Lockheed F-117 Nighthawk

Last revised January 18, 2005




The F-117A "stealth fighter" had the same general configuration of the Have Blue test aircraft, but was much larger and heavier and was provided with an offensive military capability.

The structure of the F-117A is constructed mainly of aluminum, with some titanium being used in the engine and in the exhaust systems. The main facets of the outer skin are separately fastened to a rather complex skeletal frame. Since the accurate shaping and placement of these facets is critical to achieving a low radar cross section (RCS), production tooling had to be ten times more precise than the tooling used to build conventional aircraft.

The entire outer skin of the F-117A is covered by radar absorption material (RAM). The exact composition of the RAM is classified, but it is believed to consist of a matrix of magnetic iron particles held in place by a polymer binder. Originally, RAM came in large flexible sheets, and was bonded to a metal wire mesh which was in turn glued to the airframe of the F-117A. Later, when the aircraft entered service, the Air Force built a special facility for the application of the RAM. In order to provide for uniform and accurate application, as well as to prevent people from coming into contact with the highly toxic solvents which make the RAM liquid, the process is completely automated. During the application of the RAM, the F-117A is held like some sort of gigantic chicken being roasted on an spit, and is slowly turned as the RAM is sprayed on by computer-controlled nozzles. However, minor touch-ups can be made in the field by using a hand-held spray gun.

The engines powering the F-117A are a pair of non-afterburning General Electric F404-GE-F1D2 turbofans. These were derivatives of the afterburning F404-GE-400 turbofans which power the McDonnell Douglas F/A-18 Hornet. They are housed in broad nacelles which are attached to the sides of the angular fuselage. The General Electric turbofans are fed by a pair of air intakes (one on each side of the fuselage). Two gratings with rectangular openings cover each intake. The purpose of these gratings is to prevent radar waves from traveling down the intake ducts and reaching the whirling blades of the turbofans, which would tend to produce large echoes. This works because the spacings between the grids on the grating are smaller than the wavelengths of most radars. The grating is covered with RAM, which helps to reduce the reflections even further. The small fraction of incident radar energy which does pass through the grating is absorbed by RAM mounted inside the duct. Unfortunately, these gratings also restrict airflow to the engines, so a large blow-in door is fitted atop each engine nacelle to increase airflow to the engine during taxiing, takeoffs, or low-speed flight. Ice buildup on the intake gratings is a persistent problem, which tends to clog the rectangular openings and restricts the airflow even further. In order to clear the ice, the F-117 employs a electrical heating system which removes ice during flight. A light on either side of the fuselage illuminates the intake covers so that the pilot can watch the deicing operation during night flights.

One of the more unusual aspects of the F-117 is its engine exhaust system. Like the air inlets, the exhaust outlets are mounted atop the wing chord plane, one on each side of the centerline. The engine exhausts are narrow and wide and are designed to present as low an infrared signature as possible. In addition, they are intended to mask the rear of the engine from radar illumination from the back. The exhaust ducts are round at the rear of the turbofans, but are flattened out and become flume-like by the time that they reach the front of the narrow slotted exhaust outlets at the rear of the fuselage. At the end of each of the narrow slotted exhaust ducts, there are twelve grated openings, each being about six inches square. These grated openings help to reduce unwanted radar reflection from the rear and they provide additional structural strength to the exhaust ducts. The exhaust gratings are shielded from the rear and from the bottom by the F-117's platypus-bill-shaped rear fuselage section. The extreme rear edge of the aircraft behind the exhaust slot is covered with heat-reflecting tiles. These ceramic tiles help to keep the rear of the aircraft cool, since they tend to reflect the infrared radiation emitted from the exhaust, rather than to absorb it as metals tend to do. In addition, bypass air from the engine is used to help cool down the entire metal structure of the rear of the aircraft. The exhaust system is complex, incorporating sliding elements and quartz tiles to accommodate heat expansion without changing shape. Although the system works fairly well, Lockheed has reported that the design of this exhaust system was the single most difficult item in the entire F-117A project.

