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DEPARTMENT OF THE NAVY -- NAVAL HISTORICAL CENTER
805 KIDDER BREESE SE -- WASHINGTON NAVY YARD
WASHINGTON DC 20374-5060
Radio Proximty (VT) Fuzes
Radio Proximity (VT) Fuzes and How
They Operate
Significance and Background of the Radio
Proximity Fuze (VT) in World War II
Development of Proximity Fuzes (VT) for
Projectiles - VT Fuzes MKS 32 to 60
Image of Mark 53 VT Fuse
Radio Proximity (VT) Fuzes and
How They Operate
Proximity fuzes are intended to detonate missiles automatically
upon approach to a target and at such a position along the flight
path of the missile as to inflict maximum damage to the target.
Various methods of obtaining proximity operation against a target
were investigated: electrostatic, acoustic, optical, and radio.
Prime considerations for a proximity fuze were reliability and
simplicity. The former was necessary to insure performance under
various stringent Service conditions, and the latter, to allow
the fuze to be contained in a small volume and to be produced
quickly in large quantities. Following initial exploratory investigations,
two types of fuzes, optical (photoelectric) and radio, were selected
for intensive development. The photoelectric method was selected
because it appeared a relatively easy approach to the proximity
fuze problem, although the fuzes would be limited to daytime use,
unless light sources were provided. The radio method appeared
to be more complicated, but it afforded opportunity for reliable
performance not only 24 hours a day but under a much wider variety
of other conditions than were possible with the photoelectric
fuze. The two methods were pursued in parallel until it was definitely
established that radio proximity fuzes could be produced to fulfill
all requirements. When this stage of development was reached,
work on photoelectric fuzes was terminated (October 1943), and
the radio method was prosecuted even more vigorously than before.
How a Radio Proximity Fuze Operates
Among various possible types of radio proximity fuzes,
an active-type fuzed operating on the doppler effect was selected
as being the most promising method.
In a doppler-type fuze, the actuating signal is produced by the
wave reflected from a target moving with respect to the fuze.
The frequency of the reflected wave differs from that of the transmitted
wave, because of the relative velocity of the fuze and the target.
The interference it creates with the transmitter results in a
low-frequency beat caused by the combination of the transmitted
and the reflected frequencies. The low-frequency signal can be
used to trigger an electronic switch. Selective amplification
of the low-frequency signal is generally necessary.
Operation of the fuze occurs when the output signal from the amplifier
reaches the required amplitude to fire the thyratron. For a given
orientation of the fuze and target, the amplitude of the target
signal produced in the oscillator-detector circuit is a function
of the distance between the target and the fuze. Hence, by proper
settings for the gain of the amplifier and the holding bias on
the thyratron, the distance of operation may be controlled. Distance,
however, is not the only factor which requires consideration.
Orientation or aspect is very important, particularly against
aircraft targets, since operation should occur at that point on
the trajectory when the greatest number of fragments will be directed
toward the target.
For most missiles, the greatest number of fragments are directed
upon detonation approximately at right angles to the axis of the
missile. For trajectories which would normally pass by the target
without intersecting it, there will be optimum chance of damage
if detonation of the missile occurs when the target is in the
direction of greatest fragmentation density. However, for trajectories
which would intersect the target, the missile should come as close
to the target as possible before detonation. Hence the basic requirements
for directional sensitivity of a proximity fuze for antiaircraft
use are: (1) the sensitivity should be a maximum in the direction
corresponding to maximum lateral fragmentation density of the
missile, and (2) the sensitivity should be a minimum along the
axis of the missile. Directional sensitivity of this type can
be obtained by using the missile as an antenna, with the axis
of the missile corresponding to the axis of the antenna. With
the fuze in the forward end of the missile, such antennas are
excited by means of a small electrode, or cap, on the nose of
the fuze. Additional control over the sensitivity pattern of the
fuze is possible by means of the amplifier gain characteristic.
For use against surface targets, proximity fuzes are designed
for an optimum height of burst, depending on the nature of the
target and the properties of the missile. These optimum heights
vary from 10 to 70 ft for fragmentation and blast bombs and are
of the order of a few hundred feet for chemical warfare bombs.
With a fuze intended for ground approach operation, it is desirable
to have maximum sensitivity along the axis of the bomb. A short
dipole antenna mounted in the fuze transversely to the bomb's
axis gives such sensitivity.
It was also found that fairly good ground approach performance
could be obtained from fuzes with axial antennas by designing
the amplifiers to compensate for the appreciable decrease in radiation
sensitivity in the forward direction. For example, steep angles
of approach generally mean high approach velocities with higher
doppler frequencies. Thus a loss in radiation sensitivity with
steep approach can be compensated by an increase in amplifier
gains for the higher doppler frequencies.
A miniature triode is used for the oscillator in the fuze, and
a pentode for the amplifier. Some fuzes use separate detector
circuits wit a tiny diode to provide the required rectification.
A miniature thyractron serves as the triggering agent, and a specially
developed electric detonator initiates the explosive action.
Energy for powering the electronic circuit is obtained, in the
later fuze models, from a small electric generator. This is driven
by a windmill in the airstream of the missile. A rectifier network
and voltage regulator are also essential parts of the power supply.
