Note: Descriptions are shown in the official language in which they were submitted.
1~4~S88
This invention relates to projectiles generally. More
particularly, the Lnvention relates to practice projectiles designed to
have a predictable ricochet envelope.
BACKGROUND OF TH~ INVENTION
For effective operational readiness, members of a military
force require training and exercising of their skills. Thus, persons
such as pilots, gunners, and artillery men must practice attacks on
targets. The shells or projectiles used in such practice must closely
simulate the actual live weaponry. Further, since practice ranges are
often limited in size, or location, it is important to minimize the
hazards arising from ricochet of the projectile following impact with
the target or the ground. Problems of ricochet are even more acute for
aircraft gunnery since the firing conditions are such that the aircraft
has to overfly the impact area of the projectiles after his run.
It is possible, under suitable ground conditions in the
impsct area, for the shell or projectile to penetrate the ground. In
80 doing, all of its energy i8 dissipated during penetration. On the
other hand, ground penetration i9 not always a certainty. Iocal meteoro-
logical conditions clearly are a factor on this point. The ground, for
example, tnay be wet and soft, or hard and dry, or even frozen, or con-
taminated with stones or spent projectiles. Due to these variables
which are to a large extent beyond our control, it is extremely difficult
to design a projectilé which will almost always penetrate the ground.
A further complication arises should one simply suggest a
design which causes break-up of the projectile on contact with the
target or the ground. In many instances projectiles used for gunnery
training of aircraft pilots, are not designed, nor intended, to break-up
immediately upon impact. Such projectiles are typically of a heavy wall
structure capped with a solid metal nose. Furthermore, all attempts so
3G far to design a practice round that would safely stand the launch
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forces, but that would break into fragments when impacting a soft target
at a high degree of obliquity, have failed. To maximi~e training bene-
fits, practice rounds or projectiles should closely simulate the size,
launch and flight characteristics of a live round.
In consequence of the foregoing circumstances, there has
remained in this art a need for a practice round or projectile which
affords good training, and yet minimizes any hazards due to ricochet
in or beyond a target area.
SUMMARY OF THE INVENTION
The present invention embodies a projectile which largely
overcomes the above noted problems. A projectile as envisaged herein
has eqùivalent matching of ballistics characteristics, while simultaneously
providing desirable impact properties, as regards penetration and/or
ricochet.
Accordingly, applicant has invented an improved practice
pro~ectile in which there is provided a cup-shaped tubular body circular
in cross-section, having a central longitudinal axis, inner and outer
surfaces, and a closed downstream end and an open upstream end, the
tubular body also having a relatively thick wall; an ogive-shaped nose
cap secured to the tubular body to close the open upstream end; drive
means on the exterior of the tubular body operative auring launch to
impart spin to the projectile; and stress raising means provided in the
tubular body, the stress raising means being operative to increase ten-
sile stresses in the tubular body to a predetermined value which, at
impact, causes the total stresses in the body to exceed a designed maxi-
mum value and cause break-up of the projectile.
Further, by another embodiment of this invention there is
provided a practice projectile wherein the stress raising means comprises
a plurality of axial boreholes formed in the wall of the tubular body,
the axial boreholes extending parallel to the central axis of the tubular
body and being spaced symmetrically apart relative to the axis to maintain
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mass symmetry thus resulting in flight stability. In other forms the
stress raises are axial grooves on the surface.
The axially oriented boreholes can vary in diameter, for a
20mm projectile, being preferably in the range irom about 1.7 millimeters
to about 3.3 millimeters. In a similar manner, the depth of the axially
extending grooves can be varied, with a preferred range being from about
0.9 millimeters to about 1.9 millimeters.
