The Norden Mk. XV, known as the Norden M series in Army service, was a
bombsight used by the
United States Army Air Forces
United States Army Air Forces (USAAF) and the
United States Navy
United States Navy during World War II, and the United States Air
Force in the Korean and the Vietnam Wars. It was the canonical
tachometric design, a system that allowed it to directly measure the
aircraft's ground speed and direction, which older bombsights could
only measure inaccurately with lengthy in-flight procedures. The
Norden further improved on older designs by using an analog computer
that constantly calculated the bomb's impact point based on current
flight conditions, and an autopilot that let it react quickly and
accurately to changes in the wind or other effects.
Together, these features seemed to promise unprecedented accuracy in
day bombing from high altitudes; in peacetime testing the Norden
demonstrated a circular error probable (CEP)[a] of 23 metres
(75 ft), an astonishing performance for the era. This accuracy
allowed direct attacks on ships, factories, and other point targets.
Both the Navy and the AAF saw this as a means to achieve war aims
through high-altitude bombing; for instance, destroying an invasion
fleet by air long before it could reach US shores. To achieve these
aims, the Norden was granted the utmost secrecy well into the war, and
was part of a then-unprecedented production effort on the same scale
as the Manhattan Project. Carl L. Norden, Inc. ranked 46th among
United States corporations in the value of
World War II
World War II military
In practice it was not possible to achieve the expected accuracy in
combat conditions, with the average CEP in 1943 of 370 metres
(1,200 ft) being similar to Allied and German results. Both the
Navy and Air Forces had to give up on the idea of pinpoint attacks
during the war. The Navy turned to dive bombing and skip bombing to
attack ships, while the Air Forces developed the lead bomber concept
to improve accuracy, while adopting area bombing techniques by ever
larger groups of aircraft. Nevertheless, the Norden's reputation as a
pin-point device lived on, due in no small part to Norden's own
advertising of the device after secrecy was reduced late in the war.
The Norden saw some use in the post-
World War II
World War II era, especially
during the Korean War. Post-war use was greatly reduced due to the
introduction of radar-based systems, but the need for accurate daytime
attacks kept it in service for some time. The last combat use of the
Norden was in the US Navy's
VO-67 squadron, which used them to drop
sensors onto the
Ho Chi Minh Trail
Ho Chi Minh Trail as late as 1967. The Norden remains
one of the best-known bombsights of all time.
1 History and development
1.1 Early work
1.2 First bombsight design
1.3 Initial Army interest
1.4 Fully automatic bombsight
1.6 British interest, Tizard mission
1.7 Production, production problems, and Army standardization
2 Description and operation
2.2 Basic operation
2.3 System description
3 Combat use
3.1 Early tests
3.2 Air war in Europe
4 Wartime security
4.2 Postwar analysis
4.3 Postwar use
5 See also
6 Explanatory notes
7.3 Further reading
8 External links
History and development
The Norden sight was designed by Carl Norden, a Dutch engineer
Switzerland who emigrated to the U.S. in 1904. In 1911,
Sperry Gyroscope to work on ship gyrostabilizers,[b]
and then moved to work directly for the
US Navy as a consultant. At
the Navy, Norden worked on a catapult system for a proposed flying
bomb that was never fully developed, but this work introduced various
Navy personnel to Norden's expertise with gyro stabilization.
World War I
World War I bomb sight designs had improved rapidly, with the ultimate
development being the Course Setting Bomb Sight, or CSBS. This was
essentially a large mechanical calculator that directly represented
the wind triangle using three long pieces of metal in a triangular
arrangement. The hypotenuse of the triangle was the line the aircraft
needed to fly along in order to arrive over the target in the presence
of wind, which, before the CSBS, was an intractable problem. Almost
all air forces adopted some variation of the CSBS as their standard
inter-war bomb sight, including the
US Navy and US Army, who used a
version designed by Georges Estoppey, the D-series.
It was already realized that one major source of error in bombing was
leveling the aircraft enough so the bombsight pointed straight down.
Even small errors in leveling could produce dramatic errors in
bombing, so the Navy began a series of developments to add a
gyroscopic stabilizer to various bomb sight designs. This led to
orders for such designs from Estoppey, Inglis (working with Sperry)
and Seversky. Norden was asked to provide an external stabilizer for
the Navy's existing Mark III designs.
First bombsight design
Prototype Mark XI on display at the Steven F. Udvar-Hazy Center.
Turning it around the bearing at the base indicated the desired
direction changes to the pilot.
Although the CSBS and similar designs allowed the calculation of the
proper flight angle needed to correct for windage, it was normally not
visible to the pilot. In early bombers, the bomb aimer was normally
positioned in front of the pilot and could indicate corrections by
hand signals, but as aircraft grew larger it became common for the
pilot and bomb aimer to be separated. This led to the introduction of
the pilot direction indicator, or PDI. These typically consisted of a
pair of electrical pointers mounted in a conventional aircraft
instrument panel mount (a small dial). The bombardier used switches to
move the pointer on his unit, which was repeated in the cockpit in
front of the pilot.
Norden's first attempt at an improved bombsight was actually an
advance in PDI design. His idea was to remove the manual electrical
switches used to move the pointer and use the entire bombsight itself
as the indicator. He proposed attaching a low-power sighting telescope
to a gyro platform that would keep the telescope pointed at the same
azimuth, correcting for the aircraft's movements. The bombardier would
simply rotate the telescope left or right to follow the target. This
motion would cause the gyros to precess, and this signal would drive
the PDI automatically.
To time the drop, Norden used an idea already in use on other
bombsights, the "equal distance" concept. This was based on the
observation that the time needed to travel a certain distance over the
ground would remain relatively constant during the bomb run, as the
wind would not be expected to change dramatically over a short period
of time. If you could accurately mark out a distance on the ground, or
in practice, an angle in the sky, timing the passage over that
distance would give you all the information needed to time the
In Norden's version of the system, the bombardier first looked up the
expected time it would take for the bombs to fall from the current
altitude. This time was set into a countdown stopwatch, and the
bombardier waited for the target to line up with a crosshair in the
telescope. When the target passed through the crosshair, the timer was
started, and the bombardier then rotated the telescope around its
vertical axis to track the target as they approached it. This movement
was linked to a second crosshair through a gearing system that caused
the second to move twice as fast as the first. The bombardier
continued moving the telescope until the timer ran out. The second
crosshair was now at the correct aiming angle, or range angle; the
bombardier waited for the target to pass through the second crosshair
to time the drop.
