Patent Application
20160258726
Urbanization surrounding military training areas worldwide is changing the context and parameters of military training and the military utilization of land set aside for training. The United States and NATO militaries, when deploying, set up training areas. Due to the danger of ricochet and other anomalies, military forces are required to establish “Surface Danger Zones” (SDZs) adjacent military training ranges. The necessity to establish buffers alongside firing ranges requires militaries, or their host nations, to lease, purchase or otherwise acquire large tracks of land and erect warning signs and restrict traffic in these training areas. The maximum range of a projectile determines the size of the area to be set aside as a Surface Danger Zone (SDZ). The Surface Danger Zones (SDZs) are calculated based on the maximum range of the ammunition type(s) used in training along with a myriad of other considerations that include the ricochet danger inherent in the ammunition design. In many cases, militaries also desire to convert existing ranges from one ammunition type to another (for example to re-purpose 0.50 cal ranges to allow for live fire training on medium caliber 25 mm ammunition). In this context, “Short Range Training Projectiles” (SRTP's), also known as “Short Range Training Ammunition” (SRTA), provide both direct and indirect benefits to militaries.
The following U.S. patents disclose different types of Short Range Training Projectiles (SRTPs): U.S. Pat. No. 4,128,060 to Gawlick; U.S. Pat. No. 4,140,061 to Campoli; U.S. Pat. No. 4,911,080 to Leeker and U.S. Pat. No. 5,001,986 to Meister. All of these patents describe methods for modifying air-flow over the projectile body, thereby shortening the projectile's flight path. In addition, European Patent Pub. No. 0,036,232 A1 to DeBrant discloses designs for SRTPs where the outer surface undergoes changes after set-back that induce an aero-ballistic drag that shortens the flight path of the ammunition.
Most of these disclosed methodologies induce a linear increase in aero-ballistic drag and yaw after barrel exit. The introduction of linear aerodynamic forces will increase the drag and reduce the rate of spin of the projectile. In many cases, currently available SRTPs rely on the customer accepting a very loose or inexact ballistic match definition. SRTP designs, as advertised by GDOTS (Canada), CBC (Brazil) and NAMMO (Scandinavia), have external de-spinning features on the ammunition's outer surface where the ammunition induces an immediate reduction in spin and increased drag after barrel exit. The requirement to utilize de-spinning features where the projectile's outer-diameter is modified can, in certain calibers, negatively affect ammunition feeding.
A principal objective of the present invention, therefore, is to provide a training ammunition cartridge where the flight path of its projectile initially matches the flight path of a reference projectile and subsequently loses stable flight characteristics, thus shortening the maximum range of the projectile. The shortened maximum range can reduce the Surface Danger Zone both at the end and aside of the firing range.
This objective, as well as other objectives which will become apparent from the discussion that follows, are achieved, in accordance with the present invention, by an ammunition cartridge with a projectile:
Advantageously, the projectiles according to the invention are designed to initially exhibit a very close match to reference (e.g. ball) war-shot ammunition but, at a point in the training projectile's ballistic path, the liquid and, if present, the solid material in the void induces a combination of forces that quickly destabilize the projectiles' flight.
The shortening of the maximum range of the projectiles allows for a corresponding reduction in the surface danger zone surrounding a firing range. Militaries and owners of private ranges can therefore use larger caliber ammunition on ranges originally developed for small caliber ammunition.
Alternatively, the SRTP's according to the present invention allow militaries and/or private range owners to establish training ranges on smaller parcels of land. This, in turn, allows militaries to convert land previously set aside for surface danger zones to re-utilize, and/or repurpose the land set aside for small caliber shooting to train with larger weapons.
In cases where the amount of solid mass is significantly greater than a projectile's liquid mass, the mathematical calculations regarding stability and instability are greatly simplified. The AMC Pamphlet pp. 701-165 states:
For a complete understanding of the invention, it is important to know that the void geometry of the SRTA projectile induces forces on the projectile accentuating spin decay and yaw. It is also possible to configure the geometry to shift the center of gravity of the projectile to further accentuate the projectile's yaw amplitude and frequency, thereby further degrading the flight stability. The selection of the void geometry identifies what design equations to utilize in predicting both stable flight and the projectile's transition to unstable flight.
