A novel radiative heat flux measurement technique for combustion products flowing in solid rocket motors

In order to understand the behavior of internal insulators in solid rocket motors (SRMs), it is necessary to observe them under a well-characterized thermal environment. While it is most important to know the total heat flux to the insulator, it is also valuable to know what fraction of that flux is due to its separate radiative and convective components, as each mode is governed by fundamentally different physics and, thus, affects the insulator differently. Therefore, a radiometer has been developed to measure the purely radiative component of heat flux in an SRM. In this design, a Schmidt-Boeltertype heat flux sensor is isolated from the convective flow of SRM combustion products by a sapphire window, which is mounted in a recess in the wall of the SRM. Condensed-phase particles generated by the combustion of metalized solid propellant are prevented from striking and adhering to the window by two jets of cold inert gas: one injected at an oblique angle to the combustion product flow and another injected normal to the window. The radiometer was installed in a laboratory-scale SRM burning AP/Al/PBAN composite propellant for a series of test firings, the results of which demonstrated that the inert gas jets are capable of maintaining a clean window throughout a 20 s firing duration. The radiation measurements obtained from these firings exhibit good repeatability and physically reasonable temporal behavior. Therefore, it is concluded that this radiometer design is robust and capable of providing accurate measurements of thermal radiation fluxes within an SRM.