The arming and safety features of the radio proximity fuzes are
closely tied in with the power supply. This is a natural procedure
since an electronic device is inoperative until electric energy
is supplied. Arming a radio proximity fuze (generator type) consists
of the following operations: (1) either removal of an arming wire
which frees the windmill, allowing it to turn in the airstream
(bomb fuzes), or actuation of a setback device freeing the drive
shaft of the generator, allowing it to turn (rocket and mortar
shell fuzes), (2) operation of the generator to supply energy
to the fuze circuits, (3) connection of the electric detonator
into the circuit after a predetermined number of turns of the
vane corresponding to a certain air travel, and (4) removal of
a mechanical barrier between the detonator and booster, prior
to which explosion of the detonator would not explode the booster.
Generally, operations (3) and (4) occur simultaneously by motion
of the same device.
Additional safety is provided by the fact that unless the generator
of the fuze is turning rapidly the fuze is completely inoperative.
A minimum airspeed of approximately 100 mph is required to start
the generator turning.
Source: Office of Scientific Research and Development. National
Defense Research Committee. Summary, Photoelectric Fuzes and
Miscellaneous Projects. vol. 3 of Summary Technical Report
of Division 4 [Ordnance Accessories] NDRC. (Washington DC:
1946): 2-3. [declassified 27 Oct. 1960].
Significance and Background of the Radio Proximity Fuse (VT)
in World War II
The radio proximity, or VT fuze for artillery shells represents,
as will be readily apparent, a major contribution to the successful
prosecution of the war in Europe as well as in the Pacific. Its
development, production and military use is an outstanding tribute
to continuous and effective collaboration by research groups,
industrial organizations and the military services.
In ordnance terms, a fuze is that part of an artillery projectile
which detonates the explosive charge. An ideal fuze would detonate
the shell at the most favorable position to inflict maximum damage
on the target.
Early in the war, it became disturbingly evident that speed, maneuverability
and heights attainable by modern military aircraft presented a
method of attack against which fuzes currently available for antiaircraft
guns were relatively ineffective. Even with the improvements in
directing antiaircraft gunfire made possible by radar, diminishing
probability of hitting elusive attacking planes made the problem
of defense against aircraft extremely urgent for a nation involved
in the war.
The idea of influence, or proximity fuzes is not unique and was
suggested independently by various persons in the United States
and abroad long prior to 1940. However, the obstacles in the way
of actually developing a fuze of this type seemed insurmountable.
Many technically inclined people who have witnessed an antiaircraft
demonstration have toyed with the idea of a proximity fuze. The
small target area presented by an airplane, together with its
isolation in space, practically forced a consideration of a fuze
which would detonate in the vicinity of the airplane.
The inherent disadvantages in the time fuze and the contact fuze
stimulated this type of speculation. The first type, which detonates
a projectile at a specified time after it leaves the gun, has
been widely used against airplanes and personnel. However, use
of time fuzes requires not only that time of flight from the gun
to the airplane be calculated in advance, but that each fuze be
"set" for this time. Even a slight error in setting
will cause the projectile to explode at a harmless distance from
the target.
The value of the contact fuzed projectile as an antiaircraft device
is also limited, since it must actually hit its target before
it will detonate. As range lengthens, this becomes increasingly
difficult.
It has long been recognized by ordnance experts that the efficacy
of explosive projectiles would be greatly increased if they could
be equipped with fuzes which would be actuated by the influence
of a target. For example, an antiaircraft projectile which would
automatically detonate when it came within lethal range of an
airplane would simplify fire control techniques and would be highly
effective.
Although inventors had suggested almost every possible type of
proximity fuze, in both prewar and war years, they failed to indicate
how the formidable development and engineering difficulties could
be satisfactorily overcome. Such fuzes to be useful for artillery
purposes would have to be capable of withstanding the shock of
acceleration when shot from a gun, in addition to undergoing a
high rate of rotation in flight. Many patents on proximity devices
were issued in various countries, but these also failed to indicate
how the invention would be manufactured.
British scientists were working on proximity fuze devices for
rockets and bombs at least as early as 1939. Captured documents
indicate that German work on proximity fuze development had begun
in the early 1930's, and was still in process when hostilities
ended in the European Theatre.
In brief, there is nothing unique about the "idea" of
a proximity fuze. The possibility that proximity fuzes of various
types might be feasible has been recognized for a long time. The
American achievement, accomplished by no other country, was the
actual development of a proximity fuze that would function and
that could be manufactured by mass-production techniques.
Source: The "VT" or Radio Proximity Fuze: Supplemental
Basic Information Prepared by Applied Physics Laboratory, the
Johns Hopkins University. (Silver Springs MD: The Laboratory,
1945): 3-5. [Released for publication on 20 Sep. 1945.].
Development of Proximity Fuzes (VT) for Projectiles-VT Fuzes
Mks 32 to 60
Introduction
During the summer of 1940 shortly after the formation of the NDRC
[National Defense Research Committee], work was started on the
development of a proximity fuze. The initial development was undertaken
by Section T of Division A of the NDRC. The initial project was
very broad in objective; namely, to develop a proximity fuze of
any type (radio, acoustic, photo-electric, electro-static, infra-red,
etc.) for rockets, bombs, and projectiles. Such a project was
assigned to Section T by the Navy.
