Senior Trend

In June 1977 the Air Force set up a special project office in the Pentagon; its objective, to exploit low observable technology then being demonstrated in phase one of the

XST program, and in addition, to initiate conceptual studies into a manned strike aircraft program, referred to as the Advanced Technology Aircraft (ATA) program.

Two sets of preliminary requirements for the ATA were developed: ATA ‘A’, a single scat attack aircraft, with a

5,0 lb payload and 400 n mile range; and ATA ‘B’, a two-seat bomber with a 10,000 lb payload and 1,000 n mile range.

An SI 1.1 million concept definition contract was awarded to the Skunk Works on 10 October 1977, for a one year study, based on these two sets of requirements.

As assimilation of the two proposals continued, it became increasingly apparent that ATA ‘B’ (despite being strongly favoured by Strategic Air Command, following cancellation by the Carter administration of the B-1A), w’as in the upper right corner of what was at that time considered realistically achievable.

Consequently in the summer of 1978, Air Force officials decided to terminate further studies involving ATA ‘B’ and instead, opted to proceed with ATA ‘A’ into full scale development (FSD).

Covert funds were established, and key individuals serving on various government committees were briefed on the programme. On 1 November 1978, production was authorised, the programme accorded the code name ‘Senior Trend’ and Lockheed were awarded a S340 million contract to cover the cost of building five full – scale development aircraft, plus, provide spares, support and flight testing (this amount did not include the cost of purchasing the aircraft’s General Electric engines).

The production timescales for this revolutionary aircraft program were tight; its first flight was planned for July 1980 – hence the last three digits of the prototype’s serial number, 780. Initial Operational Capability (IOC) was to be achieved in March 1982, with a planned production run of twenty aircraft. Construction of FSD1, the prototype F-l 17A, (Aircraft 780) commenced at Burbank in November 1979.

Technical Specifications

The F-117A Nighthawk is a survivable interdictor; the determinant in achieving this goal has been the develop­ment of Very Low Observable (VLO) techniques. To confound the principal detection medium – radar – design focused upon producing a low radar cross section (RCS). The reduction of an aircraft’s RCS to levels that would provide an explicit operational advantage had been the ‘holy grail’ for many military aircraft designers since the latter stages of World War Two.

Over subsequent years, development work had, by and large, been focused on producing materials capable of absorbing incident radiation to varying degrees. Although the use of Radar Absorbing Materials (RAM) certainly achieved a reduction in RCS, this was not enough to gain ‘an explicit operational advantage’; that could only be achieved when designers were able to build a shape both capable of performing an operational mission and produc­ing an RCS lower by several orders of magnitude than any current conventional aircraft. It was here that the odds were definitely stacked against the designers, as perfectly demonstrated by the radar equation which basi­cally states that, ‘detection range is proportional to the fourth root of the radar cross section’. That is to say, in order to reduce detection range by a factor of ten, it is necessarv to reduce the target aircraft’s RCS by a factor of 10,000, or 40 dBs.

Having established the required RCS signature levels from various look angles, together with the overall shape required to meet those goals, it then becomes necessary to consider other aspects of the aircraft’s design that will impact on RCS values. For a conventional jet aircraft, these include the air-intake and exhaust cavities, the aircraft’s cockpit, etc. Thus to prevent radar energy

Above All sixty F-l l7As were constructed within the Skunk Works facilities at Burbank. (Lockheed Martin)

Be/owThe F-l 17 is powered by two General Electric F404-GE – FID2 engines. (Lockheed Martin)

reflecting back from numerous corner reflectors inside the cockpit, the F-117A’s cockpit windows are metallised, much like metallised sunglasses; allowing the pilot to see out, but to all other intents, performing as a facetted panel in relation to electromagnetic radiation, reflecting energy away from its source.

The RAM coating applied over the rest of the aircraft was originally made up of 8 feet by 2 feet sheets (desig­nated BX210), which were glued onto the aircraft’s surface like linoleum tiles. The process was extremely

time consuming and expensive, costing S750,000 dollars just in labour to apply the material. As a result, a computer controlled spray coating was developed, which is environmentally safe, bonds satisfactorily to the aircraft and preserves the required radar attenuation characteris­tics. The original compound was known as BX199, but its durability and maintainability was improved upon and it evolved

In addition to producing a low RCS, the F-117A designers also paid good attention to reducing electromag­netic emissions and infrared radiation from the aircraft’s hot parts. An important feature regarding design for low observability is that in general, the design of an aircraft does not have to be compromised to negate the different ‘observables’. For example, if something is good for reducing radar returns, it can generally be made good for reducing infrared returns and vice versa. It was therefore appropriate to shield the exhaust nozzle for both radar and infrared reasons.

