carlo@gaia.gcs.oz.au (Carlo Kopp) (02/11/91)
From: Carlo Kopp <carlo@gaia.gcs.oz.au>
(C) AEROSPACE PUBLICATIONS PTY LTD 1990
P.O. Box 3105, WESTON CREEK, ACT 2611, AUSTRALIA
Ph:+616-288-1677 Fax:+616-288-2021
As published in December 1990 issue of the AUSTRALIAN AVIA-
TION journal
Lockheed F-117A Stealth Fighter
The angular F-117A is without doubt the oddest yet product
of Kelly Johnson's Skunk Works. It is however a milestone in
tactical air weapon systems design, the full implications of
which are still to be demonstrated.
Stealth technology is generally the least understood and
least appreciated of the newer generation of technologies
which are now proliferating. As a result many lay observers
have indiscriminately criticised designs such as the F-117
and larger B-2. Under closer analysis however most criti-
cisms demonstrate their irrelevence, and as such in turn
demonstrate the questionable credibility of the critics
themselves. The F-117A and its newer cousins, the B-2A and
A-12A, are strictly functional in design and appearance and
all reflect the modern theory of defence penetration.
The modern air battle is as much an electronic and optical
battle as it is a confrontation between armed aircraft and
defences. Virtually every aspect of any individual engage-
ment and any major battle is locked into electronic and opt-
ical systems, the radar and optical surveillance, fire con-
trol and guidance systems determine largely the capability
of any offensive or defensive system.
This has understandably led to a situation where a substan-
tial effort is expended on countermeasures and counter-
countermeasures. Jammers degrade every sensor and in turn
every sensor will be adapted to resist various types of jam-
mer. Some will be more effective, some less in their respec-
tive functions. Significantly though, all jamming involves
emission of energy to defeat a hostile sensor, be it passive
or active. Defeating a particular sensor but revealing your
position to another in the process is of questionable use-
fulness, the proliferation of sensitive Electronic Support
Measures and InfraRed Search and Track Systems is slowly but
surely increasing the detectability of conventional jammer
equipped penetrators.
The philosophy of defeating sensors by jamming is itself
open to question, as the power of single chip computers and
signal processors increases. Many designs are even today so
resilient that building effective jammers is difficult with
detailed knowledge of the victim system, where that
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knowledge is absent the task is all the more difficult. Jam-
ming is always most effective where it is specific to a
weakness in a victim system, where the victim is reprogramm-
able by reloading its software the useful life of any coun-
termeasure is very limited.
Stealth technology offers another degree of freedom to the
weapon system designer. Stealthy airframe and system design
vastly reduces the signatures of an aircraft and this in
turn reduces detectability while enhancing the effectiveness
of those jammers which are usable. Stealth aircraft ideally
operate under those conditions where they are difficult to
detect and thus do not require the use of jammers, which are
a last ditch fallback if cornered.
Stealth techniques today concentrate upon the reduction of
the radar cross section and infrared emissions of an air-
frame, as these parameters are critical to the performance
of radar and infrared fire control and guidance systems. As
is clearly evident from the design of the F-117A and B-2A,
the emphasis in both designs is upon defeating microwave ra-
dar and shorter wavelength infrared systems, both associated
with terminal threats rather than long range surveillance
systems. The current generation of stealth aircraft is
designed to defeat missile and fighter fire control systems
and missile guidance systems above all, this is achieved by
reducing the effective range of the sensors used to the
point, where they are of questionable tactical usefulness.
How this is achieved becomes more evident upon closer exami-
nation of a specific design.
Lockheed F-117A Stealth Fighter
The F-117A traces its origins to the early 1970s, when DARPA
took a serious interest in the idea of a tactical aircraft
designed for minimal radar cross section (RCS; see AA May
1987). By the mid seventies, five prime contractors were
asked to study configurations for a low RCS fighter,
Lockheed were apparently excluded due to a lack of recent
fighter design experience. Seeing the long term potential of
the technology Lockheed expended considerable engineering
effort and subsequently approached DARPA to enter the pro-
gram. DARPA agreed and Lockheed and Northrop were later
shortlisted for the Have Blue flying stealth technology
demonstrator aircraft program. Lockheed won the $30 million
contract for two prototypes. Tests on these two airframes
proved that the concept was technically viable and the USAF
committed to the design and production of a scaled up F-117
in late 1978.
