Propellant configuration

An ammunition cartridge has a base and a sidewall extending from the base to a mouth. The sidewall bounds an interior of the case. A projectile is secured to the case proximate the mouth and has at least an aft portion within the case interior. A propellant charge is located in a first region of the case interior at least in part surrounding the aft portion of the projectile. A second propellant charge is carried within a second region of the case interior generally aft of the first region.

BACKGROUND OF THE INVENTION 
The invention is directed to ammunition in which a portion of the 
projectile extends substantially aft into the ammunition case. The 
invention is particularly suitable for use with armor-piercing 
fin-stabilized discarding sabot (APFSDS) ammunition. 
There exists a well-developed art in the field of APFSDS (including, inter 
alia, APFSDS-T (with tracer)) ammunition. APFSDS rounds have been 
developed for both rifled barrels (e.g., the 105 mm barrel of the 
relatively old M60 tank) and smoothbore barrels (e.g., the 120 mm barrel 
of the relatively new M1A2 tank). A rifled barrel or tube functions to 
spin-stabilize a projectile encased in the sabot, a principle utilized in 
a majority of modem weapons from handguns to large naval guns. An 
exemplary muzzle velocity is from about 1,375 to about 1,650 meters per 
second. A projectile exiting the muzzle of a rifled tube typically also 
has a relatively high spin rate. Once the projectile is free of the sabot, 
it relies on its aerodynamic fins for stability at a relatively low spin 
rate (e.g., about 70 revolutions per second (rps)). Upon discard of the 
sabot, the aerodynamic interaction of the projectile's fins with the air 
angularly accelerates the projectile to ultimately induce a desired low 
rate of spin (e.g., about 70 rps). With a smoothbore tube, upon discard of 
the sabot, the aerodynamic interaction of the projectile's fins with the 
air angularly accelerates the projectile to ultimately induce a desired 
low rate of spin (e.g., about 70 rps). 
BRIEF SUMMARY OF THE INVENTION 
The inventor has observed that in conventional APFSDS ammunition 
cartridges, the propellant charge is ignited via a primer at the base of 
the cartridge and therefore burns generally from its aft end to its fore 
end. When conventional propellant (formed in extruded strands) is replaced 
with economical spheroidal propellant (typically formed as flattened 
spheres), the inventor has observed damage to the projectiles fired from 
such ammunition. This has, in particular, been observed in the 25 mm M919 
APFSDS-T round. Such globular propellant is disclosed in U.S. Pat. No. 
2,027,114, of Fredrich Olsen, the disclosure of which is incorporated 
herein by reference in its entirety, and is sold under the trademark BALL 
POWDER by Primex Technologies, Inc. of St. Marks, Fla. The inventor 
believes that compaction of the propellant near the fore end of the case 
(by the initial combustion of the propellant at the aft end) leaves an 
annulus of propellant forward of the projectile fins yet unburned when the 
combustion of propellant aft of the projectile drives the projectile 
forward. The fins therefore collide with this annulus of compacted 
unburned propellant and are damaged, degrading projectile performance. 
Accordingly, in one aspect, the invention is directed to an ammunition 
cartridge. The cartridge has a case having a base and a sidewall extending 
from the base to a mouth. The sidewall bounds an interior of the case. A 
projectile is secured to the case proximate the mouth and has at least an 
aft portion within the case interior. A propellant charge is located in a 
first region of the case interior at least in part surrounding the aft 
portion of the projectile. A second propellant charge is carried within a 
second region of the case interior generally aft of the first region. The 
second propellant charge has a burn rate slower than a burn rate of the 
first propellant charge. 
The cartridge may include a combustible containment tube containing the 
first propellant charge and separating the first propellant charge from 
the second propellant charge. The cartridge may include a flash tube 
having an aft end proximate the base of the case and a fore end coupled to 
an aft end of the containment tube. The projectile may include a body 
having a nose and a tail. The projectile may further include a plurality 
of stabilizing fins projecting from the body. The cartridge may further 
include a discardable sabot. The sabot may have an engagement portion for 
surrounding the projectile and engaging the projectile to prevent relative 
longitudinal movement of the projectile and sabot. The sabot may have a 
bourrelet portion extending outward from the engagement portion and 
engaged with the case to secure the sabot and projectile to the case. The 
containment tube may have a fore end affixed to the sabot. The first 
region may be at least in part forward of the stabilizing fins. The flash 
tube may be substantially non-combustible. 