The first of these Mark XI bombsights was delivered to the Navy's
proving grounds in Virginia in 1924. In testing, the system proved
disappointing. The circular error probable (CEP), a circle into which
50% of the bombs would fall, was 34 metres (110 ft) wide from
only 910 metres (3,000 ft) altitude. This was an error of over
3.6%, somewhat worse than existing systems. Moreover, bombardiers
universally complained that the device was far too hard to use.
Norden worked tirelessly on the design, and by 1928 the accuracy had
improved to 2% of altitude, enough that the Navy's Bureau of Ordnance
placed a $348,000 contract for the devices.
Norden was known for his confrontational and volatile nature. He often
worked 16 hour days and thought little of anyone who didn't. Navy
officers began to refer to him as "Old Man Dynamite". During
development, the Navy suggested that Norden consider taking on a
partner to handle the business and leave Norden free to develop on the
engineering side. They recommended former Army colonel Theodore Barth,
an engineer who had been in charge of gas mask production during World
War I. The match-up was excellent, as Barth had the qualities Norden
lacked: charm, diplomacy, and a head for business. The two became
Initial Army interest
In December 1927, the War Department was granted permission to use a
bridge over the
Pee Dee River
Pee Dee River in North Carolina for target practice,
as it would soon be sunk in the waters of a new dam. The 1st
Provisional Bombardment Squadron, equipped with
Keystone LB-5 bombers,
attacked the bridge over a period of five days, flying 20 missions a
day in perfect weather and attacking at altitudes from 6,000 to
8,000 feet (1,800–2,400 m). After this massive effort, the
middle section of the bridge finally fell on the last day. However,
the effort as a whole was clearly a failure in any practical sense.
About the same time as the operation was being carried out, General
James Fechet replaced General
Mason Patrick as commander of the USAAC.
He received a report on the results of the test, and on 6 January 1928
sent out a lengthy memo to Brigadier General William Gillmore, chief
of the Material Division at Wright Field, stating:
I cannot too strongly emphasize the importance of a bomb sight of
precision, since the ability of bombardment aviation to perform its
mission of destruction is almost entirely dependent upon an accurate
and practical bomb sight.
He went on to request information on every bombsight then used at
Wright, as well as "the Navy's newest design". However, the Mark XI
was so secret that Gillmore was not aware Fechet was referring to the
Norden. Gilmore produced contracts for twenty-five examples of an
improved version of the Seversky C-1, the C-3, and six prototypes of a
new design known as the Inglis L-1. The L-1 never matured, and Inglis
later helped Seversky to design the improved C-4.
But by this time the Army heard of the Mark XI in 1929 and was
eventually able to buy an example in 1931. Their testing mirrored the
Navy's experience; the gyro stabilization worked and the sight was
accurate, but it was also "entirely too complicated" to use. The
Army turned its attention to further upgraded versions of their
existing developments, replacing the older vector bombsight mechanisms
with the new synchronous method of measuring the proper dropping
Fully automatic bombsight
While the Mk. XI was reaching its final design, the Navy learned of
the Army's efforts to develop a synchronous bombsight, and asked
Norden to design one for them. Norden was initially unconvinced this
was workable, but the Navy persisted and offered him a development
contract in June 1929. Norden retreated to his mother's house in
Zurich and returned in 1930 with a working prototype. Lieutenant
Frederick Entwistle, the Navy's chief of bombsight development, judged
The new design, the Mark XV, was delivered in production quality in
the summer of 1931. In testing it proved to eliminate all of the
problems of the earlier Mk. XI design. From 1,200 metres
(4,000 ft) altitude the prototype delivered a CEP of 11 metres
(35 ft), while even the latest production Mk. XI's were 17 metres
(55 ft). At higher altitudes, a series of 80 bomb runs
demonstrated a CEP of 23 metres (75 ft). In a test on 7
October 1931, the Mk. XV dropped 50% of its bombs on a static target,
the USS Pennsylvania, while a similar aircraft with the Mk. XI had
only 20% of its bombs hit.
Moreover, the new system was dramatically simpler to use. After
locating the target in the sighting system, the bombardier simply made
fine adjustments using two control wheels throughout the bomb run.
There was no need for external calculation, lookup tables or pre-run
measurements - everything was carried out automatically through an
internal wheel-and-disc calculator. The calculator took a short time
to settle on a solution, with setups as short as 6 seconds, compared
to the 50 needed for the Mk. XI to measure its ground speed. In
most cases, the bomb run needed to be only 30 seconds long.
In spite of this success, the design also demonstrated several serious
problems. In particular, the gyroscopic platform had to be levelled
out before use using several spirit levels, and then checked and
repeatedly reset for accuracy. Worse, the gyros had a limited degree
of movement, and if the plane banked far enough the gyro would reach
its limit and have to be re-set from scratch - something that could
happen even due to strong turbulence. If the gyros were found to be
off, the levelling procedure took as long as eight minutes. Other
minor problems were the direct current electric motors which drove the
gyroscopes, whose brushes wore down quickly and left carbon dust
throughout the interior of the device, and the positioning of the
control knobs, which meant the bombardier could only adjust
side-to-side or up-and-down aim at a time, not both. But in spite of
all of these problems, the Mark XV was so superior to any other design
that the Navy ordered it into production.
Carl L. Norden Company incorporated in 1931, supplying the sights
under a dedicated source contract. In effect, the company was owned by
the Navy. In 1934 the newly-forming GHQ Air Force, the purchasing arm
US Army Air Corps, selected the Norden for their bombers as
well, referring to it as the M-1. However, due to the dedicated source
contract, the Army had to buy the sights from the Navy. This was not
only annoying for inter-service rivalry reasons, but the Air Corps'
higher-speed bombers demanded several changes to the design, notably
the ability to aim the sighting telescope further forward to give the
bombardier more time to set up. The Navy was not interested in these
changes, and would not promise to work them into the production lines.