Lockheed's Skunk Works in Burbank, California, were respon-
sible for the aircraft design and lived up to their reputa-
tion for fast design cycles at the technological cutting
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edge. A major factor was the program structure, as TAC
sought earliest possible IOC (Initial Operational Capabili-
ty). The USAF Aeronautical Systems Division at Wright
Patterson AFB carried out much of the program management
which involved the potentially risky concurrent
development/production technique.
After 31 months of effort the first aircraft flew on the
18th June, 1981, piloted by Lockheed's Hal Farley. It was a
very conservative system design using a wide range of then
current components and subsystems used in other programs,
this was necessary to confine risks to those areas which
were new - the stealth features and their aerodynamic side
effects. An added benefit was the lesser chance of
compromising the then top secret 'black' program by attract-
ing attention to development and production contracts for
unique components and subsystems.
IOC was achieved in October, 1983, by the 4450th Tactical
Air Group based at the secluded Tonopah test range airfield
in the Nevada desert. The presence of the aircraft at Tono-
pah was well known by the mid eighties, however TAC had res-
tricted all operations to night time time only and official-
ly refused to acknowledge the existence of the aircraft. Ru-
mours and conjecture concerning the configuration of the
aircraft appeared in almost all trade journals, no doubt en-
couraged by the USAF who had no interest in revealing the
true configuration of the aircraft to the public, and hence
also to the PVO and VVS. Some readers may recall the brief
program review in AA May 1987, in the light of real hard
data the aircraft is actually bigger and more capable than
estimated by AA at the time, while using an aerodynamically
less effective design strategy than assumed at the time.
The USAF contracted for 59 aircraft, the last of which was
delivered on the 12th July, this year. The total program
cost is US$6.56B of which about US$2B covered development
and the remainder production and some facilities costs. The
unit flyaway cost is US$42.6M, with a program unit cost of
US$111.2M. The difference in these figures illustrates the
magnitude of the technology development effort. It is worth
noting a Lockheed comment which stated that production rates
greater than or equal to 8 units per year maximum during the
program would bring the unit flyaway cost down to about
US$30M.
The USAF, under political pressure over the B-2 program
cost, and seeking to integrate the aircraft into regular TAC
operations, decided late last year to reveal the aircraft to
the public and commence daylight operations. As a result,
the 4450th TAG has been redesignated the 37th Tactical
Fighter Wing comprising the 415th (Nightstalker), 416th
(Ghostrider) Tactical Fighter Squadrons and 417th (Bandit)
Tactical Fighter Training Squadron, these units being his-
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torically associated with night fighter squadrons. The 37th
TFW will relocate to a permanent base at Holloman AFB in New
Mexico in early 1992. Logistical support is provided by the
Sacramento Air Logistics Center at McClellan AFB and the
aircraft are undergoing a planned weapon system improvement
program, carried out at the USAF Plant 42 at Palmdale in
California.
Airframe and Propulsion
The unique geometry of the F-117A reflects the state of the
art in RCS modelling techniques in the late 1970s, in stark
contrast to the more refined B-2A geometry. The faceting
technique derives from the use of the method of geometrical
optics (see [1] page 114) which essentially says that an
impinging ray (beam) is reflected at an angle equal to the
incident angle relative to the normal to the reflecting sur-
face (ie shine a torch beam at a mirror and see the effect).
For this to be true though the wavelength must be much
smaller than the dimensions of the reflecting flat surface
and hence it is clear that the F-117A is designed to defeat
high band microwave radars.
By breaking the area of the airframe into flat facets, the
designers sought to reflect impinging radar beams away from
the radar. This is also the reason why the external geometry
has no curved edges. Straight edges reflect principally in
directions given by the above rule, therefore by arranging
all areas to be flat and all edges to be straight, the
designers could ensure that most impinging microwave energy
is reflected away from the aircraft at angles which are
determined by the instantaneous orientation of the airframe
relative to the searching radar. As the frontal RCS is of
greatest importance tactically, the edges and surfaces of
the airframe about the frontal aspect are all arranged at
shallow angles with respect to an impinging wave.
The result is not only a weak radar return but also a con-
tinuously scintillating one, scintillation will cause prob-
lems in many target tracking systems. In this fashion by
clever shaping the RCS of the airframe was dramatically de-
creased. This alone was however inadequate as other detail
contributors to the aircraft's RCS would have dominated the
return. Hence the cockpit canopy windows were coated with an
electrically conductive layer and the inlets were covered by
a fine mesh grill, with holes smaller than the wavelength of
the victim radars. Potentially good reflectors such as the
engine fan faces and cockpit interior are thus hidden away.
