Infrared tracer for ammunition

A pyrotechnic composition that emits essentially only infrared radiation upon combustion contains at least one peroxide component, an oxidizer that is more energetic than the peroxide component, a coolant, a binder and silicon. In one embodiment, the peroxide component is a mixture of strontium peroxide and barium peroxide, the oxidizer more energetic than the peroxide component is barium nitrate, the coolant is magnesium carbonate, the binder is calcium resonate and the silicon has a minimum purity of 99.9%. The pyrotechnic composition is useful as the combustible component of an infrared tracer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a tracer composition having an infrared radiation 
output. More particularly, a combination of the tracer composition and 
particulate size extends the burn time and reduces the visible output. 
2. Description of the Related Art 
Ammunition shells containing a combustible tracer composition are used by 
gunners to determine a proper firing trajectory. One type of tracer 
composition has an infrared signature. On combustion, the tracer 
composition generates infrared radiation, with a typical wavelength of 
between 2.5 and 14 microns. Very little of the tracer combustion output is 
in the visible light range. The gunner, or an observer teamed with the 
gunner, can view the infrared emission through night vision goggles or 
other system sensitive to infrared output. The absence of an output in the 
visible spectrum makes it difficult for an enemy to determine the gunner's 
location. 
One infrared tracer composition, developed by the United States Army, is 
designated R-440. This composition, a mixture of barium peroxide, 
strontium peroxide, calcium resonate and magnesium carbonate, is disclosed 
in U.S. Pat. No. 3,677,842 to Doris, Jr. that is incorporated by reference 
in its entirety herein. 
The R-440 formulation tends to burn quite quickly. The volume of tracer 
composition contained in a standard 30 millimeter shell is consumed in 
about 5 seconds. Modem ammunition trajectories sometimes require a flight 
time in excess of 5 seconds reducing the efficacy of the R-440 tracer mix. 
Additionally, the R-440 tracer mix combustion output is partially in the 
visible spectrum, possibly placing the gunner at risk. 
U.S. Pat. No. 5,639,984 to Nielson discloses a covert infrared tracer 
composition with a combustion output that is disclosed to be essentially 
free of visible emissions. This composition contains a mixture of an 
alkaline metal compound, a burn rate catalyst, at least one peroxide and a 
binder. The Nielson patent is incorporated by reference in its entirety 
herein. 
The composition disclosed in the U.S. Pat. No. 5,639,984 is formed into 500 
micron-800 micron sized particles by a solvent evaporation process. Such a 
"wet" process is time consuming and increases the cost of the tracer mix. 
In addition, this tracer mix burns at about the same rate, or faster, than 
the R-440 tracer mix. 
There remains, therefore a need for a tracer composition that has an 
infrared output substantially free of visible light that is both 
economical to produce and has a burn rate superior to R-440. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to provide a combustible 
tracer mix that emits substantially entirely infrared radiation on 
burning. It is a feature of the invention that the tracer composition is a 
mixture of oxidizers, fuels and burn rate modifiers. Each component of the 
tracer mix is provided in a specified amount and a specified particulate 
size. Another feature of the invention is that the constituents of the 
tracer composition are preferably mixed dry, without the necessity of 
adding a solvent, and then compacted. 
Among the advantages of the tracer composition of the invention are that 
the composition has a burn time considerably longer than that of R-440 and 
an output that is substantially within the infrared spectrum. A further 
advantage is that due to the dry processing, the tracer composition may be 
manufactured economically in large quantities. 
In accordance with the invention, there is provided a pyrotechnic 
composition that has an output of substantially infrared radiation on 
combustion. By weight, the composition consists essentially of from about 
20% to about 90% of at least one peroxide component, from about 1% to 
about 20% of an oxidizer that is more energetic than the peroxide 
component, from about 5% to about 15% of a burn rate modifier, from about 
5% to about 15% of a binder and from about 0.1% to about 11% of silicon. 
The above stated objects, features and advantages will become more apparent 
from the specification and drawings that follow.

