Flame-spreading phenomena in a head-end fin-slot segment of a subscale motor simulating the space shuttle boosters

Open Access
Moore, Jeffrey David
Graduate Program:
Mechanical Engineering
Doctor of Philosophy
Document Type:
Date of Defense:
December 13, 2007
Committee Members:
  • Kenneth K Kuo, Committee Chair
  • Andre Louis Boehman, Committee Member
  • Fan Bill B Cheung, Committee Member
  • Gita Talmage, Committee Member
  • Richard A Yetter, Committee Member
  • flame-spreading
  • fin-slot motor
  • head-end segment
  • RSRM
Fin-slot solid propellant grains have been used in a variety of solid rocket propulsion systems, most notably in the Space Shuttle Reusable Solid Rocket Motor (RSRM) Boosters. Advantages that can arise from using fin-slot grains compared to conventional end-burning or center-perforated grains include a relatively constant burning surface area and thrust level, large free volume, and greater exposed surface area for reliable ignition. The influence of the igniter jet can have a profound effect on recirculating flow patterns, due to impingement angle, degree of under-expansion, and strength of the induced vortex. In order to accurately predict the overall RSRM ignition transient, it is necessary to have knowledge of the igniter induced flow, heat-transfer, and flame-spreading rates inside the fin region. This research was aimed at obtaining a better understanding of the effect of flame spreading in the fin-slot section of the rocket motor. Experimentally, a subscale (roughly 1:10) pie-shaped fin-slot motor was designed to simulate two fin-slot regions. The test rig consisted of a head-end igniter, a fin-slot propellant section, a downstream cylindrical propellant grain section, and an interchangeable external exit nozzle. Diagnostically, the motor was equipped with pressure transducers, a set of sacrificial and main viewing windows for real-time imaging, and a diagnostic holder capable of mounting an array of flush-mounted heat-flux gauges, near-IR fast-response photodetectors, acoustic emission sensors, or fine-wire thermocouples in a perpendicular orientation. The first part of this study dealt with the design and assembly of the rocket motor along with the testing and analysis of the igniter induced flow field and heat-transfer to the propellant surface. Experiments were conducted using heated air flow as well as the direct discharge of a live igniter onto an inert fin-slot propellant sample. Results were used to develop a heat-transfer correlation within the fin-slot region. Deduced heat-transfer rates from this correlation were in good agreement with the measured data within experimental error. The second part of this study was the observation and characterization of the flame-spreading phenomena across the fin-slot surface using a combination of a high-speed digital camera and non-intrusive photodetector optical methods. A dimensionless flame-spreading interval correlation was developed. The flame-spreading interval was found to be inversely proportional to the local fin-slot pressurization rate to the power of 0.62. Therefore, the greater the pressurization rate, the shorter flame-spreading interval.