Herbert T. Knight

Precision Measurement of Detonation and Strong Shock Velocity in Gases

A simple system is described for determining the velocity of detonation or strong shock waves, with temperatures above 3000°K, by using the conductivity behind the wave. Wave contact is made by two 36‐mil wires set 0.1 inch apart in a Teflon plug mounted in the experimental tube. When a wave passes, signals are produced across a 30‐K resistor in series with these wires and a 0.001 μf capacitor charged to 300 v. Any number of circuits may be paralleled across a single signal resistor if a diode is added to each circuit to prevent signal deterioration. The arrival time of a wave at a pin can be determined with an accuracy of almost 10−8 sec from an oscilloscope record of the signals. The principal advantages of this system are excellent space resolution and very simple basic circuitry. An amplifier is described which can be used with an individual pin circuit to fire a thyratron and extend the range of applicability of this system to waves with temperatures as low as 1000°K.

DISSOCIATION ENERGY OF CYANOGEN AND RELATED QUANTITIES BY X-RAY DENSITOMETRY OF SHOCK WAVES

Density ratios across shock waves in a 0.85 Kr+0.15 C2N2 mixture at an initial pressure of 50 mm Hg and room temperature, have been determined with an x‐ray densitometer as a function of shock velocity. The heat required to dissociate cyanogen into two CN radicals D(C2N2) has been determined to be 145±6 kcal/mole by comparing the experimental data with curves of density ratio vs shock velocity calculated as a function of D(C2N2). Dissociation energies of 174±3 kcal/mole for CN and 129±3 kcal/mole for HCN forming H and CN, and a heat of formation of 109±3 kcal/mole for CN, were obtained by the application of Hesss law to the appropriate chemical reactions using this value of D(C2N2) and the currently accepted values for the dissociation energy of nitrogen (225 kcal/mole) and the heat of sublimation of graphite (170 kcal/mole). The value of D(HCN) was confirmed by analogous density‐velocity measurements on shock waves in a 0.85 Kr+0.15 HCN mixture. A rate constant for the recombination of CN to form C2N2 a...

Apparatus for Precision Flash Radiography of Shock and Detonation Waves in Gases

An apparatus is described which is based on the technique introduced by Kistiakowsky utilizing the absorption of soft x‐rays to measure densities behind gaseous shock and detonation waves. Experimental conditions leading to the smallest absorption statistical uncertainty consistent with maximum sensitivity are defined. These may be approximated at reasonable pressures and tube diameters by adding a strongly absorbing rare gas diluent to the experimental gas mixture. Under such conditions measured densities are accurate to ±1%, as demonstrated by comparison of experimental and calculated densities for shock waves in krypton. At some sacrifice in accuracy, space resolution of 1 mm and time resolution of 1 μsec may be achieved.A continuously pumped, laboratory‐built, demountable, pulsed x‐ray tube with an L‐cathode and tungsten target is used at an accelerating voltage of 20–‐30 kv to obtain currents up to 0.2 amp for durations of the order of 1 msec. Copper and chromium targets were also investigated.For ca...

Precision Flash X‐Ray Determination of Density Ratio in Gaseous Detonations

An x‐ray densitometer has been used to measure the density ratio across a detonation wave in a number of different gaseous mixtures involving C2H2, C2N2, or H2 with O2 and Kr. In a 3‐in. i.d. tube at an initial pressure of 60 cm the density ratios observed were between 1.64 and 1.67. Investigations of the influence on density ratio of tube diameter at constant initial pressure and of initial pressure at constant tube diameter both indicate an extrapolated infinite diameter density ratio of 1.70 for a 0.3 C2H2 + 0.3 O2 + 0.4 Kr mixture. This result is significantly lower than the ratio 1.79 which corresponds to the tangent point on the equilibrium Hugoniot from the initial state. At the present time these observations cannot adequately be explained by detonation theory.