Patent Grant
7086629
The present invention relates to severing materials, such as an aircraft canopy made from polycarbonate, polycarbonate laminate or acrylic/polycarbonate laminate, with an explosive charge.
Most military aircraft contain an ejection seat that allows the pilot to escape the aircraft while in flight. When an ejection seat is jettisoned from the cockpit of an aircraft, it must pass through the region occupied by the transparent canopy of the aircraft. In instances where the canopy is not jettisoned prior to the ejection seat firing, the ejection seat must be capable of blasting entirely through the canopy. To reduce the risks to the pilot or other aircraft occupant attendant to forcing the-ejection seat through the canopy, canopy fracture systems have been provided to fracture the canopy and better clear a path for the ejecting occupant so as to minimize bodily impact with the canopy.
Canopy fracture systems have been effective at removing portions of canopies that are made from fragilizing materials, such as cast or stretched acrylic. Fragilizing materials are those that may be caused to shatter into a significant number of pieces on application of sufficient pressure or explosive force. These systems utilize a mild detonating charge (MDC) or linear shaped charge (LSC) placed on, in or near the transparency which, upon detonation, creates shock waves that fracture the canopy. With fragilizing canopies, such as those made from cast or even stretched acrylic, it is not necessary to fully sever the material in order to defeat its structural integrity.
Many high performance aircraft utilize polycarbonate in their canopies instead of acrylic. Polycarbonate is a nonfragilizing material, meaning that it does not shatter on application of explosive force. With polycarbonate, it is absolutely necessary to fully sever the material because fracturing to complete the break is very unreliable. Because polycarbonate has a relatively low melt point and because cutting the material generates considerable heat there also exists a potential for resealing behind the cut if the severance is not complete.
U.S. Pat. No. 5,170,004 teaches an explosive device wherein a nearly incompressible transmitting medium is placed between the explosive device and an aircraft canopy. The function of the transmitting medium is to transmit the shock wave, produced upon detonation, to the canopy with a minimum of dissipation. This device is effective on fragilizing canopies but has not been successful with thicker non-fragilizing canopies made from monolithic polycarbonate, polycarbonate laminates or acrylic/polycarbonate laminates.
U.S. Pat. No. 5,780,763 teaches a method of fracture wherein explosive cords are placed in parallel grooves on the tipper surface of a canopy and simultaneously detonated to create overlapping shock waves. This method is apparently capable of breaking a 0.75 inch thick polycarbonate in the laboratory at ambient temperature or below, but is unreliable at elevated temperatures on the order of 165° Fahrenheit or above. This method, however, requires two charges, grooves to be cut in the material and also utilizes shock waves which are not reliable on polycarbonate materials.
U.S. Pat. No. 5,954,296 also relates to an aircraft canopy fracture system. The 296 patent claims a canopy with a severable region shaped so as to inhibit passage of the severable region back through the canopy after severance. The 296 patent also refers to the use of a LSC to sever a polycarbonate canopy.
All documents, including other patents and references, referred to in this document are hereby incorporated by reference in their entirety, although no documents are admitted to render any of the claims unpatentable either alone or in combination with any other references known by the applicant.
The prior art also does not adequately deal with the problems of providing for severance around the corners of the severable portion of the target or for routing a charge over the top of another charge.
It is therefore an object of this invention to provide a severance method which severs materials such as polycarbonate, polycarbonate laminates or acrylic/polycarbonate laminates that cannot be severed by existing methods except by use of excessive amounts of explosive charge or by placing the charge inside the material to be fractured, thereby degrading the material's structural integrity.
It is another object of this invention to provide a severance method that allows for a minimum amount of explosive charge to sever a given thickness of material at high and low temperature extremes.
It is further an object of this invention to provide a severance method that allows for severance around corners and through intersecting portions of the explosive charge.
The present invention is a method for severing a non-fragilizing material such as polycarbonate with a linear shaped charge (“LSC”). Previous methods of polycarbonate, polycarbonate laminate or acrylic/polycarbonate laminate severance were either ineffective, or required imbedding explosive charges into the canopy itself. The prior art relies on the use of shock waves to fracture aircraft canopies. These methods, however, are not effective or reliable on canopies made from non-fragilizing materials such as polycarbonate, polycarbonate laminate or acrylic/polycarbonate laminate. The present invention discloses a method of using the cutting face of the explosive charges to sever a material such as polycarbonate, instead of relying on unreliable shock waves. Because the severance method of this invention is more effective and more reliable than previous severance methods, a lesser amount of explosive charge is required to effect target severance. In the case of a polycarbonate aircraft canopy target, the present invention's severance method provides pilots with a lower exposure to explosion back blast and noise.