At the time this project was started, the primary objective was
to provide better defense against aircraft. Methods of using proximity
fuzes for this purpose then being considered included use in bombs
for air-to-air bombing, use in rockets, and use in projectiles.
At about the time this project was started, it was learned that
the British had been developing proximity fuzes and had some considered
to be fairly promising for use in bombs and rockets. The British
had considered fuzes for projectiles, but felt that the technical
difficulties in making such fuzes rugged enough to withstand firing
from a gun were insurmountable, at least during World War II.
While the original project covered all types of proximity fuzes,
for rockets, bombs, and projectiles, a primary interest to the
Navy was a proximity fuze for the Navy 5"/38 projectile,
as this weapon was the Navy's principal antiaircraft weapon. Section
T at the outset considered the development of the projectile fuze
as a primary objective, and undertook investigations leading toward
achievement of sufficient ruggedness of electronic parts and the
like to permit firing from a gun. By the spring of 1941 work on
the radio type of projectile fuze had progressed to the point
where it appeared to he the most promising type of fuze, and at
that time Section T dropped its work on investigation of other
types of proximity fuzes and concentrated entirely on the radio-type
of projectile proximity fuze. This development ultimately led
to the present type of radio proximity fuze for projectiles and
is the development with which this report is concerned.
In addition to the U. S. Navy interest in the projectile proximity
fuze, the British and the U. S. Army were also interested. Agreements
were made that all projectile proximity fuze work would be carried
out by the Navy and Section T. The Army had also entered the proximity
fuze program, but in line with these agreements, the Army concentrated
on proximity fuzes for rockets and bombs. The British had also
started some work in Canada on the proximity fuze for British
projectiles, and this development was carried out cooperatively
with the Section T program. At that time the priorities for projectile
proximity fuze development were set up as follows:
(1) U. S. Navy, (2) British Navy, (3) U. S. Army, (4) British
Army.
The first development was a fuze, known as the VT fuze Mk 32,
for the Navy 5"/38. This development was followed by modifications
in design to permit adaptation of the projectile fuze to British
Navy guns, U. S. Army guns and British Army guns. At that time
the primary objective was to provide better defense against aircraft;
and hence, the fuzes being developed were all antiaircraft fuzes.
Some thought was given to use of a proximity fuze to achieve air
bursts over ground for anti-personnel work, etc., but it was not
until late in 1942 that much emphasis was placed on this use of
the proximity fuze. This use of the proximity fuze would necessarily
require enormous quantities of fuzes; and hence, its attainment
depended upon a design not only small enough to be used in Army
projectiles, but also simple enough to produce in very large quantities.
The first fuze developed, which was the Mk 32 for the 5"/38,
was too large and too difficult to produce to be used for this
purpose. Continual effort toward reduction in size and simplification
of design finally led to the development of the Mk 45 which was
suitable for such use, and from that tine on emphasis was also
placed on the field artillery use.
Development and Tests
The original development of the radio proximity fuze for projectiles
was undertaken by Section T, NDRC, at the Department of Terrestrial
Magnetism of the Carnegie Institution of Washington. The problem
of devising circuits to detect proximity of objects was simple
enough, and it appeared that fuzes could be made to operate on
the same principle, provided circuits could be made small enough
to be contained in a projectile and rugged enough to withstand
firing from a gun. Right from the start, it appeared that the
development of vacuum tubes sufficiently rugged this purpose would
be the most difficult problem to solve. Late in 1940, experiments
were made with commercial vacuum tubes mounted in blocks and dropped
on concrete or armor plate to test for ruggedness. A surprising
degree of ruggedness was evident and it seemed reasonable to hope
that the problem of developing rugged vacuum tubes was not insurmountable.
As a very small size was also required for these tubes, investigations
were made with small hearing-aid types of tubes them commercially
available. Among these were Raytheon and Hytron hearing aid types.
As glass breakage of the tube was also a problem, investigations
were started on methods of potting the tubes to protect the glass.
Likewise, work was started on improvement of electrode structures
and methods of mounting to achieve better mechanical strength.
It was soon decided that the best way to test tubes and other
components for ruggedness was to actually fire then from a gun
and recover them to examine for extent and causes of failure.
Early in 1941, experiments were carried out in which tubes were
mounted in blocks in a 5"/38 projectile arranged for parachute
recovery. Other means of recovery firing were also undertaken.
A smooth bore gun was made out of a piece of gas pipe and set
up in a farm yard for testing of tubes and components. This gun
was fired vertically and the projectiles, which were homemade,
fell back in the field where they could be recovered and disassembled.
This gun was later superseded with an Army 37mm gun used for recovery
firing.
Concurrently, circuit work was carried out in the laboratory.
Also, functioning oscillators were mounted in projectiles and
fired in attempts to get functioning in flight. Both the 5"/38
and the 37mm guns were used in these tests. Radio receivers were
used in an attempt to hear the signal from the oscillator during
flight. As a source of power for the unit in the 5"/38 projectile,
special batteries built by the National Carbon Company for the
bomb fuze were used. For the unit in the 37mm projectile, a special
battery was built using National Carbon Company's minimax cells
for B-power and pen-light cells for A-power. At about the end
of April 1941, an oscillator fired in the 37mm gun was actually
heard throughout flight.
