From 1951 to 1957 my daily work was centered around objects similar to those shown above. Our interest, at National
Northern in Hanover , was in the chemical formulations we researched for the cavity in the upper portion which is known as a 20 millimeter
projectile. The fuse was a necessary component but we seldom used the cartridge case which contains propellant.
Our objective wasn't
to develop a better 20 mm but to improve upon the high explosive formulations in use by the military at the time. We thought of the
20 mm projectile as just another test tube in the lab. Our results were regularly reported to the government ordnance facilities at
Piccatiny Arsenal in New Jersey and Aberdeen Proving Grounds in Maryland.
The Allies and their opponents, Germany and Japan , had just
gone through five or more years of combat firing at each other with bombs and other devices filled with TNT. TNT is still widely used.
has many advantages. It is easily synthesized from a petroleum product and common acids. It also has a convenient melting point of
80 C, 20 degrees below the boiling point of water. This makes it easy to concentrate large amounts by melting the TNT and pouring
the liquid into the projectile or bomb, sort of like casting candles in molds. The material is also very stable and resistant to detonation
by impact. This comes in handy when projectiles or bombs get loose and roll around the ground or decks.
TNT was the standard by which
new or researched products were compared. The British, in 1940, opted for an explosive called Tetryl. Tetryl, because of its melting
point, could not be cast but had to be compressed into projectiles .Pellets were made by an automatic press much like the manufacture
of aspirin tablets. The pellets were then placed in the projectile and compressed again. Tetryl's blast performance is slightly better
A company in Hanover, Massachusetts , National Fireworks, suspended their fireworks operation in 1940 to make 20 mm and 40
mm projectiles for the British . Hundreds of locally unemployed found work at the plant. It was easy to tell someone who worked there
because one of the effects of long exposure to the Tetryl was a yellowing of skin.
With the end of the war, the company decided to
sell to a group of scientists who were formerly employed by the government. The name was changed to National Northern and was organized
to do research and development on high explosives with government contracts. The new company needed chemists. The nature of the business
didn't appeal to many prospects. The company also had to be careful about hiring anyone who could be a potential " sparky ", a term
given to a person who has an unusually high tendency to enjoy explosions and incendiary events. Evidently , as a newly married, I
didn't fit that category and was hired.
Labs and test and storage facilities were set up, but first the old fireworks had to be disposed.
A truck loaded the materials, both finished and raw, and brought them to the newly constructed firing range. At twilight, the show
was the most spectacular I have ever seen. Think about five First Night Shows with only a finale. Lots of latent " sparky's " came-out
Technical ordnance people today, looking back at how research was done in the fifties, is like us in the fifties looking
back at a time at the turn of the century when x-rays were just discovered and the electron was a new concept. We thought we were
on the cutting edge of science with our slide rules and mechanical Burroughs calculators. The hand-held or desktop electronic calculator
was twenty years in the future. We did, however, have a variety of equipment which proved to be very useful at the time and some of
which is currently seen in labs.
Our contracts specified that we research various formulations to add to the inventory of available
explosives. Utmost in importance was safety for the worker and end-user. The new material had to meet standards for exposure to heat,
resistance to detonation from impact and other criteria. We did not synthesize new compounds but worked with mixtures.
had just developed a mixture using powdered aluminum and cyclotrimethylenetrinitramine, known as RDX .( RDX incorporated into a plasticizer
makes the well-known C 4.) RDX without the aluminum had about the same blast performance as TNT , but adding the aluminum increased
it considerably. However, RDX and aluminum, which became known as HBX, performed on a par with TNT and all others at high altitude.
Tests in high-vacuum chambers gave results for all explosives that were markedly lower than at ground level. We were looking for high
performance at altitudes not reached in WWII or for that matter not reached by planes at that time.
There were two theories about performance
at high altitudes. One stated that the lack of oxygen did not provide for the so-called" after-burning" which occurred at ground level.
Another theory held that at high altitudes there were very few molecules available to carry the shock-wave. These two theories and
the need to develop something with a big bang at low pressures kept us busy for about six years.
Occasionally we would formulate
by the seat of the pants, i.e. , somebody would have a hunch that if we tried this or that we would have an explosive that would perform
well. The hunch wasn't entirely out of the blue but not exactly based on tried and true scientific approaches. Progress came about,
as expected , when hunches were abandoned.
