Changes/variations during manufacturing and storage of free radical-polymerized hydroxyl-terminated polybutadiene resin/prepolymer (widely used binder for the composite solid propellants) are not reflected in terms of appreciable change in the hydroxyl value. As a result, cured properties of the propellant mixed using the given formulation finalized by keeping R ratio (NCO/OH ratio) within 0.7–0.9 did not yield predicted mechanical properties. Investigations carried out subsequently, by the authors, identified the root cause to be variations in molecular weight and its distribution, which were not indicated in terms of change in hydroxyl value. Authors confirmed their findings by 1H1NMR studies whereby the variation in molecular weight distribution could be explained in terms of variation in spin–spin relaxation time ( T 2) values.

Even though literature available on EPDM is abundant, the data on physical, mechanical, thermal, interface properties and ablative performance as specifically required for the design and acceptance of internal insulation of large Composite Rocket Motor Casing is scant in the plethora of literature available in the subject of EPDM insulation. This gave the impetus to take up this experimental work wherein most promising formulation of EPDM with Kevlar and silica as reinforcing fillers and with only silica as reinforcing filler were thoroughly characterized for the insulation requirements of large Composite Rocket Motor Casing and compared with the performance of proven Nitrile rubber based insulation. The significant findings which are unique, are discussed in this paper.

Natural rubber (NR)-based elastomer, by virtue of its amenability to yield shear modulus at 3.5 kg/cm2 shear stress as low as 1.6–2.0 kg/cm2, is the most preferred elastomer for the realization of flex seals that are used for the thrust vector control of large rocket motors. Despite the utmost care taken during compounding, there exists a high degree of uncertainty in achieving required shear modulus primarily because of complexity involved in compounding to obtain high degree of homogeneity of the NR compound. Assessing the quality of NR compound is a tedious and time-taking process since it involves the preparation and testing of standard quadruple lap shear (QLS) specimens. Any non-conformance in shear modulus calls for fine-tuning of NR formulation; remaking of QLS specimens to recheck shear modulus leads to huge time delay and wastage of materials. The authors have meticulously generated oscillatory disc rheometer (ODR) data in comparison with QLS test results for various NR compounds and introduced ODR as an effective quality assurance tool. This article demonstrates how, based on ODR data, shear modulus can be predicted, thereby corrective actions can be taken in NR compound quickly and the appropriate vulcanization scheme can be prescribed for the given NR compound, which will prevent reversion and ensure optimum vulcanization, thereby eliminate the causes for shear modulus increase during storage of flex seals.

Two potential formulations of sulfur cured ethylene propylene diene terpolymer (EPDM) insulation were characterized against the proven insulation based on nitrile rubber (NBR) in the earlier study. Unlike the proven insulation, the cure behavior exhibited by sulfur cured EPDM is quite different in that, the rheometer curves exhibit marching behavior even at elevated temperature for longer duration (160 C for 7 h; most severe condition). This warranted extensive cure study at different temperatures for varying durations in order to determine the optimum cure condition. The methodology adopted to find out the optimum cure condition and corroboration of the same in terms of instrumental techniques (differential scanning calorimetry, thermogravimetric analysis, and Fourier transform infrared), which are quite interesting, are discussed in this article.