STATEMENT OF GOVERNMENT SUPPORT 
     The invention was made with government support under contract W9113M-07-C-0047. The government has certain rights in the invention. 
    
    
     BACKGROUND 
     Predominant rocket body separation systems used by the aerospace industry include stage separation systems using Linear Shape Charges (LSCs) and other variations that use explosives. The explosives create a hazardous work environment, are expensive, make rocket assembly more logistically challenging, generate debris, and generate a large amount of source shock detrimental to electronic systems. The shock energy produced by explosives-based rocket body separation systems transmits through the rocket structure and into sensitive payloads that are susceptible to damage from high shock loads. 
     SUMMARY 
     A low shock rocket body separating joint for the purpose of joining and separating rocket bodies, and method of assembly thereof is disclosed. The proposed solution combines the use of a radax joint, joined by fasteners, with a flattened bladder and inflation system coupled thereto. Upon activation of the inflation system, the bladder is pressurized and exerts a separating force between the members of the radax joint, overcoming the load carrying capability of the fasteners and breaking apart the radax joint. Advantages of the disclosed technology include increased strength, reduced weight, reduced induced shock during rocket body separation, and reduced assembly hazards and logistics. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings depict example features and embodiments in accordance with this disclosure, and are not limiting of its scope. 
         FIG. 1  is a diagram illustrating an example rocket body separation system. 
         FIG. 2  is a diagram illustrating a cross section of an example rocket body separation system. 
         FIG. 3A  is a diagram illustrating a cross section of an example flattened bladder, manifold, and bladder inflation system interface. 
         FIG. 3B  is a diagram illustrating an example flattened bladder, manifold, and bladder inflation system interface. 
         FIG. 4  is a diagram illustrating a cross section of an example rocket body separation system with bladder inflation system. 
         FIG. 5A  is a diagram illustrating a cross section of an example rocket body separation system with a shaped flattened bladder. 
         FIG. 5B  is a diagram illustrating a cross section of an example rocket body separation system with a shaped flattened bladder and matching shaped platforms. 
         FIG. 6  is a diagram illustrating an example flattened bladder divided into a plurality of segments. 
         FIG. 7  is a diagram illustrating a cross section of an example rocket body separation system with a pyrotechnic piston actuator. 
         FIG. 8  is a diagram illustrating a cross section of an example rocket body separation system with a notched fastener and floating fastener receptacle. 
         FIG. 9  is a diagram illustrating a cross section of an example rocket body separation system with a detachable platform. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, may be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and made part of this disclosure. 
       FIG. 1  is a diagram illustrating an example rocket body separation system. The rocket body separation system may comprise a radax-type joint including a first radax member  2  and a second radax member  3 , each member coupleable to a rocket body such as a booster stage, payload, or other rocket body, and each member coupleable to the other member. Members of the radax joint may be coupleable to one another using fasteners such as  7 , e.g., frangible bolts, washers such as  5 , and threaded inserts such as  8 . The fastener quantity and spacing may be adjusted to accommodate the load carrying capability of the joint. 
     The radax joint may be round, as illustrated in  FIG. 1 , or may be any other shape. In general, the shape of the radax joint will conform to the shape of the rocket bodies joined by the joint. The illustrated configuration provides added benefit in that separation systems don&#39;t have to take on a cylindrical shape. The apparatus, systems and methods disclosed herein are applicable to shroud and/or nose cone deployments as well as staging of oval, square and other irregularly shaped bodies. 
     A flattened bladder  1  may be located between the first and second radax members  2  and  3 , and may be coupled to an inflation system  9 . The flattened bladder  1  may be positioned such that inflation thereof applies a low-shock separating force to the first and second radax members  2  and  3 , thereby separating the first and second members of the radax joint. The low induced shock softens the electronics design requirements for any onboard electronics, and the low complexity design of the illustrated separation system, and class 1.4 pyrotechnic system (discussed further below) improves integration with the rocket bodies and reduces the hazards and handling associated with class 1.1 explosive systems. 
