Thermocouple sensor for rapid temperature measurements during ignition and early phase combustion of packed propellant beds

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Abstract
Flame‐spreading processes during the ignition of ballistic events have long been studied by pressure measurements and visual records from high‐speed film. While optical temperature measurements have been utilized, they are not useful much below 1500 K. A fine‐wire platinum versus 10% Rh/platinum thermocouple probe was designed to measure gas temperature in a packed propellant bed over the range from the initial bed temperature (295 K) to more than 2000 K. Probe mounts were designed to provide sufficient physical support and protection for the thermocouples and yet minimally perturb the flow environment. Three diameters of fine‐wire thermocouples were tested for survivability and time response in the simulated pressure‐wave environment of ignition of a propellant bed. The largest diameter thermocouple (3 mil or 76 μm) exhibited a response time too great (3 ms). The smallest diameter (1 mil or 25 μm) exhibited the shortest response time (0.4 ms) and would be the thermocouple of choice for this experiment; however, it was not used in the current experiments due to difficulties in probe fabrication and consideration of wire strength. For this application, a 2 mil (51 μm) diameter thermocouple was chosen. The lag time for this thermocouple was estimated at approximately 1.5 ms. Corrections to the temperature for catalytic activity (<1 K) and radiative cooling (60 K in the plateau region) were estimated. Examples of temperature histories during ignition of M30A1 and M43 propellants are also included.