Worse, Norden's factories were having serious problems keeping up with
demand for the Navy alone, and in January 1936, the Navy suspended all
shipments to the Army.
Mk. XV's were initially installed with the same automatic PDI as the
earlier Mk. XI. In practice, it was found that the pilots had a very
difficult time keeping the aircraft stable enough to match the
accuracy of the bombsight. Starting in 1932 and proceeding in fits and
starts for the next six years, Norden developed the Stabilized
Bombing Approach Equipment (SBAE), a mechanical autopilot that
attached to the bombsight. However, it was not a true "autopilot",
in that it could not fly the aircraft by itself. By rotating the
bombsight in relationship to the SBAE, the SBAE could account for wind
and turbulence and calculate the appropriate directional changes
needed to bring the aircraft onto the bomb run far more precisely than
a human pilot. The minor adaptations needed on the bombsight itself
produced what the Army referred to as the M-4 model.
In 1937 the Army, faced with the continuing supply problems with the
Norden, once again turned to
Sperry Gyroscope to see if they could
come up with a solution. Their earlier models had all proved
unreliable, but they had continued working with the designs throughout
this period and had addressed many of the problems. By 1937, Orland
Esval had introduced a new AC-powered electrical gyroscope that spun
at 30,000 RPM, compared to the Norden's 7,200 , which dramatically
improved the performance of the inertial platform. The use of
three-phase AC power and inductive pickup eliminated the carbon
brushes, and further simplified the design. Carl Frische had developed
a new system to automatically level the platform, eliminating the
time-consuming process needed on the Norden. The two collaborated on a
new design, adding a second gyro to handle heading changes, and named
the result the Sperry S-1. Existing supplies of Nordens continued to
be supplied to the USAAC's B-17s, while the S-1 equipped the B-24Es
being sent to the 15th Air Force.
Some B-17s had been equipped with a simple heading-only autopilot, the
Sperry A-3. The company had also been working on an all-electronic
model, the A-5, which stabilized in all three directions. By the early
1930s, it was being used in a variety of Navy aircraft to excellent
reviews. By connecting the outputs of the S-1 bombsight to the A-5
autopilot, Sperry produced a system similar to the M-4/SBAE, but one
that was much more fast acting. The combination of the S-1 and A-5 so
impressed the Army that on 17 June 1941 they authorized the
construction of a 186.000 m² factory and noted that "in the future
all production models of bombardment airplanes be equipped with the
A-5 Automatic Pilot and have provisions permitting the installation of
either the M-Series [Norden]
Bombsight or the S-1 Bombsight".
British interest, Tizard mission
By 1938, information about the Norden had worked its way up the Royal
Air Force chain of command and was well known within that
organization. The British had been developing a similar bombsight
known as the Automatic Bomb Sight, but combat experience in 1939
demonstrated the need for it to be stabilized. Work was underway as
Stabilized Automatic Bomb Sight
Stabilized Automatic Bomb Sight (SABS), but it would not be
available until 1940 at the earliest, and likely later. Even then, it
did not feature the autopilot linkage of the Norden, and would thus
find it difficult to match the Norden's performance in anything but
smooth air. Acquiring the Norden became a major goal.
Their first attempt, in the spring of 1938, was rebuffed by the Navy.
Air Chief Marshal Edgar Ludlow-Hewitt, commanding RAF Bomber Command,
Air Ministry action, and they wrote to George Pirie, the
British air attaché in Washington, suggesting he approach the Army
with an offer of an information exchange with their own SABS. Pirie
replied that he had already looked into this, and was told that the
Army had no licensing rights to the device as it was owned by the
Navy. The matter was not helped by a minor diplomatic issue that
flared up in July when a French air observer was found to be on board
Douglas Aircraft Company
Douglas Aircraft Company bomber, forcing President Roosevelt
to promise no further information exchanges with foreign powers.
Six months later, after a change of leadership within the Navy's
Bureau of Aeronautics, on 8 March 1939 Pirie was once again instructed
to ask the Navy about the Norden, this time enhancing the deal with
offers of British power-operated turrets. However, Pirie expressed
concern as he noted the Norden had become as much political as
technical, and its relative merits were being publicly debated in
Congress weekly while the Navy continued to say the Norden was "the
United States' most closely guarded secret".
The RAF's desires were only further goaded on 13 April 1939, when
Pirie was invited to watch an air demonstration at
Fort Benning where
the painted outline of a battleship was the target:
At 1:27 while everyone was still searching [the sky for the B-17s] six
300-pound (140 kg) bombs suddenly burst at split second intervals
on the deck of the battleship, and it was at least 30 seconds later
before someone spotted the B-17 at 12,000 feet (3,700 m)
The three following B-17s also hit the target, and then a flight of a
dozen Douglas B-18 Bolos placed most of their bombs in a separate
550 m × 550 m (600 yd × 600 yd) square
outlined on the ground.
Another change of management within the
Bureau of Aeronautics had the
effect of making the Navy more friendly to British overtures, but no
one was willing to fight the political battle needed to release the
design. The Navy brass was concerned that giving the Norden to the RAF
would increase its chances of falling into German hands, which could
put the US's own fleet at risk. The
Air Ministry continued increasing
pressure on Pirie, who eventually stated there was simply no way for
him to succeed, and suggested the only way forward would be through
the highest diplomatic channels in the Foreign Office. Initial probes
in this direction were also rebuffed. When a report stated that the
Norden's results were three to four times as good as their own
Air Ministry decided to sweeten the pot and suggested
they offer information on radar in exchange. This too was
The matter eventually worked its way to Prime Minister Neville
Chamberlain, who wrote personally to President Roosevelt asking for
the Norden, but even this was rejected. The reason for these
rejections was more political than technical, but the Navy's demands
for secrecy were certainly important. They repeated that the design
would be released only if the British could demonstrate the basic
concept was common knowledge, and therefore not a concern if it fell
into German hands. The British failed to convince them, even after
offering to equip their examples with a variety of self-destruct
This may have been ameliorated by the winter of 1939, at which point a
number of articles about the Norden appeared in the US popular press
with reasonably accurate descriptions of its basic workings. But when
these were traced back to the press corps at the Army Air Corps, the
Navy was apoplectic. Instead of accepting it was now in the public
domain, any discussion about the Norden was immediately shut down.