By June 1941, circuit work had been carried to the point where
a circuit of sufficient sensitivity and small enough size to be
contained in a fuze could be made. The circuit consisted of an
oscillator, a two-stage audio frequency amplifier, a thyratron,
and an electric detonator developed by Hercules Powder Company
connected in the thyratron output in such a fashion that it would
initiate the explosive detonation. A dry battery built by the
National Carbon Company and similar to the unit used in the 37mm
test projectile was used as a source of power. Switches, known
as set-back switches and developed by Section T, were used in
the fuze to close the battery circuits upon firing of the projectile.
An electrical arming delay was incorporated in the circuit to
prevent arming of the fuze until after the tube filaments had
heated and the unit had quieted down after the initial impact
of firing. The oscillator radiated a radio frequency signal. Some
of the energy from this radiated field would be reflected back
from any target in the vicinity of the projectile in such a fashion
as to react upon the oscillator, causing an audio frequency signal
which was then amplified by the amplifier and used to trigger
the thyratron. The electric detonator in the thyratron output
circuit initiated detonation of the auxiliary detonator and hence
the explosive charge. At this time development had progressed
to the point where a complete mechanical design of a proximity
fuze was laid out.
In order to improve facilities for recovery firing, a test field
was set up at Stump Neck, Maryland, where a 57mm gun was mounted
for recovery firing. This gun was selected because it was the
smallest gun which fired a projectile large enough to contain
a fuze of the size necessary to accommodate the required components.
Special recovery projectiles were developed for this gun which
could be used to carry the complete fuze or to carry any of the
components being tested for improvements in ruggedness. The projectiles
fired from this gun were arranged to carry a small smoke puff
to indicate operation of the fuze and detonation of the electric
detonator.
By September 1941, a complete fuze had been made to ride throughout
flight and function properly at the end of the trajectory. Troubles
at this time were primarily premature functioning of the fuze
caused by mechanical breakage, by microphonic disturbances from
the tubes and the circuit, and voltage fluctuations from the battery.
Considerable vertical firing was done of tubes and refinements
were made in tube design which ultimately led to satisfactory
tubes. Circuit designs were modified by such means as shaping
the amplifier response to minimize microphonic noises. Refinements
of the battery were directed toward more rigid construction, more
positive contact, etc. to minimize spurious voltages from these
sources. The cannon primer was refined in strength so that it
could be made sufficiently rugged. This amounted primarily to
modifications in design of the bridge wire and bridge wire support.
In September 1941, tests of complete fuzes were started at Naval
Proving Grounds, Dahlgren, in the 5"/38 projectile. Early
Dahlgren tests were not very successful primarily because of extreme
premature failures. At this time a double filament triode tube
was being used as an oscillator, and it was discovered that beats
between these two filaments set up microphonic noises within the
audio frequency pass band of the amplifier and were probably the
cause of much of this premature trouble. Consequently, the oscillator
tube was then changed to a single filament type.
In the fall of 1941 the Sylvania Company was brought into the
tube program and contributed greatly toward the development of
improved types of tubes. Throughout this same period considerable
work was done toward refining quality of glass on the miniature
tubes and improved methods of potting of the tubes to overcome
glass breakage failures. During this period work was started at
RCA on the development of metal envelope miniature tubes to overcome
the glass breakage failures, However, improvements in manufacture
and in methods of mounting glass tubes eventually overcame these
tube failures and the metal tube development was subsequently
abandoned.
By January 1942 a test had been conducted at Dahlgren which gave
slightly better than 50% successful performance which was considered
to be adequate to bring a manufacturer in the program. Up until
this time all manufacture of test fuzes had been carried out by
Section T facilities and by the Erwood Company which was brought
into the program in the fall of 1941. At this time a development
contract was given to the Crosley Corporation with a view toward
ultimate production.
Throughout all this early development period, considerable question
remained in the minds of many people that the position of bursts
of proximity fuzes of this type around an airplane target might
not be properly located to cause maximum or even any damage from
projectile fragments. Accordingly, considerable study was made
of proper amplifier frequency response curves, etc., with a view
toward achieving the proper positioning of influence or proximity
bursts. Likewise, the University of Michigan had been brought
into the program and had been doing small scale model work to
study these various features. From laboratory investigations,
it appeared that proper directionality or positioning of bursts
had been achieved but in the spring of 1942 it was decided to
conduct a test against a full-scale model to ascertain the effectiveness
of proximity bursts.
Accordingly, in April 1942, a test was conducted at Parris Island,
North Carolina, in which proximity fuzes in 5"/38 projectiles
were fired against a full-scale airplane target suspended beneath
a balloon. An analysis of the results of this test appeared to
show that the fuze was functioning effectively, although some
question remained because the projectiles used were black powder
loaded which caused the bursts to be somewhat late and made the
burst pattern appear slightly behind the target. However measurements
of delay in firing caused by black powder loading seemed to clear
up this discrepancy. In later tests of this sort when black powder
loading was used, the fuze threads were relieved so that the fuze
would be blown out instantaneously rather than delay while sufficient
internal pressure was being built up within the projectile.
