USAF’s Second Generation Stealth Fighter

The USAF’s original intentions were to replace its large fleet of F-111 tactical strike aircraft with a derivative of the US Navy’s Ata or A-12A Avenger II. There was also a good chance, if the aircraft’s low observables performance was acceptable, that this aircraft would have replaced the F-117A eventually. This was not to occur, as the Bush Administration cancelled the programme before the prototypes flew, due to cost overruns which were not unexpected in the technical community, given the sheer complexity of the aircraft and its demanding mission profile.

What has been a well-kept secret is that a replacement aircraft for the A-12A, F-111 and F-117A has been quietly worked upon under the code designation of “Have Black”, since the mid-1980s. This project was “deep black”, and sponsored by the USAF as a second-generation stealth fighter to replace the F-117A and, given budget, the F-111 and F-15E. The USAF was reported to have had its doubts about the political ability of the US Navy to keep the A-12A funded, as well as having private doubts about the suitability of the A-12 design for the USAF role, which really required better radius performance and lesser disposable weapon load than the Navy specified.

The Have Black programme was designed to capitalise on the large established technology base built up during the Atb (B-2) and Atf (F-22/YF-23) programmes, while using as many off-the-shelf components and black boxes as possible. The large B-2 manufacturing programme was to have provided cover for the manufacture of the Have Black fighter, which US sources indicate has been tentatively designated the F-24A.

The aircraft is reported to have first flown in 1992, and several prototypes have been since put through very thorough flight and low observables testing. Unconfirmed reports suggest that the prime contractor is Northrop, with Grumman providing the systems integration and simulator designs. It has been conjectured that the merger between Grumman and Northrop was related to the joint development programme, as a means of better consolidating the shared development and manufacturing facilities.

The Have Black is slightly larger than an F-15E but smaller than the F-111 family of aircraft. The aircraft is a flying wing in general configuration similar to a B-2A, but with a wing sweep angle of 45 degrees designed to optimise the airframe for transonic cruise. It is reportedly fitted with two unreheated turbofans, prototypes supposedly flying with the GE F110 but with production aircraft expected to be fitted with a pair of unreheated ATF common P&W F119 fans.

The aircraft carries no less than 11,350kg (25,000lb) of internal fuel which provides a nominal combat radius of 1200nm (2225km) with 2725kg (6000lb) of internal weapon load. Empty weight is approximately 13,620kg (30,000lb), with an Mtow in the vicinity of 29,500kg (65,000lb). The aircraft is reported to have a span just over 22m (72ft) and a length of about 14m (46ft).

The crew are seated side by side, as in the F-111, on a pair of Escapac ejection seats. A two-piece aft-hinged bubble canopy is used. The cockpit is reported to use flat-panel active matrix Lcd multifunction displays, but with a pair of raster scan Huds which are common to the F-15E. Conventional centre-mounted controls are fitted, and the RH station provides full command capability by virtue of software-driven displays. It is expected that the RH station will be crewed with Wsos, the controls being provided to save the expense of a specialised combat-rated trainer version.

It has been reported that the aircraft uses a full authority quadruplex digital fly-by-wire flight control system using algorithms developed for the B-2A, but using different computer hardware. The high-pressure hydraulic system uses B-2 technology. While the use of optical Mil-Std-1773 was considered for the dual bussed main avionic system, cost pressures resulted in a conventional 1553B bus which supports the dual mission computers as well as the pair of display management processors. The complex integrated avionic system designed for the Atf was not used, due to uncertainties in funding and hence production schedules.

Avionics are conventional. The primary navigation and attack sensors are a nose-mounted steerable navigation Flir, derived from Lantirn, in a blended nose fairing and the first generation Texas Instruments Dlir ventral turret as fitted to the existing F-117A. The laser-equipped Dlir turret provides target designation and rangefinding for laser-guided bomb delivery. The dual field of view navigation Flir projects on the Huds and is equipped with video tracking facilities for engagements against airborne targets. The nav-attack system uses dual Honeywell H523 Ring Laser Gyros and production aircraft are reported to be fitted with Gps and possibly a covert Terrain Contour Matching navigation system similar to the UK Terprom system.

The aircraft uses a conventional electronic warfare system, built around the Alr-56R super heterodyne warning receiver/Esm subsystem, derived from the mature high-density Alr-56M. The newer subtype is reported to have substantially better sensitivity and additional forward-facing interferometric antennas are used for precision direction finding of emitters. It has been conjectured that the aircraft may also carry a Phase Rate of Change passive rangefinding subsystem for defence suppression and silent attack on airborne radiating targets. One report suggests that the aircraft was to have been fitted with a derivative of the Alq-165 Aspj trackbreaker, but will fly without the equipment given favourable operational experience with the F-117A in the Gulf campaign (and political problems with the Aspj).