This drove both the
Air Ministry and
Royal Navy to increasingly
anti-American attitudes when they considered sharing their own
developments, notably newer
ASDIC systems. By 1940 the situation on
scientific exchange was entirely deadlocked as a result.
Looking for ways around the deadlock,
Henry Tizard sent Archibald
Vivian Hill to the US to take a survey of US technical capability in
order to better assess what technologies the US would be willing to
exchange. This effort was the start on the path that led to the famous
Tizard Mission in late August 1940. Ironically, by the time the
Mission was being planned, the Norden had been removed from the list
of items to be discussed, and Roosevelt personally noted this was due
largely to political reasons. Ultimately, although Tizard was unable
to convince the US to release the design, he was able to request
information about its external dimensions and details on the mounting
system so it could be easily added to British bombers if it were
released in the future.
Production, production problems, and Army standardization
The conversion of the Norden company's New York City engineering lab
to a production factory was a long process. Before the war, skilled
craftsmen, most of them German or Italian immigrants, hand-made almost
every part of the 2,000-part machine. Between 1932 and 1938, the
company produced only 121 bombsights per year. During the first year
after the Attack on Pearl Harbor, Norden produced 6,900 bombsights,
three-quarters of which went to the Navy.
When Norden heard of the Army's dealings with Sperry, Theodore Barth
called a meeting with the Army and Navy at their factory in New York
City. Barth offered to build an entirely new factory just to supply
the Army, but the Navy refused this. Instead, the Army suggested that
Norden adapt their sight to work with Sperry's A-5, which Barth
refused. Norden actively attempted to make the bombsight incompatible
with the A-5, and it was not until 1942 that the impasse was finally
solved by farming out autopilot production to
Honeywell Regulator, who
combined features of the Norden-mounted SBAE with the aircraft-mounted
A-5 to produce what the Army referred to as "Automatic Flight Control
Equipment" (AFCE) the unit would later be redesigned as the C-1.
The Norden, now connected with the aircraft's built-in autopilot, had
the ability to allow the bombardier alone to fully control minor
movements of the aircraft during the bombing run.
By May 1943 the Navy was complaining that they had a surplus of
devices, and full production was turned over to the Army Air Forces.
After investing more than $100 million in Sperry bombsight
manufacturing plants, the AAF concluded that the Norden M-series was
far superior in accuracy, dependability, and design. Sperry contracts
were canceled in November 1943. When production ended a few months
later, 5,563 Sperry bombsight-autopilot combinations had been built,
most of which were installed in Consolidated B-24 Liberator
Norden bombsight production to a final total of six
factories took several years. The Army Air Forces demanded additional
production to meet their needs, and eventually arranged for the Victor
Adding Machine company to gain a manufacturing license, and then
Remington Rand. Ironically, during this period the Navy abandoned
the Norden in favour of dive bombing, reducing the demand. By the end
of the war, Norden and its subcontractors had produced 72,000 M-9
bombsights for the Army Air Force alone, costing $8,800 each.
Description and operation
A page from the Bombardier's Information
File (BIF) that describes the
components and controls of the Norden Bombsight. The separation of the
stabilizer and sight head is evident.
Typical bombsights of the pre-war era worked on the "vector bombsight"
principle introduced with the
World War I
World War I Course Setting Bomb Sight.
These systems consisted of a slide rule-type calculator that was used
to calculate the effects of the wind on the bomber based on simple
vector arithmetic. The mathematical principles are identical to those
E6B calculator used to this day.
In operation, the bombardier would first take a measurement of the
wind speed using one of a variety of methods, and then dial that speed
and direction into the bombsight. This would move the sights to
indicate the direction the plane should fly to take it directly over
the target with any cross-wind taken into account, and also set the
angle of the iron sights to account for the wind's effect on ground
These systems had two primary problems in terms of accuracy. The first
was that there were several steps that had to be carried out in
sequence in order to set up the bombsight correctly, and there was
limited time to do all of this during the bomb run. As a result, the
accuracy of the wind measurement was always limited, and errors in
setting the equipment or making the calculations were common. The
second problem was that the sight was attached to the aircraft, and
thus moved about during maneuvers, during which time the bombsight
would not point at the target. As the aircraft had to maneuver in
order to make the proper approach, this limited the time allowed to
accurately make corrections. This combination of issues demanded a
long bomb run.
Experiments had shown that adding a stabilizer system to a vector
bombsight would roughly double the accuracy of the system. This would
allow the bombsight to remain level while the aircraft maneuvered,
giving the bombardier more time to make his adjustments, as well as
reducing or eliminating mis-measurements when sighting off of
non-level sights. However, this would not have any effect on the
accuracy of the wind measurements, nor the calculation of the vectors.
The Norden attacked all of these problems.
To improve the calculation time, the Norden used a mechanical computer
inside the bombsight to calculate the range angle of the bombs. By
simply dialing in the aircraft's altitude and heading, along with
estimates of the wind speed and direction (in relation to the
aircraft), the computer would automatically, and quickly, calculate
the aim point. This not only reduced the time needed for the bombsight
setup but also dramatically reduced the chance for errors. This attack
on the accuracy problem was by no means unique; several other
bombsights of the era used similar calculators. It was the way the
Norden used these calculations that differed.
Conventional bombsights are set up pointing at a fixed angle, the
range angle, which accounts for the various effects on the trajectory
of the bomb. To the operator looking through the sights, the
crosshairs indicate the location on the ground the bombs would impact
if released at that instant. As the aircraft moves forward, the target
approaches the crosshairs from the front, moving rearward, and the
bombardier releases the bombs as the target passes through the line of
the sights. One example of a highly automated system of this type was
the RAF's Mark XIV bomb sight.
The Norden worked in an entirely different fashion, based on the
"synchronous" or "tachometric" method. Internally, the calculator
continually computed the impact point, as was the case for previous
systems. However, the resulting range angle was not displayed directly
to the bombardier or dialed into the sights. Instead, the bombardier
used the sighting telescope to locate the target long in advance of
the drop point. A separate section of the calculator used the inputs
for altitude and airspeed to determine the angular velocity of the
target, the speed at which it would be seen drifting backward due to
the forward motion of the aircraft. The output of this calculator
drove a rotating prism at that angular speed in order to keep the
target centered in the telescope. In a properly adjusted Norden, the
target remains motionless in the sights.