It is known that liquids generally exhibit nine hundred times more resistance to motion when compared to that of a gas. Liquids may also exhibit a resonance that can influence objects in flight. Prior work has shown that configurations with of a projectile's liquid filled void often had an infinite set of initial boundary conditions and projectiles have frequently been troublesomely susceptible to picking up resonances which have imparted un-predictable forces that act on the projectile in flight.
Early designers of liquid fuel rockets went to extensive efforts to understand and manage the complicated characteristics exhibited by liquid fuels in the rockets in flight.
Like a spinning top, a projectile's gyroscopic stability is achieved by optimizing the mass rotating around center of gravity and the axis of rotation. Thus in combination with other parameters cited in this reference, a designer can, in selecting materials and geometry, shift the solid mass in a projectile to further reduce a training projectile's gyroscopic stability, further shortening its range.
The U.S. Army Material Command (AMC) Pamphlet 706-165, published in April 1969 and approved for release to the public in January 1972, provides an authoritative overview of the challenges associated with designing liquid filled projectiles. The opening paragraph states “the problem of the unpredictable behavior of liquid-filled projectiles in flight has been known to designers for a long time.” This AMC Pamphlet was published to assist Army ammunition designers in producing ammunition with payloads such as white phosphorus that, under certain conditions, could liquefy and create flight instability. The AMC Pamphlet 706-165 further notes the challenge in establishing repeatable initial boundary conditions for a projectile containing a liquid. The pamphlet notes that “spin up” of the projectile in the barrel after set-back and before barrel exit often produces severe transient instability that renders a liquid-filled projectile useless in practice and can, further, render Stewartson's equations irrelevant. The feeding and handling of a projectile and its subsequent chambering in a breach creates an almost infinite set of initial boundary conditions making it almost impossible to establish a design that produces repeatable performance at barrel exit. Spin-stabilized ammunition that is fired with a liquid material retains transient spin instabilities that vastly complicate a designer's ability to reliably induce derogation of flight ballistics.
The “Miles Report on the Stability of Liquid Filled Shells” (1940) identified the basic physics for stable and unstable projectiles in flight. This mathematical formulae coupled with Miles' experimental data show that projectiles with a specified range of features were stable in flight while other projectiles were unstable. Thus it was shown that, when using a certain set of parameters, it was possible to have a stable liquid filled projectile. Thus, using the so-called “Miles” equation, a projectile can be configured to initially exhibit stable flight and, by introducing and manipulating post set-back conditions a designer can destabilize the projectile's flight. One should note that the formulae derived in the Miles report shows that liquid-filled projectiles generally exhibit either an increasing or diminishing yaw amplitude. Alternatively, it is possible to insert a material that liquefies after set-back and, in accordance with Miles formulae, produces an inherently unstable flight. By fixing the initial boundary conditions (e.g. of the material that acts as a solid until muzzle exit), the projectile exits the muzzle with six degrees of flight freedom acting as a solid.
The present invention allows a designer (1) to use the Miles equation to identify a liquid-filled projectile that will initially have stable flight and where forces in the projectile subsequently destabilize the flight, or (2) to firmly establish the initial boundary conditions of barrel exit by using a material that transitions from solid to liquid after set-back. The change from a solid to liquid may be accomplished either by a heat-induced phase change or by the use of a Non-Newtonian liquid or dilitant. In flight, the liquid in the void induces forces that destabilize the projectile's flight after an initial match period with a reference projectile.
The material contained in the void is a solid when it transits the barrel. This solid does not retain resonance frequencies as are generally induced in liquids and which are known to be detrimental when liquid-filled ammunition exits the barrel. According to the invention, however, the material rapidly liquefies after barrel exit and, interacting with the void geometry and solid projectile body, reliably increases the yaw amplitude and frequency of the projectile. This approach provides the basis for a unique design the projectile, causing it to become unstable in flight.
The selection of a void geometry and void liquid must be taken with care as liquids have known Eigenvalues (Eigen frequencies) that can induce increasing resonance in the projectile. Materials placed into the void will have a natural liquid frequency that, under certain conditions, will amplify resonance and forces creating instability. Thus a designer must take care that, when selecting void liquids, the materials must not induce an unwanted, destabilizing, liquid resonance during the projectiles barrel traverse when under high acceleration. While it is generally desired to preclude the introduction of unwanted resonance at spin up, during free flight it may be desirable to induce resonance or a combination of other characteristics that quickly amplify the projectile's yaw amplitude, causing the projectile to quickly lose its flight stability.
