Abstract:
Methods and apparatus are provided for a low shock separation joint. The separation joint comprises a male member, a female member, and an explosive device. At least one projection is formed on the male and female members. Surfaces of the at least one projection on the male and female members are mated to one another to prevent separation under compressive and tensile forces. The explosive device is placed within a cavity of said female member. A method for reducing shock in a separation joint is provided. An explosive device in the female member of the separation joint is detonated. A volume increase of the explosive device bends flanges of the female member away from one another. Surfaces in intimate contact with one another are moved out of contact with one another to decouple the male member from the female member.

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to separation joints, and more particularly relates to separation joints having an explosive ordnance.  
       BACKGROUND OF THE INVENTION  
       [0002]     A launch vehicle such as a rocket requires a tremendous amount of energy to escape Earth&#39;s gravity. Thus, a primary goal in the design of a rocket is to maximize the payload that is carried while utilizing the minimum amount of fuel. An efficient methodology that has been widely used in the aerospace industry is a staged rocket. The premise behind a staged rocket is that unneeded mass is jettisoned as soon as possible thereby increasing the payload that can be sent.  FIG. 1  is an illustration of a prior art two stage rocket  100 . Rocket  100  comprises a first rocket stage  101 , a second rocket stage  102 , and a fairing  103 . Initial acceleration of rocket  100  from standstill requires first rocket stage  101  to have high thrust engines and large propellant tanks to feed these engines. First rocket stage  101  provides thrust to accelerate the entire mass of rocket  100 . First rocket stage  101  is separated from second rocket stage  102  and fairing  103  by a separation joint  104 . Separation typically occurs at a high altitude where a large engine is no longer needed thereby greatly reducing the mass of rocket  100 .  
         [0003]     Second rocket stage  102  is enabled after separation from first rocket stage  101  to provide thrust to keep rocket  100  on its intended path. Similar to first rocket stage  101 , second rocket stage  102  is no longer needed as it nears the intended destination. A separation joint  105  separates second rocket stage  102  from fairing  103 . It also may be noted that in some circumstances, fairing  103  is separated from the rocket during first stage  101  bum in order to shed mass as soon as possible and maximize payload to orbit.  
         [0004]      FIG. 2  is an illustration of prior art two stage rocket  100  of  FIG. 1  showing first rocket stage  101 , second rocket stage  102 , and fairing  103  separated from one another. Separation of first rocket stage  101  from second rocket stage  102  exposes a rocket engine of rocket stage  102 . Fairing  103  is separated exposing a payload  107  that was housed in fairing  103 . Although rocket  100  is greatly simplified it illustrates the need for an extremely reliable separation system. A failure in any one of separation joints  104 - 106  of  FIG. 1  would result in a complete failure of the mission at a cost of time, money, and perhaps human life.  
         [0005]     Many different types of separation joints have been proven to be extremely reliable in applications similar to that described hereinabove. One type utilizes an explosive device to alter the separation joint from a fastened state to a decoupled state. In general, a separation joint comprises a first and second element. The first and second elements respectively couple to first and second structures that are to be separated under certain conditions. Typically, the first and second elements of the separation joint connect together in a manner where they do not separate under normal operating conditions. Separation of the joint is achieved when the explosive device is detonated. The most prevalent method of holding the separation joint together is to use bolts, rivets, or other mechanical fasteners.  
         [0006]      FIG. 3  is a cross-sectional view of a prior art explosive device  300  used in a separation joint. Explosive device  300  comprises a tube  301  and an explosive material  302 . In an embodiment of explosive device  300 , explosive material  302  is a mild detonation cord. The mild detonation cord is often encased in a sheath that fits within the cavity of tube  301  such that the mild detonation cord is centrally located within tube  301 . For example, the sheath may comprise silicone rubber or a shock absorbing/thermally insulating material. Contamination of the field around the separation joint is a critical issue. Tube  301  contains the debris generated from the explosion to prevent contamination of the area near the separation joint. Also, tube  301  is easily formed in a shape for a particular application. For example, a separation joint between two rocket stages is circular in shape, thus tube  301  is circumferentially placed in the joint separating the two rocket stages to provide simultaneously release when detonated.  
         [0007]      FIG. 4  is a cross-sectional view of the prior art explosive device  300  of  FIG. 4  after detonation. Detonation of explosive material  302  within tube  301  generates gases that radiate radially from the charge. Tube  301  expands under the pressure of the gases generated by the explosion but is designed not to rupture to prevent particle contamination. In an embodiment of explosive device  300 , tube  301  is formed of thin walled stainless steel. The rapidly increasing pressure created by the detonation of explosive material  302  causes tube  301  to expand to a final state as shown in  FIG. 4 .  
