First Published in Australian Aviation, April and May, 1991, Issues
(c)1991, 1997, Carlo Kopp

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.

Airframe Design

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.

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.

Propulsion

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.

Performance

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 shift 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. 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: 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. The issues surrounding the RAAF's Hornet replacement will be covered in some detail in August, September and October, 1997, issues. In hindsight, we correctly anticipated current events, the early replacement of the Hornet now a distinct possibility)

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.