A selected liquid may induce desired or undesired instability when the Eigen frequency falls near the natural frequency of the liquid or nutation frequency. Alternatively, a selected liquid may introduce a stabilizing damping effect. Fundamentally, the selection of a liquid should allow the projectile exiting the barrel to have degrees of freedom and velocities that match the desired reference projectile.
The present invention comprises a projectile containing a void and a select material contained in the void. The material is a solid or a non-Newtonian fluid at set-back that liquefies after set-back and muzzle (barrel) exit. The liquefied material initiates a combination of forces that induce instability in the projectile. An AMC Pamphlet states, “Experiments show that Stewartson's theory with viscous corrections accurately predicts the initial rate growth of amplitude at resonance.” The instability is created after a short period of stable flight where the projectile flight path closely matches the path of a reference projectile. In addition to resonance, internal geometry and characteristics of the void create friction between the liquid and solid. Properly coupled together, void geometry, liquid-solid forces and imparted resonance increase a projectile's yaw amplitude and retard the projectile's rotational frequency which, in combination, destabilize the projectile.
One should note that the fluid must act as a Non-Newtonian fluid under the high g-forces of acceleration. Many materials that exhibit normal flow liquid characteristics under nominal conditions exhibit Non-Newtonian characteristics under the high-g forces induced at acceleration. Thus where resonance might be induced on normal un-stressed liquids, certain liquids that exhibit Non-Newtonian characteristics' under g-loads may no longer exhibit Newtonian characteristics. Amplification of a liquid's natural frequency is precluded and risks associated with associated perturbations are eliminated and initial barrel exit conditions are normalized.
One can also utilize features inherent in certain liquids to changes stresses and moments under high acceleration prior to barrel exit and can also introduce liquids that change characteristics in flight. Rheopectic liquids become more viscous when shaken, agitated or stressed. Bingham plastics behave as a solid in low stress environments but exhibit viscosity under stressed conditions. Shear thickening liquids exhibit increasing viscosities with increased shear stress. Shear thinning liquids exhibit decreased viscosity as the shear stress is decreased. Thixotropic liquids become less viscous when shaken, agitated or otherwise stressed. Dilatant or shear thickening behavior is typically observed in fluids with a high concentration of small, solid particulate suspended within a liquid. Behaving like a true fluid under low shear stress conditions, dilatants then transition to a solid-like condition when a greater shear stress or force is applied. The greater the force (shear) applied to a dilatant material, the more resistance will be felt. When subjected to extremely high levels of shear stress under the high-g loads of acceleration, dilatant materials become very rigid.
The projectile can utilize the heat imparted to its driving band as it progresses through the barrel and/or it can harvest heat from the pyrotechnic propellant, transferring the heat to the material in the void. The resulting increase in temperature flows from the driving band and the propellant through the projectile body into the void. The heated material in the void undergoes a phase change from solid to liquid. The liquefied material in the void induces forces on the projectile in flight.
In addition to the foregoing methodology of harvesting heat from the driving band and the rear of the projectile, high velocity projectiles may harvest heat in flight in the vicinity of the nose. It is well known that air friction encountered by high velocity projectiles in flight transfers significant heat into the projectile's nose assembly. Therefore, in certain configurations, in is advantageous to locate a void with a liquid in the void harvesting the friction heat to induce a phase change in the material housed in the void.
Alternatively, the void can be filled with a non-Newtonian fluid which acts as a solid when exposed to high acceleration forces but exhibits the characteristics of a normal liquid in a reduced acceleration environment. Thus, at set-back and continuing through to muzzle exit during which the projectile encounters a rapid acceleratory force, the high G-forces acting on the non-Newtonian fluid cause the fluid to act as a solid mass. At barrel exit, where the projectile is suddenly in free flight in a low G environment, the non-Newtonian material acts as a liquid. This allows the design to establish a fixed set of barrel exit conditions that closely match those of a reference projectile and subsequently induce instability that shortens the projectiles flight path. In setting repeatable boundary conditions and matching the exterior form of a ball projectile, a good initial match to a ball projectile is achieved.