         [0008]     It is this change in volume of tube  301  from  FIG. 3  to  FIG. 4  after explosive material  302  is ignited that is used to produce a condition where a separation joint separates. As mentioned previously, separation joints are held together with rivets, bolts or other mechanical fasteners. The expansion of tube  301  generates an extremely high force. The force is applied in a manner to shear rivets or fracture elements of the separation joint thereby releasing the joint to separate.  
         [0009]     A significant problem with this type of separation joint are the shockwaves that are generated. The shockwaves are coupled to the attached structures of the separation joint. The problem is greatly exacerbated by the release of constrained energy due to the shearing or fracturing of components in the separation process. Shockwaves of up to 5000 g can be coupled to the attached structure. For example, NASA estimates that 45% of all first day spacecraft failures are attributed to damage caused by high dynamic environments. This problem exists today with all new proposed spacecraft designs. Spacecraft are typically ground tested to detect failures using random vibration, acoustic, and shock testing to simulate a launch environment. Perhaps more sensitive to the shockwaves generated by the separation joint is the payload within the spacecraft. The payload is often extremely sensitive or fragile to shock. The cost increases greatly to design components (in the payload) to be more shock resistant. Much of the research is focused on ways to minimize damage to the payload using isolation and damping techniques on the platform on which the payload is mounted.  
         [0010]     Accordingly, it is desirable to provide a separation joint that greatly reduces shockwaves transferred to an attached structure when separation occurs. In addition, it is desirable to provide reduce the cost of manufacture and increase the reliability of the separation system. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011]     Methods and apparatus are provided for a low shock explosive separation joint. The separation joint comprises a male member, a female member, and an explosive device. The separation joint couples a first structure and a second structure. The male member has a first major surface and a second major surface. At least one projection is formed on the first and second major surface of the male member. A female member includes a first flange and a second flange. At least one projection is formed on the first and second flange of the female member. Surfaces of the at least one projection on the first and second flange are respectively mated to surfaces of the at least one projection on the first and second major surface of the male member. The explosive device is placed within a cavity of said female member. A method for reducing shock in a separation joint is provided. A male member is coupled to a female member such that surfaces on said male member are in intimate contact with corresponding surfaces on said female member. The surfaces in intimate contact on the male and female member prevent separation of the separation joint under tensile and compressive forces. An explosive device in the female member of the separation joint is detonated. A housing of the explosive device expands from a first volume to a second volume. The volume increase of the explosive device bends flanges of the female member away from one another. The surfaces in intimate contact with one another are moved out of contact with one another to decouple the male member from the female member. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and  
         [0013]      FIG. 1  is an illustration of a prior art two stage rocket;  
         [0014]      FIG. 2  is an illustration the prior art two stage rocket of  FIG. 1  showing a first rocket stage, a second rocket stage, and a fairing separated from one another;  
         [0015]      FIG. 3  is a cross-sectional view of a prior art explosive device used in a separation joint;  
         [0016]      FIG. 4  is a cross-sectional view of the prior art explosive device of  FIG. 4  after detonation;  
         [0017]      FIG. 5  is a cross-sectional view of a prior art separation joint;  
         [0018]      FIG. 6  is a cross-sectional view of prior art separation joint  501  of  FIG. 5  after detonation of explosive device  507 ;  
         [0019]      FIG. 7  is a cross-sectional view of a separation joint in accordance with the present invention;  
         [0020]      FIG. 8  is a cross-sectional view of male member  710  of  FIG. 7  illustrating surfaces to prevent separation under compressive and tensile forces in accordance with the present invention;  
         [0021]      FIG. 9  is a cross-sectional view of female member  713  of  FIG. 7  illustrating surfaces to prevent separation under compressive and tensile forces in accordance with the present invention; and  
         [0022]      FIG. 10  is a cross-sectional view of the separation joint of  FIG. 7  separating in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description.  
         [0024]      FIG. 5  is a cross-sectional view of a prior art separation joint  501 . In general, separation joint  501  comprises a single female joint structure. Separation joint  501  utilizes an explosive device  507  to impart a force that fractures separation joint  501  in a predetermined manner. Separation joint  501  comprises a main body  510 , a mounting flange  503 , explosive device  507 , a flange  504 , and a flange  505 . Flanges  504  and  505  extend from main body  510  substantially parallel to one another forming a clevis. Explosive device  507  is fitted within the clevis. A structure  502  is also fitted within the clevis and is rigidly coupled to separation joint  501  by a bolt  508  and nut  509 . Bolt  508  extends through flange  504 , structure  502 , and flange  505 . Structure  502  is one of the components that are separated by separation joint  501  when explosive device  507  is detonated. The other structural component is not shown in  FIG. 5  but attaches to flange  503  distal to main body  510 .  
