Download Air Force Radar History PDF

TitleAir Force Radar History
TagsFighter Aircraft Anti Aircraft Warfare Bomber United States Air Force Aerial Warfare
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Total Pages60
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A MITCHELL INSTITUTE STUDY

The Radar Game
Understanding Stealth and Aircraft Survivability

By Rebecca Grant
September 2010

A mitchell inStitute Study

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On September 12, 1918 at St. Mihiel in France, Col. Wil-
liam Mitchell became the first person ever to command
a major force of allied aircraft in a combined-arms opera-
tion. This battle was the debut of the US Army fighting
under a single American commander on European soil.
Under Mitchell’s control, more than 1,100 allied aircraft
worked in unison with ground forces in a broad offen-
sive—one encompassing not only the advance of ground
troops but also direct air attacks on enemy strategic tar-
gets, aircraft, communications, logistics, and forces beyond the front lines.

Mitchell was promoted to Brigadier General by order of Gen. John J. Pershing,
commander of the American Expeditionary Force, in recognition of his com-
mand accomplishments during the St. Mihiel offensive and the subsequent
Meuse-Argonne offensive.

After World War I, General Mitchell served in Washington and then became
Commander, First Provisional Air Brigade, in 1921. That summer, he led joint
Army and Navy demonstration attacks as bombs delivered from aircraft sank
several captured German vessels, including the SS Ostfriesland.

His determination to speak the truth about airpower and its importance to
America led to a court-martial trial in 1925. Mitchell was convicted, and re-
signed from the service in February 1926.

Mitchell, through personal example and through his writing, inspired and en-
couraged a cadre of younger airmen. These included future General of the Air
Force Henry H. Arnold, who led the two million-man Army Air Forces in World
War II; Gen. Ira Eaker, who commanded the first bomber forces in Europe in
1942; and Gen. Carl Spaatz, who became the first Chief of Staff of the United
States Air Force upon its charter of independence in 1947.

Mitchell died in 1936. One of the pallbearers at his funeral in Wisconsin was
George Catlett Marshall, who was the chief ground-force planner for the St.
Mihiel offensive.

ABOUT THE MITCHELL INSTITUTE FOR AIRPOWER STUDIES: The Mitchell Institute for
Airpower Studies, founded by the Air Force Association, seeks to honor the
leadership of Brig. Gen. William Mitchell through timely and high-quality re-
search and writing on airpower and its role in the security of this nation.

Brig. Gen. Billy Mitchell

Published by Mitchell Institute Press
© 2010 Air Force Association
Design by Darcy Harris

ABOUT THE AUTHOR: Dr. Rebecca Grant is an airpower analyst with 20 years of
experience in Washington, D.C. She is President of IRIS Independent Research
and serves as director, Mitchell Institute, for the Air Force Association. She has
written extensively on airpower and among her most recent publications are
several Mitchell reports, including The Vanishing Arsenal of Airpower (2009),
The Tanker Imperative (2009), and Airpower in Afghanistan (2009).

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THE RADAR GAME: Understanding Stealth and Aircraft Survivability

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expected, or perhaps not at all, thereby giving the aircraft an
added measure of surprise.
Across the Channel, an unusual German aircraft de-
sign had also grasped at radar-defeating features. In 1943,
the shortage of metal led Walter and Reimer Horten to de-
sign an aircraft with improved performance, plus shaping
and coatings that might have reduced its radar return. The
Horten twin-engine flying wing bomber/reconnaissance air-
craft used plywood and charcoal materials that efficiently
absorbed the long centimetric wavelengths of the period.
The Hortens’ initial interest in the flying wing design stemmed
from their desire to eliminate sources of parasitic drag. There
were no vertical surfaces on the plane, and the cockpit and
BMW 003 turbojet power plants were housed entirely within
the wing itself.52 The center section of the wing housed the
engines and cockpit and was made of conventional welded
steel-tube construction. The rest of the plane was covered
with plywood sandwiched around a center of charcoal ex-
cept for the engine exhausts, which were coated with met-
al.53

In early flight tests, the Horten, also designated the Go 229,
attained a level speed of 497 mph but the prototype crashed
during landing and was destroyed. Aircraft maker Gotha still
had production prototypes in work when US troops captured
the Friedrichsroda plant in late April 1945. At that time, one Go
229 prototype was being prepared for flight testing, and sever-
al others were in various stages of production. Had the aircraft
gone into service, its estimated maximum speed would have
been a formidable 607 mph and its maximum ceiling about
52,500 feet, with a range of 1,181 miles.54

Reducing radar return was not forgotten after the war. In
March 1953, when the Air Force drew up specifications for a
new reconnaissance aircraft, Paragraph 2(g) of the specifi-
cations stipulated that “consideration will be given in the de-
sign of the vehicle to minimizing the detectability to enemy
radar.”55 Within a decade, designers would take the first steps
toward a low observable aircraft.

