EXPERIMENTAL EVALUATION OF RESISTOJET THRUSTER

and operational cost. A 48 percent decrease in total propulsion system life cycle cost over a ten year period was predicted over a The exhaust of an engineering model resistojet has been stand-alone system. However, as with the high thrust propulsion investigated using rotary pitot probes and a rotary quartz crystal system and other sources of effluents, the exhaust flow field of microbalance. The resistojet operated on CO2 propellant at a mass the resistojet must be well defined prior to integration because of flow rate of 0.29 g/sec in both heated and unheated flows, the potential impacts that the plume may have on station science Measurements of local flow angles in the near field of a conical and technology activities. Impacts of primary concern include plume shield indicated that the shield was not wholly effective in induced environmental contamination during quiescent periods and confining the flow to the region upstream of its exit plane, mass deposition on sensitive surfaces. Thermal loading and However, the absolute levels of the measured mass flux into the disturbance torques caused by plume impingement may also backflow region were very low, on the order of 7 x 10-7 g/cm 2 become important, depending on the space station architecture. sec or less. The use of a circular disk at the exit plane of the existing conical shield showed some benefit in decreasing the A number of diagnostic techniques for the measurement of amount of backflow by a factor of 2. Lastly, a detached shield fundamental plume properties are well established. 2 . 3 placed upstream of the resistojet exit plane demonstrated a small Experimental studies of pure gas, conical nozzle expansions into degree of local shielding for the region directly behind it. hard vacuum have investigated the effects of nozzle area ratio and lip geometry, facility pressure, condensation, and inadequate LIST OF SYMBOLS cryopumping in the backflow region. Further, these experiments demonstrated that conventional numerical techniques alone are A Nozzle area, m insufficient to accurately predict plume flow field characteristics, D' Nozzle throat diameter, m particularly at the high angle regions of interest for contamination De Nozzle exit diameter, m studies. Preliminary investigations of the resistojet plume have Dp Pitot probe orifice diameter, m also been reported previously. Mass flux measurements in the Ds Conical shield exit diameter, m exhaust of a laboratory resistojet operating on CO 2 propellant at K Constant for Equation (2) 298 K indicated that the flow expanded beyond a calculated M Mach number limiting turning angle using continuum methods. The amount, M Mass flux, g/cm -sec however, represented only a very small fraction of the total m Mass flow rate, g/sec thruster throughput P Gas pressure, N/m2 P,' Measured pitot pressure, N/m2 The purpose of the experimental work described herein r Radial distance from thruster exit plane to center of was to gain more insight into the backflow issue of resistojet QCM crystal, m plumes. An experimental investigation was conducted at the R Radius from plume centerline, m Arnold Engineering Development Center (AEDC) to measure the T Gas temperature, K level of backflux from an engineering model resistojet developed U Gas velocity, m/sec for space station application and also to evaluate the effectiveness z Axial distance from conical shield exit plane, m of plume shields in preventing or minimizing the backflux. Three y Specific heat ratio specific shield types were evaluated: (1) a large, conical plume 0 Angle from plume centerline, deg shield integrated in the design of the engineering model thruster 6 ; A Viscosity, N-sec/m2 (2) circular disks positioned at the exit plane of the conical shield p Gas density, number/m3 with the intent of simulating an infinitely thick lip; and (3) a flat 0 Nozzle half angle, deg plate placed upstream of the thruster exit, well into the free Q Solid angle, steradians molecule flow regime of the resistojet plume. Subor Superscript Notation This paper presents an initial evaluation of the experimental data obtained for the resistojet plume. Details of the measurement e Denotes exit plane conditions techniques and experimental hardware are provided, along with a i Denotes inlet conditions brief description of the test facility. The near field plume character max Denotes maximum of the resistojet is described in terms of dynamic pressure profiles o Denotes stagnation conditions and local flow angle variation for the different shield * Denotes sonic conditions configurations. Other data to be presented include estimates of Denotes freestream static Mach number variation along plume centerline, radial dynamic 2 Denotes conditions downstream of normal shock pressure profiles for heated and unheated flow, and mass flux variation in the plume as a function of radial and axial distance. Absolute values of measured mass flux are also compared with experimental results of related investigations. INTRODUCTION APPARATUS The multipropellant resistojet thruster has been baselined for auxiliary propulsion applications onboard the Space Station Engineering Model Resistojet Freedom (Level B Space Station Control Board Directive). It The resistojet thruster used in this investigation was an offers low thrust propulsion for drag make-up and also the engineering model ( Figure 1 ), designed and fabricated by a capability for propulsive, or non-propulsive venting of waste Rocketdyne-Technion industrial team under contract to the NASA gases. A NASA-sponsored studyl has indicated that use of the Lewis Research Center. 6 In the space station application, the resistojet in conjunction with a hydrogen/oxygen primary resistojet propulsion system would utilize waste gases from the propulsion system is advantageous in terms of both development