         [0025]     In an embodiment of separation joint  501 , one side of explosive device  507  abuts main body  510 . An opposing side of explosive device  507  abuts structure  502  such that explosive device  507  does not move during normal operation of separation joint  501 . Notches  506  are formed in flanges  504  and  505  to create a weak point in separation joint  501  that will readily fracture when explosive device  507  is detonated. Notches  506  are placed in a location where flanges  504  and  505  adjoin main body  510 . Notches  506  are also placed near explosive device  506 . It should be noted that separation joint  501  is made of a light weight but high strength material such as aluminum to rigidly couple a structure coupled to flange  503  to structure  502 . Notches  506  do not compromise the requirement of separation joint  501  to withstand torsional, compressive, and tensile forces under normal operating conditions.  
         [0026]      FIG. 6  is a cross-sectional view of prior art separation joint  501  of  FIG. 5  after detonation of explosive device  507 . Explosive device  507  is a tube type ordnance where the tube is an expandable housing. As shown in  FIG. 5 , explosive device  507  has a first volume prior to detonation that fits within the clevis of separation joint  501 . The expandable tube of explosive device  507  is made of thinned wall metal that does not rupture upon detonation of the explosive material housed within. Preventing contamination of the area near separation joint  501  is highly desirable. Explosive device  507  is designed to expand to a second volume when detonated. The explosive material generates gas that radially expands explosive device  507  to a second volume. Explosive device  507  is constrained from expanding by flange  504 , flange  505 , main body  510 , and structure  502 . Notches  506  of  FIG. 5  are the designed weak point of separation joint  501  that fractures or breaks when the force applied by explosive device  507  exceeds the material strength constraining explosive device  507  from expanding. An enormous amount of constrained energy is released upon the fracture of notches  506 . The rapid volume increase of explosive device  507  bends flanges  504  and  505  outward as shown. Main body  510  is no longer coupled to flanges  504  and  505  when notches  506  of  FIG. 5  fracture. Thus, main body  510  is free to separate.  
         [0027]     Separation joint  501  has proven to be an extremely reliable separation component that has been widely used in the aerospace industry. One aspect of separation joint  501  is the generation of high magnitude shockwaves. Shockwaves are a concern because components coupled to separation joint  501  can be damaged by vibrations generated during separation. A significant amount of research and development has been dedicated to enhancing the structural integrity of components to withstand the shock as well as the creation of isolation/damping strategies to reduce vibration coupled to components. Typically, these modifications add weight, cost, and complexity to the design of the system. Moreover, these changes are directly related to the design of the separation joint and would not be needed if the shockwave could be reduced.  
         [0028]     The shockwave is a high magnitude impulse because the release of constrained energy is instantaneous upon fracture of separation joint  501 . For example, the upper limit for a separation joint used in a rocket assembly is a shockwave of approximately 5000 g. The shockwave travels in either direction through separation joint  501 . A shockwave  601  is coupled through main body  510  to flange  503  where it is coupled to the attached structure. Similarly, a shockwave  602  is coupled through flanges  504  and  505  to structure  502 . In general, separation methodologies that break an element holding the joint together create potentially damaging shockwaves when the constrained energy is released. Another example that is well known utilizes rivets to hold a separation joint together. An explosive device produces a force on the separation joint that shears the rivets holding the joint together which produces a similar high magnitude shockwave.  
         [0029]      FIG. 7  is a cross-sectional view of a separation joint  701  in accordance with the present invention. The approach used in separation joint  701  eliminates the release of constrained energy due to fracturing or breaking of components during separation since all components remain intact and unbroken. The shockwave produced by separation joint  701  is reduced by an order of magnitude or more when compared with fracturing type separation joints. Separation joint  701  comprises a male member  710  and a female member  713 . Male member  710  includes surfaces for preventing separation of separation joint  701  under compressive and tensile forces. In an embodiment, of separation joint  701 , male member  710  further includes a flange  703  for attachment to a structure (not shown). The particular shape of flange  703  is not critical to the design of male member  710  but can be modified or adapted for the attachment needs of the particular structure it is being coupled to.  