Steps Toward Low Observables
By the 1960s, speed and high-altitude performance
were not enough to evade the newest generation of guided
missiles. Low observable (LO) technology grew out of the
need to minimize the amount of radar reflected back from
the aircraft. In theory, it was widely understood that special
coatings, materials, and shapes could make objects less easy
to detect. Both the U-2 and especially, the SR-71, explored
these concepts even while putting the primary emphasis on
altitude and speed for survivability.
The SR-71 was the first aircraft to incorporate low ob-
servables as a design feature. Lockheed engineers kept the
tails of the SR-71 as small as possible and constructed them
with as much radar-absorbing material (RAM) as possible.
Designers of the aircraft also modified the rounded fuselage

by adding a chine—a lateral sloped surface that gave the
fuselage the appearance of a cobra—that left the belly of
the aircraft essentially flat. This reduced radar cross section
by as much as 90 percent.
In addition, Lockheed made extensive use of RAM in all
the sharp horizontal edges of the aircraft that might be hit
by radar waves, including chines, wing leading edges, and
elevons. The RAM consisted of a plastic honeycomb mate-
rial that made up 20 percent of the aircraft’s structural wing
area. Although the SR-71 was 108 feet long and weighed
140,000 pounds, it had the RCS of a Piper Cub. In fact, the
SR-71 had lower radar cross section than the B-1 bomber
built two decades later.56

As tantalizing as their promise appeared, low observ-
ables depended on capturing so many variables in the
aircraft’s signature that it took a revolution in computing
technologies to make the engineering tasks feasible. Bal-
anced signature reduction ultimately came to include not
only radar return, but infrared, visual, acoustic, and laser cross
section reduction, and reducing emissions from the aircraft
radar. However, the first challenge was to understand and
quantify the behavior of radar waves as they encountered
the many different shapes and surfaces on an aircraft.

Calculating Radar Return
More than 30 years passed between the British hypoth-
eses about blending aircraft into background radiation,
and the events that made shaping an aircraft to lower its
observability to radar a reality. A major breakthrough came
when Lockheed Skunk Works engineer Denys Overholser saw
something in the work of a Russian radar scientist. The Russian,
Pyotr Ufimtsev, had rediscovered that the equations of Scot-
tish physicist James Clerk Maxwell could be used to predict
how a certain geometric shape would reflect electromag-
netic waves. In a paper first published in the mid-1960s, Ufimt-
sev applied this principle to calculating the sum of the radar
cross sections of different geometric shapes.
Calculations of radar return depend on laws governing
the properties of electromagnetic radiation. Electromagnet-
ic waves behave the same whether their wavelength puts
them in the radar or optical light regions. Maxwell’s equa-
tions, formulated in the late 19th century, established bound-
ary conditions for the behavior of electromagnetic waves.
Ufimtsev postulated that Maxwell’s equations would make it
possible to calculate the behavior of radar waves retransmit-
ted from a reflective object. The radar return would depend
in part on the shape of the object. By treating an aircraft as a
group of geometric shapes, each with its own radar-reflect-
ing properties, it would be possible to calculate the RCS of
the aircraft as a whole. Then, in theory, an air vehicle could
be designed with geometry that minimized the radar return.
Building on the direction suggested by Ufimtsev, RCS
engineering used principles from physical optics to estimate

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A MITCHELL INSTITUTE STUDY

the type of scattered field that would be created
when radar waves encountered an aircraft. The radar
range equation and the equation for calculating RCS
are both based on physical optics methods.
The sum of the major reflective components of
the aircraft’s shape is defined as radar cross section.
RCS is the area (width and length) of the scattered
wave field being returned toward the radar. Generally,
the size of the radar cross section of a conventional
aircraft is much larger than the physical size of the air-
craft. The RCS of an aircraft determines the amount of
the sending radar’s power that is reflected back for
the sender to receive. An RCS is typically measured in
square meters or in decibels per square meter, often
abbreviated as dBSM.
Radar waves reflect and scatter in many different
ways. Each feature of an aircraft, carefully designed
for its power, strength, and aerodynamic qualities, can
have quite a different meaning when considered from
the standpoint of radar cross section, as shown in the
middle chart.
To design low observable aircraft, engineers had
to reliably predict and control the multiple forms of ra-
dar return that add up to the RCS.
Principles of physical optics define several differ-
ent types of radar wave reflection and scatter that
form the aircraft’s radar return. For example, specular
reflection occurs when waves are reflected back at a
known angle. Diffraction takes place when waves en-
counter an edge—like a wingtip—and are diffracted
in all directions around a cone. Traveling waves can
be created that are not reflected or diffracted imme-
diately, but travel on a long, thin body nose-on to the
incoming wave. Similarly, creeping waves are like trav-
eling waves that propagate around the shadow area
or back of the target. Waves generated in the object
by the emitted radar waves are kept in check by the
object’s electromagnetic currents. When waves hit a
crack, slope, or different material, they scatter.
Waves do not simply bounce off and return from
surfaces. Waves may bounce around in cavities and
ducts and generate additional return. Similarly, radar
waves can scatter inside a cavity such as a cockpit or
engine inlet.
The principal contributors to radar cross section
each demand distinct analysis of their behavior. Exam-
ining each of these mechanisms in turn demonstrates
the many different variables that designers must con-
trol to achieve a reduced RCS.

Specular Reflection
Specular reflection is the major source of radar re-
turn, and minimizing specular reflection is the first task

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A MITCHELL INSTITUTE STUDY

The Air Force Association, founded in 1946, exists to promote Air Force air-
power.

We educate the public about the critical role of aerospace power in the
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