It is, therefore, an object of this invention to provide a
practice projectile having improved properties as a training round. The
present projectile possesses good matching of ballistics characteristics
while minimizing uncontrolled ricochet in the target area. These and
other features of this invention will become apparent from the following
detailed description. Such description is to be read in conjunction with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIGURE 1 is an elevation view, taken partly in longitudinal
cross-section, of one embodiment of a practice projectile envisaged by
thls invention;
FIGURE 2 is an end view taken in cross-section at line 2-2
of FIGURE l;
FIGURES 3, and 3a are also end views taken as fragmentary
cross-sections to illustrate preferred forms of stress raising means in
the practice projectile of this invention; and
FIGURE 4 is a graphical representation of the time history
of the principal stresses in a projectile encompassed by this invention,
as compared to a conventional practice round.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Turning now to the drawings, Figure 1 shows at 10 a typical
practice round or projectile embodying this invention. The projectile
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10 comprises a cup-shaped tubular body 12 usually made of steel, having
a closed downstream end 14 and an open upstream end 16. This open end
16 is closed by an ogive or conically-shaped nose cap 18 typically made
of a metal such as aluminum. For purposes of a lower cost practice
round, however, the nose cap 18 may be made of a polymeric material, such
as polyethylene or nylon, especially if climatic conditions allow
acceptable compatibility. As seen from the partial cutaway of Figure 1,
the nose cap 18 is secured to the main tubular body 12. The embodiment
shown in Figure 1 uses a threaded interconnection of cap 18 and body 12, -
as seen at 20. The cap 18 is thus releasably secured to the body 12.
Other connecting arrangements can also be used, as will be evident to
persons skilled in this art.
Each of the nose cap 18 and tubular body 12 has walls shown
respectively at 22 and 24. These walls 22 and 24 are considered "thick
walls", it generally being taken that a thick-walled cylinder has a
ratio of~wall thickne6s to diameter greater than 1/10. Thus walls 22
and 24 range in thickness from about 2mm to about 6mm for a 20mm practice
round. These figures will also apply to a practice round of 30mm diameter.
The tubular body 12 has an outer surface 26, tapered smoothly radially
inwardly towards the open end 16, thus providing the body wall 24 with
a thinner forward portion. The outer surface 26 joins smoothly to the
outer surface of the nose cap 18, the latter being of a conical-shape
as noted above.
The rear portion of outer surface 26 of body 12 has at least
one external channel 28 formed therein. The channel 28 extends circum-
ferentially of the tubular body 12, and is in a plane perpendicular to
the central axis of the body 12. The channel 28 is adapted to receive
and retain a driving band 30, usually made of copper, another such metal
or a polymeric material. The driving band 30 is of a relatively soft
material to coact with rifling etc. in the barrel of a gun or launch
tube, not shown. Thus, a predetermined rate of spin is imparted to the
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p~ojectile 10 when it is "fired". The spin rate is predetermined, since,
for a projectile of given material, the largely spin-created tensile
stresses are to approach, but remain below, the yield strength of that
material. This is further described below.
In accordance with one embodiment of the invention, the body
wall 24 of projectile 10 is formed with a plurality of stress raising
means, or stress raisers shown in Figures 1 and 2 as axially extending
blind boreholes 32. These boreholes 32 extend para`llel to the longitudinal
central axis of projectile 10. Moreover, the boreholes 32 are spaced
symmetrically of that central axis at equal distances therefrom. Figure
2 shows the boreholes 32 at a 90 spacing; 120 or some other symmetrical
configuration is also possible. Each borehole in a particular projectile
10 is of the same diameter. In Figures 1 and 2, the boreholes 32
are of circular cross-section. These boreholes 32 can also vary in
diameter, preferably, being in the range from about 1.7 millimeters to
about 3.3 millimeters for a 20 millimeter round. In addition, these
boreholes are at distances from the central axis which can vary somewhat,
i.e. at different diameters, as from one projectile 10 to another. Such
diameters can vary, for example, from about 11.5 millimeters to about
13.1 millimeters.
It will be realized by persons knowledgeable in this art
that tensile stresses in the projectile 10 are mainly generated by the
rotation of the projectile, and that a high rate of spin is needed to
confer good stability in free flight. Even with the high spin rate
involved, the level of the tensile stresses, without stress raising means
in the projectile, are relatively low as compared to compressive stresses
in the latter. Compressive stress are generated during launching by the
propelling gases behind the projectile. The addition of stress raising
means extending longitudinally and axially of the projectile has a great
influence on the level of tensile stresses. These do not, ho~lever,
significantly affect the level of either compressive or shear stresses
due to the longitudinal orientation of such stress raising means.