Throughout all this early development, one important item was
that of providing an adequate safety feature to prevent functioning
of the fuze until it had traveled a safe distance beyond the muzzle
of the gun. The use of an auxdet [auxiliary detonator], of course,
provided safety against bore bursts, but was no assurance against
bursts just outside the muzzle. This safety feature was to be
achieved by incorporating a mechanical clock as one component
of the fuze arranged to prevent functioning of the fuze within
a time interval of about 3/10 to 5/10 second after firing. All
early tests were conducted using fuzes which contained only the
RC delay. It was not until the middle of 1942 that a satisfactory
safety clock had finally been developed. The design finally developed,
known as the Mk 1 clock, was more or less a modification of the
Mk 18 time fuze movement. When this development was finally achieved,
all the essential components for a satisfactory proximity fuze
were available. Accordingly, plans were them made to carry out
actual drone firings from a Navy ship.
This test firing of proximity fuzed 5"/38 projectiles against
drones was carried out in August 1942 aboard the cruiser [USS]
Cleveland [CL-55]. Results of this test were entirely satisfactory
and accordingly, full-scale production of proximity fuzes was
initiated at the Crosley Corporation in September 1942. Early
production was plagued with numerous difficulties but satisfactory
material was finally produced. This fuze, which was designated
the Mk 32, was delivered to the Fleet during November and December
1942, and the first Japanese plane was shot down with proximity
fuzed projectiles by the cruiser [USS] Helena [CL-50] in
January 1943.
Early in 1942 it became apparent that the complexity of the proximity
fuze was such that to a large extent its successful manufacture
would require very careful and extensive quality control procedures.
While the Bureau of Ordnance was setting up an organization to
handle the proximity fuze program, it did not possess facilities
to handle adequately this phase of the program, Consequently,
Section T was requested to assume responsibility for quality control
as well as for the engineering and development of proximity fuzes.
Section T undertook to carry this responsibility and as a result
had to expand its facilities considerably. In May 1942, Section
T was divorced from the NDRC and was placed directly under the
OSRD [Office of Scientific Research and Development], becoming
Section T, OSRD. At this time Section T also was moved from the
Department of Terrestrial Magnetism of the Carnegie Institution
of Washington, which did not have adequate facilities for such
an expansion, and was set up in a new laboratory in Silver Spring,
Maryland, under The Johns Hopkins University which took a contract
with the OSRD to carry the proximity fuze work. This laboratory
became known as the Applied Physics Laboratory of The Johns Hopkins
University.
As the second priority for proximity fuze development was an antiaircraft
fuze for British Navy projectiles, work had been underway since
the summer of 1942 for providing a fuze for this use. As British
projectiles were smaller in diameter at the nose end where the
fuze was contained, this problem amounted primarily to that of
shrinking down the size of the Mk 32 fuze to a smaller diameter.
In the original British requirement, it was intended to include
in the fuze an adjustable self-destruction mechanism in order
to avoid having duds fall back down around friendly installations.
For this purpose, design was started on a mechanical clock which
incorporated this feature. As a result of this requirement, the
original mechanical design of the British fuze, which was finally
termed the Mk 33, was somewhat different from the mechanical design
of the Mk 32 although the fundamental assembly of the various
parts was pretty similar to that of the Mk 32. This clock development
did not progress very rapidly and consequently it was finally
abandoned and the British Mk 33 fuze was produced without this
feature being included.
In general, all work on the British fuze paralleled the work on
the Navy's Mk 32 fuze. About the fall of 1942 a contract was placed
with the Radio Corporation of America for production of these
fuzes, and shortly after the first of the year 1943, Eastman Kodak
Company was also brought into the program on this fuze. Early
work on the Mk 33 was rather unsatisfactory and although production
was carried along at a small rate, acceptable material was not
available for sometime. In about May 1943 an emergency program
was set up to iron out the remaining difficulties in the Mk 33
fuze with the hope of obtaining satisfactory material before the
end of the summer of 1943. By September of that year the fuze
was in fairly satisfactory production and deliveries were commenced
to the British. This fuze was designed specifically for the British
4".5 gun which was carried aboard aircraft carriers. In addition,
it was contemplated that the fuze would also work in the British
5".25 Navy gun, but because of more severe treatment of the
fuze in this gun, the fuze was not at that time satisfactory for
use in the 5".25 British gun.
In addition to the development of the Mk 33 fuze for the British,
another fuze known as the Mk 41 was also produced. This latter
fuze was designed primarily for the British 4" gun carried
aboard destroyers, and differed from the Mk 33 in that its size
was still smaller. This was necessary because the 4" projectile
was too small to accommodate the Mk 33 fuze and still leave sufficient
quantity of explosive. The design of the fuze was more or less
similar to that of the Mk 33 except that the mechanical clock
rear fitting safety device was replaced by a newer fitting which
contained a mercury switch to provide the arming delay. Likewise,
the firing condenser which provided the RC electrical delay in
the firing circuit of the thyratron and which had been mounted
in a block down with the mechanical clock rear fitting had to
be placed up in the amplifier section of this fuze.
