Abstract:
In a rocket motor having an insulative coating on selected portions of thexterior casing, one or more ribs are used to structurally strengthen portions of the casing and to reinforce thermal stress patterns which will cause failure venting at a predetermined point of the rocket motor as a cook-off safety feature.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to rocket motors. More particularly, this invention relates to a solid propellant rocket motor. Additionally, this invention relates to a solid propellant rocket motor which produces mild burning reactions rather than explosions when exposed to external fires. 
     2. Description of the Prior Art 
     Navy carrier operations especially provide the potential for aircraft fuel fires to occur in the vicinity of weapons and ordnance. Many rocket motors react after about one minute of exposure to external fires and flames. The reaction can vary from a mild burning to a violent case rupture. 
     Past efforts to improve the heat resisting capability of ordnance items have included placing a thermal barrier on the exterior of the rocket motor casing or warhead. By thermal insulation of the rocket motor propellant or explosive, the length of time the ordance item can be exposed to fire without reaction is increased. If the fire is not extinguished within a short period of time, the internal temperature will increase and the ordnance item may ignite and explode. 
     Explosion and violent rupture of a heat weakened motor can occur when the propellant grain is ignited along the central void in the grain. If combustion can be limited to the outside of the grain and properly vented, the severity of the reaction is lessened. 
     SUMMARY OF THE INVENTION 
     This invention overcomes the problems of the prior art by providing a rocket motor resistant to violent explosions. While portions of the rocket motor are structurally strengthened and thermally protected, other selected stress points in the casing are left unprotected. This permits buckling of the casing at the unprotected point and a venting of the rocket motor can occur. By proper venting, a violent rupture and explosion can be averted. 
     OBJECTS OF THE INVENTION 
     Accordingly, it is an object of this invention to provide an improved solid propellant rocket motor. 
     A further object of this invention is to provide a fire resistant rocket motor which may be safely used in areas prone to fires. Another object of this invention is to provide a rocket motor which will undergo a small-locus case rupture and produce a mild burning reaction to prevent a dangerous build-up of interior pressure. 
     These and other objects of the invention will become more readily apparent from the ensuing specification when taken with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view of the device shown in its operational environment; 
     FIG. 2 is a view of the rocket motor case sector under thermally induced compression showing a pattern of stress &#34;lines&#34;; and 
     FIG. 3 is a view of the rocket motor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a rocket motor 10 is seen mounted under an aircraft aboard a ship 8. In this environment, aircraft fuel spillage can result in occasional fires as shown at 7. Rocket motor 10 can then endanger the crew and damage the ship, if it should react violently to a fire. 
     Referring to FIG. 2, a portion of rocket motor 10 is shown represented by a cylinder. When such a cylinder is subjected to longitudinal loading, as caused by thermal expansion, stress &#34;lines&#34; 11 develop in a criss-cross pattern. The intersections of these stress &#34;lines&#34; produce points 12 at which failure will occur, in the cylinder or rocket motor 10. By emphasizing the thermally induced stresses, a failure point is engineered into the construction of the rocket motor. 
     Referring to FIG. 3, a rocket motor 10 is shown as including a cylindrical casing 13 constructed from steel or another suitable metal. Along the length of casing 13 run ribs 14 which, preferably, are unitarily constructed therewith as shown in FIG. 3 and are, therefore, fixedly connected to casing 13. Optionally, one large rib can be used in place of several smaller ribs. Ribs 14 are located over a predetermined portion of the rocket motor and serve to structurally strengthen or stiffen that portion of casing 13. As seen in FIG. 3, said portion is segmental in shape and extends axially of casing 13. This stiffening will help provide the compressional stress when thermal expansion results from heating. Other stiffening patterns may also be used. Various attachments and skirts connected to the rocket motor can provide additional stiffening by keeping portions of the metal cooler. 
     A coating 16 covers portions of the outside of rocket motor 10. The coating may be of an insulating paint or an intumescent material and should be reflective of fuel fire radiation to protect those portions of the motor from external heat. As shown in FIG. 3 a peripheral casing area or bare patch 17 is located opposite of ribs 14 on casing 13 and is, therefore, diametrically opposite of the casing from said portion thereof over which the ribs are located. As is apparent from FIG. 3, area 17 is, in a direction axially of casing 13, disposed centrally of said portion over which ribs 14 are located. It is also apparent from FIG. 3 that ribs 14 stiffen said portion against bending in a plane passing through said portion and patch 17. Area 17 does not receive any thermal coating protection. Alternatively, this area can be coated to be thermally absorptive to radiation to enhance differential heating. Any fire will most likely occur beneath the rocket motor. Bare patch 17 should be situated on the bottom or underneath side. 
     When rocket motor 10 undergoes external fuel fire heating, the stiffening or strengthening provided by ribs 14 along the top of casing 13, combined with bare patch 17 along the bottom of casing 13 will generate stress at a predetermined stress point. 
     FIG. 3 shows that casing 13 surround a mass of propellant of grain 21. As is conventional, grain 21 has a central void 22 with a plurality of radially extended slots 23. Four slots 23 are shown in this star grain, but any number of radial or longitudinal extensions may be used in accordance with good motor design techniques. The shape of void 22 controls combustion characteristics of the rocket motor. When ignition occurs along this void in a heat weakened motor, destructive explosion or combustion is likely unless venting is sufficient. 
     Bare patch 17 is preferably located, circumferentially of the casing, between a pair of the radial slots 23 as is apparent from FIG. 3, so that casing 13 will rupture at a stress point 11 in a small region between the radial slots. Then, grain 21 will be externally ignited on a small area and burn in a manner to torch a large hole in casing 13, before the grain burns through a slot 23 or void 22. The resultant venting can prevent interior pressure from reaching a dangerous level and causing explosive destruction. 
     The foregoing description taken together with the appended claims constitute a disclosure such as to enable a person skilled in rocket motor arts and having the benefit of the teachings contained therein to make and use the invention. Further the structure herein described meets the objects of the invention and generally constitutes a meritorious advance in the art unobvious to such a person not having the benefit of these teachings. 
     Obviously many modifications and variations of this invention are possible, and, it is therefore understood that within the scope of the disclosed inventive concept, the invention may be practiced otherwise than specifically described.