DETAILED DESCRIPTION 
The pyrotechnic composition of the invention has, as a first constituent, 
at least one peroxide component. Suitable peroxide components include 
strontium peroxide, barium peroxide, potassium peroxide, ammonium 
peroxide, sodium peroxide and mixtures thereof, with strontium peroxide, 
barium peroxide and mixtures thereof being preferred. The peroxide 
component content should be at least about 35% by weight and less than 
about 90% by weight. If the peroxide component is present in an amount of 
either less than about 35% or more than about 90%, then the oxygen to fuel 
ratio will not support proper ignition or burn characteristics. The 
peroxide component may be made up of more than one peroxide such as a 
mixture of strontium peroxide and barium peroxide. One exemplary tracer 
composition contains strontium peroxide and barium peroxide in a weight 
percent ratio of about 1:1. 
A second component is an oxidizer that is more energetic than the peroxide 
component. The energetic oxidizer increases the reliability of the burn 
without increasing the visible output of the tracer and without providing 
an unacceptably large increase in infrared output that could overwhelm the 
infrared detection system. A preferred energetic oxidizer is barium 
nitrate. Other suitable energetic oxidizers include ammonium perchlorate, 
potassium perchlorate, sodium nitrate, ammonium nitrate, guanidine nitrate 
and strontium nitrate. 
The minimum quantity of energetic oxidizer is about 1%. In concentrations 
less than about 1% the ignition reliability becomes suspect. When the 
concentration of the energetic oxidizer exceeds about 20%, then the 
combustion products become visible. 
The next constituent of the pyrotechnic composition is a coolant present in 
an amount of from about 5% to about 15%. One preferred coolant is 
magnesium carbonate. Other suitable coolants include the oxalate family of 
compounds such as ammonium oxalate, strontium oxalate, sodium oxalate, 
barium oxalate, calcium oxalate and mixtures thereof. 
When the coolant content is less than about 5%, the combustion products are 
visible. When the coolant content exceeds about 15%, the tracer 
composition has poor burning characteristics. 
A binder maintains the other constituents of the pyrotechnic composition, 
that are provided in particle form, together. The binder is selected to 
conform to the other constituents at a pressure of less than about 85,000 
psi. One preferred binder is calcium resonate. Other suitable binders 
include polymers such as polyurethanes and epoxies. These binders increase 
the structural integrity of the tracer material. 
The binder is present in an amount of from about 5% to about 15%. When the 
binder content is either below about 5% or exceeds about 15%, the binder 
does not maintain the integrity of the tracer composition in flight and 
break-up of the tracer may result. 
Another component of the pyrotechnic composition is silicon having a purity 
of at least 98%, by weight, and preferably having a purity in excess of 
99.9%, by weight. The silicon is preferably in an amorphous form. High 
purity silicon is required since impurities in the silicon tend to produce 
visible emissions on combustion. 
The silicon effectively increases the burn intensity of the tracer 
composition, improving burn reliability. Preferably, the silicon is 
present in an amount of from about 0.1% to about 15%. When the silicon 
content is less than about 0.1% the tracer composition burns erratically. 
When the content exceeds 15% the tracer composition burns to quickly. 
The constituents of the pyrotechnic composition are provided as relatively 
small particles. Since the particles are of random shape, the particle 
size is determined by passing the particulate through a sieve and 
identifying the largest number sieve through which the particles would 
fall. For the peroxide component, the particles should pass through a 
number 100 sieve, maximum dimension about 149 microns. The oxidizer that 
is more energetic than the peroxide component should be slightly smaller 
than the peroxide component and pass through a 140 mesh sieve, for a 
maximum particle size of about 105 microns. The coolant may be somewhat 
larger, passing through a 35 mesh sieve for a maximum particle size of 
about 400 microns. The binder should pass through an 80 mesh sieve for a 
maximum particle size of about 177 microns and the silicon should pass 
through a 100 mesh screen for maximum particle size of about 149 microns. 
The use of relatively small particulate for the constituents of the 
pyrotechnic composition facilitates both intimate mixing of the 
constituents and a consistent burn rate throughout the entire pyrotechnic 
composition. The constituents are combined in their desired proportions 
and mixed in a cone blender, or equivalent, until a substantially 
homogeneous composition is achieved. Typically mixing for from about 30 
minutes to about 2 hours will achieve the desired degree of homogeneity. 
