Inhibition of Composite Propellants

The inhibition of composite propellants is somewhat easier than that of DB propellants. The binders used for composite propellants (with or without fillers) have been reported for inhibition of composite propellants. Such inhibition systems possess stronger bonds with composite propellants and prove to be more compatible coupled with better shelf-life of the inhibited propellants. However, epoxy or novolac epoxy resin with or without inert fillers is generally preferred for the inhibition of composite propellants due to a combination of properties possessed by them. The inhibition is usually done by casting technique and inhibition thickness is usually required on higher side in order to make the missions successful. In India, thread winding technique or inhibitor sleeve technique is preferred where 2.5-3.0mm inhibition thickness is sufficient as against 3.5-4.0 mm in case of inhibition by casting technique . 

Considering thermodynamics of adhesion, epoxy/novolac epoxy resins play a vital role for bonding applications especially for inhibition of composite propellants. In view of this fact, it is considered worthwhile to discuss the chemistry of epoxy/novolac epoxy resins in this section before we discuss other systems for this purpose. 

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Kay and Fust postulated the use of epoxy resin for inhibition of composite propellants [335] and as a consequence, epoxy resins were tried for the first time for the inhibition of HTPB-based composite propellants at Thiokol Corporation, USA. Subsequently, use of the amido-amine hardened modified bisphenol-A-based epoxy resin was reported as inhibitor for fluorocarbon-based composite propellants [336]. Epoxy resins are the most versatile resins for bonding applications for a variety of substrates. This is because of the following characteristics. 

High adhesive properties As explained earlier, electromagnetic bonding forces, chemical reaction and relatively less shrinkage lead to strong bonds between epoxy resins and composite propellants and that is why epoxy resins are preferred for inhibition of composite propellants [337-339]. 

Epoxy-Liquid Polysulfide Blends Epoxy resins in combination with liquid PSs appear to possess many added advantages as the elastomeric PS segments in epoxy chains impart permanent flexibility [18]. Epoxy-liquid PS blends have been reported as binders for Army illuminating formulations [8]. Similarly, novolac epoxy-liquid PS blends have been reported for inhibition of composite propellants [19]. 

Novolac epoxy flexibilizers Improve low and high temperature characteristics of novolac epoxy resins for inhibition of composite rocket propellants [164]. 

The choice of material for use as inhibitor depends mainly upon the type of propellant, that is, DB, CMDB, Composite and Fuel-rich and also on the ingredients in their formulations. For double-base propellants, cellulosic materials such a cellulose acetate, ethyl cellulose and different filled or unfilled flexible polyesters are used while fuel or binder material filled with inert substances such as asbestos, mica, silica, etc. in fine powder form is used for composite propellants. Since nitroglycerine is present in CMDB propellants also as in DB propellants, the materials used for DB propellants may also be used with minor modifications for the inhibition of CMDB propellants. 

The major ingredients of composite propellants are ammonium perchlorate (AP-68%), metal powder (-16%) and polymeric binders like CTPB, HTPB and Thiokol. As total solids loading in composite propellants is -85-88%, polyesters which are commonly used for inhibition of DB propellants, have a relatively weak bond with composite propellants. This problem is further aggravated because of higher shrinkage in the case of polyesters. 

The combustion temperatures are much higher in CMDB propellants than that of DB and composite propellants and therefore, inhibition systems for CMDB propellants should have better BS and ablative properties. As major ingredients of CMDB propellants are NC and NG similar to DB propellants, it is logical to use polyesters which have already proved their potential for DB propellants for inhibition of CMDB propellants. However, the unsaturated polyesters which function... 

The mechanical and physical properties of the inhibiting material must be fairly similar to those of the propellant in order to minimize the differential expansion. In this respect the use of several layers with possibly different compositions is favorable. With long burning timers (more than 20 sec) the development of a reliable inhibitor poses a fairly difficult problem, especially with end-burning grains. 

Similar to composite propellants, flexibilized epoxy or novolac epoxy resins reinforced with fillers or fibers are used for inhibition of fuel-rich propellants. 

Agrawal, J.P., Deo, S.S., Tapaswi, M.A., and Marathe, M.M. (1986) Process Schedule for Inhibition of HTPB Based Composite Propellants for Trishul. 

Two-way analysis of variance (and higher classifications) leads to the presence of interactions. If, for example, an additive A is added to a lube oil stock to improve its resistance to oxidation and another additive, B, is added to inhibit corrosion by the stock under load or stress, it is entirely possible that the performance of the lube oil in a standard ball-and-socket wear test will be different from that expected if only one additive has present. In other words, the presence of one additive may adversely or helpfully affect the action of the other additive in modifying the properties of the lube oil. The same phenomenon is clearly evident in a composite rocket propellant where the catalyst effect on burning rate of the propellant drastically depends on the influence of fine oxidizer particles. These are termed antagonistic and synergistic effects, respectively. It is important to consider the presence of such interactions in any treatment of multiply classified data. To do this, the two-way analysis of variance table is set up as shown in Table 1.24. 

The burning rate characteristics of AP composite propellants with and without LiF are shown in Fig. 7-13. Though the burning rate without LiF increases as pressure increases, with LiF the burning rate decreases as the concentration of LiF increases at constant pressure. Further increase of the LiF concentration at a given pressure eventually results in self-extinction of the propellant1151. Thus, one can state that LiF not only decreases the burning rate but also inhibits steady-state combustion at or below a certain pressure. 

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