Vapor pressure of tektite glass and its bearing on tektite trajectories determined from aerodynamic analysis

Various experiments have been conducted to resolve a large discrepancy between two measurements that have been reported for the vapor pressure of tektite glass. This discrepancy significantly affects the trajectories and mode of tektite origin as determined from calculations of aerodynamic ablation. Measurements in a furnace of vaporization rate relative to that of four different standards (SiO2, TiO2, Au, Ag) yield mutually concordant results for tektite vapor pressure. Also, measurements in an arc jet of the stagnation temperature, the rate of surface recession, and the mass loss during aerodynamic ablation are in mutual agreement with the results from vaporization rate measurements. Further, observations of flash boiling occurring under certain experimental conditions, and not occurring under others, provide a bracketing of tektite vapor pressure that is also compatible with the other measurements. Over the temperature range of the present experiments, 1700 to 3000°K, a least-squares fit to the data is represented by Pν(atm) = exp[18.5 - 57,400/T(°K)]. These various experimental results demonstrate that previous measurements by other investigators of the pressure for incipient bubbling do not represent tektite vapor pressure but, instead, some bubble pressure that at certain temperatures is as much as a million times higher than the vapor pressure. Some recent aerodynamic calculations of very shallow, grazing tektite trajectories, which were based on bubble-pressure measurements, are therefore invalid; other calculations of relatively steep, direct-entry trajectories are not affected significantly by the new vapor-pressure data.