The Norden thus calculated two angles: the range angle based on the
altitude, airspeed and ballistics; and the current angle to the
target, based on the ground speed and heading of the aircraft. The
difference between these two angles represented the "correction" that
needed to be applied to bring the aircraft over the proper drop point.
If the aircraft was properly aligned with the target on the bomb run,
the difference between the range and target angles would be
continually reduced, eventually to zero (within the accuracy of the
mechanisms). At this moment the Norden automatically dropped the
In practice, the target failed to stay centered in the sighting
telescope when it was first set up. Instead, due to inaccuracies in
the estimated wind speed and direction, the target would drift in the
sight. To correct for this, the bombardier would use fine-tuning
controls to slowly cancel out any motion through trial and error.
These adjustments had the effect of updating the measured ground speed
used to calculate the motion of the prisms, slowing the visible drift.
Over a short period of time of continual adjustments, the drift would
stop, and the bombsight would now hold an extremely accurate
measurement of the exact ground speed and heading. Better yet, these
measurements were being carried out on the bomb run, not before it,
and helped eliminate inaccuracies due to changes in the conditions as
the aircraft moved. And by eliminating the manual calculations, the
bombardier was left with much more time to adjust his measurements,
and thus settle at a much more accurate result.
The angular speed of the prism changes with the range of the target:
consider the reverse situation, the apparent high angular speed of an
aircraft passing overhead compared to its apparent speed when it is
seen at a longer distance. In order to properly account for this
non-linear effect, the Norden used a system of slip-disks similar to
those used in differential analysers. However, this slow change at
long distances made it difficult to fine-tune the drift early in the
bomb run. In practice, bombardiers would often set up their ground
speed measurements in advance of approaching the target area by
selecting a convenient "target" on the ground that was closer to the
bomber and thus had more obvious motion in the sight. These values
would then be used as the initial setting when the target was later
Norden bombsight consisted of two primary parts, the gyroscopic
stabilization platform on the left side, and the mechanical calculator
and sighting head on the right side. They were essentially separate
instruments, connecting through the sighting prism. The sighting
eyepiece was located in the middle, between the two, in a less than
convenient location that required some dexterity to use.
Before use, the Norden's stabilization platform had to be righted, as
it slowly drifted over time and no longer kept the sight pointed
vertically. Righting was accomplished through a time consuming process
of comparing the platform's attitude to small spirit levels seen
through a glass window on the front of the stabilizer. In practice,
this could take as long as eight and a half minutes. This problem was
made worse by the fact that the platform's range of motion was
limited, and could be tumbled even by strong turbulence, requiring it
to be reset again. This problem seriously upset the usefulness of the
Norden, and led the RAF to reject it once they received examples in
1942. Some versions included a system that quickly righted the
platform, but this "Automatic Gyro Leveling Device" proved to be a
maintenance problem, and was removed from later examples.
Once the stabilizer was righted, the bombardier would then dial in the
initial setup for altitude, speed, and direction. The prism would then
be "clutched out" of the computer, allowing it to be moved rapidly to
search for the target on the ground. Later Nordens were equipped with
a reflector sight to aid in this step. Once the target was located the
computer was clutched in and started moving the prism to follow the
target. The bombardier would begin making adjustments to the aim. As
all of the controls were located on the right, and had to be operated
while sighting through the telescope, another problem with the Norden
is that the bombardier could only adjust either the vertical or
horizontal aim at a given time, his other arm was normally busy
holding himself up above the telescope.
On top of the device, to the right of the sight, were two final
controls. The first was the setting for "trail", which was pre-set at
the start of the mission for the type of bombs being used. The second
was the "index window" which displayed the aim point in numerical
form. The bombsight calculated the current aim point internally and
displayed this as a sliding pointer on the index. The current sighting
point, where the prism was aimed, was also displayed against the same
scale. In operation, the sight would be set far in advance of the aim
point, and as the bomber approached the target the sighting point
indicator would slowly slide toward the aim point. When the two met,
the bombs were automatically released. The aircraft was moving over
110 metres per second (350 ft/s), so even minor interruptions in
timing could dramatically affect aim.
Early examples, and most used by the Navy, had an output that directly
drove a Pilot Direction Indicator meter in the cockpit. This
eliminated the need to manually signal the pilot, as well as
eliminating the possibility of error.
In U.S. Army Air Forces use, the
Norden bombsight was attached to its
autopilot base, which was in turn connected with the aircraft's
Honeywell C-1 autopilot could be used as an autopilot
by the flight crew during the journey to the target area through a
control panel in the cockpit, but was more commonly used under direct
command of the bombardier. The Norden's box-like autopilot unit sat
behind and below the sight and attached to it at a single rotating
pivot. After control of the aircraft was passed to the bombardier
during the bomb run, he would first rotate the entire Norden so the
vertical line in the sight passed through the target. From that point
on, the autopilot would attempt to guide the bomber so it followed the
course of the bombsight, and pointed the heading to zero out the drift
rate, fed to it through a coupling. As the aircraft turned onto the
correct angle, a belt and pulley system rotated the sight back to
match the changing heading. The autopilot was another reason for the
Norden's accuracy, as it ensured the aircraft quickly followed the
correct course and kept it on that course much more accurately than
the pilots could.
Later in the war, the Norden was combined with other systems to widen
the conditions for successful bombing. Notable among these was the
radar system called the H2X (Mickey), which were used directly with
the Norden bombsight. The radar proved most accurate in coastal
regions, as the water surface and the coastline produced a distinctive
Norden bombsight was developed during a period of United States
non-interventionism when the dominant U.S. military strategy was the
defense of the U.S. and its possessions. A considerable amount of this
strategy was based on stopping attempted invasions by sea, both with
direct naval power, and starting in the 1930s, with USAAC
airpower. Most air forces of the era invested heavily in dive
bombers or torpedo bombers for these roles, but these aircraft
generally had limited range; long-range strategic reach would require
the use of an aircraft carrier. The Army felt the combination of the
B-17 Flying Fortress
B-17 Flying Fortress presented an alternate solution,
believing that small formations of B-17s could successfully attack
shipping at long distances from the USAAC's widespread bases. The high
altitudes the Norden allowed would help increase the range of the
aircraft, especially if equipped with a turbocharger, as with each of
the four Wright Cyclone 9 radial engines of the B-17.