A typical fighter has a head-on RCS of about five square meters, which is technical language for saying that it seems as large on radar as a perfectly-reflective sphere of the same cross-sectional area. However, if critical flat surfaces or whirling turbine blades happen to be exposed to the radar, the RCS can be much larger. Reportedly, the combination of faceting and the application of RAM gives the F-117A an effective radar cross section of somewhere between 0.01 and 0.001 square meters, which makes the F-117A appear to be no larger than a small bird on a radar screen. This means that a typical radar will not be able to detect an F-117A at a range any greater than 8-16 miles.

Directional stability and control of the F-117A is provided by a pair of all-flying tails that are mounted on the aircraft's central spine and oriented in a V arrangement, reminiscent of the tail of the Beechcraft Bonanza. Unlike most V-tails, however, they have no pitch-control function. Each vertical tail consists of a a fixed stub, and an all-flying rudder which pivots around a fixed shaft. However, the hinge line between stub and moveable tail is Z-shaped rather than straight, in accord with the stealth principle of the avoidance of any straight edges. Both the fixed stub and the all-flying rudder are faceted in order to further reduce radar reflectivity.

On the Have Blue test aircraft, the vertical tails were mounted further outboard on the wings and were canted inward rather than outward. The purpose of the inward-canted vertical tails on the Have Blue was to shield the upward-facing platypus exhaust nozzles from infrared detectors above the aircraft. In practice, however, these tails tended to act as reflectors for infrared radiation, bouncing the rays toward the ground and making the aircraft more visible from below. Originally, the basic stealth design philosophy was to have the lowest observability from the bottom and from the front, with the upper hemisphere having less stringent requirements. Consequently, on the F-117A aircraft, the tails were moved back further on the fuselage so that they are no longer directly over the exhaust. In addition, the Have Blue tails were in effect mounted on twin booms which was a structurally inefficient arrangement.

The leading edge wing sweep on the Have Blue was 72.5 degrees, and the resulting low aspect ratio gave a rather poor payload-range performance. In order to improve the performance, the wing sweep was reduced to 67.5 degrees on the F-117A. The flying surfaces on the F-117A consist of four elevons on the wing trailing edge (two inboard and two outboard) and two all-flying rudders mounted in a V arrangement on the rear fuselage. The elevons and the rudder are all faceted in order to reduce their radar signature, and the hinge lines between the wings and the elevons are sealed with flexible RAM. The four elevons can deflect upward or downward by 60 degrees, and the rudders can deflect 30 degrees left or right. The elevons act in the pitch and roll axes, whereas the rudders act in the yaw axis. The angle of attack during landing is about 9 degrees. The elevons do not double as flaps, which makes the landing speed of the F-117A rather high.

The Have Blue cockpit canopy windshield had a center bow, reminiscent of that of the F-102/F-106 interceptor. The F-117A replaced this windshield with a center flat panel since a heads-up display would not work very well with a center bow blocking the view. This resulted in a change in the shape of the nose to a steep downward-sloping section for good downward visibility with a sharp, pyramidal-shaped nose cap for aerodynamics and stealth. This change made the F-117 slightly more observable by radar than the Have Blue.

The cockpit of the F-117 is covered by a large and heavy hood-like canopy with five separate flat transparencies (one on either side and three in front). The visibility from the cockpit is rather limited upward, downward, and to the rear. The canopy opens to the rear and has serrated edges in order to limit the radar reflectivity of the joint between canopy and fuselage when the canopy is closed. The five flat transparent panels are specially treated to further reduce the aircraft's RCS. The windshield is coated with a special gold film layer to prevent the pilot's helmet from being detected by radar. This was found to be an important problem during early tests. The pilot sits on a McDonnell Douglas ACES II ejector seat, the same type of seat that is fitted to the F-15C/D.

The cockpit is equipped with a Heads-Up Display (HUD). The main control panel has two five inch CRTs, while the main FLIR/DLIR CRT had a twelve-inch screen. For nighttime operations in clear weather at low altitudes, the aircraft is primarily flow by using the FLIR/DLIR CRT.

The F-117A, like the Have Blue before it, is unstable about all three axes and requires a fly-by-wire system in order to be able to fly at all. The fly-by-wire system is similar to that in the F-16, and is quadruply redundant. There are four independent channels which each control the same function. The signals from each of the channels are constantly being compared with each other, and if one signal is found to differ from the other three, its channel is assumed to have failed and is automatically shut down. In the unlikely event that all four channels manage to fail at the same time, the aircraft cannot be flown and the pilot would be forced to eject.

