A training projectile that utilizes flutes or flats to augment roll damping characteristics and thereby cause the projectile to crossover into a gyroscopically unstable trajectory pattern at a predetermined time. Prior to the crossover, the training projectile maintains a gyroscopically stable trajectory, which enables extrapolation to ascertain the trajectory of a non-training projectile that does not have an augmented roll damping section. The unstable trajectory pattern substantially reduces the distance the training projectile can traverse, thereby reducing the amount of area required for a training range.
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1. A spin-stabilizer projectile comprising:
an ogival nose portion; a posterior portion; and a midportion disposed between said nose portion and said posterior portion, said projectile defining an elongated, aerodynamically-shaped body having an axis of rotation; said midportion comprising a longitudinally extending roll damping augmentation section disposed in a circumferential surface recess thereof, said roll damping augmentation section comprising a plurality of members which extend outwardly no more than about the depth of said recess and at least twice as high as the projectile boundary layer momentum height, for causing said projectile to become gyroscopically unstable at a predetermined range and continuously gyroscopically unstable thereafter .
2. The spin-stabilized projectile as described in
3. The spin-stabilized projectile as described in
4. The spin-stabilized projectile as described in
5. The spin-stabilized projectile of
6. The spin-stabilized projectile as described in
7. The spin-stabilized projectile of
8. The spin-stabilized projectile as described in
9. The spin-stabilized projectile as described in
10. The spin-stabilized projectile as described in
11. The spin-stabilized projectile as described in
12. The spin-stabilized projectile as described in
13. The spin-stabilized projectile as described in
14. The spin-stabilized projectile as described in
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The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of DAAAZI-90-C-0096 awarded by the Department of the Army.
1. Field of the Invention
This invention relates to a new and improved training projectile that has a predetermined range limited trajectory characteristic. More particularly, radially distributed flats induce the onset of gyroscopic instability at a predetermined range, thereby reducing the overall flight path of the projectile.
2. Description of the Art
U.S. Pat. No. 4,063,511 (Bullard) discloses a spinning shotgun projectile with grooves to streamline the projectile body thereby decreasing air resistance during flight.
U.S. Pat. No. 4,520,972 (Diesinger et al.) discloses a spin-stabilized training projectile, which changes its axial stability by operation of a stabilizer mounted at the rear end of the projectile.
U.S. Pat. No. 4,708,065 (Schilling et al.) discloses a training projectile with an annular recess around its circumference but does not use roll damping to truncate the normal trajectory of the projectile.
U.S. Pat. No. 4,905,602 (Buckland) discloses a spin-damped training projectile, which has an array of spin-damping fins mounted on the nose of the projectile.
Ranges for testing the trajectory of large caliber ammunition require a great deal of area for obvious safety reasons. A typical range for a 25-mm projectile has a length of approximately 14-km because projectiles of 25-mm typically travel a distance of 12-km. These distances change depending on the size of the projectile. The larger projectiles require a proportionally larger area. Many ordnance applications, i.e., target practice rounds, require projectiles to satisfy two conflicting objectives: 1) achieve a high performance flat trajectory to a specified range and 2) abruptly decelerate and thereby not exceed a specified range limit. Conventional spin stabilized projectiles, due to their pointed cylindrical shape, are severely limited in the degree to which they can satisfy these two conflicting requirements.
The problem is that high initial velocities result in excessively long carry ranges; or alternatively, if the specified range limitation is met, the initial trajectory performance is inadequate.
The present invention solves this problem by providing a training projectile with a roll damping augmentation section that causes the projectile to become gyroscopically unstable after a traveling a predetermined distance. The gyroscopically unstable trajectory causes the projectile to begin high yaw and thereby reduces the distance the projectile will ultimately travel.
This invention relates to a projectile that achieves a flat trajectory to a specified predetermined distance and upon reaching that distance abruptly becomes gyroscopically unstable. Accordingly, one embodiment is drawn to a projectile having an ogival nose portion; a posterior portion; and a midportion.
