Power Source for a 1-Kw Arc Engine Test Capsule

Abstract

The potential application of arc jet engines for propulsion in space is linked closely with the type and characteristics of the power supply available. Discussed herein is a power supply designed to power a 1-kw arc engine for use with the Scout Electric Rocket Test (SERT) program. The most significant features of the power supply are the excellent control, stability, increased engine life, and efficiency. Data obtained from actual arc engine tests are included. The conclusions drawn are based on preliminary analysis of prototype engine and power supply test runs. Although the feasibility of the system has been demonstrated, the development is not considered complete. IntroductionT HE original power supply proposed for the 1-kw arc jet engine was a 112-v battery, current-limited by a ballast resistor. Despite the simplicity of the system, several disadvantages were encountered. The power losses in the ballast resistor limited the efficiency to 30 to 50%; the resistor required a large heat sink; arc stability was relatively poor; and the voltage requirement was not compatible with the power system of the SERT capsule.Test data established a typical steady-state electrical characteristic for the prototype engines. 1 Figure 1 illustrates this characteristic as determined by varying the resistance in series with the engine and battery. Figure 1 also shows that engine characteristics can be described reasonably by the classical expression for an arc discharge which iswhere A is the electrode-potential drop in volts, B is a constant dependent upon the geometry of the engine and the arc length, 7 is the arc current in amperes, and n is a constant dependent on the type and condition of the gas. The values of the constants for the theoretical curve were A = 30 v, B = 222, and n = 0.5.It is recognized that such a characteristic curve represents a specific set of engine parameters that are subject to such disturbances as variations in gas flow, electrode erosion, and temperature. It also is recognized that the dynamic electrical characteristics essential for design of a power supply