Solid propellant mechanical properties

Solid Propellant Mechanical Properties Testing, Failure Criteria, and Aging... 

The urethane reaction is particularly useful for solid propellant applications because of its quantitative nature, convenient rate which can be adjusted by proper choice of catalysts, and the availability of many suitable hydroxyl compounds which permit the tailoring of propellant mechanical properties. Despite the quantitative nature and apparent simplicity of the urethane reaction R NCO + ROH - R NHCOOR, its exact course has not been fully explored yet. It does not follow simple second-order kinetics as the above formula would suggest since its second-order rate constant depends on many factors, such as concentration of reactants and the nature of the solvent. Baker and co-workers (2) proposed that the reaction is initiated through the attack of an alkoxide ion on the carbon atom of the isocyanate group... 

H. J. Hoffman, Moisture Effects on Mechanical Properties of Solid Propellants, CPTR 84-29, CPIA Pubhcations, Johns Hopkins University, Laurel, Md., Feb. 1984. 

Fig 9 Effect of Aging (70°) and Average Humidity During Processing/Testing Upon thp Mechanical Properties of PBAA (83% Solids) Propellant... 

Hercopel a unique all-epoxide cure composite solid propellant with excellent mechanical and ballistic properties. Its outstanding performance in extended environments makes it well suited for tactical missiles Double-Base Solid Propellants a wide variety of physical and ballistic properties which can be tailored to meet specific performance requirements. Their high specific impulse and excellent reproducibility are two of the many reasons Hercules double-base propellants are found in many of our rocket motors and gas generators used for both military and space applications... 

The key to the successful application of high performance, pourable nitrocellulose plastisols lies in a reasonably priced, high quality source of fine-particle, at least partially colloided, spheroidal nitrocellulose. Here we are speaking of particles much finer than the well-known ball powder, produced by the Olin Mathieson Chemical Co. for small arms for over 30 years (7). Actually, particles on the order of 5-50/x diameter appear to be required to assure a reasonable continuum of uniformly plasticized nitrocellulose binder in a propellant containing 45% or more of combined crystalline oxidizer and powdered metal fuel. Such a continuum of binder is necessary to assure acceptable mechanical properties and reproducible burning characteristics of the finished propellant. Preincorporation of a certain content of the water-insoluble solids within the nitrocellulose microspheres is an effective means of helping to assure this continuum of binder and alleviates the requirements for extremely small ball size. The use of a total of 45% or more of crystalline oxidizer and (generally) metal fuel is essential if the propellant is to be competitive with other modern propellants now in service. 

With only small differences in (Is)max the choice of the binder system is influenced by processability, physical properties, and propellant density. Thus, with the polyether binder an Is of 247 is reached with about 14% binder, but with the polyester the same Is is obtained with 11.5% binder, which is a definite disadvantage in terms of processability and mechanical properties. The higher Is with the polybutadiene binder is realized only at high solids loadings, but owing to its lower density, processability is still satisfactory. 

Effects of Curing Agent Type. Epoxide-Cured Propellant. Carboxyl-terminated polybutadiene is a linear, difunctional molecule that requires the use of a polyfunctional crosslinker to achieve a gel. The crosslinkers used in most epoxide-cured propellants are summarized in Table IV and consist of Epon X-801, ERLA-0510, or Epotuf. DER-332, a high-purity diepoxide that exhibits a minimum of side reactions in the presence of the ammonium perchlorate oxidizer, can be used to provide chain extension for further modification of the mechanical properties. A typical study to adjust and optimize the crosslinker level and compensate for side reactions and achieve the best balance of uniaxial tensile properties for a CTPB propellant is shown in Table V. These results are characteristic of epoxide-cured propellants at this solids level and show the effects of curing agent type and plasticizer level on the mechanical properties of propellants. 

Prepolymer and Propellant Quality Control. Prepolymer Control. The characterization of the functionally terminated prepolymers was an important factor in the control of these materials required to assure satisfactory reproducibility of solid propellant properties. To characterize these materials adequately, it was necessary to develop new analytical methods or to improve existing methods to provide the desired level of control. It was ultimately demonstrated that (1) propellant mechanical... 

The several solid propellant manufacturing systems have been reviewed along with two systems that have not reached routine production. A comparative system of establishing the processability of new propellants has been suggested along with the instrumental analysis plan. The requirements of the product systems from a mechanical properties standpoint are mentioned in terms of the environment, and special steps to insure minimum variation between batches are described to illustrate the stringent tolerances that are met. 

Although the uniaxial test has traditionally received the most attention, such tests alone may be insufficient to characterize adequately the mechanical capability of solid propellants. This is especially true for ultimate property determinations where a change in load application from one axis to several at once may strongly affect the relative ranking of propellants according to their breaking strains. Since the conditions usually encountered in solid rocket motors lead to the development of multiaxial stress fields, tests which attempt to simulate these stress fields may be expected to represent more closely the true capability of the material. 

While mechanical testing of all types provides a general description of the bulk properties of solid propellants, it is difficult to make generalizations or even extrapolations which may be used in a predictive fashion. When the content or type of solid filler is changed, or curative ratios are altered, there is no simple corresponding material property change which can be defined based on mechanical testing experience. If filler content... 

Different classes of solid propellants DB, CMDB, and fuel rich (FR) have been developed in order to meet the requirements of various missions in terms of specific impulse (Lsp) and wide range of burn rates with low pressure index (n). High density, low temperature sensitivity and good mechanical properties constitute other essential requirements of these propellants. The salient features of such performance parameters are ... 

BAMO is perhaps the most prominent among the azido oxetanes class in terms of the number of polymers and copolymers reported so far. Due to its symmetrical azido groups, it assumes special significance as a hard block repeating unit in a thermoplastic elastomer. However, the homopolymer is solid and cannot be used directly for binder applications because of its crystal-tine nature. Also, poly(BAMO) shows relatively poor mechanical properties as a binder for solid rocket propellants [153]. Many copolymers of BAMO with non-energetic co-monomers tike tetrahydrofuran (THF) have been reported. The BAMO-THF copolymer is an excellent candidate for binder applications with its energetic BAMO content coupled with the THF block which affords... 

Solid composite propellants are usually rich in combustible ingredients and the amount of oxidant is usually limited by the mechanical properties of the mixtures. Careful c hoice of components is needed to obtain high loadings without jeopardizing fluidity in mixing and creating discontinuities in the binder. A common procedure consists of using oxidants in two or more sizes. 

T.D. Phillips, Effect of Gamma Radiation from Cobalt 60 on the Mechanical Properties of AHH and ARP Double-Base Solid Propellants , NPP TMR-159, Naval Powder Factory, Indian Head... 

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