     One or more members of the radax joint may be configured with a housing for insertion of one or more pyrotechnic piston actuators. Pyrotechnic piston actuator  6  is illustrated in a housing. A retention collar or other fastener such as  4  may serve to hold a pyrotechnic piston actuator in place in the housing. Pyrotechnic piston actuators may be located within or proximal to the radax joint and positioned to increase separation velocity of the first and second radax members upon activation of the one or more pyrotechnic piston actuators. Pyrotechnic piston actuators may for example be employed to supplement the separation force provided by the flattened bladder  1 , and to impart additional separation velocity to separated rocket bodies. Such embodiments may enable high-Q stage separation, using pyrotechnic piston actuators that incorporate class 1.4 pyrotechnics. The use of class 1.4 pyrotechnics reduces the hazards and handling associated with class 1.1 explosive systems. High-Q separation capability provides improved separation performance, for the purpose of, for example, early motor separations that occur in the atmosphere and have a risk of re-contacting rocket bodies after separation. 
     It should be noted that the term “rocket” as used herein refers to any missile, spacecraft, aircraft or other vehicle, whether manned or unmanned, that obtains thrust from a rocket engine. A rocket engine is any engine that ejects propellant mass in a rearward direction in order to produce thrust in a forward direction. Also, the terms rocket and missile are used interchangeably herein. 
     The term “rocket body” refers to any part of a rocket. In a traditional rocket-based embodiment, a rocket body may for example comprise a rocket stage such as a booster stage or payload stage. Some embodiments of the low-shock rocket body separation technologies described herein may comprise low-shock stage separation systems. However, the potential uses of the described technologies are not limited to rocket stage separation—any number of other rocket bodies may benefit from the low-shock rocket body separation technologies described herein. 
     Furthermore, the technologies described herein may also be applicable in certain non-rocket applications. In some embodiments, the low-shock rocket body separation technologies described herein may be applied to separation of any first body from any second body, whether the first and second bodies are rocket bodies or otherwise. 
       FIG. 2  is a diagram illustrating a cross section of an example rocket body separation system.  FIG. 2  includes a first radax member  200  and a second radax member  201 . The first radax member  200  is coupleable to the second radax member  201  for example by inserting a fastener into fastener guide  270 . Fastener guide  270  extends in both the first and second radax members  200  and  201 , as illustrated by the dotted lines. 
     In general, radax joints are characterized in that they comprise an angled contact plane  202  where the male and female radax members make contact, as illustrated, with a fastener that couples the radax members extending through the angled contact plane  202 . Radax joints are very strong, stiff and assemble easily, while being low mass. High stiffness provides rocket/missile guidance and control avionics packages desirable system responsiveness and controllability. Building on this flight proven technology, embodiments of this disclosure integrate an innovative flattened bladder design and inflation system, while modifying the radax joint to make it a separable system. 
     The first radax member  200  is coupleable to a rocket body sidewall  250  for example by inserting a fastener into fastener guide  280 , which may extend into the coupling member  260 . The coupling member  260  is a portion of the first radax member  200  which may be configured as appropriate for coupling to the sidewall  250 . In some embodiments, it will be appreciated that coupling member  260  may be integral to the sidewall  250 , whereby the coupling member  260  and sidewall  250  are formed as a single piece, reducing the part count and eliminating a need for a fastener guide and/or fastener. The option of integrating radax members with rocket body sidewalls is an advantage of embodiments of this disclosure. Unlike previous rocket body separations systems, especially those using a Linear Shaped Charge (LSC) for stage separation, the disclosed rocket body separation system does not require an inter-stage. Embodiments without an inter-stage can be lower-weight. Furthermore, the disclosed rocket body separation system does not preclude the use of an inter-stage in some embodiments. 
     Likewise, the second radax member  201  is coupleable to a rocket body sidewall  251  for example by inserting a fastener into a fastener guide, which may extend into the coupling member  261 . The coupling member  261  is a portion of the second radax member  201  which may be configured as appropriate for coupling to the sidewall  251 . In some embodiments, it will be appreciated that coupling member  261  may be integral to the sidewall  251 , as described with reference to the coupling member  260  and sidewall  250 . 
