development programsrespectively.

Amajor system level requirement was supportability in the field and veryhigh reliability, the latter a must in view of the complexity of theaircraft. The scale of effort in this area is reflected by arequirementfor a combat turnaround of 15 minutes (cf 35 minutes for F-15), arequirement for 9 support personnel/airframe and 6.8 C-141 loads ofsupport equipment, in comparison with the existing 17 for a 24 aircraftTAC squadron.

TheYF-23 avionic system is built around a core integrated system usingUnisys 32 bit GPPE (General Purpose Processing Element) modules. Theoriginal 3 CPU 1750 architecture mission computer arrangement wasdiscarded as the support hardware requirements were excessive, and thecomputational power inferior.

Signal processing is done with a single dedicated processor, slicedbetween two large physically separated 75 card racks, with redundantfunctional modules spread between the racks to enhance survivability.

TheYF-22 avionic system is built around Hughes CIP (Common IntegratedProcessor) modules. Lockheed/B/GD have apparently opted forunconventional liquid cooling of the processor modules to reducehardware operating temperatures.

Weapon system software is to be implemented in US DoD standard ADAlanguage, it is not clear whether the production code will be tocurrentADA or revised ADA 9X standard.

TheINEWS electronic warfare systems are being developed by two contractorteams, TRW/Westinghouse/Tracor/Perkin Elmer for the YF-23 andSanders/GE/Motorola/HRB for the YF-22.

Thesensor suite will be dominated by an active phased array radar. Theradar will employ electronic antenna scan exploiting over 1,000transmit/receive/phase-shifter elements, each of which is a wholly selfcontained module. This arrangement results in a highly robust designwhich doesn't require mechanical pointing, as main lobe shape anddirection are controlled electronically, and which gracefully degradesin performance as modules fail. The use of electronic beamshaping/pointing provides major advantages as this class of radar maytimeshare its antenna between modes, optimise lobe shapes to modes,tolerate violent manoeuvring and also selectively direct nulls attroublesome jammers as a potent ECCM. Both contenders would employ aWestinghouse/TI radar design, initially flown in 1989 and derived fromthe URR (Ultra Reliable Radar) program.

Thisradar is the most radical step in fighter air intercept radar designsince the first pulse Doppler sets were introduced in the earlyseventies, and offers diverse upgrade paths through software changes inthe beam control subsystem and the signal processing subsystem.

Theradar is to later be supplemented by an EOSS (Electro-Optical SensorSuite) which is essentially an advanced IRS&T set. Both contendersare to employ a Martin Marietta/GE system using focal plane array (FPA-see earlier TE) technology. The advantage in a FPA design is highersensitivity and the absence of moving parts, scanning beingaccomplishedelectronically. At the time of writing it was unclear as to whether acheaper mid-infrared PtSi or InSb design would be adopted, or whether along-infrared HgCdTe design would be employed. While the latter candetect airframe skin friction, it is more demanding in cooling andsignal processing. The difficult requirement is to detect and tracktargets against an IR background at low level, at altitude thebackground environment is easier to deal with. At the time of writingthe EOSS was deferred as it was considered too immature for a low riskproduction design.

Theco*ckpits of both the YF-22 and YF-23 will be conventional 'glass'arrangements, although Lockheed/B/GD have opted for LCD technology inpreference to CRT displays. The YF-22 uses no less than 6 LCDs,typically providing 512x512 pixel resolution with 4,096 colours. Anadvanced HUD will be employed, as will the USAF's new G-suit technologycurrently being introduced on the F-15. Both contenders are reported touse conventional control layouts, the sidestick controller not beingused.

Flight testing of both avionic suites has taken place on dedicatedtestbed aircraft, Boeing using a 757 and Northrop/MDC a well reworkedBAC-111.

TheATF will be armed primarily with the AIM-120 AMRAAM ARH BVR missile,supplemented by a short range all aspect heatseeker, the AIM-9M at thistime. A design requirement is the carriage of four Amraams, these mustbe ejected from internal bays at launch. An internal gun will beemployed, although it appears that the gun is absent on all prototypes.

Performance

It is clear from published accounts that the ATF is an enormous stepforward in aerodynamic and low observables performance in comparisonwith the teen series fighters and their Soviet counterparts. Bothcompetitors have repeatedly supercruised on dry thrust with speeds of1.58 Mach reported for both airframes with YF-120 powerplants. Inaddition the YF-23 attained 1.8 Mach in afterburner and reportsindicatethat the final maximum speed figures have been classified by the USAF.

