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REDt;C":;:,'I0.0l' OF R.A.DAR CROSS SEC1'I0.0l'
OF URGE HIGH ALTITUDE AIRCRAFT
Clarence L. "Kelly" Johnson
Lockheed Aircraft Corporation
This report is a swnrnary of some of the work done by the Advanced
Development Projects Division (ADP) of Lockheed Aircraft Corporation
(known more widely as the Skunk Works) in reducing the radar cross
section of various aircraft designs during the period 1956 to 1974.
The work was sponsored by the U. S. Government under the
cognizance of Mr. R. M. BisselL
The ADP activities were supervised by the writer. A number of
consulting groups played important parts in the overall program - such as
Lincoln Laboratories, Scientific Engineering Company, L"lc. (SEI), Pratt
& Whitney Aircraft Corporation, Edgerton, Germeshausen and Grier, Inc.
(EG&G), Westinghouse, General Electric and a number of individuals
from universities and industry.
The general activity was instigated in the time period when the
Lockheed U-2 entered service in 1956 making numerous overflights of the
Russian land mass. The Soviet reaction to these flights led them to
accelerate their developments in radar (Tall King, Fan Song, etc.) and
missiles. leading to the SA -2 ty-pe, which finally shot down Francis Gary
Powers on May 1, 1960.
During the per:od 1956 t.~rough 1960, ADP t.:ndertook many studies
of various aircraft des1gns intended to reduce the radar cross section as
well as to improve the cruising speed, altitude, and range of reconnaissance aircraft. Shape factors lead to consideration of all forms of aircraft
including flying saucers, which fundamentally have a low radar reflection
from low viewing angles. Materials, such as rubber, plastics, inflated
~1yla r, carbon loaded honeycomb, etc. , were used to configure many
different designs. After the Powers incident, work on reducing the
vulnerability of the U -2 accelerated to meet the threat. Electronic
countermeasures - EC~i - for warning the pilot against ::adar sightings,
!or jamrrting missile guidance systerns, and aircraft gunnery radars were
given the highest poss1ole priority. (EC::V1 developments and reduction of
h altitude optical v1sibility will :lOt be reported herein.)
Page 2
The basic ::-adar :requencies usea tn evaluation of the aircraft radar
cross section were 70, 170, 2 850, and 5000 megahertz with each frequency
varied from its normal ·.ralue given by plus or minus 501o to account for
actual frequency variations which might be encountered in the field. The
lower frequencies were those of the early warning systems, and the higher
ones were actually both ground based and airborne radars used by fighter
aircraft. It was necessary to consider the wide range of frequencies so as
not to be misled by the fact that its relatively easy to reduce cross sections
of aircraft at a single specific narrow band frequency.
Summarv
By combining the effect of shapes, resistively loaded plastics and
other design features, the radar cross section of aircraft such as the
SR-71 was reduced very significantly over a wide range of radar frequencies, beam polarizations and look angles, compared to normal design
practices. With all these factors, however, it was not possible to create
an aircraft invisible to early warning or missile guidance radars, but in
combination with high speed, high altitude, maneuvering and electronic
countermea:;ures. a~very l;,ig'!:t,rll'!g~ee o~~urvivability has been obtained in
service.
The very difficult problems of reducing the return of the air inlet
on the engines, from canopies, camera windows, radomes, and antenna
installations were satisfactorily achieved after thousands of tests both at
model and full scale. Test techniques were successfully developed for
both ground and flight conditions, as were manufacturing methods for
producing ~igh temperature plastics, loaded foam, and structural honeycomo components.
Page 3
The E:fiect of Basic Shaoe
In order to evaluate the effect of overall shape on ::adar :::ross
section, tests were run over a 10 year period of many different aircraft
designs. These were run in a small radar anechoic chamber having
diinensions of 12' x 12' x 30' and later in the new large ADP anechoic
chamber shown in Figures i, 8, 9 and 10 which is much larger. Some
tests were n:.n on an outside radar range located on a dry lake, but in
general, most of the early tests were run in the small box. Elementary
shapes were evaluated, such as disks of different contours, streamline
shapes, and various bodies designed for producing low radar sections without consideration of their aerodynamic feasibility as aircraft. Fundamentally, a shape similar to flying saucers with a sharp edge and no protuberances has a very low radar crass section without any anti-radar t:-eatment
at all, particularly against vertical polarization of the radar beam when
viewed at small angles of incidence. Attempts were rnade stmultaneously
to develop power plant installations and control systems which would make
some of the low radar c::::oss ection o ects
satisfactonly.
?age
4,
A substantial amount of effort went into development of all plastic
designs, inflated Mylar and rubber aircraft and combinations thereof.