Electrical discontinuities associated with panel edges and
control surfaces at angles close to normal to frontal aspect
beams could also make a measurable contribution to frontal
RCS, therefore the canopy edges, weapon bay, undercarriage
doors and Flir bay have serrated edges. The angles used in
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the detail features are again shallow with respect to fron-
tal aspect beams.
Shaping has thus been the principal RCS reduction technique
used in the F-117A design. In addition, radar absorbent ma-
terials were used for some panels and radar absorbent coat-
ing over the area of the aircraft. The RCS of the aircraft
has been estimated in the range of 0.001-0.01 square metres,
which is incidently between 1% to 10 % of the RCS of a typi-
cal chicken [1].
The aerodynamic penalties incurred by airframe shaping to
minimise RCS have been considerable. Sharp edges and flat
surfaces create vortices and thus severely disturb laminar
flow causing parasitic drag. The large sweepback angle and
low aspect ratio results in a shallower lift-curve slope
which forces a higher nose attitude in landing configura-
tion, this is confirmed by the high position of the canopy
which in turn incurs an additional drag penalty. Another
consequence of this effect is limited lift on takeoff re-
quiring taller undercarriage to facililate the required AoA
on rotation. Highly swept wings are also poor performers at
low speed, producing considerable lift induced drag, the F-
117A will almost certainly have a narrow range of optimal
high subsonic operating speeds where the parasitic and lift
induced drag terms appropriately balance.
Natural stability and handling characteristics have also
been severely compromised by the configuration of the air-
craft. The swept wing will cause poor Dutch roll performance
and limited control effectiveness, while the beavertail rear
fuselage will contribute little to yawing stability. As a
result, fly-by-wire control and artificial stability was an
absolute must for the design, the system used was derived
from the Lear Astronics design fitted to the F-16, quadruply
redundant digital with an interface to the navigation sys-
tem. The F-117A uses fully movable V-tail surfaces and split
full span trailing edge flaps. It is unclear as to how the
surfaces are used, a reasonable guess suggests the tail is
used differentially with the wing surfaces used either as
dedicated flaps or flaperons. The presence of fly by wire
control opens endless possibilities insofar as how the con-
trol surfaces are used.
The structural design of the airframe is unclear from the
available material. It is very likely that the internal
weapon bay structure forms a rigid box with structural
members running fore and aft to support the tail and nose.
The wing spars and engines would attach to this central
structure, with radar absorbent external shells or panels
fastened on to the structure. The absence of precedents
makes any analysis in this area very much guesswork.
The aircraft is powered by a pair of non-afterburning F404-
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GE-F1D1 low bypass ratio turbofans which were developed from
the F/A-18 powerplant in early 1980. These powerplants
deliver about 11,000 lb of thrust in military power, with a
pressure ratio of 25 and SFC around 0.8. How well they per-
form throttled behind a inlet mesh and constipated by an in-
frared suppressing exhaust is not clear. The inlet mesh may
reduce sensitivity to inlet AoA and sideslip angle, but it
is not easy to estimate how well the configuration performs
due to the interaction with the wing leading edge which at
that point probably functions more like a strake/chine,
shedding vortices. Vortex flow has been used in the past to
improve the performance of dorsal inlets [2].
The exhaust geometry is typical for a stealth design (cf AA
May 87) in that it uses a narrow horizontal slit to confine
the infrared radiation pattern of the exhaust into a very
narrow range of angles in elevation (ie a beavertail lobe
shape). Because of the swept trailing edge of the exhaust,
it was necessary to fit vanes to limit radiation to the
sides, which also conveniently reduce the azimuth over which
the exhaust can be sighted. The cumulative effect of this
geometry is to confine the volume of space where the exhaust
is directly visible to dead astern and slightly above the
aircraft. Even a gentle turn by the F-117A will immediately
hide the exhaust from a previously well positioned observer.
In practice it means that a short wavelength infrared search
and track set cannot lock on to or successfully track the
scintillating emissions from the F-117, even from astern.
Systems and Performance
The F-117A designers exploited as much off the shelf tech-
nology as practical to reduce design risks and keep costs
and design turnaround times down. The aircraft uses an Al-
lied Signal environmental control system adapted from the
C-130, Lear flight controls from the F-16, Loral brakes from
the F-15, a TAC standard ACES-2 ejection seat common to the
F-15, F-16 and A-10, and nav/comm equipment used in other
TAC aircraft. US sources indicate that 95% of the ground
support equipment used for the aircraft is common to other
types, thus facilitating deployments and cutting life
cycle/logistical costs.
The F-117A employs a navigation Flir system mounted in a re-
cess below the windshield, a radar reflective cover is used.