     Each radax member  200 ,  201  may further comprise a flange or platform extending therefrom. In  FIG. 2 , the first radax member  200  comprises first platform  210 , and the second radax member  201  comprises second platform  211 . The platforms  210 ,  211  may extend from the radax members  200 ,  201  such that, when the first and second radax members are coupled together, the first platform  210  on the first radax member  200  is proximal to the second platform  211  on the second radax member  201 . 
     A flattened bladder  220  may be located between the first and second platforms  210 ,  211 , wherein the flattened bladder  220  is positioned such that inflation thereof applies a separating force to the first and second platforms  210 ,  211 , separating the first and second radax members  200 ,  201  of the radax joint. 
     In some embodiments, the flattened bladder  220  may be made of a metal such as steel. An example flattened bladder  220  may comprise a plurality of sections of steel tube, welded together, and each section formed in an appropriate shape as desired for a particular radax joint. A flattened bladder  220  may for example have a sidewall thickness between 0.010-0.080 inches. When flattened, a compressed height of the flattened bladder  220  may for example be in the range of 0.050-0.500 inches. A compressed height of 0.150 inches and sidewall thickness of 0.040 inches would leave an example compressed internal volume thickness of 0.070 inches. 
     In some embodiments, the first and/or second platforms may extend around substantially an entire inner perimeter of the first and/or second radax members. For example referring back to  FIG. 1 , the platforms may extend around the entire circumference of radax members  2  and  3 , with the platforms extending inwardly, e.g. extending into the area that is circumscribed by the radax members. Embodiments in which the platforms extend outwardly may also be adapted for some configurations. Furthermore, in some embodiments, the platforms may extend inwardly in some locations, and outwardly in other locations. Platform placement may be configured as needed to avoid obstacles and accommodate system requirements, while also providing a continuous surface for the flattened bladder  220 , such that when the flattened bladder  220  expands, a symmetrical pressure is produced which allows for separation without rotational forces on the separated rocket bodies. Likewise, in some embodiments, the flattened bladder may extend around substantially an entire length of the first and/or second platforms, as illustrated in  FIG. 1 , in which the flattened bladder  1  extends around the full circumference of radax members  2  and  3 . 
     The flattened bladder  220  may fit in a groove  240  within at least one of the platforms. The groove  240  may be defined by at least one sidewall, e.g., the groove sidewall proximal to the angled contact plane  202 . A bottom surface of the groove  240  is illustrated in  FIG. 2  within the platform  210 . Groove  240  may, but need not, also be defined a sidewall formed by a platform lip  230  on an edge of a platform. Platform lip  230  adds strength to the platform  210 , but is not required for rocket body separation. A lip  230  may also prevent debris from getting into the vicinity of the flattened bladder  220  and may serve to prevent malfunctions. 
       FIG. 3A  is a diagram illustrating a cross section of an example flattened bladder  220 , manifold  301 , and bladder inflation system interface  310 .  FIG. 3B  is a diagram illustrating a side view of an example flattened bladder  220 , manifold  301 , and bladder inflation system interface  310 .  FIG. 3A  and  FIG. 3B  provide an example of how a rocket body separation system as disclosed herein may be equipped with a bladder inflation system interface  310 . The flattened bladder  220  may be configured with a manifold  301  on the bladder  220  at a location defined by the connection between the bladder  220  and the bladder inflation system interface  310 . 
     In some embodiments, a flattened bladder  220  may be equipped with a plurality of bladder inflation system interfaces such as  310 . Providing more than one interface such as  310  provides redundancy so that failure of an inflation system does not lead to failure of the separation system as a whole. For example,  FIG. 1  illustrates two inflation systems such as  9 , connecting to two interfaces such as  310 . 
       FIG. 4  is a diagram illustrating a cross section of an example rocket body separation system with bladder inflation system  400 . In  FIG. 4 , the bladder inflation system interface  310  is configured to extend from the manifold  301  in a platform in the radax joint, away from the radax joint to allow attaching a bladder inflation system  400  to the bladder inflation system interface  310  after coupling of the first and second radax members to one another and to their respective rocket stages. In the illustrated example, the bladder inflation system  400  may be accessed via the sidewall door  410 , allowing fastening or otherwise coupling of the inflation system  400  to the interface  310 . 
