F-22A Raptor - Supercruise and Stealth Technology
By any measure, the development of the F-22A was revolutionary - in the technology employed and from a stealth design point standpoint - and this produced a combat aircraft that has no equal.
Being able to carry weapons internally was essential to its stealth capability, and efficient supercruise. Given the role away from dedicated air superiority, rather than incur the costs of resizing the compact main internal bays of the F-22A, sized initially around four early AIM-120A AMRAAMs, the US Air Force embarked on the 'Small Smart Bomb' program centred on the idea of carrying a payload of multiple 250 lb class bombs. Today's 385 lb GBU-39/B and GBU-40/B Small Diameter Bombs (SDB) were sized around the F-22A weapon bay. For the intended role of busting an opponent's airfields, mobile missile batteries, and command posts, the mix of either two GBU-32 1,000 lb JDAMs or eight GBU-39/40 SDBs is an excellent fit. With all-aspect stealth and penetrating supersonic at 50,000 ft AGL, the F-22A remains virtually unstoppable by surface-to-air missiles or fighters. It is likely that operational F-22As will be used far more frequently to break down an opponent's air defences than in the classical air superiority role, despite the aircraft retaining the full air superiority capabilities envisaged for the ATF.
The 1990s saw the production configuration of the F-22A refined, with a new fuselage shape, revised larger span wing planform, smaller vertical tails positioned aft, relocated cockpit, and production configuration avionics architected.
Stealth shaping for the F-22A design was unique as it used edge-aligned inlets and thrust-vectoring nozzles, by virtue of edge length capable of defeating radars with greater wavelengths than any other stealthy fighter inlet and nozzle designs in existence. Built thus for all-aspect relatively wideband stealth, this is a distinct optimisation for deep-penetration and air combat, and the most capable stealth design other than the specialised F-117A and B-2A.
The raw aerodynamic performance of the F-22A was without precedent. In military (dry) thrust setting the F-22A could cover the whole afterburning performance envelope of the F-15 - or advanced Sukhois, both still the highest performing energy fighters widely deployed. The F-22A was rated for 9G at combat weights.
With 20,650 lb of internal fuel, the F-22A internally carried 88 per cent of the fuel in a CFT-equipped F-15E, with no drag penalty, yet with four 592 USG drop tanks, a total fuel load of 36,515 lb could be carried, 6 per cent more than the internal fuel of the larger F-111.
This uncalibrated chart compares the speed/altitude envelope of the F-22A in military power (dry) thrust rating against the F-15C in afterburning (maximum) thrust rating. The combination of F119-PW-100 supercruising engine and optimised supersonic aerodynamics results in a revolutionary advance in performance, evidenced by repeated one vs many engagements against F-15Cs during Opeval going to the F-22A (US Air Force / Author).
Refined supersonic aerodynamics allowed the F-22A to exceed Mach 1.5 in military thrust at altitude - the exact top speed in dry thrust has never been disclosed. In early trials, F-15 chase aircraft could not keep up, and test pilots soon reported instances where even modest heading changes by F-22A prototypes in head-to-head engagement geometries caused opposing teen series fighters to abort engagements entirely - an experience historically seen only in engagements against Foxbats and Foxhounds.
In the simplest of terms, the supercruising F-22A kinematically defeated all opposing fighters, and even without stealth would kinematically defeat most existing surface-to-air missile types. The only design with the potential to kinematically challenge today's F-22A are advanced derivatives of the Su-30 fitted with supercruising AL-41F fans, the Russian equivalent to the F119-PW-100 engine in the F-22A, and an LRIP production item since 2004.
The unchallenged aerodynamic performance of the F-22A design required considerable design innovation, and extensive use of new materials techniques. At nearly 40 per cent of total empty weight, the F-22A had the highest fraction of Titanium alloy in any US design since the SR-71A, which compares closely to the Russian Sukhois. Resin Transfer Molded (RTM) thermoset composites, specifically epoxy and high temperature bismaleimide (BMI) composites, made up 24 per cent of total empty weight. New processes, such as Hot Isostatic Pressed (HIP) casting and vacuum chamber electron beam welding were introduced to allow complex high strength shapes to be fabricated from Titanium alloys, primarily Ti-64 and Ti-62222, minimising the number of fasteners used. Only 16 per cent of the F-22A's empty weight comprised Aluminium alloys. Like the B-2A, geometrical accuracy is critical to stealth performance, and the F-22A required similar production tolerancing.