In 1940, Barth claimed that "we do not regard a 15 square feet
(1.4 m2) ... as being a very difficult target to hit from an
altitude of 30,000 feet (9,100 m)". At some point the company
started using the pickle barrel imagery, to reinforce the bombsight's
reputation. After the device became known about publicly in 1942, the
Norden company in 1943 rented
Madison Square Garden
Madison Square Garden and folded their
own show in between the presentations of the Ringling Bros. and Barnum
& Bailey Circus. Their show involved dropping a wooden "bomb" into
a pickle barrel, at which point a pickle popped out.
These claims were greatly exaggerated; in 1940 the average score for
an Air Corps bombardier was a circular error of 120 metres
(400 ft) from 4,600 metres (15,000 ft), not 4.6 m from
9,100 m. Real-world performance was poor enough that the Navy
de-emphasized level attacks in favor of dive bombing almost
Grumman TBF Avenger
Grumman TBF Avenger could mount the Norden, like
the preceding Douglas TBD Devastator, but combat use was
disappointing and eventually described as "hopeless" during the
Guadalcanal Campaign. In spite of giving up on the device in 1942,
bureaucratic inertia meant they were supplied as standard equipment
USAAF anti-shipping operations in the Far East were generally
unsuccessful. In early operations during the Battle of the
Philippines, B-17s claimed to have sunk one minesweeper and damaged
two Japanese transports, the cruiser Naka, and the destroyer
Murasame. However, all of these ships are known to have suffered
no damage from air attack during that period. In other early battles,
Battle of Coral Sea
Battle of Coral Sea or Battle of Midway, no claims were
made at all, although some hits were seen on docked targets.
The USAAF eventually replaced all of their anti-shipping B-17s with
other aircraft, and came to use the skip bombing technique in direct
Air war in Europe
As U.S. participation in the war started, the U.S. Army Air Forces
drew up widespread and comprehensive bombing plans based on the
Norden. They believed the B-17 had a 1.2% probability of hitting a 30
metres (100 ft) target from 6,100 metres (20,000 ft),
meaning that 220 bombers would be needed for a 93% probability of one
or more hits. This was not considered a problem, and the AAF forecast
the need for 251 combat groups to provide enough bombers to fulfill
their comprehensive pre-war plans.
After earlier combat trials proved troublesome, the Norden bombsight
and its associated AFCE were used on a wide scale for the first time
on the 18 March 1943 mission to Bremen-Vegesack, Germany; The 303d
Bombardment Group dropped 76% of its load within a 300 metres
(1,000 ft) ring, representing a CEP well under 300 m
(1,000 ft) As at sea, many early missions over Europe
demonstrated varied results; on wider inspection, only 50% of American
bombs fell within a 400 metres (1⁄4 mi) of the target, and
American flyers estimated that as many as 90% of bombs could miss
their targets. The average CEP in 1943 was 370 metres
(1,200 ft), meaning that only 16% of the bombs fell within 300
metres (1,000 ft) of the aiming point. A 230-kilogram
(500 lb) bomb, standard for precision missions after 1943, had a
lethal radius of only 18 to 27 metres (60 to 90 ft).
Faced with these poor results,
Curtis LeMay started a series of
reforms in an effort to address the problems. In particular, he
introduced the "combat box" formation in order to provide maximum
defensive firepower by densely packing the bombers. As part of this
change, he identified the best bombardiers in his command and assigned
them to the lead bomber of each box. Instead of every bomber in the
box using their Norden individually, the lead bombardiers were the
only ones actively using the Norden, and the rest of the box followed
in formation and then dropped their bombs when they saw the lead's
leaving his aircraft. Although this spread the bombs over the area
of the combat box, this could still improve accuracy over individual
efforts. It also helped stop a problem where various aircraft, all
slaved to their autopilots on the same target, would drift into each
other. These changes did improve accuracy, which suggests that much of
the problem is attributable to the bombardier. However, precision
attacks still proved difficult or impossible.
Jimmy Doolittle took over command of the
8th Air Force
8th Air Force from Ira
Eaker in early 1944, precision bombing attempts were dropped. Area
bombing, like the RAF efforts, were widely used with 750 and then 1000
bomber raids against large targets. The main targets were railroad
marshaling yards (27.4% of the bomb tonnage dropped), airfields
(11.6%), oil refineries (9.5%), and military installations (8.8%).
To some degree the targets were secondary missions; Doolittle used the
bombers as an irresistible target to draw up
Luftwaffe fighters into
the ever-increasing swarms of Allied long-distance fighters. As these
missions broke the Luftwaffe, missions were able to be carried out at
lower altitudes or especially in bad weather when the
H2X radar could
be used. In spite of abandoning precision attacks, accuracy
nevertheless improved. By 1945, the 8th was putting up to 60% of its
bombs within 300 metres (1,000 ft), a CEP of about 270 metres
Still pursuing precision attack, various remotely guided weapons were
developed, notably the
AZON and R
AZON bombs and similar weapons.
The Norden operated by mechanically turning the viewpoint so the
target remained stationary in the display. The mechanism was designed
for the low angular rate encountered at high altitudes, and thus had a
relatively low range of operational speeds. The Norden could not
rotate the sight fast enough for bombing at low altitude, for
instance. Typically this was solved by removing the Norden completely
and replacing it with simpler sighting systems.
A good example of its replacement was the refitting of the Doolittle
Raiders with a simple iron sight. Designed by Capt. C. Ross Greening,
the sight was mounted to the existing pilot direction indicator,
allowing the bombardier to make corrections remotely, like the
bombsights of an earlier era.
However, the Norden combined two functions, aiming and stabilization.
While the former was not useful at low altitudes, the latter could be
even more useful, especially if flying in rough air near the surface.
