6 А. Л. Пумпянский 81

to take place by a single shock along the Hugoniot, by a series of compressions along a curve approximating anadiabat, or even by means of a clearcut shock reflection. Regardless of these details, the temperature can be altered at least 500° С by choosing gases with widely different hydrodynamic and thermodynamic properties. In order to achieve extremes in this sort of behavior, we have evacuated wedges and replaced the air either with argon, to produce high gas temperatures, or with methane, to produce low gas temperatures. The results of these tests are shown in Fig. 6; the fact that these depths of initiation agree withinexperimental error with the values for air is taken as strong indication that the temperature of the inter-stital gas has nothing to do with the mechanism of initiation. Temperature differences of several hundred degrees would have a profound effect on the rate of grain burning or on the rate of chemical reaction if such temperature changes could be brought to bear on these processes. It is possible that a change in efficiency of //zeheat transfer process compensates for the change in temperature when different gases are used.

An experiment was also performed in which the pressure of the interstitial air was reduced to the range between 50—100. The temperature of the compressed residual air in this case was not much different from that achieved at normal density, but the total amount of energy available for transfer from gas to PETN was lowered by a factor of about 10*. This low-pressure shot is also shown in Fig. 6, and again demonstrates that the interstitial gases did not affect the initiation process ^г.

The Elastomer Section

For this discussion, static compression stress or static shear stress can be considered the stress which is exerted on the elastomer by normal rated torque delivery. Dynamic stress is that which is produced by cyclical forces or excitations originating from the power source or driven unit, or by excursions caused when misalignment is accommodated by the coupling.

Static stress, if held to practical limits, is not the major criterion for determining service life of the elastomer. Fatigue life of elastomers is a function of the severity and frequency of dynamic strain. Important, too, in obtaining maximum service life, is prevention of a return to zero strain once the initial strain due to normal operation is applied. When possible, the coupling design should permit static strain large enough to avoid a return to zero strain during deflection in the elastomer due to dynamic operation.

Selection of a specific elastomer to perform the specific duties must include consideration of many factors. There is no one material which will satisfy all requirements in all applications. Good fatigue

¹ Journal of Applied Physics, June 1961, p. 1097. 82

life, extreme-temperature resistance, and inherent damping are properties which can be «built-in» to an elastomer when required !.

Determination of Real-Gas Stagnation Temperature Based on Mass-Flow Consideration

One of the major problems encountered in the operation of high-enthalpy test facilities is the determination of high stagnation temperatures (T = 4,000° R). Since these temperatures normally exceed the capabilities of available thermocouples, it is often necessary to resort to one of the various optical methods — such as pyromeiry or the sodium-line reversal technique — for the determination of these temperatures. A method has been presented whereby real-gas stagnation properties in a high-enthalpy system can be determined if the mass-flow rate through the system and the stagnation pressure are known. In the present analysis, this method was applied to determine stagnation temperature for stagnation pressures ranging from 100 to 1,000 atm. Nitrogen is considered to be the flow medium and the expansion of the gas from the stagnation chamber is assumed to be an isentropic and steady-state process. The maximum temperature dealt with (5,000° R) was low enough so that the gas remained in an imdissociated state; however, the method may be extended to other gases which may or may not be dissociated, provided the thormodynarnic properties of the gas in the desired pressure and temperature range are known.

A one-dimensional analysis of the flow is used to evaluate the mass flow per unit area at the throat of a nozzle for various stagnation pressures and temperatures. The evaluation of mass flow per unit area at the throat of a nozzle is a trial-and-error procedure and was performed in the following manner. Starting with an assumed stagnation pressure and temperature, an initial value of stagnation entropy was obtained from published data. The flow in the system was expanded by reducing the stagnation temperature at a constant value of stagnation entropy. At each assumed temperature the values of pressure, density, and enthalpy were obtained from Ref. 2 or 3; the velocity at each point was calculated from the energy equation.

The result of the analysis is presented in Fig. 1, wherein mass flow per unit throat area is plotted against stagnation temperature. The dashed lines represent ideal-gas calculations. The spread between the ideal-gas and real-gas curves becomes larger as the stagnation pressure is increased. It may be seen from Fig. 1 that a significant error in stagnation temperature could result by using the ideal-gas relationship ².

¹ Design, September 1961, p. 155.

² Journal of Aerospace Science, 1961, p. 742.

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