The Advanced Tactical Fighter
by Carlo Kopp
First Published in Australian Aviation, April and May, 1991, Issues
(c)1991, 1997, Carlo Kopp
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.
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. Watch for Part 2 on Friday.
MDC F-4B 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.
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