Since the aircraft cannot use any sort of radar navigation system, the fly-by-wire system relys on information about airspeed and angle of attack from four individual static pitot probes of diamond section with pyramid-like tips mounted in the extreme nose. Each of the four-sided pitot heads have tiny holes on each facet, and differential readings from each hole provide airspeed, pitch and yaw information to the flight control system. The design of these four nose sensors, plus the requirement that they not produce any unwanted radar reflections, was one of the more difficult engineering problems that the Lockheed team had to solve.

The F-117A also differed from the Have Blue in having a weapons bay. Since external hardpoints for bombs or fuel tanks are taboo for a stealth attack aircraft, all stores must be carried internally. The weapons bay is located in the belly on the centerline. It has two wells, each covered by an inboard-opening door. The outer edges of the weapons bay doors have serrated edges that are designed to reduce the radar reflection from the joint between the doors and the fuselage belly. The weapons bay can accompany up to 5000 pounds of ordinance (2500 pounds in each well).

Some of the loads which can be accommodated in the F-117A's weapons bay include two laser-guided MK84 2000-pound bombs, two laser-guided GBU-10 Paveway II 2000-pound bombs, two laser-guided GBU-12 Paveway II 500-pound bombs, two laser-guided GBU-27 Paveway III 2000-pound bombs, two BLU 109 deep-penetration bombs, or two AGM 130s. The GBU-10 Paveway II laser-guided bomb consists of a special nose and tail section attached to a standard 2000-pound Mk 84 high-explosive bomb. The tail section of the bomb consists of a set of folding aerodynamic surfaces which permit the bomb to glide, whereas the nose section includes a laser light seeker, guidance electronics, and control fins. The GBU-24 Paveway III is a more advanced version of the Paveway II with a larger tail surface and a more efficient guidance system which permits it to be used at lower altitudes and at greater distances from the target. The BLU-109 deep-penetration bomb has a forged casing of hardened steel which permits it to pierce more than six feet of reinforced concrete before exploding. When dropped on softer targets, the BLU-109 can bury itself deep into the ground before exploding, destroying its target by sending earthquake-like shock waves rippling through the ground. The F-117A can also carry up to two Mark 61 nuclear weapons, although the aircraft does not actually have an assigned nuclear mission. For long-range ferry flights, fuel tanks can be installed in the weapons bays in the place of bombs.

The F-117A has no air-to-air capability, or at least none that has been announced to the general public. It has no radar, it does not carry an internal cannon, and is not equipped to carry or launch air-to-air missiles. However, I suppose that the F-117A could in principle launch an infrared homer, provided that the missile could be dropped from an extendable rack so that its seeker could acquire the target before launch.

The F-117A cannot rely on radar for navigation, weapons aiming, or weapons delivery because the transmission of a radar signal would tend to give away the location of the aircraft and hence defeat the whole purpose of stealth. For low-level navigation and weapons-aiming purposes, the F-117 aircraft is provided with forward-looking infrared (FLIR) and downward-looking infrared (DLIR) systems. Both systems are built by Texas Instruments. The FLIR is mounted in a recess just ahead of the cockpit front windshield. It is located in a steerable turret containing a dual field of view sensor. When the FLIR is not in use, it is rotated 180 degrees to keep debris from damaging the sensor. The DLIR sensor system is located in a recess mounted underneath the forward fuselage and to the right of the nose landing gear well. Both the FLIR and the DLIR recesses are covered by a RAM mesh screen to prevent unwanted radar reflections from the active elements, and the edges of the recesses are serrated, with the fasteners being covered with RAM putty.

The DLIR is provided with a bore-sighted laser for illuminating the target for attack by laser-guided weapons. Together, these systems form the infrared acquisition and designation system (IRADS). The laser is slaved to the IRADS and is an integral part of the infrared system. The spot size of the laser on the ground is about 12-18 inches, and the spot is stabilized in position by the IRADS.