An LSC is placed in proximity to the material to be severed at a distance sufficient to generate an explosive cutting face or “jet” adequate for cutting the target thickness. The LSC is held in place and at the sufficient distance by a retainer that surrounds the back of the charge. The retainer is adhered or attached to the target, again maintaining the appropriate distance between the charge and the target. Upon detonation, the charge severs the target. The charge and retainer may be tooled to provide for effective severance with minimum explosive force around corners. The present invention also provides for various methods of charge intersection, crossover and detonation transfer.
a–d are tables showing performance test results of varying grain sizes of various LSCs on polycarbonates of varying thickness.
a is a depiction of a side view of the crossover method.
b is a depiction of a side view of the crossover method where a portion of the sheathing on the lower length of LSC has been cut away.
c is a depiction of a side view of the crossover method where a solid anvil is placed under the lower length of LSC to aid in detonation transfer.
a is a side view of the piggyback method.
a is a side view of the miter method.
a is a side view of the miter method used at a right angle.
Referring to the drawings by reference numbers,
The LSC 10 includes an explosive core 11 and a sheath 12. The best results are obtained when the LSC is shaped like a chevron with the opening facing and parallel to the target 13. There is open space between the LSC and the target. The LSC is surrounded by a retainer 14 that is attached to the target by some form of adhesive. The retainer attaches the LSC to the target such that the LSC is offset from the target by a distance 15 that allows the LSC to generate an explosive blast of sufficient strength to sever the target.
In a preferred embodiment the retainer is made from rubber and attached to the canopy with an epoxy or other adhesive. The retainer not only serves to hold the LSC in place at an appropriate distance from the canopy, it also acts to attenuate the back blast and noise from the explosion to protect the aircrew (in the case of a canopy target). The retainer may also be made from a other type rubber products, an epoxy composite or other compounds and still achieve satisfactory results.
LSC sheathing is made of metal, typically tin, lead, aluminum, silver or copper. For severance of polycarbonates, tin, lead, and silver are preferable, but silver is rarely used due to high material costs. Testing was performed using PBXN-SILead, PBXN-5/Tin, FINS/Lead and FINS/Tin LSC of varying coreloads. Some of those test results are set forth in
Because of the lack of success with previous canopy fracturing and severance methods in polycarbonate canopies, a great deal of testing was performed to determine the proper LSC coreload and stand off necessary to sever a set thickness of polycarbonate. In the present invention the coreload and stand-off necessary to sever polycarbonates of varying thickness is disclosed. As would be expected, the thicker the polycarbonate to be severed, the greater the coreload required. The LSC is more effective if setoff a certain distance from the target. A tabular representation of the test results achieved on varying polycarbonate thicknesses with varying LSC coreload, powder sheathing, back ups and stand-off is set forth below:
Previous methods of aircraft canopy fracturing and severance relied on the use of shock waves generated from an explosive charge. In order to facilitate shock wave transfer from the explosive charge to the target, U.S. Pat. No. 5,170,004 taught the use of a nearly incompressible medium placed between the charge and target. While this method is effective on acrylic and other fragilizing type materials, it is not reliable with polycarbonate, polycarbonate laminate or acrylic/polycarbonate laminates. With polycarbonate type materials, the use of an incompressible or other medium between the LSC and the target degrades rather than improves results. This is because polycarbonates are not easily fractured and therefore cutting using the explosive “jet” blast of the LSC is preferable to fracturing using shock waves. Because severance is preferable to fracturing, it is important that the area between the LSC and the target be clear of debris. This goes against the teaching of the 004 patent which taught the use of some form of nearly incompressible medium between the charge and the target in order to facilitate transmission of shock waves.
U.S. Pat. No. 5,780,763 taught a method of fracture wherein explosive charges are placed in parallel grooves cut in the upper surface of the material and simultaneously detonated to create overlapping shock waves. Again, because this method relies on shock waves which are unreliable to sever polycarbonate materials it is inferior to the present invention that uses the unimpeded explosive “jet” blast of the LSC to sever the material.
As revealed by the test results set forth above, the best severance results are obtained by use of a rubber back up, a PBXN-5 powder and a lead or tin sheath. Ideal set off was in the range of 0.100 to 0.300 inch for coreloads of 12 to 40 grains per foot.
Laminates of polycarbonate, acrylic, stretched acrylic and polyurethane can be severed with less overall energy than pure polycarbonate. The target thickness for the laminate is the total thickness from the LSC side through the final layer of polycarbonate or polyurethane.