     The bladder inflation system  400  may generally be any system that causes the flattened bladder  220  to inflate. Example systems that may be used as a bladder inflation system  400  are a hot gas generator, which may be electrically initiated and forces hot gas into the flattened bladder  220 , and a Rapid Deflagrating Cord (RDC) or mild detonation cord system comprising a cord inside the flattened bladder  220 , and an electrically initiated igniter which may access the cord via the inflation system interface  310  to ignite the cord. Either of the above example systems may utilize a class 1.4 pyrotechnic system, which reduces shock via a lower energy explosion than that created by the class 1.1 explosive systems which are commonly used in today&#39;s separation systems. 
     The bladder inflation system interface  310  may be configured to couple with a bladder inflation system  400 . For example, when the bladder inflation system  400  is a hot gas generator, the inflation system interface  310  may be a tube of appropriate diameter and sidewall thickness to allow coupling to an output of a hot gas generator, and flow of hot gas from the hot gas generator into the flattened bladder  220  under controlled conditions. 
     In  FIG. 4 , the first radax member  200  is a female radax member, and the second radax member  201  is a male radax member. The first radax member  200  is coupleable to a determined rocket body, defined in part by sidewall  250 , and the second radax member is coupleable to a subsequent rocket body defined in part by sidewall  251 . The “determined” rocket body may be any determined rocket body, e.g. a booster stage such as a first booster stage, second booster stage, etc., a payload stage, or other rocket body. In some embodiments, a determined rocket body may comprise a spent booster stage. The determined rocket body may also be referred to as an aft rocket body. 
     The subsequent rocket body is any rocket body subsequent to the determined rocket body, for example, if the determined rocket body is a first booster stage, then the subsequent rocket body may be a second booster stage, a payload stage, or other rocket body; if the determined rocket body is a second booster stage, then the subsequent rocket body may be a third booster stage, a payload stage, etc. The subsequent rocket body may also be referred to herein as a forward rocket body. 
     The flattened bladder has a bladder inflation system interface  310  extending toward the determined rocket body defined in part by sidewall  250 , to allow attaching a bladder inflation system  400  within the determined rocket stage. Upon separation of the radax joint, the first radax member  200 , the inflation system  400 , and the bladder (shown inside manifold  310 ) are configured to separate from the second radax member  201  and the subsequent rocket body defined in part by sidewall  251 . 
       FIG. 4  may be used to describe a method of assembling a rocket. The rocket may comprise a first radax member  200  coupled to a determined rocket body  250  and a second radax member  201  coupled to a subsequent rocket body  251 . Each radax member may comprise a platform extending therefrom such that, when the first and second radax members are coupled together, a first platform on the first radax member is proximal to a second platform on the second radax member, as illustrated in  FIG. 4  and also described with reference to  FIG. 2 . The method of assembling a rocket may comprise: inserting a flattened bladder between the first radax member  200  and second radax member  201 , for example by manually positioning the flattened bladder in a platform groove designed to receive the flattened bladder, manifolds and any inflation system interfaces; coupling the first radax member  200  and second radax member  201  together to join the determined rocket body  250  with the subsequent rocket body  251 , for example, by inserting fasteners into the fastener guides intersecting the angled contact plane of the radax joint; and after coupling the first radax member  200  and second radax member  201  together, attaching a bladder inflation system  400  to a bladder inflation system interface  310  extending from the flattened bladder (shown inside the manifold  310  in  FIG. 4 ). The bladder inflation system  400  may be attached for example by inserting inflation system  400  through a rocket body sidewall door  410 , and welding, screwing, clipping, clamping or otherwise attaching inflation system  400  to interface  310 . 
     An advantage of a method as described above is that it does not require pyrotechnics installation until the final stages of rocket assembly. Pyrotechnics installation may be deferred even to a time after a full up rocket is assembled, for example, if side panels exist on the rocket motor skin as illustrated in  FIG. 4 . This ability to defer installation of pyrotechnics is referred to as “isolation”, and the separation pyrotechnics are an “isolated” system. By isolating the pyrotechnics, assembly and workplace safety can be less hazardous and, as a result, cheaper and less complex. 