The midportion includes a longitudinally extending roll damping augmentation section disposed in a recess and extending outwardly no more than approximately the depth of the recess. The roll damping augmentation section has flats of flutes defining grooves which interact with oncoming air causing the projectile to become gyroscopically unstable at a predetermined range and continuously gyroscopically unstable thereafter.
FIG. 8(a) and (b) shows variations in the number of flutes or flats respectively.
This invention relates to training rounds for which a range limitation mechanism has no substantial effect on the trajectory with a specified range, but acts to curtail the range thereafter, preventing the training rounds from exceeding the boundaries of the training area.
Typically, a rotating projectile has stable flight when the gyroscopic stability factor, which enables a projectile to have an aerodynamic stabilized flight trajectory, is greater than 1.0 and the dynamic stability factor, which represents the ability of a projectile to maintain a stable trajectory, is between zero and 2∅
A rotating projectile has a stable flight trajectory when Sg>1 where:
V is the velocity of the undisturbed oncoming air flow; Ix is the axial moment in inertia of the projectile; P is the air density; d is the reference diameter of the projectile; Iy is the transverse moment of inertia of the projectile; ω is the angular velocity about the longitudinal axis of the projectile and Cmα is the aerodynamic moment slope.
Standard International units may be used for consistency.
As d, Ix and Iy are fixed and P and Cmα only vary slightly for low angle, high velocity trajectories, the primary factor governing projectile stability is the ratio of angular velocity to forward velocity (ω/v).
The present invention seeks to achieve a range limited projectile through augmented roll damping which causes the spin rate to decay faster than the forward velocity. In the course of a normal trajectory, the velocity decay is greater than the spin rate decay thus the projectile becomes more stable. If the spin damping of the projectile is increased sufficiently for the spin rate decay to exceed the velocity decay, Sg will decrease during flight and a projectile, which started off stable can have instability induced after travelling a critical distance. It is important that the roll damping mechanism does not increase the projectile drag, nor introduce extraneous pitching moment changes nor alter the Magnus moments in a manner that would adversely affect the capability of the training round to resemble as closely as possible a combat round that it is intended to simulate. The present training round design does not interfere with normal operation of full caliber projectiles on the use of subcaliber projectiles using sabots.
The instant invention enables a projectile to have a first segment of its trajectory gyroscopically stable and thus, correlate to a regular cartridge projectile. The flight characteristics of the first segment of the trajectory can be observed and recorded. The data gathered from observing the first segment of the trajectory may be used to extrapolate the trajectory the projectile would have if the roll damping augmentation feature were not present.
The first section of the trajectory has a flight velocity imparted from a muzzle with a Mach number. The firing also imparts an angular velocity proportional to the barrel rifling twist angle. As the projectile proceeds along its flight trajectory, the flight velocity begins to decrease at a faster rate than the angular velocity. This decrease necessitates the inventive augmented roll damping section as shown in
A second segment of the trajectory is gyroscopically unstable due to an increase in the rotational pitching moment caused by the interface of air and the augmented roll damping section of the projectile. The gyroscopic instability causes the projectile to assume high yaw angles. These high yaw angles provide high drag that decreases the distance the projectile will travel. One purpose of the recessed roll damping section 100 is to allow the design to be used in full caliber projectiles, fired from conventional gun barrels; or for the adaptation of existing sub-caliber projectile/sabot configurations without the need to modify the structurally critical subprojectile aft end/pusher base interface or the sabot manufacturing and/or molding process.
A rotating projectile used as a training round has a flight velocity (V) which drops more rapidly than the angular velocity (ω). Thus, as the projectile slows down, the flight pattern becomes more stable. The present invention, by the use of an augmented roll damping section in the midportion of the projectile, causes the projectile to experience a moment about its rotational axis which causes
to decrease.
This causes the projectile to become gyroscopically unstable and to begin a high yaw and/or tumbling trajectory.
