A study of the ignition transient in large aspect ratio solid rocket motors

A computer model was developed to predict the ignition transient for solid rocket motors with high length-to-diameter (L/D) ratios and head-end finned sections like the Space Shuttle's Redesigned Solid Rocket Motor (RSRM), the Titan 34D, the Titan IV, and the Titan Solid Rocket Motor Upgrade (SRMU). In keeping with the goal of developing an engineering design tool, the ignition transient code utilizes the wealth of empirical data currently available to reduce the numerical complexity of the model and, therefore, the required run times to less than a few hours of CPU on a PC. This was achieved by building a time-dependent, 1-D, multi-species, finite-volume flow solver to represent the bulk fluid motion in the motor and coupling it with a unique, 2-D heat transfer and flame spreading model to predict the ignition and subsequent flame spread rate into the deep fin-slots in the head-end of the motor.; After successfully validating the new ignition transient code via the usual idealized 1-D theoretical problems which possess analytical solutions for comparisons to the numerical solutions, the code was used to model the Space Shuttle's RSRM and the Titan SRMU to verify the code's ability to predict the very different characteristics of the respective start transients of these two motors. The new numerical model successfully reproduced the severe start transient of the RSRM, which produces head-end pressure rise rates twice as high as predicted by the classical igniter design theory and modeling methods, and also accurately reproduced the more benign start transient of the Titan SRMU. This was followed by a parametric study conducted for both the RSRM and the SRMU which revealed that the cause of the high pressure rise rate in the head-end of the RSRM was a direct consequence of the propellant grain configuration in the head-end of this motor. This conclusion was arrived at after the original three theories postulated to explain the RSRM behavior: (1) igniter over-driving the system, (2) in-depth heating, and (3) complex pressure wave interactions, were shown to not predict the major physical effect.