The containment tube may include a straight aft tubular boss portion for 
receiving the fore end of the flash tube. The containment tube may further 
include a straight mediate tubular portion surrounding the stabilizing 
fins and a flange portion coupling the aft tubular portion to the mediate 
tubular portion. A fore tubular portion may couple the mediate tubular 
portion to the sabot. The fore tubular portion may have a general 
aft-to-fore taper. The containment tube may be adhered to the bourrelet 
portion. 
The cartridge may further include a percussion primer having a primer 
charge and mounted in a primer pocket of the base. The primer pocket may 
be aligned with and coupled to the aft end of the flash tube so that 
ignition of the primer charge ignites a flash tube charge, which in turn 
ignites the first propellant charge. The second propellant charge may 
constitute the majority of total propellant within the case. The first 
propellant charge may be substantially laterally surrounded by a fore 
portion of the second propellant charge. The first propellant charge may 
be predominantly formed of a first spheroidal propellant and the second 
charge may be predominantly formed of a second spheroidal propellant. 
In another aspect, the invention is directed to a method for manufacturing 
a fin-stabilized discarding-sabot ammunition cartridge. The method 
includes providing a saboted projectile. The saboted projectile includes a 
subcaliber penetrator having an elongate body and a plurality of 
stabilizing fins projecting from the body. The discardable sabot comprises 
an engagement portion for surrounding the penetrator and engaging the 
penetrator to prevent relative longitudinal movement. The sabot further 
includes a bourrelet portion extending outward from the engagement 
portion. A container is provided, sized to surround a portion of the 
penetrator aft of the bourrelet. A first propellant is introduced into the 
container. A case is provided extending from a mouth to a base. A second 
propellant charge is introduced into the case. An aft portion of the 
saboted projectile is inserted through the mouth and into the case. The 
aft portion is surrounded by the container and the first propellant 
charge. The case is secured to the bourrelet. 
The securing step may include crimping the case to the bourrelet. The 
container may be provided as a combustible containment tube for containing 
the first propellant charge. A fore end of the containment tube may be 
secured to the sabot such as via an adhesive. The insertion of the aft 
portion of the saboted projectile through the mouth and into the case may 
cause a fore end of the flash tube to rupture a membrane sealing the 
containment tube proximate an aft end of the containment tube. The first 
propellant charge may be provided having a first burn rate and the second 
propellant charge may be provided having a second burn rate which is lower 
than the first burn rate. The second propellant charge may consist 
essentially of a spheroidal propellant. 
In another aspect, the invention is directed to an improvement in an 
ammunition cartridge of the type having a projectile with an aft portion 
of the projectile extending into a cartridge case. The improvement 
includes a propellant and a propellant ignition system configured so that 
the ignition system initiates combustion of a fore portion of the 
propellant prior to combustion of a major portion of the propellant. The 
fore portion of the propellant initially blocks exit of the aft portion of 
the projectile from a mouth of the case. The fore portion combusts by a 
time at which the combustion of the major portion drives the aft portion 
of the projectile through a space initially occupied by the fore portion 
of the propellant. 
Other aspects of the present invention will be readily apparent upon 
reading the following detailed description of the invention, and from the 
drawing and the claims.

Like reference numbers and designations in the several views indicate like 
elements. 
DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a weapon 10 having a tube 12 extending from a chamber 14 at 
the aft end of the tube to a muzzle 16 formed by a fore end of the tube. 
The tube extends along a central longitudinal axis 200 and has a rifled 
bore or inner surface 18 with a groove-to-groove diameter and a 
land-to-land diameter, which in the exemplary embodiment are dimensioned 
to accommodate and fire M919 ammunition. As shown in FIG. 1, the rifling 
19 has a right hand twist as is common for weapons of U.S. manufacture 
although the invention is equally amenable to use with left hand twist and 
smoothbore tubes. 