As shown in
The flutes 120 and flats 220 can have planar or twisted and/or curved surfaces thereby causing the air to have a greater or lesser effect on the trajectory of the projectile. The cumulative effect of a plurality of longitudinally elongated flutes 120 or flats 220, deflecting air currents, causes the moment forces to overcome the tendency for the projectile to become more gyroscopically stable as it decelerates.
The flutes 120 or flats 220 are recessed in the midportion of the projectile 110 such that they do not extend substantially past the ogival surface of the projectile. The depth of the flutes 120 or flats 220 is approximately equal to the depth of the recess in the midportion and should be at least twice as high as the boundary layer momentum height so they do not become submerged in the boundary layer. The boundary layer is an area that surrounds a moving projectile and exerts forces on the projectile.
The flutes 120 and flats 220 may define air cavities 180, which are filled with on-coming air. The air cavities 180 may be of virtually any depth, however, a depth of 2.5% to 7.5% of the projectile body diameter is preferred with 5.7% of the projectile body diameter being most preferred.
The flutes 120 are aligned along the longitudinal axis 710 of the projectile 10 as shown in FIG. 7. The flutes 120 can be placed at varying degrees in relation to the axis and can vary in shape.
As shown in
The roll damping augmentation section 100 which includes the flutes 120 or flats 220 is placed in the midsection 110 of the projectile 10, which is near the center of gravity, thereby reducing undesired perturbations in the flight trajectory. The roll damping augmentation section can be the entire length of the projectile or up to 2.0 times the body diameter of the projectile. A preferred length is between 1 and 1.75 times the body diameter of the projectile. The most preferred length is 1.33 times the body diameter of the projectile.
The projectile 10 as shown in
Through variations in the size, number and/or twist angles of the roll damping flutes 120 or flats 220, the projectile's aerodynamic roll damping torques can be tuned to control the time of onset for the high drag condition, thereby providing vast improvements in tailoring the respective fast and slow portions of the trajectory. The deeper the flutes 120 or flats 220 into the midportion 110, the sooner the projectile 10 will become gyroscopically unstable. The flutes 120 or flats 220 redirect the air flow around the surface of the projectile 10 because the flutes 120 and flats 220 cause the aerodynamic forces operating in opposite directions to produce a moment about the rotational axis, which decreases the gyroscopic stability and causes the projectile to being a high yaw and/or tumbling trajectory. As shown in FIGS. 8(a) and (b), the roll damping section segments 100 can vary. Any number of segments would work. However, a preferred number of segments are between 4 and 12 equally spaced around the circumference of the projectile.
As shown in
As shown in
The cone-cylinder groove height of 5.7% of the body diameter provided four times the roll damping of a bi-conic groove height of 3.5% of the body diameter.
A preferred embodiment of the invention utilizes recessed flutes 120 having a flat vertical surface extending outward from the longitudinal axis. This configuration increases the effective surface area of the flutes 120. The flutes 120 have a length to height ratio of 15:1. A tungsten cylinder as the midportion 110 allows tailoring of gyroscopic stability to ensure cross-over and range truncation regardless of the ambient air temperature. The crossover rate will be unaffected in muzzle temperatures ranging from +150°C C. to -60°C C. when a tungsten cylinder is utilized. The flutes 120 or flats 220 may be molded into the tungsten cylinder or may be carved into the tungsten cylinder.
While preferred embodiments of the present invention have been illustrated in detail, it should be apparent that modifications and adaptations to those embodiments may occur to one skilled in the art without departing from the scope of the present invention as set forth in the following claims.
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Executed on | Assignor | Assignee | Conveyance | Frame | Reel | Doc |
Jan 29 2001 | PRIMEX TECHNOLOGIES, INC | GENERAL DYNAMICS ORDNANCE AND TACTICAL SYSTEMS, INC | CHANGE OF NAME SEE DOCUMENT FOR DETAILS | 012469 | /0729 | |
Jul 31 2001 | General Dynamic Ordnance and Tactical System, Inc. | (assignment on the face of the patent) | / |
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