The F119-PW-100 supercruising thrust-vectoring engine proved no less challenging. The 'short and fat' F119 engine was built with integrally bladed rotors, using high strength long chord fan and compressor blades, floatwall combustors exploiting high Cobalt content alloys. Heat resistant Titanium Alloy C was used extensively in compressor stators, the afterburner and nozzles. The part count in the F119 was reduced by 40 per cent against the earlier F100/F110 engines to improve reliability and maintainability. Extensive self diagnostic capabilities were incorporated to reduce personnel and test equipment demands on deployment by 50 per cent against the F-15. The F119-PW-100 is typically cited in the 35,000 lb static SL thrust class. The nozzles provide 20-degree deflection, used for manouevre and for supersonic cruise trim drag reduction. An Airframe Mounted Accessory Drive (AMAD) is used to couple engine power to generators, hydraulic pumps and shaft power to the engines from the Air Turbine Starter System (ATSS).
Unlike earlier designs, the F-22A introduced a digital Vehicle Management System (VMS), which integrates primary flight controls, leading edge flap controls, engine controls and thrust vector controls. The complex software at the heart of the VMS absorbed a large fraction of development costs, but provides the aircraft with unrestricted 'carefree' handling throughout the envelope, and very high angular rates in manoeuvre. The F-22A uses rudder tow-in to provide the speedbrake function.
Aircraft utility systems also saw much innovation. The aircraft uses an On-Board Oxygen Generating System (OBOGS) to provide breathable oxygen, but also cockpit pressurisation and defogging. An On-Board Inert Gas Generation System (OBIGGS) is used to produce nitrogen for fuel tank inerting, and a Halon gas extinguisher system was introduced to protect the engine bays, APU, and most large cavities in the airframe. The Allied Signal Aerospace Auxiliary Power Generation System (APGS) is built around a 450 SHP G-250 turbine APU, considered the highest power density design in production, coupled to a Stored Energy System (SES) using compressed air bottles for self-starting.
Avionics cooling in the F-22A also departed from convention, using a liquid cooling system to dump heat out of the core avionic suite, especially the APG-77 radar and Common Integrated Processors (CIP). Polyalphaolefin (PAO) coolant is cycled through the avionics in two loops, then through the air cycle cooler (bleed air driven) and then through heat exchangers in the wing tanks, dumping waste heat into the wing tank fuel. The fuel acts as a heat sink, but itself is cooled by the Thermal Management System (TMS), using an air inlet between the fuselage and inboard inlet edge to dump heat from a fuel system heat exchanger.
The fuel system comprises the F-2, F-1A and F-1B forward fuselage tanks, the paired A-2L/R wing internal tanks, the paired A-3L/R centre fuselage tanks, the pair A-1L/R outboard aft fuselage tanks, and optional drop tanks, totalling 36,515 lbs of JP-8 fuel. A refuelling receptacle under clamshell doors is used.
The 270 Volt DC electrical system is powered by two 65 KiloWatt generators, and a redundant 4,000 psi hydraulic system is used. A retractable arrestor hook was included for short field recoveries.
The forward fuselage module is built mostly of composites and aluminium, and is structurally built around chined composite side beams and upper longerons. The clamshell canopy uses a single piece Sierracin 0.75" thick polycarbonate canopy, designed with Zone 1 optical quality through the full field of view. An improved ACES-II ejection seat is used. The cockpit uses six full colour AMLCD multifunction displays, the Primary Multi-Function Display (PMFD) being 8x8", the three Secondary Multi-Function Displays (SMFDs) being 6.25x6.25", and two Up-Front Displays (UFDs) being 3"x4" in size. A GEC Head Up Display is used. The pilot is equipped with an integrated breathing regulator/anti-g valve (BRAG) controlling air to the mask and the G-suit, the latter acting as a partial pressure suit at high altitude, and including an integrated air-cooling garment. Full NBC capability is provided.