         [0030]     Female member  713  comprises a flange  704 , a flange  705 , a flange  711 , and a flange  712 . Female member  713  also includes a cavity  706  for housing an explosive device  707 . In an embodiment of separation joint  701 , female member  713  is formed having two symmetrical halves. A first half of female member  713  includes flanges  704  and  711  on opposing ends. A second half of female member  713  includes flanges  705  and  712  on opposing ends. The halves of female member  713  align such that flanges  711  and  712  oppose one another and flanges  704  and  705  oppose one another. Flanges  711  and  712  include surfaces that correspond to the surfaces of male member  710  for preventing separation of separation joint  701  under compressive and tensile forces.  
         [0031]     Separation joint  701  is assembled by placing the two halves of female member  713  on opposing sides of male member  710 . Male member  710  is positioned and aligned such that surfaces for preventing separation of separation joint  701  under compressive and tensile forces on each half of female member  713  will mate with corresponding surfaces on male member  710 . Explosive device  707  is aligned to female member  713  to fit in cavity  706  that is formed when the halves are placed together.  
         [0032]     Placing the halves of female member  713  together retains male member  710  and explosive device  707 . In an embodiment of separation joint  701 , flanges  704  and  705  combine to form a clevis. A portion of structure  702  fits within the clevis. Structure  702  is fastened to flanges  704  and  705  by a fastening mechanism such as a bolt  708  and a nut  709 . Tightening bolt  708  not only rigidly fastens structure  702  to female member  713  but holds separation joint  701  together for use under normal operating conditions. In general, normal operating conditions occur when separation joint  701  fastens structure  702  to another structure (not shown) that is coupled to flange  703 . In other words, the normal condition occurs when separation join  701  is not separated and holds structures together. A separation event is not a normal operating condition. Tightening bolt  708  also forces male member  710  in contact with female member  713 . In particular, the surfaces on male member  710  are placed in intimate contact with corresponding surfaces of flanges  711  and  712 . Male member  710  is inhibited from moving within female member  713 . Similarly, explosive device  707  is held under a slight clamping pressure applied by the walls of cavity  706 .  
         [0033]     Separation joint  701  is shown in cross-section allowing only a single fastening device to be illustrated. In reality, separation joint  701  may have a substantial length. For example, in a rocket assembly, separation joint  701  would be placed between two rocket stages around the entire circumference to couple the rocket stages together. Male member  710  would couple to one of the rocket stages and female member  713  to the other rocket stage. Fastening mechanisms such as bolts and nuts would be required periodically around the circumference to hold separation joint  701  together.  
         [0034]     Note that the fastening mechanism holds the halves of female member  701  together on a side comprising flanges  704  and  705 . There is no fastening mechanism holding flanges  711  and  712  to male member  710 . Each half of female member  713  is made of sufficiently strong material to hold or retain male member  710  under torsional, compressive, and tensile forces imposed on separation joint  701  during normal operation. In general, female member  713  and male member  710  are made of metal. For example, separation joint  701  can be formed from 7075 or 6061 aluminum for an application in space flight where weight is critical. Magnesium is an example of another light weight metal that can be used. The compressive and tensile forces seen by separation joint  701  in a space flight is in the range of 900 to 5000 pounds per square inch. The thickness of flanges  704 ,  705 ,  711 ,  712 , and male member  710  to meet these specifications is approximately 0.125 inches or greater.  
         [0035]      FIG. 8  is a cross-sectional view of male member  710  of  FIG. 7  illustrating surfaces  801  and  802  to prevent separation under compressive and tensile forces in accordance with the present invention. Male member  710  includes a flange  703  for attaching to a structure (not shown) and a plurality of flanges or projections that extend horizontally from a main body. The flanges are on opposing sides of male member  710  and correspond to flanges or projections on female member  713  of  FIG. 7 . Each projection on male member  710  has an upper surface  801  and a lower surface  802 . Upper surfaces  801  prevent separation of male member  710  from female member  713  when tensile forces are applied to separation joint  701 . Conversely, lower surfaces  802  prevent separation of male member  710  from female member  713  when compressive forces are applied to separation joint  701 . In either case, upper surfaces  801  and lower surfaces  802  are in contact with corresponding surfaces of female member  713 . The design of the projection and the amount of upper and lower surface area placed on each projection is a function of the loading placed on separation joint  701 . A single projection on either side may be sufficient on the main body of male member  710  for light loads while multiple projections as shown in  FIG. 8  are required for high load applications such as a separation joint between rocket stages. In an embodiment of separation joint  701 , upper surfaces  801  and lower surfaces  802  are angled in a manner that prevents locking with the corresponding surfaces of female member  713 . A separation event of separation joint  701  moves the surfaces of female member  713  out of contact with upper surfaces  801  and lower surfaces  802  thus separation is more repeatable and reliable if made non-locking.  
