Calculation of Blowout Gun Nozzle Temperatures

: Small laboratory devices such as the 37mm blowout gun at the BRL have a long history in unraveling the factors influencing gun barrel wear. Such devices are particularly useful when testing scarce and expensive experimental propellants. An unresolved question is whether relative propellant erosivity measured in such laboratory devices correlates with large-caliber gun wear. To try to answer the question, Nordheim's scheme for computing heat transfer in guns and erosion vents was used to calculate bore surface temperatures in a 17.3 mm diameter nozzle for a series of five propellants for which the wear had already been measured. Empirical expressions for estimating wear in guns suggested the wear should increase exponentially with peak bore surface temperature if the nozzles mimicked wear in guns. A linear-least squares of natural log of wear vs. peak temperature showed the wear in nozzles could be fit to such an expression, specifically n(w)= - 9.7 + 0.0094 T, where T is the peak surface temperature in Kelvin and wear is expressed as micron/shot. To test how well the dependence of wear on bore surface temperature compares to large caliber guns, bore surface temperatures computed by Calspan from heat input measurements in the 155m M185 cannon were plotted against wear. The slope was 0. 0076/K suggesting the blowout gun wear may exaggerate relative wear expected in a large-caliber gun by ten percent.