       FIG. 5A  is a diagram illustrating a cross section of an example rocket body separation system with a shaped flattened bladder  501 . The shaped flattened bladder  501  comprises a flattened middle portion with a compressed height that is less than that of the flattened outer portions, as shown. The illustrated shape has an advantage of not “pinching” the outer portions, which can stress the bladder material (e.g. steel), while also thinning the middle portion to provide enhanced separation force. In some embodiments, a shaped flattened bladder  501  (or a flattened bladder of any shape) may be annealed subsequent to flattening and/or otherwise shaping the flattened bladder. 
       FIG. 5B  is a diagram illustrating a cross section of an example rocket body separation system with a shaped flattened bladder  501  and matching shaped platforms  510  and  511 . The shape of the platforms  510  and  511  may fit the shape of the flattened bladder  501 , to allow the flattened bladder  501  to impart maximal separation force to the platforms  510  and  511 . 
       FIG. 6  is a diagram illustrating a top view of a flattened bladder  600 . The flattened bladder  600  is dividable into example bladder segments A, B, C, D, E, F, G, and H. In some embodiments, a flattened bladder  600  may be characterized in that it comprises at least one first bladder segment with a compressed height that is thinner (less) than the compressed height of at least one second bladder segment. For example, segments A, C, E, and G may have smaller compressed heights than segments B, D, F, and H. Platform segments may also be shaped to match corresponding segments of the bladder  600 . When the bladder  600  inflates, radax joint fasteners near the thinner bladder segments (those of smaller compressed height) may break before fasteners at the thicker (larger compressed height) bladder segments—providing a cascading or multi-stage fastener break. The cascading or multi-stage fastener break may produce lower shock than breaking all of the radax joint fasteners at once. 
       FIG. 7  is a diagram illustrating a cross section of an example rocket body separation system with a pyrotechnic piston actuator  700 . In the illustrated embodiment, the pyrotechnic piston actuator  700  is located within the radax joint by placing the piston actuator  700  in a piston support housing  720 , which is attached to one radax member. A moving piston element  701  may make contact with a piston contact surface  710 , which is attached to another radax member. Activating the pyrotechnic piston actuator  700  causes the moving piston element  701  to move away from the body of the pyrotechnic piston actuator  700 , thereby producing a separating force between the members of the radax joint. 
     A pyrotechnic piston actuator  700  may be configured to establish prior contact between the moving piston element  701  and the piston contact surface  710 , prior to activation of the actuator  700 . A variety of mechanisms/configurations may be useful for establishing prior contact, including for example fasteners, springs, air pressure or any number of other approaches as will be appreciated by those of skill in the art, with the benefit of this disclosure. Establishing prior contact reduces shock caused by activating the pyrotechnic piston actuator  700 . When the moving piston element  701  is in prior contact with the piston contact surface  710 , the moving piston element  701  does not build velocity prior to contacting the piston contact surface  710 , and therefore the shock of initially striking the piston contact surface  710  may be avoided. 
       FIG. 8  is a diagram illustrating a cross section of an example rocket body separation system with a frangible fastener  800  and floating fastener receptacle  811 . Either or both of a frangible fastener  800  and floating fastener receptacle  811  may be used to reduce shock caused by separation of the radax joint in some embodiments. While one frangible fastener  800  and one floating fastener receptacle  811  are illustrated in  FIG. 8 , it will be appreciated that a plurality of frangible fasteners may couple the first and second radax members, and a plurality of floating fastener receptacles  811  may be employed. 
     In some embodiments, frangible fastener  800  may comprise a frangible bolt with a notch  801  defined by a necked down section that creates a stress concentration at the plane of separation of the radax joint. The fastener  800  and notch  801  may be sized accordingly for a load carrying capability of the joint. The depth and width of the notch  801  may be tailored to allow the fastener  800  to break at a desired load. The fastener  800  may include threaded sections above and below the notch  801 , to allow the fastener  800  to attach to each of the radax members. In some embodiments, a frangible fastener  800  may be produced for example by drilling a hole down the length of the fastener  800 , or by drilling hole perpendicularly through the fastener  800 . 