A highly-accurate Inertial Navigation System (INS) backs up the sensors. This system uses an electrostatically-suspended gyro as the primary means of guidance. The INS guides the aircraft to the immediate target area and points the FLIR's wide angle field of view toward the general location of the target. As the aircraft approaches the target, the pilot monitors the view presented by the FLIR on the heads-up display screen. When the specific target is identified, the pilot switches to the narrow view on the FLIR, and locks the screen of the display onto the target. As the target disappears underneath the aircraft, control is handed over to the the DLIR, which acquires the target and continues to track it. When the pilot decides to attack, he releases a laser-guided Paveway bomb. Approximately 7 to 10 seconds before bomb impact, the DLIR's laser is turned on and illuminates the target, and the bomb homes onto the reflected infrared laser light reflected from the target. Video tapes from the FLIR/DLIR displays that have been released to the public by the DOD have shown that the F-117A flying during a clear night can hit a target only one meter in size from an altitude of 25,000 feet.

A parachute braking system is provided, since the lack of flaps makes the landing speed quite high (160 knots, or 185 mph). The braking parachute is housed behind split doors atop the rear fuselage. The braking chute is deployed as soon as the nosewheel makes contact with the runway. The parachute can also double as an emergency antispin device if needed.

An inflight refuelling receptacle is added behind the pilot's cockpit. A small light is mounted near the receptacle to guide the refuelling boom operator in nighttime refuelling operations. Midair refuelling is one of the more difficult aspects of F-117A flight, since it is always done at night and the F-117A pilot's upward vision is blocked by the canopy so that he cannot actually see the boom of the refuelling aircraft.

The landing gear is of the standard tricycle type, with single wheels and tires that retract forward. The landing gear doors have serrated edges that help to reduce the radar cross section.

A set of retractable communications antenna are fitted to the upper fuselage just behind the pilot. These are deployed during day flights, but are retracted for stealth missions at night. Detachable radar reflectors can be mounted on the fuselage sides so that local air traffic control can track the aircraft when it is not in stealth mode. Some of the reports of the F-117A being tracked by radar during Desert Storm may have been due to the mounting of these reflectors.

In 1991, persistent problems with the unorthodox exhaust system led to a decision to fit a new type of engine exhaust system involving the use of a new bottomside to the shelf-like extension over which the exhaust passes. The modification involves the use of new heat shields, better seals, new airflow paths, and new high-temperature thermal protection at the edge of the exhaust system. Most of these changes were designed to improve the maintainability of the exhaust system, which had proven to be a persistent problem.

During the production run of the F-117A, the two metallic all-moving tail fins were replaced with ones made of graphite thermoplastic materials. This change resulted from the loss of one fin and rudder from a F-117 in 1987 during a flight test. The aircraft landed safely despite the loss of the fin. The retrofit program was interrupted by Persian Gulf deployment, so most of the F-117s deployed to Saudi Arabia had the original metallic tail fins.

The cockpit of the F-117A has been recently updated and improved in accordance with advances in electronics and display technology. The original navigation system of the F-117A was the SPN/GEAS inertial navigation system. Later, this was replaced by a ring laser gyro and a global positioning system receiver. In order to improve the pilot's situational awareness, a Honeywell color multi-function display was fitted which had the capability of integrating a Harris digital moving map. Two cathode ray tube-based multifunction displays are used to call up digital maps, target photos, or target identification diagrams. A liquid crystal display data entry panel allows the pilot to select from 256 avionics functions. The new cockpit equipment is designed to minimize the chance of pilot disorientation at nighttime, which was suspected as the primary cause of two accidents involving operational F-117s.

In the early 1990s, autothrottles were added to provide the capability for arrival at a precise predetermined time over a target. This innovation was, however, not available in time for Desert Storm.

When the F-117 first emerged into the "white" world, it carried a three-digit serial number on the tail. The numbers are assigned in sequence, beginning with 780 and ending with 844. These were probably Lockheed construction numbers rather than official USAF serial numbers. Aircraft 780 through 784 were full-scale development (FSD) aircraft, whereas 785 through 844 were production aircraft. The presentation of these numbers on the tail of the F-117A is sort of unusual, since the serial numbers of Air Force aircraft are typically presented as a combination of the last two numbers of the fiscal year in which the aircraft was ordered, followed by the last three digits of the aircraft's USAF serial number. Eventually, however, the serial number presentation evolved into the standard format used by other USAF combat aircraft. It seems that the three-digit Lockheed construction numbers were used to "build" new USAF serial numbers, and these were fit by one means or another into "gaps" in the existing system, which led to F-117 serials being spread out over several different fiscal years, sometimes not in strict numerical order. The exact serials (particularly the FY allocations) are still somewhat uncertain, but here's what I have. I would appreciate receiving any updates or corrections.