A second difference between these fuzes and the U. S. Navy Mk
32 was that the dry battery which was used in the Mk 32 fuze could
not be made small enough to fit into the Mk 33 and Mk 41 fuzes
and still retain adequate life characteristics. Such a small dry
battery was produced and used experimentally, but its shelf life
was not greater than two or three months, which was entirely inadequate
for this fuze.
Accordingly, along with fuze development for the British, work
was also started on a new type of battery known as the reserve
battery which was a wet battery containing the active electrolyte
in a glass ampoule [small bulbous glass vessel hermetically sealed]
within the battery which did not permit the battery to become
activated until that ampoule had been broken at setback and the
electrolyte had been distributed over the battery plates. This
development was carried out at the National Carbon Company under
supervision of Section T, OSRD. One of the principal problems
in the development of this battery was to provide a means of supporting
the ampoule so that the battery would be sufficiently rugged for
normal handling and yet permit the ampoule to break on setback.
This was ultimately accomplished by means of various sorts of
breaker mechanisms mounted below the battery ampoule.
The development of the Mk 41 fuze was also somewhat later in being
accomplished than that of the Mk 33 fuze because the British 4"
gun for which this fuze was designed subjected the fuze to appreciably
higher setback than did the other British guns. Mechanical difficulties
in this fuze were finally overcome by November 1943, and these
fuzes were also supplied to the British.
Among the various problems peculiar to the fuze development for
the British were the following: The fuze was originally designed
so that it depended to a considerable extent upon the potting
medium, namely, cerese wax for support of the oscillator bundle.
This seemed to work quite satisfactorily in cold weather when
the wax was hard, but under high temperatures the wax softened
enough so that this support was inadequate. This was overcome
by rigidly fastening the oscillator assembly to the plastic nose.
The rear fitting safety device used in the Mk 33 fuze, which was
essentially a scaled-down version of the Mk 1 safety device used
in the Mk 32, did not operate at the projectile spins encountered.
Testing of these rear fittings had all been done at higher spins
than were encountered in projectiles and did not show up this
trouble. Later when the failure was discovered, a slight modification
of the fittings and a change in testing procedure to check the
units at spins comparable to projectile spins overcame this trouble.
Raytheon tubes were used in the models built by Eastman Kodak
Company and were giving considerable trouble. This was overcome
by improvements in supporting of the tube elements in the micas,
by improved welding techniques, and by other similar modifications.
It was also discovered that upon being fired, the soldered connection
between the antenna cap of the fuze and the radio circuit frequently
melted permitting the connection to separate, causing intermittent
connections which resulted in prematures. It was subsequently
learned, although not at this time fully appreciated, that this
melting was caused by generation of heat from air friction as
the projectile traveled through the air. At this tine a procedure
for waterproofing this fuze was tried which consisted of coating
the fuze nose with a wax made of a mixture of cerese wax and vistanex.
This wax coating protected the solder joint from heat sufficiently
so that this trouble was overcome.
Throughout this entire period of development, work was also underway
on reducing the size of the fuze still further. By the spring
of 1943 a new model, smaller than the Mk 41, had been designed
and work was underway toward proving it in for ultimate use. This
new smaller fuze, which was later termed the Mk 45, appeared to
be small enough and simple enough to manufacture to meet the requirements
of use in Army field artillery. Consequently the Army began to
become quite interested in using this fuze for obtaining air bursts
against personnel, etc. with howitzers in addition to use as an
antiaircraft weapon. By September 1943 successful tests had been
achieved with this fuze and it was started in production at the
Crosley Corporation.
The first model being produced was a model for the Army 90mm antiaircraft
gun. In addition, models were being developed for use in all various
Army howitzers. Production facilities were being expanded in order
to produce the enormous quantities required for such Army uses
and quantity production eventually got underway on these various
models of the Mark 45. Up until this time, because of security
considerations, it had been decided that proximity fuzes should
not be used where there was any chance of a dud falling on enemy
territory and being recovered by the enemy. Consequently, stock
piles were being accumulated with the intent of committing these
fuzes to use at some future opportune tine. Finally, in December
1944, a decision was reached to release proximity fuzes for general
unrestricted use and these fuzes were committed to use in Europe
with outstanding effectiveness.
In parallel with all these developments on the British and the
Army fuzes, continual refinements were being made in U. S. Navy
fuzes. One of the very first problems to become evident on the
U. S. Navy Mk 32 fuze was that its life in storage was inadequate.
Most of this life failure resulted from the fact that the dry
batteries used in this fuze deteriorated after several months
in storage under hot, tropical conditions. Consequently, steps
were taken to use the so-called reserve battery which has a more
permanent life in this particular fuze. In about May 1944, Mk
32 fuzes including the reserve battery were made ready for delivery
to the Fleet.
In addition to this life problem, difficulties also became apparent
in the use of these fuzes against torpedo plane attacks low over
waves. It was apparent that signals returned back from waves to
the fuze when the fuze was fired at low trajectories could also
trigger the fuze prematurely and under certain conditions could
even prevent the fuze from arming at all. To overcome this problem,
a circuit was devised known as the wave suppression feature to
reduce the sensitivity of the fuze to these spurious wave noises.