This led James "Buck" Dozier to mount a Doolittle-like sight on top of
the stabilizer in the place of the sighting head in order to attack
German submarines in the Caribbean Sea. This proved extraordinarily
useful and was soon used throughout the fleet.
Photo of the AFCE and
Bombsight shop ground crew in the 463rd Sub
Depot affiliated with the USAAF 389th Bomb Group based at Hethel,
Since the Norden was considered a critical wartime instrument,
bombardiers were required to take an oath during their training
stating that they would defend its secret with their own life if
necessary. In case the bomber plane should make an emergency landing
on enemy territory, the bombardier would have to shoot the important
parts of the Norden with a gun to disable it. As this method still
would leave a nearly intact apparatus to the enemy, a thermite grenade
was installed; the heat of the chemical reaction would melt the Norden
into a lump of metal. The
Douglas TBD Devastator
Douglas TBD Devastator torpedo bomber
was originally equipped with flotation bags in the wings to aid the
aircrew's escape after ditching, but they were removed once the
Pacific War began; this ensured that the aircraft would sink, taking
the Norden with it.
After each completed mission, bomber crews left the aircraft with a
bag which they deposited in a safe ("the Bomb Vault"). This secure
facility ("the AFCE and
Bombsight Shop") was typically in one of the
Nissen hut (Quonset hut) support buildings. The
was manned by enlisted men who were members of a Supply Depot Service
Group ("Sub Depot") attached to each USAAF bombardment group. These
shops not only guarded the bombsights but performed critical
maintenance on the Norden and related control equipment. This was
probably the most technically skilled ground-echelon job, and
certainly the most secret, of all the work performed by Sub Depot
personnel. The non-commissioned officer in charge and his staff had to
have a high aptitude for understanding and working with mechanical
As the end of
World War II
World War II neared, the bombsight was gradually
downgraded in its secrecy; however, it was not until 1944 that the
first public display of the instrument occurred.
Herman W. Lang (FBI file photo)
Main article: Duquesne Spy Ring
In spite of the security precautions, the entire Norden system had
been passed to the Germans before the war started. Herman W. Lang, a
German spy, had been employed by the Carl L. Norden Company. During a
visit to Germany in 1938, Lang conferred with German military
authorities and reconstructed plans of the confidential materials from
memory. In 1941, Lang, along with the 32 other German agents of the
Duquesne Spy Ring, was arrested by the FBI and convicted in the
largest espionage prosecution in U.S. history. He received a sentence
of 18 years in prison on espionage charges and a two-year concurrent
sentence under the Foreign Agents Registration Act.
German instruments were actually fairly similar to the Norden, even
before World War II. A similar set of gyroscopes provided a stabilized
platform for the bombardier to sight through, although the complex
interaction between the bombsight and autopilot was not used. The Carl
Zeiss Lotfernrohr 7, or Lotfe 7, was an advanced mechanical system
similar to the Norden bombsight, although in form it was more similar
to the Sperry S-1. It started replacing the simpler Lotfernrohr 3 and
BZG 2 in 1942, and emerged as the primary late-war bombsight used in
Luftwaffe level bombers. The use of the autopilot allowed
single-handed operation, and was key to bombing use of the
single-crewed Arado Ar 234.
Postwar analysis placed the overall accuracy of daylight precision
attacks with the Norden at about the same level as radar bombing
8th Air Force
8th Air Force put 31.8% of its bombs within 300 metres
(1,000 ft) from an average altitude of 6,400 metres
(21,000 ft), the 15th Air Force averaged 30.78% from 6,200 metres
(20,500 ft), and the 20th Air Force against Japan averaged 31%
from 5,000 metres (16,500 ft).
Many factors have been put forth to explain the Norden's poor
real-world performance. Over Europe, the cloud cover was a common
explanation, although performance did not improve even in favorable
conditions. Over Japan, bomber crews soon discovered strong winds at
high altitudes, the so-called jet streams, but the Norden bombsight
worked only for wind speeds with minimal wind shear. Additionally, the
bombing altitude over Japan reached up to 9,100 metres
(30,000 ft), but most of the testing had been done well below
6,100 metres (20,000 ft). This extra altitude compounded factors
that could previously be ignored; the shape and even the paint of the
bomb mantle greatly changed the aerodynamic properties of the weapon,
and, at that time, nobody knew how to calculate the trajectory of
bombs that reached supersonic speeds during their fall.
Unable to obtain the Norden, the RAF continued development of their
own designs. Having moved to night bombing, where visual accuracy was
difficult under even the best conditions, they introduced the much
simpler Mark XIV bomb sight. This was designed not for accuracy above
all, but ease of use in operational conditions. In testing in 1944, it
was found to offer a CEP of 270 metres (890 ft), about what the
Norden was offering at that time. This led to a debate within the RAF
whether to use their own tachometric design, the Stabilized Automatic
Bomb Sight, or use the Mk. XIV on future bombers. The Mk. XIV
ultimately served into the 1960s while the SABS was removed from
service almost immediately after the war.
In the postwar era, the development of new precision bombsights
essentially ended. At first this was due to the military drawdown, but
as budgets increased again during the opening of the Cold War, the
bomber mission had passed to nuclear weapons. These required
accuracies on the order of 2,700 metres (3,000 yd), well within
the capabilities of existing radar bombing systems. Only one major
bombsight of note was developed, the Y-4 developed on the Boeing B-47
Stratojet. This sight combined the images of the radar and a lens
system in front of the aircraft, allowing them to be directly compared
at once through a binocular eyepiece.
Bombsights on older aircraft, like the
Boeing B-29 Superfortress
Boeing B-29 Superfortress and
the later B-50, were left in their wartime state. When the Korean War
opened, these aircraft were pressed into service and the Norden once
again became the USAF's primary bombsight. This occurred again when
Vietnam War started; in this case retired
World War II
World War II technicians
had to be called up in order to make the bombsights operational again.
Its last use in combat was by the Naval Air Observation Squadron
Sixty-Seven (VO-67), during the Vietnam War. The bombsights were used
Operation Igloo White
Operation Igloo White for implanting Air-Delivered Seismic
Intrusion Detectors (ADSID) along the Ho Chi Minh Trail.