Bothairframes offer 1.4:1 class combat thrust/weight ratio performance andcombat wing loadings well below 60 lb/sqft, therefore the energymanoeuvrability performance will equal if not exceed that of the F-15.Controllability at high AoA has been reported as excellent for bothtypes, in the absence of hard data it is therefore difficult toestimatewhether Lockheed/B/GD's claimed advantage in manoeuvrability will bedecisive.

Tactical radius and cruise speed are also critical parameters for themission, in both areas the ATF is well ahead of the teen seriesfighters. Again in the absence of hard figures it is difficult toestablish whether Northrop/MDC's greater speed and radius performanceare a decisive advantage. Certainly the ATF's 25,000 lb class fuelcapacity must offer a major gain in radius in comparison with the13,000lb class F-15, how much more will depend on the flight profile. Reportssuggest the YF-22 consumes 30% less fuel in supercruise than an F-15 inafterburner, suggesting an SFC of about 1.5 lb/lbt/hr which is abouttwice the dry SFC of an F100-PW-100. Therefore on a purely supercruisemission profile the additional fuel may not offer a gain in radius,however a mixed subsonic/supercruise profile almost certainly would,thegain inversely proportional to the ratio of time spent in supercruisevssubsonic cruise. Both airframes are designed for boom refuelling.

Thecombined effects of the airframe and powerplant designs will see atoward supersonic engagements, where current generation aircraftoptimised for transonic/subsonic manoeuvring with afterburner cannotmeasure up. The current generation fighter will suffer shortfalls inpersistence due increased fuel flow and sustained manoeuvringperformance due aerodynamics optimised for turning at lower speeds.

Lowobservables performance is an area where Northrop/MDC will almostcertainly win out over Lockheed/B/GD, due to the effort expended on thedesign of the rear fuselage exhaust area and due to the use of blendingand lobing techniques which offer far lower numbers of airframediscontinuities. Any discontinuity promotes surface wave scattering,therefore the smoother the design the lesser the scattered return. Asthe RCS figures are classified, it is not clear how great a performancemargin exists. Tail aspect radar and infrared performance must besuperior in the Northrop/MDC design simply as a result of the geometryused.

Lockheed/B/GD compromised low observables performance to achievegreateragility, whereas Northrop/MDC focussed on stealth, speed and radiusperformance [Editor's Note 2005: more recent disclosures indicate thatthe YF-23 was capable of achieving all specified agility points withoutthrust vectoring, unlike the YF-22]. The USAF's decision will clearlyillustrate which of these parameters are considered of greater value inthe projected strategic air war of the future.

The Perspective View

To the Australian observer the ATF underscores the revolution under wayin tactical air warfare, with stealthiness, radius and agility growingsignificantly against the existing generation of aircraft. The ATF willbe substantially more expensive than smaller multirole fighters such asthe F/A-18A, but also offers vastly superior performance in the longrange air superiority mission.

[Editor's Note 1997: since the time we published this item in 1991, theoutcome predicted in this article has indeed come to pass, with the PRCand India about to deploy large numbers of advanced Flankers, and evenMalaysia deploying the potent MiG-29SE with the deadly Archermissile. 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 wentonline in 1997, Malaysia has opted for the Su-30MKM, Indonesia amix 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]

Inthe current regional air defence environment the F/A-18A has no seriousrival. This could however change with a regional acquisition of theFulcrum and Flanker. While the Fulcrum could be readily tackled in BVRand visual engagements, the larger Flanker would present a seriousproblem particularly in extended range BVR engagements due to itssuperior radius and radar performance. Well flown Flankers couldpresenta serious problem for the RAAF as they have greater persistence, superbmanoeuvring performance and a larger envelope for firing radar guidedmissiles.

Asthe Gulf war demonstrated, modern radar guided missiles are far morelethal than their Vietnam era predecessors and the initial pre-mergephase of an engagement has thus become far more dangerous. Closingfighters now have the option of a head-on BVR missile shot, a situationwhere radar and RCS performance are critical. Evading an inboundmissilecan severely disadvantage the defending fighter in terms of geometryandenergy state, this in turn penalises it once the merge occurs and aturning dogfight is initiated. A Flanker with its powerful radar andBVRmissiles has thus a major advantage over an F/A-18A, which can onlyemploy its manoeuvrability and weapon system to an advantage once aturning engagement has been entered. In a close in turning fight it hasthe advantage of smaller size and better dogfighting radar modes, butwill suffer an energy disadvantage if the Flanker is flying at a lowerfuel state. Similarly the Flanker will have an advantage inpersistance,given fuel state.