The low observable composite radome covering the APG-77 phased array is one of the most sensitive - and expensive - components in the forward fuselage. The APG-77 itself with cca 1500 X-band active solid state TR modules is the most powerful and sophisticated radar ever installed in a combat aircraft, providing conventional weapon modes, LPI modes, and an ISAR capability to image the shape of a target aircraft to facilitate early recognition in combat. The forward fuselage includes structural provisions for growth in the radar via paired sidelooking phased arrays. Processing for the baseline F-22A's APG-77 was performed in a package of up to three Common Integrated Processor assemblies - built around the Intel i960 chip and VHSIC arrays. Full production aircraft will use COTS technology processors, reflecting the obsolescence created by politically mandated production delays.
The mid fuselage comprises three modules, and is the structural core of the airframe. It contains the two main fuselage weapon bays, the side weapon bays, much of the fuel storage, the APU, the 20 mm M61A2 gun, the main gear bays and the inlet tunnels. Serrated upper fuselage doors are used to dump excess air from the inlet subsystem, and an APU exhaust and inlet are mounted at the left wing root - the gun occupying that volume in the right wing root.
The aft fuselage mounts the engines, TVC nozzles, and tail surfaces. It has the highest fraction of structural titanium alloy, at 67 per cent, 25 per cent of its weight being in the lightweight high strength paired electron-beam-welded tail booms. The vertical tails use composite rudders, skins and edges, and HIP titanium castings in the actuator. The stabilators are largely honeycomb, but using composite edges, and a unique lightweight composite actuator shaft.
The wings of the F-22A were no less innovative, and aerodynamically optimised for supersonic cruise and high G manoeuvre, but with excellent transonic performance. Structurally by weight the wing uses 42 per cent Titanium, 35 per cent composites and 23 per cent aluminium and other materials. Sinewave spars are used, with 75 per cent of spars composite, and 25 per cent Titanium alloy to improve ballistic damage tolerance.
The avionic system in the F-22A accounted for a very large fraction of development and production costs, in a large part due to the first large scale use of active phased array technology in the radar, and PAO liquid cooled avionic hardware - much of the rationale behind the Joint Strike Fighter was to find development investment and production volume to drive down the cost of radar modules, avionic components and engine components to eventually be common with mature production F-22As.
The avionic suite is the most highly integrated to date, with virtually all processing performed in the CIPs. The APG-77 radar, ALR-94 Electronic Support Measures system and AAR-56 Missile Approach Warning System are departures from classical 'federated' architecture avionics. Optical fibre links are used to significantly increase data transfer rates between the radar's high frequency components and the CIPs. The baseline F-22A was to have included an advanced long range longwave Infrared Search and Track sensor, but this was removed in the interim as a cost saving measure. Navigation reference is provided by redundant Litton LN-100F ring laser gyros and a GPS receiver. An ALE-52 dispenser for expendable countermeasures is located under doors, forward of the main wheel wells.
As a 'software centric' system with millions of lines of code, the F-22A introduced the concept of a system where virtually all functions and processing were built to rapidly evolve by software growth. The pilot of an F-22A was presented with a level of cockpit automation without precedent: sensor modes, such as radar, were automatically chosen by software, hiding complexity from the pilot. The aim was to free the pilot from the mind-numbing information saturation problem seen in most contemporary combat aircraft. The combination of the LPI radar and passive ESM provides an autonomous long-range detection footprint larger than that of any contemporary combat aircraft. A new LPI Inter/Intra-Flight Data Link (IFDL) was included to permit flights of F-22As to transparently exchange data in combat, including fuel states, weapons remaining, and targets being engaged.
There can be no doubt that the F-22A will remain the pinnacle of modern fighter technology for decades to come.
The F-22A has transitioned from LRIP to full rate production, incorporating a range of enhancements developed during the latter phase of the development program. An extensive roadmap now exists for future incremental upgrades.
Wind tunnel testing of a stealthy external stores pod, designed to carry weapons such as the GBU-39/B and GBU-40/B Small Diameter Bomb. The pylons are rated for 5,000 lb stores (US Air Force photo).
The baseline F-22A Block 10 will be capable of air superiority, air defence / cruise missile defence and deep-penetration strike roles. Subsequent Block 20, 30 and 40 configurations will progressively expand air to air and strike capabilities, and exploit the aircraft's survivability to perform ISR roles. The F-22A thus becomes the 'broadest' multirole fighter in service.
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