An ammunition round 20 is provided having a steel case 22 accommodated 
within the chamber 14. The case has a sidewall 23 extending from a base 24 
to a mouth 26 and has an interior which may be filled with propellant as 
described below. A saboted projectile 30 is accommodated within the mouth 
26 of the case 22, an aft portion 32 extending into the case 22 and a fore 
portion 34 extending into the tube 12. The projectile, shown as a long rod 
penetrator, includes a body 36 formed primarily of a high-density metal 
such as tungsten and/or depleted uranium. The body 36 extends from a nose 
38 (formed as an aerodynamic ballistic tip) to a tail 40 and bears a 
plurality of (for example, four) fins 42 extending generally radially 
outward proximate the tail 40. Centrally along the body, the penetrator 
bears interlocking features 44 (FIG. 2) engageable with mating 
interlocking features 46 of the sabot 48. The interlocking features may be 
formed as screw-like threads or as annular thread-like grooves/protrusions 
engaged with each other so as to be effective to prevent relative 
longitudinal movement of the penetrator and sabot body. 
The sabot is substantially formed in two segments or petals which, when 
assembled, define a sabot body. The petals are identical to each other 
which facilitates a balanced sabot and smooth discard of the sabot. The 
petals are separated from each other along a planar interface. The 
assembled sabot fully encircles a major portion of the penetrator body. 
Referring back to FIG. 1, the sabot body includes fore and aft 
protuberances 50 and 52 dimensioned to cooperate with the bore 18 so as to 
maintain the projectile substantially centered along the axis 200. In the 
exemplary embodiment, the petals, and thus the sabot body, are primarily 
formed of aluminum or another light weight metal. A composite material may 
alternatively be used. Suitable composite materials include: carbon and/or 
aramid fiber in an epoxy or other resinous matrix. 
The fore protuberance 50 is formed as an annular scoop. Along the 
forward-facing rim of the scoop, an annular frangible band (not shown) is 
secured. At the fore protuberance, the sabot has an external diameter, 
which, in the exemplary embodiment, is slightly under 1 in (2.54 cm), e.g. 
approximately 0.95 in (2.4 cm). 
The aft protuberance 52 is longitudinally broader than the fore 
protuberance or scoop 50, forming a bulkhead which largely retains 
propellant gases behind it and provides the principal positioning of the 
saboted projectile along the axis 200. The aft protuberance or bulkhead 52 
serves as a bourrelet to guide the projectile as it travels the length of 
the tube. The sabot body includes a saddle area between protuberances and 
tapers from the fore protuberance to the aft end of the sabot. The 
bulkhead 52 has a forward cylindrical surface portion 60 having an 
external diameter which is approximately equal to the land-to-land 
diameter (e.g., about 0.97 in (2.5 cm) in the exemplary embodiment). In 
the exemplary embodiment, the forward cylindrical surface 60 extends 
approximately 0.15 in (0.4 cm). An annular compliant obturator 62 about a 
sealing band (not shown) is carried by a channel aft of the surface 60 
along the bulkhead. An aft surface 64, aft of the obturator, is of similar 
overall diameter and length to the front surface 60 and bears an annular 
crimping groove 66 to which the case 22 may be crimped about its mouth. At 
the aft end of sabot, a frangible annular band (not shown) further secures 
the petals in their assembled condition. With the round chambered in the 
weapon, an annular saddle space 68 is defined along the saddle between the 
fore and aft protuberances. The saddle and saddle space are so named due 
to the saddle-like sectional profile of the sabot body between the 
protuberances. 
