Propellants, Composite

Many compounds explode when triggered by a suitable stimulus however, most are either too sensitive or fail to meet cost and production-scale standards, requirements for safety in transportation, and storage stability. Propellants and explosives in large-scale use are based mosdy on a relatively small number of well-proven iagredients. Propellants and explosives for military systems are manufactured ia the United States primarily ia government owned plants where they are also loaded iato munitions. Composite propellants for large rockets are produced mainly by private iadustry, as are small arms propellants for sporting weapons. 

Polymer-based rocket propellants are generally referred to as composite propellants, and often identified by the elastomer used, eg, urethane propellants or carboxy- (CTPB) or hydroxy- (HTPB) terrninated polybutadiene propellants. The cross-linked polymers act as a viscoelastic matrix to provide mechanical strength, and as a fuel to react with the oxidizers present. Ammonium perchlorate and ammonium nitrate are the most common oxidizers used nitramines such as HMX or RDX may be added to react with the fuels and increase the impulse produced. Many other substances may be added including metallic fuels, plasticizers, stabilizers, catalysts, ballistic modifiers, and bonding agents. Typical components are Hsted in Table 1. 

Fig. 1. The postulated flame stmcture for an AP composite propellant, showing A, the primary flame, where gases are from AP decomposition and fuel pyrolysis, the temperature is presumably the propellant flame temperature, and heat transfer is three-dimensional followed by B, the final diffusion flame, where gases are O2 from the AP flame reacting with products from fuel pyrolysis, the temperature is the propellant flame temperature, and heat transfer is three-dimensional and C, the AP monopropellant flame where gases are products from the AP surface decomposition, the temperature is the adiabatic flame temperature for pure AP, and heat transfer is approximately one-dimensional. AP = ammonium perchlorate.

Oxidizers. The characteristics of the oxidizer affect the baUistic and mechanical properties of a composite propellant as well as the processibihty. Oxidizers are selected to provide the best combination of available oxygen, high density, low heat of formation, and maximum gas volume in reaction with binders. Increases in oxidizer content increase the density, the adiabatic flame temperature, and the specific impulse of a propellant up to a maximum. The most commonly used inorganic oxidizer in both composite and nitroceUulose-based rocket propellant is ammonium perchlorate. The primary combustion products of an ammonium perchlorate propellant and a polymeric binder containing C, H, and O are CO2, H2, O2, and HCl. Ammonium nitrate has been used in slow burning propellants, and where a smokeless exhaust is requited. Nitramines such as RDX and HMX have also been used where maximum energy is essential. 

Table 11. Composition and Properties of Polymer-Based Cast Composite Propellants...

Pollution Prevention. Procedures haven been developed for recovery of composite ammonium perchlorate propellant from rocket motors, and the treatment of scrap and recovered propellant to reclaim ingredients. These include the use of high pressure water jets or compounds such as ammonia, which form fluids under pressure at elevated temperature, to remove the propellant from the motor, extraction of the ammonium perchlorate with solvents such as water or ammonia as a critical fluid, recrystalli2ation of the perchlorate and reuse in composite propellant or in slurry explosives or conversion to perchloric acid (166,167). 

J. A. Stein, P. L. Stang, and M. Summerfield, The Burning Mechanism ofMmmonium Perchlorate-Based Composite Propellants, Aerospace and Mechanical Sciences Report 830, Princeton University, N.J., 1969. 

J. E. Tormey, "Processing and Manufactuie of Composite Propellants," Proceedings of the AGARD Colloquim, Advances in Tactical Rocket Propulsion, CIRA Pub., Pelham, New York, 1968. 

J. T. Gxsxe glmproved Specifications for Composite Propellant Binders for Army Weapon Systems,TR-T-19-16, A-aayMssAe Command, Huntsville, Ala.,July 1979. 

D.E. Coats et al, Analysis of a Solid Propellant Gas Generator Using an Ammonium Perchlorate-Butadiene Composite Propellant , Report No AFRPL TR-72-58, Contract F04611-71-C-0058, Ultrasystems Inc, Irvine (1972)... 

M.B. Frankel et al, Smokeless Composite Propellants Containing Carboxy- or Hydroxy-Terminated Polymers and a Nitroorganic Oxidizer , USP 3853646 (1974) CA 83, 12972 (1975) 37) A. Reichel et al, High Explo-... 

PMMA finds ordn usage in several areas in ballistic or impact shields for missiles or airplanes also as windows, windshields or canopies in aircraft (Refs 7 and 22) as a Laser Q switch host using an organic Ni complex dye (Ref 22) and in proplnts as fuel (with A1 and NG as cofuels — Ref 20) and Amm perchlorate or K perchlorate as oxidizers (Refs 2, 4, 8—11, 13,14 16—20). Also see under Aeroplex Propellants in Vol 1, A108-R and under Composite Propellants in Vol 3, C464-L to C474-L Refs 1) Beil 2, [398] and (1279 1283 ... 

Polystyrene. A thermoplastic used as a binder and fuel in expls and rocket propints. See Plastic fuels (Vol 3, C465-L) under Composite Propellants also Dinitropolystyrene in this Vol, N143-L to N144-R under Nitro Polymers and Propellants, Solid , also in this Vol... 

Under increasing strain the propint volume increases from the voids created around the unbonded solid particles. Nonlinearities in Young s modulus and Poisson s ratio then occur. Francis (Ref 50) shows this effect for a carboxy-terminated polybutadiene composite propellant with 14% binder as in Figure 12. He concludes that nonlinearities in low-temperature properties reduce the predicted stress and strain values upon cooling a solid motor, and therefore a structural analysis that neglects these effects will be conservative. However, when the predictions are extended to a pressurized fiberglas motor case, the nonlinearities in properties produce greater strains than those predicted with linear analysis... 

Madison, Application of Large Vertical Helical Mixer for Mixing Composite Propellants , Paper AlChE 60th Meeting, Atlantic City, NJ... 

Although the thermal-ignition theory was developed for double-base propellants, several investigators have attempted to correlate the ignition characteristics of composite propellants using this approach. Baer and Ryan (Bl) have correlated ignition data for a polysulfide-ammonium perchlorate... 

Inspection of the numerical solutions of the equations shows that, with the exception of Es= 0 kcal/mole, the rate of surface temperature increase with time is very large once the surface temperature reaches approximately 420°K—on the order of 108°K/sec. Because typical autoignition temperatures are of the order of 625°K for composite propellants, the particular value of the ignition temperature does not affect the computed numerical value of the ignition-delay time. 

Fig. 7. Best-fit agreement between experimental and calculated ignition data for PBAA/AN composite propellant (P8). Key O, pure-oxidizer environment A, oxidizer-nitrogen environment at 55 psia total pressure.

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