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Finally, it will be clear that the level of the tensile stresses, in
accordance with this invention will be as high as practlcable at launch
and during flight, while not exceeding the yield strength of the material
of the projectile. At impact with a target, or the ground, however, it
is indeed desirable that such yield strength be exceeded.
Thus, in keeping with this invention one should create and then
maintain tensile stresses in the projectile during launch and in free flight,
below, but close to the yield strength of the projectile. The extra or
additional load imposed on the projectile on impact will cause the yield
strength to be exceeded. That in turn causes fragmentation and break-up
of the projectile 10. The small weight, size and non-aerodynamic shape of
such fragments causes the latter to have a very restricted, and acceptable,
range as far as ricochet beyond the target area is concerned. So long
as the projectile 10 breaks up into, for example, four or more pieces,
~for a 20mm round), such pieces will not be large enough to be a hazard as
regards ricochet beyond an acceptable ricochet envelope.
Figures 3 and 3(a) illustrate preferred embodiments of prac-
tice projectiles incorporating the present invention. Thus, in Figure 3
there is shown a cross-sectional view of a tubular body 40 of a practice
projectile. The body 40 has an inner surface 42 and an outer surface 44.
Stress raising means in the form of grooves 46 are provided on the inner sur-
face 42. Such grooves 46 are V-shaped in cross-section, and are positioned
symmetrically at equal radii from the central longitudinal axis of the
body 40. This configuration is most preferred from the point of view of
aerodynamic behavior and also feasibility of mass production.
In Figure 3(a), wherein the same reference numerals are used
to identify parts common to Figure 3, the stress raising means are in the
form of axially extending grooves 50, formed in the outer surface 44.
Again, the grooves 50 are parallel to the central axis of the tubular body
40, and are located symmetrically relative to the same. Here also, the
grooves 50 are V-shaped in cross-section.
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The grooves 46 and 50 can vary in depth, although groove~s on
the outer surface 44 will tend to be deeper than grooves on the inner
surface 42. Preferably, the grooves 46 and 50 will be of a depth in the
range of from about 0.7 millimeters to about 2.6 millimeters. Further
yet, although grooves 46 and 50 are shown as being of V-shaped cross-
section, other cross-sections are possible, such as square, rectangular,
semi-circular and the like. If a stress concentration such as a sharp
corner is present, this may have greater effect than changes in depth
- only.
Turning to Pigure 4, there is plotted here the time history
of the principal stresses in a practice projectile, both during launch
and in free flight. It is particularly noted that although the compressive,
shear and tensile stresses reach a maximum at different times during the
firing cycle, the former two stresses have dropped off virtually to zero
at the muzzle, whereas only the tensile stress remain significantly high
in free flight. It is seen that with the stress raising means envisaged
herein provided in the tubular body of the projectile, the tensile stress
is increased predictably to a level approaching the yield strength of the
material of which it is made. More importantly, the tensile stress remains
high during free flight. Thus, with the additional loading caused by
impact of the projectile on a target or the ground, the stress on said
projectile exceeds its yield strength causing it to break up into fragments.
The foregoing has described some preferred embodiments of this
invention. Certain modifications and/or variations have also been
suggested. Further variations are also possible. The present invention
can be practiced in larger diameter rounds, as well, i.e., in 30mm and
even larger. A stress analysis of a particular size and shape of stress
raiser in a particular size of projectile will indicate just how closely
the yield strength of the material of that projectile is approached. Thus,
the present invention can reliably and predictably be embodied in a wide
variety of projectiles, whenever break-up of the projectile upon impact
is desired.
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Accordingly, it is intended by this invention to encompass
all such changes and variations as will be evident to persons skilled
in this art and falling within the scope of the claims below.
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