Abstract:
A solid propellant restrictor for preventing erosive burning in solid  prolant grain critical areas is described. The restrictor is a mixture of hydroxyl-terminated polybutadiene with 1% by weight of dissolved 2,2&#39;-methylene-bis(4-methyl-6-tert-butyl) phenol, dioctyl adipate, triethanolamine, ferric acetylacetonate, carbon black, aluminum oxide, silicon oxide, and toluene diisocyanate, curing at room temperature and illustrates superior hardening, adhesive, and application properties.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a process for making a solid rocket motor propellant grain restrictor and its application and use in the prevention of erosive burning and pressure buildup (spiking) upon ignition. 
     2. Description of the Prior Art 
     An earlier epoxy-resin-based restrictor for use with a carboxyl-terminated polybutadiene (CTPB) propellant grain for the MK56 rocket motor tends to harden excessively upon aging. This reduces restrictor strain capability and bond strength, often resulting in separation of the restrictor from the propellant. The epoxy-based restrictor also causes the CTPB propellant grain to harden, resulting in grain cracking. 
     These defects can cause catastrophic failure when the motor is fired. Furthermore, the epoxy-based restrictor is excessively fluid and tends to &#34;run&#34; after its application and before it solidifies, i.e. &#34;cures&#34;. In order to prevent such &#34;running&#34;, it is necessary to rotate the motor during cure, or to apply and cure the restrictor on one &#34;flat&#34; or &#34;ray&#34; of the grain perforation at a time. This results in increased man-hours for the application of the restrictor to the propellant grain. 
     There is a need for a restrictor which is easy to apply, cures at room temperature, and prevents cracking and hardening of the propellant grain. Such a restrictor, having superior strain and adhesive properties, is not found in the previous art. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method for preparing a solid rocket motor propellant grain restrictor which prevents chamber pressure buildup (spiking) and erosive burning upon ignition. This restrictor can be used on any hydroxyl-terminated polybutadiene (HTPB) propellant grain and on various polyurethane propellant grains which employ an isocyanate cure. 
     OBJECTS OF THE INVENTION 
     One object of the present invention is to restrict the critical areas of a solid propellant grain by coating these areas with a relatively noncombustible material that will adhere to these areas and prevent them from igniting until neighboring uncoated areas have ignited and are burning steadily. 
     Still another object is to make a restrictor which has a thixotropic nature, so that it does not &#34;run&#34; while it is being applied to the propellant grain nor during its cure. 
     Another object is to make a restrictor which cures at room temperatures (70°-75° F.), thus eliminating the need for a hot oven. 
     And still another object is to make a restrictor which does not excessively harden, nor cause the propellant grain to crack, and which does not lose strain capability and adhesive strength properties upon aging. 
     These and other objects and advantages of this invention are apparent in the following detailed description. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The restrictor composition disclosed herein solves the aforementioned problems. The following Table I lists the ingredients, their preferred weight percentages, and the order and method for mixing them. Table II enunciates and illustrates the invention&#39;s physical properties. 
     This restrictor was developed for use in the new MK56 standard missile rocket motor which employs a solid propellant grain consisting of aluminum fuel, ammonium perchlorate oxidizer, and a diisocyanate-cured hydroxyl-terminated polybutadiene (HTPB) binder. However, it can be used with other propellant grains as previously mentioned. 
     
                       TABLE I______________________________________PREMIX:                  Percentage (%)Material               by weight______________________________________hydroxyl-terminated polybutadiene                  32.932(HTPB) binder (R45M type) with1% by weight of dissolved2,2-methylene-bis(4-methyl-6-tert-butyl)phenoldioctyl adipate (DOA) plasticizer                  12.173triethanolamine (TEA) x-linker                  0.244ferric acetylacetonate (FeAA) catalyst                  0.235carbon black           0.469aluminum oxide (Al.sub.2 O.sub.3), hydrated                  48.787Cabosil (SiO&#39;.sub.2 colloidal)                  5.160                  100.000______________________________________INGREDIENT ADDITION ORDER:            Temp °F.                       Vacuum        Mixing             mixer inchesMaterial     Time      material jacket                                 Hg______________________________________HTPB, DOA,   1      min.   by hand in beakerTEA, FeAAHTPB, DOA,   5      min.   123    134   24&#34;TEA, FeAAAl.sub.2 O.sub.3, 1/3rd        2      min.   123    134   24&#34;Al.sub.2 O.sub.3, 1/3rd        2      min.   123    134   24&#34;Al.sub.2 O.sub.3, 1/3rd        2      min.   123    134   24&#34;Carbon Black 2      min.   123    134   26&#34;Scrapedown and mix        5      min.   127    134   26&#34;Cabosil, 1/4th        1      min.   without vacuum  +                      scrapedown        2      min.   with vacuum;Cabosil, 1/4th        1      min.   without vacuum +                      scrapedown        2      min.   with vacuum;Cabosil, 1/4th        1      min.   without vacuum +                      scrapedown        2      min.   with vacuum;Cabosil, 1/4th        1      min.   without vacuum +                      scrapedown        2      min.   with vacuum;Scrapedown &amp; mix        5      min.   128    134   26&#34;Scrapedown &amp; mix        10     min.   128    134   26&#34;Mixing is done at a moderate speed, the equivalentof a setting at 1 on a one-gallon Baker Perkinstype standard vertical mixer.______________________________________CATALYST EQUIVALENCIES:HTPB (R45M type) with 1% 2,2-methylene-                  =     1264.00   ewbis(4-methyl-6-tert-butyl)phenolTEA                    =     49.73     ewTEA/HTPB equivalent ratio                  =     0.19For NCO/OH (equivalent ratio of                  =     1.15,isocyanate to hydroxyl) use 0.310 g. toluenediisocyanate (TDI) for each 10 g. ofpremix, along with 0.55 g. additionalCabosil to eliminate flow.(TDI) toluene diisocyanate equivalent                  =     87.08weightFor NCO/OH             =     1.10,0.296 g. TDI are usedCure time*  is 7 days at 75° F.**______________________________________ g = grams ew = equivalent weight *Complete cure takes 2 to 3 weeks at 75° F., but restrictor is tough and rubbery after 7 days, with NCO/OH = 1.15 **At 75° F., the complete restrictor is thixotropic and will not flow. At temperature below 70° F., cure becomes sluggish, more tim is required, and the cure quality is reduced somewhat. 
    
