The Importance of Structural Integrity in Grain Design

Abstract

During the past several years solid propellant power units have changed in many ways. Notwithstanding the increase in specific impulse, probably the most striking of all is the remarkable growth in its physical size. This past and probably future growth has been accompanied by a proportionate rise in the cost of propellant units. The point of reviewing these facts is to stress an obvious but important fact. Grains are not cheap. The corollary is that it behooves the rocket engineer to bend every effort to design a grain which will not abort. He should carefully consider the use of scale models, laboratory testing, and analytical procedures. Turning specifically to the problem of structural design, we find that there is a fundamental yet simple distinction between the liquid and solid propellant fuel. the liquid fuel supports only hydrostatic compression; the solid fuel, on the other hand, not only withstands the same loading, but also varying amounts of tension and shear stress. From the mechanical standpoint this is a mixed blessing. If the grain material will take load, it is inefficient not to use this capability. But the material will also tend to absorb load, irrespective of the designers wishes. As a minimum requirement, therefore, a grain must be designed such that its proclivity to absorb load does not contribute to a rocket failure. Then, if possible, design it in such a way that its ability to absorb load contributes to a higher loading fraction, lower system weight, or improved overall performance. To reiterate, the structural integrity of a solid grain has a much stronger interaction with the overall system design than the liquid fuel, because a structural failure in the solid grain can contribute to off-design burning or even catastrophic ignition. With these introductory remarks, it is desired to emphasize the economic and technical feasibility of conducting more sophisticated structural analyses of the grain than has heretofore been customary.