Range specifications of ATA ‘A’ dictated planning for the aircraft to be in theatre, which immediately identified the principal radar types to be deceived in order to significantly enhance survivability. These were airborne intercept and SAM radars, which typically operate on a wave length of between 3 and 10 centimetres. It was soon determined that flying at supersonic speed didn’t enhance survivability. Indeed, flying at high subsonic speeds actu-


Above Taken during a training sortie in the flight simulator, the cockpit layout is pre-OCIP phase 3. (Lockheed Martin)

Below The current, post-OCIP phase 3 cockpit includes an active liquid crystal display, incorporated in the Heads-Up Display (HUD). (Lockheed Martin)

ally increased survivability by reducing a defender’s abili­ty of detecting, and tracking the aircraft using infrared systems. It was therefore decided that the platform would be powered by non afterburning engines, which also reduced airframe temperatures, further lowering its IR signature.

Optimum weapon effectiveness was achieved by placing the aircraft at medium altitude, which, for a subsonic aircraft, touting a modest performance envelope, would be utter suicide – were it not for stealth. The aspect which presents a defender with the greatest chance of a success­ful intercept is the frontal zone. If the threshold of detection, by radars using wavelengths of between 3 and 10 cm, can be foiled to a point where the aircraft is just one minute flying time (about ten miles), from the radar head, then there is a good chance of avoiding a successful intercept. Pulling all the strands together therefore, an F – 117A, flying at an altitude of 12,000 feet and 500 knots, will achieve that one minute detection goal parameter by being at its most ‘stealthy’, head on, 25 degrees look down, and 25 degrees look up.

Powered by two General Electric F404-GE F1D2 two shaft, low-bvpass-ratio turbofans the F-l 17A Nighthawk

Right Target acquisition is achieved using this Forward Looking Infra red (FLIR) turret. As the ‘look-angle’ increases, the target is ‘handed-off’ to the Downward Looking Infra red (DLIR) turret, located within the aircraft’s underside, for final target tracking. Together, the two units are referred to as the Infrared Acquisition and Detection System (IRADS). (Paul Crickmore)

Below The F-1 17 is capable of hauling a wide variety of hard­ware, including the B61 nuclear weapon. (Lockheed Martin)

has a maximum sea level thrust rating of 10,8001bs. The engine gearbox drives the main fuel pump, the oil pump assembly, the engine alternator and the PTO shaft, which powers the Airframe Mounted Accessory Drive (AM AD). Total fuel capacitv is approximately 19,000 lbs or 2,800 US gallons of JP-8.

Senior Trend’s original avionics package was based around three Delco M362 F computers with 32k words of 16 bit core memory, as used in the F-16. However, in 1984, its avionics architecture was the subject of a three phase Offensive Capability Improvement Program (OCIP). Phase 1, the Weapon System Computational Subsystem (WSCS) upgrade program was initiated to replace the Delco M362F’s with IBM ЛР-102 MIL-STD-1750A computers. These new units boosted the capability of 1 million instructions per second, 16 bit CPU with 128k words of 16 bit memory expandable to 256k.

Phase II of OCIP, afforded greater situational aware­ness, and reduced pilot workload, by allowing a 4D Flight Management System to fly complex profiles auto­matically, providing speed and time over target (TOT) control. Also included in this phase was the installation of Colour Multi functional Display Indicators and a Digital Tactical Situation Display or moving map; a new Data Entry Panel, a Display Processor, an Auto Throttle System and a Pilot Activated Automatic Recovery System (PAARS).

OCIP phase III saw the replacement of the ageing SPN-GEANS, INS system, with a new Honeywell H – 423/Е Ring Laser Gyro (RLG). The original acronym for this programme was to have been RNIP, which stands for Ring Laser Gryro, Navigation Improvement Programme. However, the system was supplemented with a Rockwell – Collins Global Positioning System (GPS) thereby giving rise to the title RNIP plus. The new INS vastly reduces
alignment time from 43 minutes for SPN-GEANS, to just 9 minutes and considerably enhances overall reliabili­ty, increasing the mean time between failure from 400 to

2,0 hours. In itself, the H-423 may not boost enhanced accuracy (still believed to be 0.12 n m/h), however, when used in association with GPS, the system represents a significant advance in navigational accuracy.