Another shortcoming in the prior art was difficulty in safely severing polycarbonates and other materials around corners. To create a passageway for an ejecting or egressing aircrew it is necessary that the LSC circumscribe some pattern sufficient to allow egress. Severance problems arise when the LSC is bent around the corners of this pattern. These problems are attributable to at least two factors. First, in bending the LSC the coreload may be disbursed to less than an optimum strength, especially on the outside of the bend. Second, when the LSC and its retainer are bent around corners, the open end of the chevron may be shifted from parallel to the target surface. Optimal results are achieved when the open end of the chevron is parallel to the target surface and the explosive force is therefore perpendicular to that surface. Any alteration from parallel lessens the explosive force striking the target and impedes severance.
Therefore, in a preferred embodiment of this invention, bends in the LSC are gentle, with turn radii kept on the order of 2 inches or above. In this way the coreload of the LSC is kept consistent. Additionally, both the LSC and its retainer are tooled so as to keep the open end of the LSC chevron parallel to, and the apex of the chevron and resulting explosive force perpendicular to, the target. Through these methods, the full force of the explosive charge is expended directly on and perpendicular to the target. This allows severance with a minimum of coreload, thereby reducing noise and back blast.
The LSC must be placed on the target in a pattern that will effect severance of a portion of the target. In the prior art the LSC was not able to crossover another length of LSC. Instead the LSC would be bent around as it approached another length of LSC.
a shows the second length of LSC 21 flattened over the first length of LSC 20. By flattening the second length of LSC 21 its cutting force is reduced and it will detonate, instead of sever, the first length of LSC 20. It may also be desirable to cut away some of the metal sheathing on the top of the first length of LSC 20 to assist detonation by reducing the amount of metal sheathing that the second length of LSC 21 must to detonate through in order to detonate the first length of LSC 20.
Another method of ensuring detonation of the first length of LSC by the second length is to insert a solid material under the first length of LSC so that upon detonation of the second length of LSC the first length is forced against the solid material to assist in detonation transfer.
A variation on the crossover method is shown in
An additional method of routing LSC through intersections in a pattern is the mitered joint.
The retainer is not shown in the intersection figures, but those skilled in the art will recognize that a charge holder can be designed to take account of the LSC intersection or crossover.
The severance method of the present invention may be utilized in a system designed so as to be seated proximal to a sill portion of the aircraft and the system may include, in addition to the above-described arrangement for freeing a severable region of the canopy, one or more explosive charge positioned on or in close proximity to at least a portion of the sill and that will, on detonation, free substantially the entire transparent portion of the canopy from the aircraft. The present system also may be designed so as to selectively initiate either the detonation of the explosive charges mounted on or near the sill so as to sever substantially the entire transparent portion of the canopy or the severance of the smaller, severable region of the canopy. Detonation of the explosive charges mounted on or near the sill may be preferred over severing the severable region of the canopy in situations where the aircraft is on the ground or is otherwise not in flight and immediate egress from the cockpit is necessary.
The present severance method may also be designed so that the one or more explosive charges mounted on or near the sill detonate and free a large portion of the canopy from the aircraft simultaneous with the fracturing of the smaller, severable region to allow egress from the aircraft. Alternately, the present severance method may be used in a system to provide for the detonation of the one or more explosive charges mounted on or near the sill after the fracturing and severance of the smaller, severable region.
Various other details of the design and implementation of aircraft canopy fracturing systems will be apparent to those of ordinary skill in the art and, therefore, are not provided herein. Such details are in part provided in, for example, “A Systems Engineering Design Guide to Aircraft Explosive Canopy Fracturing”, December 1993, Teledyne Ryan Aeronautical, McCormick Selph Ordinance, the entire content of which is incorporated herein by reference.
Those of ordinary skill in the art also will appreciate that various changes in the details and arrangements of parts and materials which have been herein described and illustrated in order to teach the nature of the invention may be made by those skilled in the art. Any such modifications remain within the principle and scope of the invention as expressed in its claims.
This continuation application makes reference to, incorporates the same herein, and claims all benefits including under 35 U.S.C. § 121 from a co-pending application entitled SEVERANCE OF POLYCARBONATES AND POLYCARBONATE LAMINATES WITH LINEAR SHAPED CHARGE filed in the United States Patent & Trademark Office on the 22th of Dec. 1999 and there duly assigned Ser. No. 09/470,222, now U.S. Pat. No. 6,609,464 B1 issued on the 26th of Aug. 2003.
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| Number | Date | Country | |
|---|---|---|---|
| 20030189133 A1 | Oct 2003 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 09470222 | Dec 1999 | US |
| Child | 10387442 | US |