TheATF with its low frontal RCS has a distinct advantage over any currentopponent in any such engagement, allowing it to ruin its opponent'sentry into the engagement, and then apply its supersonic agility andpersistence to force the engagement on its terms. The reduced RCS andinthe YF-23 IR signature, will also reduce the usable radius of itsopponent's weapons, while allowing the ATF to disengage more readily,itself not suffering any penalties in missile guidance effectiveness.

Werethe RAAF confronted with the Flanker, it would have little option otherthan to consider a two tier force employing the ATF as the long rangeair superiority element. This in turn however raises questions aboutwhether our political leadership would be prepared to acquire suchaircraft, even in limited numbers, given the expense and perceivedspecialised role. Numbers are a major issue in this context, a verysmall number of top tier aircraft may not yield the desired effect butwill incur the fixed overheads resulting from supporting the type. Alarge number would be costly, and this would result in interservicepolitical problems.

Hopefully this question will not need to be considered during theprojected lifetime of the F/A-18A force, allowing the RAAF to look atcheaper second tier follow-on fighters employing the technologyadvancescurrently seen in the ATF.

TheUSAF at the time of writing envisaged about 500 ATFs to replace thefrontline elements of the F-15C force, a reduced requirement againsttheoriginal 750 airframes, with an additional 450 Navy airframes. The sizeof the production run would have a major impact on unit costs, giventhesubstantial R&D overhead. Political debate on the usefulness of theaircraft has been heated, as many US politicians consider it to be aspecialised asset targeted at defeating Soviet air power in a NATOtheatre conflict. While this is clearly not the case, laymen of suchcalibre seldom allow facts to interfere with their righteous crusades.

Thereality is that both the Fulcrum and Flanker if flown by competentpilots and applied appropriately, could successfully contest teenseriesfighters. The sheer incompetence of the Iraqi air force in the Gulfshould not colour perceptions of the worth of the Fulcrum and Flanker.They are serious players and the high production rate of the Fulcrumreflects its status as one of the USSR's hottest exports, almostcertainly supplanting the Fishbed as the Third World's premier tacticalaircraft.

Withshrinking budgets the US will be stretched to meet its commitments andthis will reflect in lesser numbers of tactical aircraft available forbushfire conflicts such as the Gulf campaign. Both TAC and the Navywillrequire a new air superiority fighter by the turn of the century,simplydue to airframe fatigue. Both the F-14 and F-15 are in the process ofproduction windup and shutdown.

Thequestion of course remains, will sanity win out ? The ATFs are bothquite clearly good implementations of this class of aircraft, unlikethestillborn A-12 which was killed off earlier this year as it could notmeet its design specification. US observers repeatedly commented earlythis year that the A-12 was in a more secure position politically thanthe ATF as the Navy's A-6E force is now block obsolescent and almostoutof life. The F-14 and F-15 have at least a decade of useful life leftinthem.

Alternatives proposed to the ATF vary from F-15s reengined with ATFpowerplants to the revival of the F-16XL, although the latter wouldrequire a major redesign to provide some measure of stealthiness.

Themost reasonable outcome would be low rate production of the ATF to belater supplemented by a smaller fighter, in the same fashion as theF-15/F-16 programs developed. Whether this eventually occurs remains tobe seen, and thus the ultimate fate of the superb ATF contenders isunclear.

Reference Data

Table 1. PerformanceComparison - YF-22A, YF-23A, F-15C, F/A-18A

Type	YF-22A (F-22A Raptor)	YF-23A	F-15C Eagle	F/A-18A Hornet
RegionalUsers	USAF (F-22A)	None	USAF (C/D) JASDF (CJ/DJ)	RAAF,USN(C/D), RMAF(D)
Crew	1	1	1 (2 - D)	1 (2 - B/D)
Dimensions [ft] Span Length Height Wing Area [sq]	- 43.0 64.2 17.7 830.0	- 43.6 67.4 13.9 950.0	- 42.8 63.8 18.5 608.0	- 40.4 56.0 15.3 400.0
Weights [lb] Basic Empty MTOW Combat	- 34,000 - 28,820	- 37,000 - 51,320	- 32,050 68,000 39,800	- 23,000 37,500 30,000
Internal Fuel [lb]	25,000	24,000	13,450	11,000
PropulsionManf Type Thrust,Dry [lb] Thrust,A/B [lb]	GE orP&W YF120 or YF119 ~25,000 ~35,000	GE orP&W YF120 or YF119 ~25,000 ~35,000	GE F110-GE100 18,300 28,000	GE F404-GE400 11,000 16,000
Performance Max.Sp.Alt [Mach] Cruise Sp. Alt [Mach] Combat T/W Dry [-] Combat T/W A/B [-] Combat Wg Ldg[lb/ft2] Combat Radius [NM] Inflight Refuelling	- > 1.8 ~1.58 1.02 ~1.43 58.8 > 1000 Boom	- > 1.8 ~1.58 0.97 ~1.36 54.0 > 1000 Boom	- 2.5 (1.78) subsonic 0.92 1.41 63.2 ~600 Boom	- 1.78 subsonic 0.733 1.07 75.0 405 Probe
Weapon Load (Air/Air) Gun IR AAM BVR AAM (A) BVRAAM (B)	- M-61A1 2 x AIM-9M 4 x AIM-120 -	- M-61A1 2 x AIM-9M 4 x AIM-120 -	- M-61A1 4 x AIM-9M 4 x AIM-7M 4 x AIM-120	- M-61A1 2 x AIM-9M 2 x AIM-7M 2 x AIM-120