Contained within the case interior are first and second propellant charges 
70 and 72, respectively. The first propellant charge 70 is located in a 
first region of the case interior at least in part surrounding the aft 
portion of the projectile. In the illustrated embodiment, the first 
propellant charge substantially entirely surrounds that portion of the 
projectile aft of the aft protuberance or sabot bulkhead. The second 
propellant charge 72 is carried within a second region of the case 
interior generally aft of the first region. In the illustrated embodiment, 
the second propellant charge has a burn rate slower than a burn rate of 
the first propellant charge. By way of example, the first propellant 
charge 70 may be a fine grain propellant while the second propellant 
charge 72 may be regular grain propellant having a larger typical grain 
size than the fine grain propellant. In the illustrated embodiment, the 
first region is at least in part forward of the stabilizing fins 42 and a 
fore portion 80 of the second propellant charge substantially surrounds 
the first propellant charge. By way of example, the second propellant 
charge 72 may consist essentially of BALL POWDER propellant having a 
characteristic web thickness of 0.026 in (0.066 cm) having average linear 
burn rates of 7.5 in/s (19 cm/s) at 50,000 psi (345 MPa) and 1.1 in/s (2.8 
cm/s) at 5,000 psi (34.5 MPa). The first propellant charge 70 may then 
consist essentially of BALL POWDER propellant having a characteristic web 
thickness of 0.019 in (0.048 cm) and average linear burn rates of 11.2 
in/s (28.5 cm/s) and 1.5 in/s (3.8 cm/s) at 50,000 psi (345 MPa) and 5,000 
psi (34.5 MPa), respectively. Advantageously, the average linear burn rate 
of the propellant in the first propellant charge 70 may be at least 1.2 
times that of the propellant in the second propellant charge 72 over a 
range of pressures from about 5,000 psi (34.5 MPa) to about 50,000 psi 
(345 MPa). Such ratio of burn rates may advantageously be in a range of 
from about 1.2:1 to about 2.0:1 over such pressure range; more preferably, 
from about 1.35:1 to about 1.50:1. Advantageous ranges of a web thickness 
ratio of the propellant in the second propellant charge relative to that 
in the first propellant charge may be from about 1.2:1 to about 1.8:1, 
however this may be less significant than the result achieved in terms of 
burn rates. 
A container 82 in the form of a combustible containment tube contains the 
first propellant charge 70 and separates the first propellant charge from 
the second propellant charge 72. The containment tube is formed of a thin 
combustible material such as nitrocellulose paper having an aft end 90 and 
fore end 92. From aft-to-fore, the containment tube includes a straight 
aft tubular boss portion 94 which, at its own fore end, diverges to form a 
flat annular flange portion 96. The flange portion 96 extends from a 
central aperture at the boss to a circular perimeter at the aft end of a 
straight mediate tubular portion 98 which surrounds the fins 42. A fore 
tubular portion 100 extends forward from the fore end of the mediate 
tubular portion 98 and couples the mediate portion to the sabot at the 
containment tube's fore end 92. In the exemplary embodiment, the boss 94 
has an internal diameter of 0.25 in (0.64 cm) and a length of 0.36 in (0.9 
cm), the mediate tubular portion 98 has an internal diameter of about 0.89 
in (2.3 cm) and a length of about 1.62 in (4.1 cm), and the fore tubular 
portion 100 has a general aft-to-fore taper of about seven degrees and a 
length of about 0.75 in (1.9 cm), thus having at its fore end a diameter 
of about 0.69 in (1.75 cm). In the exemplary embodiment, the diameter of 
the mediate portion is chosen to closely accommodate the fins 42. The 
flange portion 96 is positioned slightly behind the aft end of the 
projectile (e.g., by about 0.28 in (0.7 cm)). The first propellant charge 
substantially fills the space within the containment tube around the 
projectile from the sabot to the flange portion 96. 
The containment tube is dimensioned and sized to mate with a flash tube 
104. The flash tube 104 includes an annular flange 106 abutting the base 
108 of a cylindrical primer pocket 110 formed in the case base 24. The 
flash tube is preferably imperforate, having openings only at its ends. A 
cup-type percussion primer 112 is accommodated in the primer pocket 110 by 
press fit. The flash tube includes an elongate tubular body 114 extending 
forward from the flange 106 to a fore end 116. The body 114 tapers 
slightly toward the fore end 116 with the tapered portion being received 
and accommodated within an aft portion of the boss 94. In the exemplary 
embodiment, the flash tube has a length of about 1.7 in (4.3 cm) and outer 
and inner diameters of about 0.2 in (0.5 cm) and about 0.1 in (0.25 cm) 
along the major portion of its length. 