It is unclear whether additional optical equipment such as
low light level TV (LLLTV) is also fitted, the presence of a
single visible turret suggests either a single Flir slaved
to the pilot's line of sight and projecting imagery on a
helmet mounted sight or a look-into-turn Flir projecting on
the HUD. A fixed forward HUD projection Flir would not re-
quire the kind of bay used.
Target acquisition and weapon delivery is carried out with a
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ventral Flir/laser turret, to the right of the nose wheel
bay. Due to absence of radar for ranging to bomb release,
the laser will provide both rangefinding and designation for
weapons, its ventral position providing a similar field of
view to established Flir/laser targeting systems such as
Pave Tack.
The navigation/attack system is fully digital and built
around IBM mission computers and MIL-STD-1553B busses, in-
tegrated with Honeywell inertial nav equipment. The cockpit
employs a Kaiser HUD derived from the F-18's AVQ-28,
Honeywell colour MultiFunction Displays coupled to a Harris
digital tactical (moving map) display system. The USAF has
revealed the existence of a ground based automated mission
planning system which would if typical use a groundbased
computer, graphics consoles and the aircraft's Delco 1553
bussed cartridge tape system for uploading mission particu-
lars into the nav/attack system. Flight instrumentation in-
cludes a Honeywell radar altimeter and air data computer.
The USAF have stated that the aircraft is typically armed
with a pair of 2,000 lb laser guided bombs, and that the
_full_ _range_ of tactical fighter ordnance may be carried
[Auth: as per USAF press releases]. A weapon bay size of
15.5 ft x 5.75 ft provides comfortable stowage for a pair of
Mk.84 retarded or laser guided 2000 lb bombs with folded
wings (cf. AA May 87), a pair of AGM-65 Maverick missiles, a
pair of AGM-88 HARM antiradiation missiles or a pair of
CBUs. These would all be typical weapons for the precision
strike and defence suppression role performed by the air-
craft. The USAF have also stated that the aircraft has a
self-defensive capability, although no specifics were
released. It is likely that Sidewinders are carried on a re-
tractable weapon bay launch rail.
Nothing has been stated by the USAF or Lockheed on the
aircraft's defensive avionic suite. It can be safely assumed
that a capable RHAW/ESM is carried with enough sensitivity
to allow the pilot to avoid or attack air defence radars as
required. The aircraft will almost certainly carry a track-
breaker ECM system to defeat close-in terminal threats,
although this equipment would be used only if absolutely
necessary. Concealment would be preferred to deafening the
opponent's radars.
The performance of the aircraft is a well kept secret. With
a length of 65.9 ft and span of 43.3 ft weighing in at
52,500 lb MTOW the F-117A is about the size and weight of an
F-4 Phantom. Assuming an empty weight of about 33,000 lb, a
weapon load of 5,000 lb, this yields a fuel capacity of
about 15,000 lb and hence a combat weight of about 45,000
lb. With 22-24,000 lb of installed thrust this yields a com-
bat thrust/weight ratio of about 0.5 which is at the least
not spectacular, but again compares closely to the F-4 on
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dry thrust. Maximum speed is apparently mildly supersonic
although in practical situations the aircraft would operate
at high subsonic speeds. Radius performance is also unknown
given the unusual aerodynamics, but the aircraft is fitted
with a refuelling receptacle on its upper fuselage.
The performance of the F-117A is clearly typical for a
sixties/seventies tactical strike aircraft, it is in hind-
sight curious that the USAF had chosen much like with the
F-111 an F-series designation rather than a more appropriate
A-series number.
Mission Profile
The F-117A is tasked with precision strike against high
value targets in dense threat environments. In practical
terms this translates into attacks upon C3
(command/communications/control) facilities such as command
posts, communications relay centres, critical fuel or muni-
tion dumps, airfields and parked aircraft. Other items in
this category include air defence radars and their associat-
ed command posts.
The USAF have been less open with specifics on how such mis-
sions would be flown. Established doctrine, the known RCS
performance of the aircraft and observations of its flying
activity in Nevada suggest that the style of missions will
differ little from those flown by the F-111, ie hostile air-
space is penetrated at low altitude at night, heavy defences
are avoided where possible and the target is attacked from
low level with laser guided bombs.
The USAF have openly conceded that the B-2 is detectable by
high power low band VHF surveillance radars, therefore it
follows that the less sophisticated F-117A will also be
detectable by such systems. HF radar such as Jindalee or VHF
radar such as many geriatric Soviet systems uses wavelengths
comparable in size to the aircraft itself, hence the
scattering mechanism which occurs (Rayleigh) is different
and a solid return is seen. VHF radars are however generally
considered to be inaccurate and very poor performers against
low altitude targets of any kind, therefore the sanctuary of
low altitude is clearly available to the stealth aircraft.