The tracer constituents can be mixed dry, or wet--with the inclusion of a 
solvent to be subsequently evaporated. 
The tracer mixture is then compacted into an ammunition shell of a desired 
caliber. Preferably, the ammunition shell is steel and compaction is by a 
hydraulic press in a two-stage process. Approximately one half of the 
tracer mixture is compacted at a first pressure and then the second half 
is added to the shell and compacted at a second, lower, pressure. 
Increasing the pressure of the first portion decreases the burn rate of 
that portion. Exemplary compaction pressures are 85,000 psi for the first 
portion and 72,000 psi for the second portion. When the shell is fired, 
the tracer composition is ignited by hot gases emitted by a propellant. 
A preferred pyrotechnic composition having an infrared output consists 
essentially, by weight, of: 
strontium peroxide: 19.5%-49.5% 
barium peroxide: 19.5%-49.5% 
magnesium carbonate: 5%-15% 
calcium resonate: 5%-15% 
silicon: 0.1%-11% 
barium nitrate: 1%-20% 
A more preferred composition for the tracer is, by weight: 
strontium peroxide: 32%-36% 
barium peroxide: 32%-36% 
magnesium carbonate: 8%-12% 
calcium resonate: 8%-12% 
silicon: 0.5%-1.5% 
barium nitrate: 8%-12% 
The advantages of the tracer composition of the invention will become more 
apparent from the Example that follows: 
EXAMPLE 
A homogeneous mixture of tracer composition having the nominal composition, 
by weight, of: 
strontium peroxide 34.5%; barium peroxide 34.4%; magnesium carbonate 10%; 
calcium resonate 10%; silicon 1% and barium nitrate 10%, with particle 
sizes as specified above, was hydraulically loaded into 30 millimeter 
steel ammunition shells. Each shell weighed 240 grams and contained about 
5.5 grams of tracer composition. One group of 20 shells was heated to 
+60.degree. C. (+140.degree. F.) and a second group of 20 shells was 
cooled to -32.degree. C. (-25.degree. F.) and maintained at temperature 
for a minimum of 2 hours to achieve equilibrium. The shells were then 
fired and the percentage of no-fires recorded. 
The burn time was then determined by spotters wearing infrared goggles 
using a stop watch to time the interval from the appearance of an infrared 
emission to the end of the emission. Another spotter, without infrared 
goggles would determine the presence of any emission in the visible 
spectrum. The results are as indicated in Table 1. 
TABLE 1 
______________________________________ 
VISIBLE 
TEMPERATURE 
BURN TIME NO-FIRES EMISSION 
______________________________________ 
140.degree. F. 
8.34 seconds 
0 no 
-25.degree. F. 
11.18 seconds 
0 no 
______________________________________ 
The tracer composition of the invention was then compared to both the R-440 
and a composition as described in U.S. Pat. No. 5,639,984. As indicated in 
Table 2, the tracer composition of the present invention is an improvement 
over both of the prior compositions. 
TABLE 2 
__________________________________________________________________________ 
TRACER VISIBLE 
COMPOSITION 
TEMPERATURE 
BURN TIME 
NO-FIRES 
OUTPUT 
__________________________________________________________________________ 
R-440 140.degree. F. 
5.47 seconds 
25% yes 
R-440 -25.degree. F. 
4.72 seconds 
40% yes 
5,639,984 
140.degree. F. 
1.62 seconds 
0 no 
5,639,984 
-25.degree. F. 
3.35 seconds 
0 no 
__________________________________________________________________________ 
It is apparent that there has been provided in accordance with this 
invention a tracer composition that fully satisfies the objects, features 
and advantages set forth hereinabove. While the invention has been 
described in combination with specific embodiments thereof, it is evident 
that many alternatives, modifications and variations will be apparent to 
those skilled in the art in light of the foregoing description. 
Accordingly, it is intended to embrace all such alternatives, 
modifications and variations as fall within the spirit and broad scope of 
the appended claims.