To assemble the cartridge 20, the sabot may first be applied to the 
projectile. The containment tube is then assembled to the saboted 
projectile by adhering the fore end of the tube to the bulkhead 52. In the 
illustrated embodiment, the fore end of the tube is adhered to a concave, 
approximately semitoroidal aft-facing surface of the bulkhead 52. The 
first propellant charge is then introduced into the containment tube 
through the boss. The boss is then sealed such as by cementing a thin 
piece of onion skin paper 120 over the aft end 90 of the containment tube. 
The flash tube 104 which, in the illustrated embodiment, is formed of a 
noncombustible material such as steel or another metal, is sealed at its 
fore end such as by cementing a thin onion skin paper over the fore end of 
the tube. The flash tube is at least partially filled with a flash charge 
of flash powder. The flash tube is then inserted through a cylindrical 
hole between the primer pocket 110 and the case interior. Once inserted, 
the primer 112 is inserted and press fit into the pocket 110 and bears 
against the flange 106 to securely hold the flash tube in place. 
The second propellant charge is then introduced into the case through the 
case mouth. The onion skin paper over the fore end of the flash tube 
prevents the second propellant charge from entering the flash tube. The 
aft portion of the saboted projectile surrounded by the containment tube 
and first propellant charge is then inserted through the case mouth 26 and 
into the case. When the exterior surface of the containment tube initially 
comes into contact with the second propellant charge, the containment tube 
may be gently rotated in a reciprocal fashion or otherwise manipulated by 
the assembler to ease its immersion into the second propellant charge, 
allowing the second propellant charge to surround the first propellant 
charge. During such insertion, the fore end of the flash tube comes into 
contact with the onion skin paper membrane 120 sealing the containment 
tube. Further insertion causes the fore end of the flash tube to rupture 
the membrane 120 thereby improving communication from the flash tube to 
the first propellant charge. When the insertion is complete so that the 
case mouth surrounds the bulkhead, the case is crimped to the bulkhead 
about the crimping groove 66 to secure the saboted projectile to the case. 
In use, the round is chambered in the associated weapon and the primer 112 
is actuated to ignite the primer charge. Ignition of the primer charge 
causes the flash charge to ignite with ignition migrating from the aft end 
of the flash tube to the fore end of the flash tube. When the combustion 
reaches the fore end of the flash tube, it then enters the containment 
tube, causing rapid ignition of the first propellant charge 70. Ignition 
of the first propellant charge increases the pressure within the case, 
producing a pressure on the aft surface of the bulkhead which tends to 
drive the saboted projectile longitudinally down the bore of the weapon 
overcoming engagement of the crimped case and bulkhead. The ignition of 
the first propellant charge causes combustion of the containment tube and 
initiates ignition of the second propellant charge. Ignition of the second 
propellant charge further increases pressure within the case and further 
accelerates the saboted projectile down the tube. 
The first propellant charge had initially represented a fore portion of the 
total propellant charge blocking exit of the fins 42 from the mouth of the 
case. Its prior ignition clears the space it initially occupied. 
Combustion of the major portion of the total propellant, represented by 
the second propellant charge, is thus free to drive the saboted projectile 
so that the fins 42 pass through the vacated space without damage. 
Although one or more embodiments of the present invention have been 
described, it will nevertheless be understood that various modifications 
may be made without departing from the spirit and scope of the invention. 
For example, although applied to a particular configuration of push-type 
sabot, the principles of the invention may be applied to other push-type 
sabots and to other type of sabots including pull-type sabots wherein the 
obturator is located in a relatively forward location (e.g., on the 
forward protuberance or flange). Although shown applied to a APFSDS round, 
principles of the invention may be applied to non-saboted rounds, 
including a variety of fin-stabilized explosive rounds and other rounds 
wherein any portion of the projectile extends within the case at least 
partially aft of some portion of the propellant contained within the case. 
Although shown as a one-piece steel case, the case may be otherwise 
formed, such as by the combination of a steel base and a combustible 
sidewall. 
Accordingly, other embodiments are within the scope of the following 
claims.