Subject: Number 117 in the Series--Lockheed F-117A (2 of 3)

From: [email-masked]
Date: 23 Mar 94 17:58:26 GMT

>From [email-masked]

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*.  

The F-117A carries a three-digit serial number on the tail.  The 
numbers are assigned in sequence, beginning with 780 and ending with 
844.  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.  According to Sweetman and 
Goodall, the tail numbers on the F-117A are NOT standard USAF serial 
numbers, that is, 780 is not a shorthand for something like 80-0780 
They are part of a different sequence of serials that are used by the 
Defense Department for some special programs.  However, Steve Pace 
claims that the tail numbers ARE standard USAF serial numbers, and he 
includes a list of them in the appendix of his book.  They are: 

79-1780/1784 	Lockheed F-117A FSD 
80-1785/1791 	Lockheed F-117A 
81-1792/1798 	Lockheed F-117A 
82-1799/1807 	Lockheed F-117A 
83-1808 	Lockheed F-117A 
84-1809/1812 	Lockheed F-117A 
85-1813/1824 	Lockheed F-117A 
84-1825/1828 	Lockheed F-117A 
85-1829/1836 	Lockheed F-117A 
86-1837/1840 	Lockheed F-117A 
87-1841/1844 	Lockheed F-117A 

However, it is rather odd that all of these different fiscal year 
numbers are in exact sequence!  In addition, note that 1825/1828 falls
in an earlier fiscal year than 1813/1824.  I'm sure that we don't have
the whole story here!

Specification of the F-117A (estimated): 

Two General Electric F404-GE-F1D2 non-afterburning turbofans, 10,800 
lb.s.t. each.  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.  Up to 5000 pounds of ordinance can be carried in two 
internal weapons bays 

Sources:

  F-117A's Performance Boosts Wide Range of Improvements, David Hughes 
  and Jeffrey M. Lenorovitz, Aviation Week and Space Technology Vol.  
  134, No 25, p 20 

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

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

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

  Observers Aircraft, William Green and Gordon Swanborough, Frederick 
  Warne, 1992.  

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

  The Illustrated Encyclopedia of Aircraft Armament, Bill Gunston, 
  Orion, 1988 

Joe Baugher		AT&T Bell Laboratories   	
2000 North Naperville Road      Naperville, Illinois 60566-7033