Lotfernrohr 7, a German equivalent.
Mary Babnik Brown, whose hair was used for the bombsight crosshairs.
^ CEP is a circle into which 50% of the bombs should fall.
^ Different sources disagree on Norden's time at Sperry. Most place
him there between 1911 and 1915, Moy and Sherman state he left in
1913, and Moy implies he worked there since 1904.
^ Peck, Merton J. & Scherer, Frederic M. The Weapons Acquisition
Process: An Economic Analysis (1962)
Harvard Business School
Harvard Business School p.619
^ a b c d e f g h Sherman 1995.
^ a b c Moy 2001, p. 84.
^ Moy 2001, p. 82.
^ a b c d e Moy 2001, p. 85.
^ a b c Moy 2001, p. 86.
^ Libbey 2013, pp. 86-87.
^ Libbey 2013, p. 87.
^ Libbey 2013, p. 88.
^ Moy 2001, p. 83.
^ Moy 2001, p. 87.
^ a b Moy 2001, p. 88.
^ "Naval Aviation Chronology 1930–1939", US Navy, 30 June 1997
^ "Precision Bombing: sample mission shows details that make it work",
Life, 30 August 1943, p. 97
^ Searle 1989, p. 61.
^ a b Searle 1989, p. 62.
^ Flight, August 1945, p. 180
^ a b c Searle 1989, p. 64.
^ Zimmerman 1996, p. 34.
^ a b Zimmerman 1996, p. 35.
^ Zimmerman 1996, p. 36.
^ a b Zimmerman 1996, p. 37.
^ a b c d Zimmerman 1996, p. 38.
^ Zimmerman 1996, p. 50.
^ Zimmerman 1996, p. 99.
^ "Business & Finance: A Bomb on Norden". TIME magazine.
1945-01-01. [T]he Norden company, ordered by the Navy Department to
turn over bombsight plans to
Remington Rand Inc., which was to build
8,500 "football units" (the main computing part), [...]
^ a b c Ross: Strategic Bombing by the United States in World War II
^ a b c d Correll 2008, p. 61.
^ a b Correll 2008, p. 60.
^ "New York Bomb", Life, 26 April 1943, p. 27
^ Alvin Kernan, Donald Kagan & Frederick Kagan, "The Unknown
Battle of Midway", Yale University Press, 2007, p. 51
^ Barrett Tillman, "Avenger at War", Ian Allan, 1979, p. 53
^ Robert Cressman, "The Official Chronology of the U.S. Navy in World
War II", Naval Institute Press, 2000, p. 62
^ Gene Eric Salecker, "Fortress Against the Sun", Da Capo Press, 2001,
^ "Midway-based Bomber Attacks on the Japanese Carrier Striking Force,
4 June 1942", US Navy, 20 April 1999
^ Neillands, Robin (2001). The Bomber War: The Allied Air Offensive
against Nazi Germany. The Overlook Press, p. 169.
^ Geoffery Perrett, "There's a War to Be Won: The United States Army
in World War II" (1991) p. 405
^ Edward K. Eckert, "In War and Peace: An American Military History
Anthology" (1990) p. 260
^ Michael C.C. Adams, "The Best War Ever: America in World War Two"
^ Correll 2008, p. 62.
^ a b Correll 2008, p. 63.
^ a b "Doolittle Raid". National Museum of the United States Air Force
11 June 2015
^ Ira V. Matthews, "Eighty-one War Stories: Buck Dozier's Bombsight"
^ "The Aviation Factfile: Aircraft of World War II" (2004) p.79
^ "Federal Bureau of Investigation: Frederick Duquesne Interesting
Case Write-up" (PDF).
Federal Bureau of Investigation
Federal Bureau of Investigation (publicly
released on March 12, 1985 under the Freedom of Information Act).
^ Correll 2008, p. 64.
^ Wakelam, Randall Thomas (2009). The Science of Bombing: Operational
Research in RAF Bomber Command. University of Toronto Press.
p. 123. ISBN 9781442693432.
^ Y-4 Horizontal Periscopic Bombsight. National Museum of the United
States Air Force. 2 June 2015
^ "Norden: Last Combat Use", Observation Squadron Sixty-Seven (VO-67),
Correll, John (October 2008). "Daylight Precision Bombing" (PDF).
Airforce Magazine: 60–64.
Libbey, James (2013). Alexander P. de Seversky and the Quest for Air
Power. Potomac Books. JSTOR j.ctt1ddr8nb.
Sherman, Don (February–March 1995). "The Secret Weapon". Air &
Space Magazine. Archived from the original on 2006-05-17.
Moy, Timothy (2001). War Machines: transforming technologies in the
U.S. military, 1920-1940. Texas A&M University Press.
Searle, Loyd (September 1989). "The
Bombsight War: Norden vs. Sperry"
(PDF). IEEE Spectrum: 60–64.
Zimmerman, David (1996). Top Secret Exchange: the
Tizard Mission and
the Scientific War. McGill-Queen's Press.
Stewart Halsey Ross: "Strategic Bombing by the United States in World
Albert L. Pardini: "The Legendary Norden Bombsight"
ISBN 0-7643-0723-1, Schiffer Publishing, 1999.
"Bombardier: A History", Turner Publishing, 1998
"The Norden Bombsight
"Bombing – Students' Manual"
"Bombardier's Information File"
Stephen McFarland: "America's Pursuit of Precision Bombing, 1910-1945"
Burroughs Corporation Records. Pasinski Family Papers,
1912-1984". Charles Babbage Institute, University of Minnesota.
Pasinski produced the prototype for the bombsight. He designed
production tools and supervised production of the bombsight at
Burroughs Corporation Records.
World War II
World War II Era Records, 1931-1946,
Charles Babbage Institute, University of Minnesota. Information on the
Norden bombsight, which Burroughs produced beginning in 1942.
Wikimedia Commons has media related to Norden bombsights.
Flight 1945 Norden Bomb Sight
How the Norden
Bombsight Does Its Job by V. Torrey, June 1945 Popular
Bombsight That Thinks." Popular Mechanics, February 1945,
Norden bombsight images and information from twinbeech.com
File (March 1945)