Some interesting results were obtained in these tests. It was found for
instance, that when the plastic parts, such as those that might be designed
in wing beams, or heavy structural rings exceeded 1 I 4 or l /2 inch in
thickness, these members might just as well be metal. An unexpected
result showed up in one model (which used very thin plastic for wing panels)
which proved to be totally useless in the fuel tank region. A vibrating tank
created a strong radar return from the surface of the fuel itself. It became obvious also that if a plastic fuselage which would be transparent to
radar was used, the radar beam saw the engine and its associate accessories, plumbing, et al, which then provided hundreds of corner reflections
and provided large radar returns. Figures 11 through 18 show a few of the
models tested during this early evaluation at ADP. The net result of our
tests on aircraft shapes leads the writer to conclude that the most desirable
shape for an advanced, high altitude reconnaissance aircraft should have the
following characteristics:
a.
A wing of very low thickness ratio.
b.
A cross section of the fuselage and engines blended into the
wing with the shape of the engine nacelles and fuselage
approaching a flying saucer as closely as possible!
c.
The number of tail surfaces should be minimal and the
vertical tails (if dual) tilted inward to reflect the radar returns
off into space. These surfaces should be constructed of
loaded plastic honeycomb combined with surface treatments.
d.
Both the engine air inlet and exhaust outlets would require
extensive development and some unique approaches would be
required to reduce the radar cross section from fore and aft
viewing angles.
e.
There must be no external antennas or other such protuberances, and radomes for such items as side looking radar
must be given very special attention.
The ability of the
installed radar to look out of the fuselage at the same time
not allowing a search radar to look in, is required.
f.
Steps would have to be taken to reduce the radar cross
section of a man's head for instance, which alone has a
return of something like two-tenths of a square meter at
S-band. Flying helmets and canopies can be treated to
reduce this effect fortunately.
Pa e 5
In order to evaluate full scale aircraft as well as model configurations at different radar frequencies, a radar test range was developed on
a dry lake in the desert. This range provided for a one mile separation
between model and the radar test equipment with intermediate mountings
provided for different scale models. ADP designed the large rotating
pedestal (Figures 19 and 20) which was able to raise a flyable SR- 71 over
60 feet in the air with the ability to tilt the model and rotate it in the
horizontal plane. Edgerton,Germeshausen and Grier, Incorporated installed the radars used for this testing, as well as the test equipment used,
in collaboration with Lincoln Laboratories and Government personnel. The
full scale model of the SR-71 was installed inverted to prevent ground
reflection effects and the proper viewing angles could be obtained simulating
the cruising angle of attack at the line of sight from an early warning radar
approximately 300 miles away. This was the grazing angle of incidence for
the radar beam with the Blackbird flying at its design altitude.
Dr. Frank Rodgers invented a device which we called the "railroad"
which was a traveling corner reflector mounted on the rail alongside of the
model. In essence, it was used to determine radar "hot spots" by determining the phase relationships from the corner reflector and the aircraft
item, allowing one to draw two lines of bearing which would intersect at
the source causing the high return. This was a most useful tool throughout
the whole series of full scale tests.
Pressurized Mylar as well as neoprene bags were used to support
a 1 I 8th scale model closer to the test center to study radar cross section
at frequencies around 70 to 200 megahertz.
Considerable difficulty was encountered in the early tests because
of returns from the post supporting the model and ground reflections.
These were overcome by providing a retractable shield around the post
which could go up and down with the model. The ground returns had to bci
controlled by placing carbon loaded hair pads on the ground under the model.
Page
1
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Design A. R. Goals at Various Time Periods
for ADP Frog rams
7
SR-i 1 Model in ADP Radar Anechoic Chamber
8
SR-71 Model in ADP Radar Anechoic Chamber
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Full Scale SA-2 Missile Model in ADP Radar
Anec:C.oic Chamber
10
U -2 Model in ADP Anechoic Chamber
11
Flying Saucer Model with all Plastic Wings Added
12
Item 11 with Wing Anti-Radar Treating Being
Added
13
Large Scale Flying Wing - A. R. Treated
14
Various Aircraft Designs Tested
15
Various Aircraft Designs Tested
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Various Aircraft Des
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ns Tested
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Fil.;!ure List
(Contd)
Figure No.
Title
18
Loaded Plastic Absorber Developments
19
Mechanism for Large Pole Support to Hold
Full Scale Aircraft 60 Feet Above Ground
20
Full Scale Model on Post - l/8 Scale Model
on >l'eoprene Bag Support
21
22
SR- 71 in Flight Testing for Anti- Radar
Evaluation
GOALS
1959
70 MC
·s. BAND
TOOLS-DEVELOPED TO DATE
INSTRUMENTATION
6m 2
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TECHNIQUES
1960
170
• MODEL RANGES
• FLIGHT TEST RANGE
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REQUIREMENTS
1965
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•SHAPE SELECTION
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• FUEL ADDITIVES
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Del>ign A. R. Goals at Various Time Periods for ADP Program8
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