This circuit, which is in effect an automatic volume control (or
AVC) circuit, was incorporated in the Mk 32 fuze. Mk 32 fuzes
incorporating this feature were produced during May 1944 and delivered
to the Fleet. The first fuzes including AVC were made with the
original type of dry battery subsequent to this, a new model fuze
known as the Mk 40 was supplied which incorporated both AVC and
the reserve battery.
As development of the Mk 45 fuze progressed, it became apparent
that would be desirable to utilize this fundamental design in
certain of the U. S. Navy fuzes. This fuze was the first fuze
small enough to be included in the U. S. Navy 3"/50 projectile.
Consequently, a Mk 45 fuze was produced for the 3"/50 in
about May of 1944. This fuze was delivered to the Fleet, but was
never very satisfactory and its production was ultimately discontinued.
A new fuze, known as the Mk 58, was designed for the 3"/50
which contained more or less the standard Mk 45 design with the
addition of a wave suppression feature to permit use of this fuze
low over waves. The Mk 58 fuze was delivered to the Fleet in November
1944.
Likewise, work was being carried out toward adapting the Mk 45
design fuze to the U. S. Navy 5"/38. Such a fuze, known as
the Mk 53, was finally developed for the 5"/38. When production
of this fuze was commenced in about August 1944, considerable
difficulty was experienced with melting of the solder on the nose
cap similar to that trouble encountered on the British Mk 33 and
Mk 41 fuzes. On this fuze, no water-proofing wax could be used
because of difficulties of use of such wax in fuze pots, etc.
Consequently, the nose connection failure could not be cured in
such a simple fashion. A number of schemes were devised to overcome
this difficulty. One method which enjoyed only moderate success
was to cement a plastic cap over the external tip of the fuze
nose, thus covering the soldered joint and insulating it from
heat. Another scheme which was worked out was to use a welded
connection at that joint. This seemed like a straight-forward
solution, but welding techniques which would not damage the plastic
had to be devised. A third method which has been the most successful
was to devise an oscillator circuit which did not use an antenna
cap but contained only a small wire loop, entirely enclosed in
the plastic nose, for the antenna. This model was known as the
"capless model" and that design principle-was later
extended to practically all of the other fuzes. This so-called
"capless" design was very satisfactory but had some
limitations with regard to frequency ranges which can be achieved.
Although production of the British Mk 33 and Mk 41 fuzes was terminated
early in 1944, it was contemplated that British requirements would
again need to be met along toward the end of 1944. The British
were particularly anxious to get new models of fuzes incorporating
the refinements included in U. S. Navy fuzes and particularly
the AVC wave protection. Consequently, development was also undertaken
on two more fuzes known as the Mk 56 and the Mk 60 which were
modifications of the fundamental Mk 45 design and were intended
specifically for the British Navy guns. The Mk 56 was to be used
in the British Navy 4" and 5".25 guns and the Mk 60
to be used in the British Navy 4" guns and some of the 4".7
guns. These fuzes were essentially similar to the Mk 53 and the
Mk 58 U. S. Navy fuzes and the development program here amounted
primarily to am extension of the development on the corresponding
Navy type. The Mk 56 fuze was finally produced and delivered in
the fall of 1944. This fuze was also plagued with the nose cap
solder-melting failure of the original Mk 53 and in the British
case first attempts at curing this were made by using the cerese
and vistanex wax coating and later by trying cemented-on plastic
caps. The Mk 60 fuze was delivered to the British in the so-called
"capless" design and hence was free of this difficulty.
Additional Navy developments led to the Mk 47 fuze and the Mk
59 fuze. Both of these were again extensions of the basic Mk 53
design to a different caliber projectile. The Mk 47 was designed
for the U. S. Navy 6"/47 gun and differed from the Mk 53
only in its circuit which had to be modified slightly from the
Mk 53 to match the different projectile size. The Mk 59 fuze was
designed specifically for the U. S. Navy 5"/54 projectile
and again differed from the Mk 53 only by a slightly different
contour and a somewhat different rear fitting safety device to
permit operation at the 5"/54 projectile spin.
After the development of the Mk 45 and the extension of that design
to all other types of projectiles, the primary differences in
the different types of fuzes lay in different circuit arrangements
necessitated by different projectile sizes and in different operating
characteristics of the rear fitting safety device to accommodate
different projectile spins. In addition, there were two distinct
types of batteries used in all these fuzes, One being an AA [Anti
Aircraft] battery, the other being a howitzer battery. The difference
here was that the howitzer battery had to be capable of being
activated at considerably lower setbacks than was required in
the AA type of fuze. Later on, battery development progressed
to the point where a universal battery was available that was
sufficiently rugged for handling in any projectile, and yet capable
of having its ampoule broken and the battery being activated at
even the low charges of the Army howitzers.
Present Status
At the present time the U. S. Navy has radio proximity projectile
fuzes for the 5"/38, 6"/47, 5"/54 and 3"/50
guns. These fuzes are all basically the same design, differing
only in the minor modifications required to adapt then to the
different projectile sizes. All are primarily antiaircraft fuzes
although they may be used for shore bombardment to produce air
bursts. The 5"/38 fuze is the only one that has seem very
extensive use in service, but the effectiveness of the other Navy
fuzes is probably comparable to that of the 5"/38 fuze. Analyses
of action reports indicate that the effectiveness of radio proximity
fuzes as compared to mechanical time fuzes in antiaircraft fire
is about the order of a 3-to-1 improvement for the proximity fuzes.