Definitions:

• MTOW - Maximum TakeOff Weight
• Combat Weight - 50% internal fuel, typical AAMload
• IR AAM - InfraRed, ie heatseeking Air-Air Missile
• BVR AAM - Beyond Visual Range Air-Air Missile,usually radar guided
• Combat Parameter - taken at combat weight,typical weapon load, at altitude
• Missile Weights: AIM-9=200 lb, AIM-7=500 lb,AIM-120=330 lb,

Author's note:

Given the early stage ofthe ATF development program and the secrecy imposed by theUSAF, many figures in this table are estimates. Inparticular speeds, weights and thrust figures must be treated withcaution, as many of these are nominal rather than actual. Note thecited 2.5M max speed of the F-15 is clean, with a full missile loadoutthis drops to 1.78M.

Table 2. ATF ProgramSchedules

28th July, 1986 - Request forProposals issued by USAF

31st October, 1986 - Team Selectionsfor 50 month Demonstration/Validation program, both to build twoprototypes. A Lockheed/Boeing/General Dynamics team is to compete witha Northrop/McDonnell Douglas team.

Mid 1990 - L/B/GDC YF-22A and N/MDCYF-23A enter Dem/Val flight test program

The ill fated A-12A Avenger II was to be a stealthy interdictorreplacing the A-6E and F-111 family. The US Navy envisaged the use ofthe long range ATA in conjunction with the navalised ATF to provide a1000 NM + power projection capability. The ATA was cancelled earlierthis year, as it had become severely overweight and could not meetdesign performance requirements with a pair of 12,000 lb class F404engines. The expense of a major redesign with 18,000 lb class F110engines was substantial and the US DoD killed the program, leaving theNavy with a fleet of obsolescent A-6Es. A short term fix is theadoptionof an enlarged strike derivative of the F/A-18, supplemented by strikecapable 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 resultingloss instealthiness. The general layout is similar to the F-15 and F/A-18, butmuch larger. RCS reduction is achieved largely through planform shapingand faceting, resulting in a multiple lobe design. Thrust vectoring isemployed to improve pitch response.

The YF-23 was optimised for speed, range and stealth at some expense inagility, compared to its rival. The general layout is unique andexploits much of the design technique developed in the B-2A ATBprogram.RCS is reduced through careful planform shaping and blending, with aunique low drag tail which conceals dorsal exhausts in troughs toreduceboth RCS and IR emissions (USAF).

The ATF is designed for a 1:1 class dry thrust/weight ratio andsupersonic dry cruise. This provides it with a major energy advantageover a teen series (or teenski series) opponent, which it canoutmanoeuvre and outlast in a supersonic engagement (USAF).

The ATF will carry its missiles internally to minimiseRCS. Both theAIM-120 Amraam and AIM-9 are ejected from their bays at launch, so thatthe increase in RCS due open bays is transient and thus cannot allowtracking. 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 majoradvantage in the high noon shootout pre-merge phase of an engagement. Aconventional opponent cannot shoot until a lock is acquired, and thusislikely to get hit in the face with an Amraam fired by the closing ATFbefore he can acquire the ATF. Once a turning engagement is entered,thehigh dry thrust/weight ratio of the ATF will confer a major energyadvantage. A measure of this is a Lockheed report which indicates theYF-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 internalfuel capacity [Editor's Note 2005: actual production F/A-22A vehiclecapacity is closer to 21,000 lb] supplemented by inflight refuelling.This provides it with phenomenal range in subsonic cruise and excellentpersistence in supercruise. In strategic air warfare terms, the ATF canpenetrate deep into hostile airspace to defeat defending fighteraircraft and disrupt any attempts at offensive air operations, theultimate application of Lanchester's laws (USAF).