Through the use of various ballistic methodologies, including well-known McCoy 6DOF calculations, and adjusting the exit velocity to offset differences in projectile mass a designer can establish required barrel exit parameters that allow a close ballistic match to reference ammunition. By using cited formulae and in selecting a combination features that includes a cavity geometry coupled with:
After selecting a void geometry, a solid-to-projectile mass ratio, and a liquid fill, the designer can use corresponding equations for stability and instability. Again, the selection of a material for post set-back liquefaction and the corresponding Eigenvalue of the liquid are important design criteria. Gyroscopic stability of the solid mass should be considered. Table 1 below identifies stability and instability formulas for corresponding void geometries. One may categorize voids and approaches with reference to their symmetry (or lack of symmetry) about the projectile's axis of spin. Mathematical equations that are verified by observation correspond to each approach.
Cylindrical cavities are useful when producing ammunition since most projectiles have a basic cylindrical form with the cylinder capped by a conical nose. Forming processes for cup-shaped forms have long been a cost effective method of metal forming in ammunition manufacture. Therefore, it is practical to produce cylindrical voids during ammunition production. Stewartson's equations, published in 1959, provided mathematical solutions to induce instability when a liquid is housed in a cylindrical cavity. The set of equations allows designers to design ammunition that induces predictable instability. Karpov's publication of “Dynamics of Liquid Filled Shell: Resonances in Modified Cylindrical Cavities” was published in 1966 and added to this body of work.
The stability and instability problem for a filled spheroidal cavity was solved by Greenhill in 1880. While cylindrical voids would generally be preferred to spheroid cavities in projectiles, the formation of spheroidal cavities can be readily introduced in production designs.
While the equations for non-symmetric cavities have less confirmatory experimentation, the basic formulas provide for a method to construct voids the induce forces to destabilize the projectile upon liquefaction of the void material. A non-symmetric cavity may be designed to quickly shift the center of gravity away from the axis of rotation.
The designer can modify the internal geometry and surface of the void to induce either laminar or non-laminar flow of the liquid in the void. This flow increases liquid-to-solid friction, reducing the projectile's spin rate and increasing the instability in an SRTP.
It is, in certain circumstances, advantageous to select material combinations and geometry that induce center of gravity shifts after a short period of free flight. Center of gravity shifts, off-center from the axis of rotation, accentuate yaw amplitude and degrade the projectile's flight stability. Suspending a dense solid in a lower density material that liquefies after set-back allows a designer the ability to shift the center of gravity of the projectile, thus inducing increased yaw.
In many circumstances, it is advantageous to select materials housing the liquid and construct the projectile so that the projectile is frangible in nature. The frangibility of the selected materials will reduce the risk of ricochet and reduce the SDZ of the projectile.
While frangible ammunition is frequently preferred, material selection and size can preclude use of frangible projectile bodies. Ricochet dangers extend the SDZ of training ranges although it is generally desired to reduce the size of SDZ's set aside because of the risk of ricochet. Certain liquid-filled voids will align the rotation of liquid and solid spin along the same axis. Where a ricochet occurs, the projectile's solid body will deflect and continue its flight. The disclosed configuration, with an initially aligned liquid and solid axis of rotation where, after deflection, the changed axis of solid rotation and the liquids in the void exert forces on the projectile that rapidly degrade and shorten the deflected projectiles flight path.
For a full understanding of the present invention, reference should now be made to the following detailed description of the preferred embodiments of the invention as illustrated in the accompanying drawings.
The preferred embodiments of the present invention will now be described with reference to
Embodiments of the present invention provide for a projectile that has an excellent ballistic match (flight path) with respect to reference ammunition for the initial stage of free flight. After a set period of transit, a liquefied material in the SRTPs void imparts forces on the projectile that rapidly degrade the SRTP's flight characteristics thus shortening the projectile's maximum range.
The liquid in the projectile void may include a non-Newtonian liquid, and/or a liquid characterized as a Hershel-Buckley, a Bingham and pseudo plastic liquid.
There has thus been shown and described a novel ammunition cartridge which fulfills all the objects and advantages sought therefor. Many changes, modifications, variations and other uses and applications of the subject invention will, however, become apparent to those skilled in the art after considering this specification and the accompanying drawings which disclose the preferred embodiments thereof. All such changes, modifications, variations and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention.
This application claims priority from Provisional Application No. 61/950,270, filed Mar. 10, 2015, entitled “AMMUNITION WITH INDUCED INSTABILITY AT A PRE-SET RANGE.”
| Number | Date | Country | |
|---|---|---|---|
| 61950270 | Mar 2014 | US |
