EXPERIMENTAL STRAIN AND STRESS ANALYSIS OF SOLID PROPELLANT ROCKET MOTORS

Abstract A review is made of the methods used to strain-analyze solid propellant rocket motor shells and grains when subjected to different loading conditions. The review includes methods directed at the determination of strains in actual rockets and also at the determination of strains in rocket models. The surveyed methods include two- and three-dimensional photo-elasticity, brittle coatings, electrical strain gages, moire, grids, etc. Preface Few engineering problems have presented in shorter time a greater amount of interest and more difficulties in their solution than the strain and stress analysis of solid rocket propellants when subjected to the numerous loading conditions to which they may be subjected either in flight, in storage, or in transportation. The difficulties in the analysis are, to a large extent, a consequence of the fact that the motor is a composite structure, and that the materials used in this composite structure are relatively new, exhibiting properties appreciably different from the properties of the more commonly used engineering materials such as steel, aluminum, and concrete. Difficulties also arise because few structures are designed to work under a state of strain and stress so near to the one associated with failure. As difficulties have developed in the functioning of solid rocket propellants, great emphasis has been given to studies directed at improving the quality of the propellant material, and even greater emphasis has been placed on the so-called characterization of these materials, or, in other words, on the study of their viscoelastic properties. There has been a strong belief for many years that stress analysis methods were sufficiently advanced to be used in the solution of rocket problems, but that they could not actually be applied because of the lack of knowledge of the required physical constants of the materials. This belief was not universally shared, * but was predominant in several leading circles and was responsible for an extraordinary concentration of interest in mechanical properties of solid propellants and a correspondingly extraordinary development of viscoelasticity theory and testing. As a consequence of this concentration, studies in both theoretical and experimental stress analyses of grains and motors have been underemphasized for a long time. In recent years some important developments in theoretical stress analysis have taken place, † but the number of experimental stress analysis contributions is still extremely small. It is estimated that of the approximately 700 papers dealing with solid rocket structural integrity that have been abstracted and classified recently, no more than 2 per cent deal with experimental strain and stress analysis methods applied to rocket grains and cases. As an example of the emphasis on material testing, consider the large program of experimental stress analysis which was published under the title “Study of Mechanical Properties of Solid Rocket Propellants”. It seems that the underemphasis on strain analysis, in general, and in particular on the application of experimental strain analysis techniques to grain and case research and design should be carefully re-evaluated. This review should not be considered exhaustive, but rather informative. It covers a broad part of the field in a systematic manner and attempts to give the reader a critical picture of the state of the art. A large amount of the information related to the field, however, has been published in reports, many of which are not easily available. Some parts of the review have been deliberately emphasized, either because they have been considered more important or because they refer to new methods not yet adequately covered in the literature. The preparation of this review was sponsored by the Office of Naval Research. The author is particularly grateful to Dr. H. Liebowitz and to Mr. J. Crowley for their constant support and encouragement. Several of the photographs and drawings used have already been published in reports submitted to Aerojet-General Corporation, Hercules Powder Company, and Thiokol Chemical Corporation. It is a pleasure to acknowledge the opportunity given by these companies to develop the strain analysis methods in their application to solid propellant grain problems. The manuscript has been read by Mr. V. J. Parks and Dr. R. N. Vaishnav, who made many important contributions. The collaboration of Mrs. J. Maio in the reproduction of the manuscript is also gratefully acknowledged.