     
                       TABLE II______________________________________     Behavior     before Aged 32 days                       Aged 32 days     start of            at room temp                        at 140° F.     aging  in air     in air  under N.sub.2______________________________________1.  Unrestricted    propellant:    σ.sub.max, psi.sup.1           116                     132    ε.sub.max, %           35                      35    ε.sub.break, %           39                      38    E, psi      722                     7732.  Restrictor    alone:    σ.sub.max, psi           208      264      329   235    ε.sub.max, %           66       71       89    65    ε.sub.break, %           *        *        *     *    E, psi      380      454      468   4363.  Propellant    with    restrictor:.sup.2    σ.sub.max, psi           127      136            145    ε.sub.max, %           32       31             32    ε.sub.break, %           34       32             34    E, psi      816      930            9514.  Restrictor-    to-propellant    adhesive peel    strength, in.-    lbs. torque    per inch    20.8              16.2______________________________________ .sup.1 σ.sub. max = maximum tensile strength ε.sub.max = elongation at maximum tensile strength ε.sub.break = elongation at break E = initial modulus (Young&#39;s modulus) *specimens slipped out of holders before breaking .sup.2 restrictor removed from propellant prior to tensile testing 
    
     This new restrictor cures at room temperatures (70°-75° F.) after application. Pot life is about three hours at room temperature. Twenty-four hours after application, the restrictor is firm enough so that such tasks as motor nozzle assembly can be accomplished without fear of restrictor damage by inadvertent bumping or touching of restricted areas. After five to seven days, the restrictor is almost completely cured to a tough, pliable rubber ready for motor firing. Complete cure takes three to four weeks at room temperature (70°-75° F.). 
     In normal use, the restrictor is first made in the form of a premix (see Table I). All of the ingredients except the toluene diisocyanate (TDI) curative and some of the Cabosil (SiO 2 ), a colloidal silica, are combined and mixed in a suitable mixer in the order outlined in Table I. A one-gallon vertical mixer of the type used to process solid propellants is satisfactory for this purpose. 
     The resulting premix is then packed in an air-tight container, and can be stored indefinitely until needed, at which time the TDI and a small amount of Cabosil (SiO 2 ) are mixed either by hand or with a simple mixer to obtain the restrictor, ready to apply to a rocket motor propellant grain. 
     Application can be done by hand, with a spatula on a long rod. The restrictor composition is spread over the critical surfaces of the propellant grain in about thirty minutes. A one-tenth inch thickness of restrictor has proven adequate for motor firings. The thickness need not be critically maintained; thin spots and runovers, which are bound to occur with this simple method of application, have produced no detectable effects on motor performance during firing. 
     A faster, more sophisticated technique of applying the resistor is possible with a pneumatic caulking gun. 
     No excessive hardening or loss of adhesion occurs with this restrictor on propellant during aging. For example, see Table II. The restrictor modulus always remains much lower than the unrestricted propellant modulus throughout the aging period, while restrictor strain capability remains well in excess of the propellant&#39;s capability. Furthermore, there is no excessive hardening of the propellant under the restrictor upon aging, and the restrictor-to-propellant adhesive peel strength remains high throughout aging. 
     All of the aforementioned factors indicate high restrictor reliability upon aging and motor firing. 
     The foregoing illustrates the advantages and new features of the present invention to one skilled in the art. However, this invention is not limited by the description, but only by the claims.