Someone suggested that we bring more oxygen to the explosion in the mixture. RDX, TNT .
Tetryl , PETN and other standards were limited in the amount of oxygen they carried as part of their structures. We made a big discovery
first tried , all be it at ground level , by the Chinese with their black powder hundreds of years previously. Potassium Nitrate,
known as saltpeter , has the oxygen that the charcoal and sulfur use in the reaction. We wanted something with more oxygen and found
it in a common chemical , ammonium perchlorate. When we substituted most of the RDX from HBX with ammonium perchlorate ,we had something
that worked. Ammonium perchlorate was chosen by a thorough search in the literature of every oxygen containing inorganic compound.
Three criteria were involved, melting point ( solid at room temperature ), percentage of oxygen and heat of formation ( ability to
release oxygen at reasonable temperatures). We conducted the search by flipping through pages of reports, books and articles. Notebooks
and pens were readily available. A computer that could do this was thirty years in the future. Our final formulation, Powdered aluminum
, ammonium perchlorate and only six percent high explosive RDX produced a mixture( MOX ) that was adopted by the military and is currently
in use. Evidence of its wide use is the prevalence of ammonium perchlorate in the environment near military bases.
test of a newly mixed formulation was impact sensitivity. I expect that the test we used in the fifties is still used today. A small
amount of the mixture , about the amount that would cover your little fingernail, was placed on a steel plate. A carpenter's framing
hammer was dropped onto the mixture from various heights. If the mixture detonated with a drop of a few inches, which happened frequently,
it was considered too sensitive for consideration. The standard was TNT which would require an actual swing from about 12 inches to
detonate. RDX was about a 3 to 4 inch drop. When coated with a special wax the impact sensitivity improved to about that of TNT. More
meaningful data was then obtained by using impact sensitivity equipment accepted by the military.
Testing ( Continued )
There were three
types of testing for performance. All testing was static unless a special request by the military necessitated firing from a 20 mm
gun into a target. The static testing examined performance by three methods: pressure increase in a chamber, shock-wave propagation
( chamber and range, Fastax Photography ), target damage ( chamber and range ).
We had two steel chambers. One was a 3 ft box and the
other a 6 ft box. Both were constructed of armor plate 1 in thick. The 3 ft chamber had a port on top . The 6 ft chamber had a door
that covered the front side and ports on the other sides. Both were exhausted through the roof of the building.
Loading Of Projectiles
20 mm projectile is about 3 inches long and a little larger than 3/4 inch diameter. Threads extended 9/16 inch to accommodate a brass
impact fuse. The cavity was filled with the formulation by casting or hydraulic press. Casting was used primarily for TNT.
were made from the mixture with a hand press. The projectile would take five pellets about 1/2 in long. After inserting each pellet,
they were compressed into the round. These operations, as one would expect, were carried about behind a safety shield. In six years
of operation, no accidental explosion occurred with this process. Although, word had it that press accidents did occur at National
Fireworks during WWII production of Tetryl loaded 20 mm's.
The brass fuse required modification before insertion. The fuse was placed
into a lathe chuck and a 1/4 inch hole drilled through the brass to the cavity that contained the initiation explosive. There is about
a 1/16 in leeway before hitting the initiator. This was not a frequent occurrence but it did occur from time to time. When it occurred
, it was disconcerting to others who may be in the vicinity. A 20 mm fuse detonation inside a building makes a lot of noise. Now a
blasting cap can be inserted into the hole , taped in and the round is ready for testing.
Fastax High Speed Photography
The Fastax camera
was first used by EG&G ( Egerton, Germeshausen and Grier ) at MIT. You may have seen some of their photos of milk drops, projectiles
penetrating a playing card and the golf swing. Their most important contribution was at Los Alamos with the test of the first atomic
bomb . (See photo on first page ). They went on, in the fifties, to develop Schleirin and Stroboscopic photography at much higher
speed. More than once we found it necessary to load up the camera and its supporting equipment and bring them to MIT for trouble-shooting.