         [0036]      FIG. 9  is a cross-sectional view of female member  713  of  FIG. 7  illustrating surfaces  901  and  902  to prevent separation under compressive and tensile forces in accordance with the present invention. Female member  713  is shown in an assembled state fastened to structure  702  without male member  710  or explosive device  707  of  FIG. 7 . Flanges  704  and  705  are rigidly held to structure  702  by bolt  708  and nut  709 . Flanges  711  and  712  each include a plurality of projections that extend horizontally from flanges  711  and  712  towards one another. Male member  710  as described in  FIG. 8  has corresponding surfaces  801  and  802 . Each projection on flanges  711  and  712  has an upper surface  901  and a lower surface  902 . Upper surfaces  901  prevent separation of female member  713  from male member  710  under compressive forces. Conversely, lower surfaces  902  prevent separation of female member  713  from male member  710  under tensile forces. Flanges  711  and  712  aligns with male member  710  when fastened together such that upper surfaces  901  mate with surfaces  802  of male member  710 . Similarly, lower surfaces  902  mate with surfaces  801  of male member  710 .  
         [0037]      FIG. 10  is a cross-sectional view of separation joint  701  of  FIG. 7  separating in accordance with the present invention. Prior to detonation, explosive device  707  has a first volume. Explosive device  707  is placed within cavity  706  when female member  713  is assembled around male member  710 . Normal operating conditions occur when separation joint  701  holds a structure attached to flange  703  to structure  702 . Separation joint  701  is designed to withstand and not separate under compressive, tensile, and torsional forces that occur during the specific application. An example of a normal operating condition is in a rocket assembly where separation joint  701  holds a first rocket stage to a second rocket stage. In this example, female member  713  couples to the first rocket stage and male member  710  couples to the second rocket stage. The first rocket stage lifts the rocket assembly into the upper atmosphere. The first stage is jettisoned as the rocket assembly reaches an elevation that can be better serviced by the lighter and smaller second rocket stage. Losing the first stage greatly reduces the weight of the remaining rocket assembly and thus is much more efficient to propel the rocket further on its destination.  
         [0038]     Explosive device  707  is detonated to separate male member  710  from female member  713 , for example, separating the first rocket stage from the second rocket stage. A separation event is not a normal operating condition of separation joint  701  but a one time event that physically modifies female member  713  to ensure decoupling from male member  710 . Explosive material within explosive device  707  is detonated which creates a high pressure gas that increases the volume of explosive device  707 . In general, explosive device  707  is housed within a tube that expands but does not rupture upon detonation. It is highly desirable to prevent any particles generated from detonating explosive device  707  from contaminating an area near separation joint  701 . The casing or tube of explosive device  707  retains all detonated explosive material.  
         [0039]     Cavity  706  is located between flanges  711  and  712 . When detonated, explosive device  707  starts to expand placing an extremely high force on flanges  711  and  712 . The force bends flanges  711  and  712  away from one another. Flanges  711  and  712  move in an arc when forced apart by explosive device  707 . All components of separation joint  701  are intact after detonation of explosive device  707 , thus no shockwave is generated due to the release of constrained energy by fracturing. The movement of flanges  711  and  712  away from one another places surfaces of male member  710  and female member  713  out of contact with one another. In particular, surfaces  801  and  802  of male member  710  ( FIG. 8 ) and surfaces  901  and  902  of female member  713  ( FIG. 9 ) that prevent separation of separation joint  701  under compressive and tensile forces are no longer in contact with one another. As mentioned previously, surfaces  801  and  802  of male member  710  and surface  901  and  902  of female member  713  are mated but are not locking surfaces which aids in separation of separation joint  701 . In the example using separation joint  701  in a rocket assembly, male member  710  is attached to the second stage of a rocket and continues on its intended flight path. Female member  713  is attached to the first rocket stage (structure  702 ) and either falls back to Earth where it can be retrieved or bums up in the atmosphere. A difference in the speeds of the male member  710  and female member  713  after detonation of explosive device  707  is enough to vertically move them the small distance required to move male member  710  from between flanges  711  and  712  thereby achieving separation. Alternately, a thrusting device could be added that provides a force that moves male member  710  and female member  713  away from one another. Separation joint  701  is low cost, simple to manufacture, easy to assemble, and reliable. Furthermore, the elimination of shockwaves due to the release of constrained energy will all allow more sensitive equipment to be part of a rocket payload and reduce the total mass of the rocket by not requiring components previously needed to damp the shockwave or isolate components from the shockwave.  
         [0040]     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.