These effectiveness figures, of course, are dependent upon such
matters as fire control accuracy, etc. and represent the effectiveness
obtained with presently used equipment.
The U. S. Army has radio proximity projectile fuzes for the Army
90mm gun, the 75mm, 105mm, 155mm, 8", and 240mm howitzers.
Models have been developed for the 120mm gun, 155mm gun, and 75mm
AA gun. The AA guns have AA fuzes incorporating a self-destruction
feature. The howitzers all have fuzes which are very effective
for producing air bursts.
The British Navy has been supplied with AA fuzes for its principal
AA guns. The British Army has been supplied with AA fuzes for
its 3".7 AA gun and with artillery fuzes for its principal
howitzers.
The principal weakness of all of the U. S. Navy fuzes is that
their life in storage is not adequate. Replacement of the original
dry battery used in the Mk 32 fuze by the reserve battery has
overcome the weaknesses in life characteristics caused by battery
deterioration. However, the fuze itself deteriorates because of
inadequate protection against moisture and humidity, particularly
under tropical storage conditions. This failure is pretty definitely
pinned down to deterioration of condensers used in the electrical
circuits, resulting from entrance of moisture. A primary job now
underway is that of improving fuze waterproofing in order to overcome
this source of life failure. A second weakness in these fuses
is that the AVC or wave suppression feature is not as completely
successful as would be desired. The present AVC fuzes do permit
use moderately low over waves, but further improvements in wave
protection are desirable. A third feature of these fuzes which
needs to be improved is that the present rear fitting safety device
incorporating the mercury switch rear fitting is very dependent
upon projectile spins for operating time limits; and hence, none
of the present rear fittings can be used in different projectiles
at widely different projectile spins. This requires a special
rear fitting for each particular use and also prevents one fitting
in one fuze being used in a single projectile at different charges;
for example, the 5"/38 at service or reduced charges. Likewise,
the problem of holding manufacturing tolerances is quite critical.
Overall performance scores on these fuzes, when new, average from
70% to 80%, but premature failures are still higher than would
be desirable.
Production on U. S. Army fuzes has now been terminated. However,
considerable developments are still underway. The Army has on
hand a large backlog of current types of fuzes which are usable
but possess some inherent limitations which these new developments
hope to overcome. One of the principal limitations of U. S. Army
fuzes is that there is no positive means of preventing prematures
from occurring over the heads of Army troops. Likewise, air observation
planes are in constant danger when proximity-fuzes are fired near
them. These fuzes, likewise, are all relatively vulnerable to
enemy countermeasures. While this failure has not been encountered
yet, it no doubt will in the future and considerable work is underway
toward improving the protection of these fuzes against enemy countermeasures.
Some models were produced which were relatively less vulnerable
than the first models, but even these are still more vulnerable
than they should be.
All work has been terminated on fuzes for the British. The British
themselves are undertaking considerable work on proximity fuzes
of their own, but if the U. S. Navy should again undertake development
of proximity fuzes for British projectiles, these developments
would closely parallel developments in U. S. Navy projectiles.
From the standpoint of production, one of the primary problems
is that very careful and extensive quality control procedures
are necessary to maintain satisfactory fuze quality. This is true
because present manufacturing specifications, laboratory tests,
etc. are not adequate to guarantee satisfactory fuze performance
in service use. In addition to all laboratory tests, actual firing
tests are relied upon to determine fuze performance; but it has
not yet been possible to --completely simulate all aspects of
service conditions in laboratory, handling and stowage, or firing
tests. As a result, it is necessary to exercise very careful control
over component quality, and general manufacturing quality, in
addition to the meeting of all manufacturing specifications and
tests, in order to assure fuze quality.
Because of these difficulties, fuzes have thus far been procured
on a basis of payment for all fuzes produced in accordance with
the present specifications, regardless of whether ultimate service
performance could be guaranteed, and quality has been maintained
by careful follow-up of the quality control procedures. These
quality control procedures have thus far been carried out by the
Applied Physics Laboratory, The Johns Hopkins University, but
steps have been taken to transfer this responsibility plus responsibility
for recommending acceptance for service use of fuzes to the Naval
Ordnance Laboratory. In addition, work is also underway to devise
specifications which will guarantee ultimate fuze performance,
so that the responsibility for producing fuzes satisfactory for
ultimate service use can be placed upon the manufacturer.
Note: British gun bore usage places the decimal directly
under the inch symbol, a combination not readily available. In
the above text the combination, as in 4".5 gun, will be used
to most closely approximate the original.
Source: Dilley, N. E. "Development of Proximity Fuzes
(VT) for Projectiles - VT Fuzes MKS 32 to 60, Inclusive (General
Description)." chapter 1 of The World War II Proximity
Fuze: A Compilation of Naval Ordnance Reports by the Johns Hopkins
University Applied Physics Laboratory. (Silver Spring MD:
The Laboratory, 1950): 1-12. [declassified 16 Jun. 1976].
22 March 2000