79-10780/10785	Lockheed F-117A Nighthawk
				full-scale development production aircraft
				10780 on display at Nellis AFB.
				10781 is in WPAFB museum.
				10785 might be 79-0785 or 80-0785.  Crashed Apr 20, 1982
					before acceptance by USAF on first flight at Groom
					Lake, NV, pilot Robert Ridenhauer survived.
80-0786/0791		Lockheed F-117A Nighthawk 
81-10792/10798	Lockheed F-117A Nighthawk
				Originally serialed 81-0792/0798 when they first came into the
					"white" world, but reserialed when it was found that the
					numbers conflicted with a batch of F-16s.
				0792 w/o July 11, 1986, near Bakersfield, CA.
					Pilot Ross E. Mulhare killed.  This plane had the original
					serial of 81-0792, before the serials were changed by
					adding a 1 to remove the conflict with F-16s.
				10793 w/o Sept 14, 1997, Middle River, MD. during 
					airshow.  Pilot Maj. Bryan Knight ejected safely.
82-0799/0806		Lockheed F-117A Nighthawk
				0801 of 49th FW/416th FS w/o Aug 5, 1992 Alamogordo, NM.
					Pilot Capt. John Mills ejected safely.
				0806 downed over Serbia Mar 27, 1999.  Pilot ejected
					safely and was rescued.
83-0807/0808		Lockheed F-117A Nighthawk
84-0809/0812		Lockheed F-117A Nighthawk
				Some references have these as 1809/1812.  Other
				references have these as 82-809/812.
84-1825/1828		Lockheed F-117A Nighthawk
				Serials may be 85-0825/0828.
				1825 crash landed at Holloman AFB Jun 4, 1997
					and burned out.  Aircraft may be repaired.  Has been seen Nov 2003 so must
					have been repaired.
85-0813/0824		Lockheed F-117A Nighthawk
				some references have these as 85-1813/1824 and have
					85-824 being 84-824.
				0815 w/o Oct 14, 1987, Nellis Range.  Maj. Michael Stewart killed
85-0829/0836		Lockheed F-117A Nighthawk
				some references have these as 85-1829/1836
				0835 in use (January 2004) by 53rd TEG/Det 1 at Holloman AFB, NM in 
					two-tone grey colour scheme to ascertain suitability for possible daylight 
					use. Nicknamed 'The Dragon'
86-0821/0823		Lockheed F-117A Nighthawk
				Some references have these in FY 1985.
				0822 w/o May 10, 1995,near Zuni, NM. Capt Kenneth Levens 
				killed
86-0837/0840		Lockheed F-117A Nighthawk
88-0841/0843		Lockheed F-117A Nighthawk
				0843 reportedly burned out in 1996 at Holloman AFB, NM.
				Aircraft might be repaired.  Seen at 1998 MCAS Miramar airshow.

Specification of the F-117A (estimated):

Engines: Two General Electric F404-GE-F1D2 non-afterburning turbofans, 10,800 lb.s.t. each. Performance: Maximum speed 700 mph at sea level (Mach 0.92). Normal maximum operating speed is 648 mph at 5000 feet (Mach 0.87). Combat radius 900 miles (unrefuelled) Weights: 30,000 pounds empty, 52,500 pounds maximum takeoff. Dimensions: wingspan 43 feet 4 inches, length 65 feet 11 inches, height 12 feet 5 inches, wing area 1140 square feet. Weapons:Up to 5000 pounds of ordinance can be carried in two internal weapons bays

Sources:

Sources:


  1. The F-117A Stealth Fighter, Steve Pace, Tab Aero, 1992.

  2. F-117A Stealth Fighter, Andy Sun, Concord Publications Co., 1990.

  3. F-117 Stealth in Action, Jim Goodall, Squadron/Signal Publications, 1991.

  4. Lockheed F-117A, Bill Sweetman and James Goodall, Motorbooks International, 1990.