In practical terms the ability of a low band radar to detect
an inbound stealth aircraft may be of little real value, as
the radar cannot be accurate enough to target anything but a
nuclear armed SAM. SAM, AAM and fighter radars all operate
in the upper G-J bands where they are effectively defeated
by the stealth aircraft's unique capabilities. A stealth
aircraft penetrating at low level can defeat VHF radar by
terrain masking and all other radar with its airframe
design. The use of the RHAW to detect threats at several
times the detection range by the threat makes avoidance of
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radars a fairly straightforward exercise.
Infrared search and track equipment is most effective at
high altitudes under VFR conditions. Under typical night IFR
conditions IR is relatively ineffective and is therefore
never used by area defence weapon systems.
In a typical scenario the F-117A would avoid detection for
as long as possible and only where the target was physically
close to a radar would it expose itself during its run in to
weapon release. At that point it would have the option of
jamming which is most effective in comparison to the conven-
tional situation. Once the target is hit the F-117A would
egress the target area at low level again using its RHAW to
keep out of the detection range of threat radars. Fighters
with even capable lookdown-shootdown radars have very little
chance of detecting the F-117A in ground clutter, given its
inherently weak returns.
The USAF's intention to integrate the F-117A into regular
TAC operations suggests that it will spend much of its time
flying defence suppression sorties to open corridors through
hostile SAM belts, thus allowing conventional strike air-
craft to penetrate. Used in concert with aircraft such as
the EF-111A tacjammer and the F-4G Weasel the F-117A is a
potent asset. The ability to attack the C3 nodes of an air
defence system just prior to a jammer supported strike para-
lyses the air defence system and thus most effectively di-
lutes concentration of fire [Auth: which we have just seen
happen over Iraq...].
In perspective the F-117A is with all of its limitations a
capable weapon system and a major milestone in tactical air-
craft design. It reflects the current Western philosophy of
defence penetration and illustrates clearly the basic prin-
ciples behind stealthly designs. Many of the lessons learned
in its design and development are evident in the newer B-2A
and will no doubt be seen in the soon to be revealed A-12A
Avenger. The question does remain, though, will tactical air
ever be the same again ?
--------------------------------------------
REFERENCES:
[1] Knott E.F. et al ,'Radar Cross Section', Artech
House, 1985
[2] Whitford R.,'Design for Air Combat', Janes, 1987
Picture Caption 1 (inflight refuelling with KC-10)
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The F-117A is a large tactical strike aircraft, comparable
in size and gross weight to an F-4 Phantom. It has an inter-
nal weapon bay large enough to accommodate at least two
2,000 lb bombs and with inflight refuelling has an appreci-
able tactical radius, as demonstrated by the Nevada to Pana-
ma non-stop raid flown during the recent US invasion of that
country.
Picture Caption 2 (frontal view)
The principal role of the F-117A is precision strike against
heavily defended targets. This is achieved by low altitude
penetration at high subsonic speeds, with navigation assist-
ed by a forward facing Flir turret hidden beneath a radar
reflective cover. Target acquisition, ranging for weapon
delivery and tracking/designation during attack is provided
by a ventral Flir/laser turret, visible to the left of the
nosewheel. This device has a comparable field of view and
function to the Pave Tack carried by the F-111. Note the
serrated panel edges about the nav Flir.
Picture Caption 3 (exhausts visible)
The exhaust geometry of the F-117A is designed to conceal
the turbine/tailpipe of the engine from all but a narrow
range of angles. The slit aperture flattens the exhaust in-
frared radiation pattern into a beavertail lobe shape, the
vanes in the exhaust narrow the shape of this lobe. A short
wavelength infrared device can detect the F-117A only from
dead astern and slightly above, and would have great diffi-
culty in maintaining a track.
Picture Caption 4
The unusual faceted geometry of the F-117A airframe is
designed to scatter impinging microwave energy in any direc-
tion but that from where the energy arrived from. In addi-
tion, panel boundaries and surface details which could re-
flect toward the front of the aircraft are serrated to
scatter energy to the sides of the aircraft. The engine in-
lets are covered with reflective mesh to hide the
fan/compressor face and the cockpit canopy is plated with a
conductive layer to hide the cockpit interior. The result is
a radar cross section quoted at 1-10% the size of a medium
sized bird.
February 9, 1991