The Fastax camera has two variable voltage synchronized DC motors and a revolving prism. The camera and event (explosive detonation)
are controlled by a "black box" unit about the size of a large suitcase, called a" Goose".It was this large because the circuitry
included vacuum tubes .The operator inserts a 100 ft roll of high speed 16 mm motion picture film onto the drive motor, then next
to the octagonal prism and onto the take-up motor. The prism rotates behind an open shutter. At maximum speed , the camera will drive
and take-up the 100 ft roll in 0.8 second. This speed corresponds to an exposure of 14,000 pictures per second. After compensating
for the blasting cap and fuse ignition delay, the operator sets the Goose to run almost 80 feet of film before detonation. The last
few feet of the film shows the explosion. It would be helpful to show photos but they are not available. It was interesting to observe
detonation in slow motion and individual frames. Fragment velocity could be determined by examining punctures in nearby aluminum plates
at various distances as they occurred. Most photos showed detonation at the fuse and base before sides of the projectile exploded.
Chamber ( 3 ft )
The small chamber was equipped with a Foxboro Pressure Gauge. The round was suspended in the center of the
chamber and detonated. The pressure buildup was pen-recorded on the gauge. Ammonium Perchlorate formulations gave results about eight
times that of standard TNT, RDX and TETRYL. To test for high altitude performance, a vacuum pump exhausted the chamber. The pump provided
a pressure corresponding to 100,000 feet. Performance for all explosives dropped markedly. The same relative comparisons among all
persisted. It was decided to test the after-burning theory by exhausting the chamber and then drawing in pure oxygen until the chamber
was at ground level( atmospheric pressure ). If the poor performance at high altitude was due to the lack of surrounding oxygen, as
postulated, the performance at ground level with pure oxygen should provide a considerable increase in performance. Those who did
not like the after-burning theory thought the blast would be comparable to regular air at ground level and decided to take no special
precautions. Those who liked the after-burning theory, decided to leave the test chamber building. Some speculated there would be
so much pressure built up in the chamber, the port would dislodge and blow through the roof of the building or the sides of the chamber
would peel open. I joined the group who left the building, not because I supported the after-burning theory but who wants to defend
any theory with his life? The results showed that detonating a 20 mm projectile in almost pure oxygen , whether with a pure explosive
like TNT ,RDX , Tetryl or Aluminized like MOX, is no different than ordinary air. So much for the "after-burners", who felt sort of
sheepish after the test.
Target Testing At The Range
A target consisted of a 24 inch angle iron box fitted with removable aluminum plates
on all six sides. The plates were standard thicknesses and strength as specified by the military. The plates were bolted to the frame
with a standard torque . The box was set on a stand and the test round placed inside the center of the box target. These tests sort
of reminded me of the days when we were kids and placed cherry bombs inside the center of cans. The routine for wiring , whether from
a Ten Shot with a handle or direct wiring to a battery , required the raising of the wire-ends to show they weren't connected to anything
before making the connection at the round. The damage to the target was assessed and rated from very low performance, where the sides
remained in tact with little or no outward bulge to high performance where the sides were blown completely off the target. The low
performers were evaluated further by examining the number and nature of fragment holes. TNT, RDX and Tetryl bulged the sides a few
inches, with varying fragmentation performances. Our aluminum, ammonium perchlorate, RDX formulation ( MOX ) blew all sides completely
off the target. The military people , who witnessed some testing , were quite impressed. Contracts were renewed.
Target testing was
also carried out in the large chamber under various altitudes and ambient atmospheres. Shock Wave Testing The demise of the after-burning
theory led to testing to correlate shock wave propagation in various media and altitudes. The gauge was about the size and shape of
a spark plug. The surface was a thin metallic material known as a catenary diaghram. When exposed to a sudden push inward , as it
would be when hit with a shock wave , a change in orientation of the inner wiring would result in a resistance change and an EMF.
The slight voltage was amplified and observed on an oscilloscope. We screwed the gauge into a side port of the large chamber and tested
our standard and experimental formulations. We tried to get many tests from one gauge but we never knew when a fragment would hit
the diaghram. A baffle to protect it would interfere with the shock wave. A photographic record of each test was obtained by a Polaroid
camera attached to the oscilloscope. The trace was carefully studied to show maximum amplitude( wave pressure ), slope to maximum
, time of wave and negative rebound. Brisance was clearly evident when a shock wave had a high amplitude, steep slope but little duration.
It was shown that the density of the surrounding medium, whether it be air or any gas, correlated with the shock wave produced by
the explosion. As expected,performance on targets correlated well with shock wave and our MOX surpassed all others.