Three ventral weapon bays are used. The central, No. 2 bay is the largest and will accommodate three Gbu-24 2000lb (910kg) laser-guided weapons, Agm-88 Harm anti-radiation missiles, or (planned) specialised pallets with either reconnaissance cameras or high-powered jammers used for the escort jamming role. An additional fuel cell may be fitted above the recce pallet. The smaller outboard No. 1 and No. 3 bays carry pairs of either Aim-120 or Aim-9 missiles on a telescoping extendable dual launch rail. All weapon stations use Mil-Std-1760 weapon control bus interfaces. The Aams as well as the Harm may be cued to targets using the Esm subsystem, which has improved detection range and Df accuracy for this specific purpose.

The airframe design shows unmistakable similarity to the B-2, and the general configuration is very close to that of the B-2A. The dorsal exhausts employ shark tooth trailing edges used to scatter impinging radiation to either side of the aircraft. The configuration bears much resemblance to the Yf-23 Atf prototypes, and a similar active cooling system is used to reduce secondary infrared radiation from the exhaust channels. A moving fuselage beavertail surface is used for active pitch trim, and split outboard ailerons/speedbrakes provide for low-speed roll and yaw control as with the B-2. The engine inlets were mounted ventrally to ensure clean airflow at increased AoA, which also indicates that the aircraft is expected to sustain a respectable turn rate.

The fuselage centre section houses a forward avionics bay, the cockpit, an avionics bay in the hump behind the cockpit, and the central weapon bay, about which is wrapped the centre-section fuel cell, which carries 4540kg (10,000lb) of Jp-5. An in-flight refuelling receptacle is directly behind the cockpit, and a single-point refuelling attachment is inside the main weapon bay. The engines are situated immediately outboard of the central weapon bay area, with the outboard weapon bays to either side inboard of the engine bays. The main undercarriage legs retract outboard, pivoting off the central structural box which mounts the weapon bays and engines; the main wheel well doors are hinged outboard. The nosewheel is a conventional dual arrangement pivoting aft, with a single left-hinging door which mounts the ground support diagnostics panel.

Engine accessory drives on both powerplants provide electrical and hydraulic power. A Garrett Gtc36 series Apu is embedded in the fuselage, exhausting into the Rhs exhaust channel inlet duct. The Apu provides compressed air for unassisted engine start, hydraulic pressure capable of sustaining the flight controls, and electrical power to drive the core avionics. An Obogs oxygen system is also carried. The fitting of the Apu and Obogs was reportedly intended to provide for operational deployments with a minimum of ground support equipment.

Dual main spars attach to the central box forward of the engines and are reinforced by an aft load-carrying member which attaches at the rear of the central box. The design is reported to be structurally rated at 7.5G, although the aircraft is unlikely to pull this load frequently given its mission profile. Fuel is split into five main cells: the central saddle tank and two outboard integral tanks in the wings.

Low observable performance is reported to be similar to that of the B-2 Spirit, given that the aircraft uses much the same configuration, materials, and blended contours. The leading edge employs embedded Ram (Radar Absorbent Material), with a compressed air de-icing system. The air, bled from the compressors, is cycled through leading edge cavities to the wingtips, from where it is returned via plumbing to the exhaust bay cooling system, where it is dumped into the airflow used to blanket the titanium exhaust bay liners.

Informed sources indicate that the aircraft is capable of achieving low supersonic dash speed, estimated at Mach 1.2, although the actual speed has not been disclosed. Sustained cruise speed is high subsonic and constrained by the wing profile. The aircraft is very efficient aerodynamically, as is the B-2, and has an excellent climb rate and “fighter-like” handling, according to test pilots. The structural G-rating will result, as with the B-2, in a very rigid aeroplane with high roll rate performance and, given the short length, high pitch rate performance. The very low wing loading at combat weight translates into excellent climb, acceleration, and turn performance.

The unreheated combat thrust-to-weight ratio of about 1:1 is by any standards excellent. Indications are that the flight control system envelope is still constrained to 22 degrees AoA max. High AoA handling is a potential problem area given the absence of vertical stabiliser surfaces, and the aircraft is not intended to be a dogfighter.

The reported mission for the aircraft is all-weather precision strike, Sead (Wild Weasel), theatre imagery and electronic recce, and covert air-to-air intruder missions primarily against high-value assets such as Aew aircraft, airborne command posts, tankers, and transports in rear areas. With its very low signature and respectable aerodynamic performance, it can stealthily penetrate deep, engage its target, and egress equally stealthily. As it has been equipped with very accurate passive sensors, under non-visual conditions it is a serious threat to conventional air superiority fighters.

The F-24A represents a quantum leap in the evolution of tactical strike aircraft and will provide the USAF with an excellent replacement for the F-117A, F-111, and F-15E, should sufficient numbers be acquired. The aircraft has substantial potential to perform in a number of tactical roles and we can hope that this type will be used to replace the Ef-111A with a credible new-technology airframe.

Needless to say, this intriguing aircraft would also make an excellent replacement for the Raaf’s F-111 Strike/Recce force, and hopefully will come under consideration as an eventual successor to the Pig.

Unconfirmed reports indicate the USAF’s planned Ioc for the aircraft is April 1, 1997. (Ed. … and pigs might fly — we hope you enjoyed our first of April canard …)