     The floating fastener receptacle  811  may be configured to receive a fastener  800  that couples the radax members, while the floating fastener receptacle  811  also allows movement of the fastener  800  (or section of the fastener  800 ) upon breakage of the fastener  800 . Receptacle  811  may for example comprise a threaded nut. When the fastener  800  breaks, the section of the fastener  800  that remains with the radax member having the receptacle  811  can move with respect to the radax member, thereby reducing the shock to the radax member caused by breakage of the fastener  800 . 
     The receptacle  811  may be positioned in a chamber  810  that is wider and/or longer than the diameter and length of the receptacle  811 , allowing the receptacle  811  to “float” (move) within the chamber  810 . A shoulder  812  allows the fastener  800  to pass through radax member  201  and couple the first and second radax members. The shoulder  812  may be sufficiently sturdy to ensure adequate load-carrying capacity of the radax joint. A gate  813  may open to allow insertion of the receptacle  811 , and may close to prevent the receptacle  811  from flying out of the chamber  810  when the fastener  800  breaks, thereby reducing the debris byproducts of separation. Radax member  200  may also be configured with a chamber and gate to retain a free fastener head, further preventing separation debris. 
     It should be emphasized that frangible fasteners and floating fastener receptacles are not required in all embodiments. For example, some embodiments may employ a threaded insert, e.g., a nut that is threaded on both the outside and inside of the nut, in place of the floating fastener receptacle  811 . Unlike the floating fastener receptacle  811 , a threaded insert would not be capable of moving with respect to the chamber  800  upon breaking of the fastener  800 . Also, some embodiments may be configured according to  FIG. 2 , with either a threaded fastener guide  270 , or a nut that is used to hold an opposite end of a fastener in place. 
       FIG. 9  is a diagram illustrating a cross section of an example rocket body separation system with a detachable platform.  FIG. 9  comprises first and second radax members  200  and  201 , wherein the second radax member  201  is configured with a platform interface  930  that mates with a detachable second platform  911 . The detachable second platform  911  is designed to detach from the radax member  201  after separation of the rocket bodies. A tether  901  may attach the platform  911  to first platform  210 , or otherwise to the radax member  200  or a rocket body associated with radax member  200 . Upon separation of the rocket bodies, the tether  900  may keep the detachable second platform  911  with the radax member  200  and associated rocket body, reducing debris and reducing the weight of the radax member  201 . An additional benefit of detachable platform configurations is clearing of obstructions (such as the detachable platform) for the follow on stage. Clearing obstructions may reduce unwanted plume interactions that may otherwise be produced by such obstructions. In some embodiments, removing obstructions may yield improved performance of various systems, e.g., improved Thrust Vector Control (TVC) maneuvering, and improved operation of an attitude control system. 
       FIG. 9  illustrates platform supports  920  and  921 . Platform supports  920  and  921  may comprise support braces that are spaced some distance apart, supporting the load bearing capacity of the platforms  910  and  911 , while providing a reduced platform weight.  FIG. 9  also illustrates a lip located on the detachable second platform  911  instead of the first platform  910 , which is an alternative configuration as will be appreciated with the benefit of this disclosure. 
     While certain example techniques have been described and shown herein using various methods, devices and systems, it should be understood by those skilled in the art that various other modifications may be made, and equivalents may be substituted, without departing from claimed subject matter. Additionally, many modifications may be made to adapt a particular situation to the teachings of claimed subject matter without departing from the central concept described herein. Therefore, it is intended that claimed subject matter not be limited to the particular examples disclosed, but that such claimed subject matter also may include all implementations falling within the scope of the appended claims, and equivalents thereof.
The present disclosure generally relates a high strength, low weight, and low shock rocket body separating joint for the purpose of joining rocket bodies, and method of assembly thereof. The solution combines a radax joint, joined by fasteners, with a flattened bladder and inflation system coupled thereto. Upon activation of the inflation system, the bladder is pressurized and exerts a separating force between the members of the radax joint, overcoming the load carrying capability of the fasteners and breaking apart the radax joint.