Improvement and Testing of a Micronewton Thrust Balance for an Autonomous Arcjet Development Test Bench

The arcjet development at the Institute of Space Systems (IRS) at Stuttgart is intended to deorbit satellites at appreciably lower power levels. To characterize the thruster, the IRS is developing a thrust stand with an expected thrust resolution lower than 0.1 mN with the maximum measurable thrust levels of up to 0.5 N. Taking the advantage of the high sensitivity as well as the compatibility with plasma diagnostics, an inverted pendulum thrust stand shall be a part of a fully autonomous test bench that allows the evaluation of thruster performance, plasma diagnostics, and thermal analysis, simultaneously. The novelty of the thrust stand lies in its adaptable pendulum arm design as its height can be adjusted within its allowable range, and thus, the sensitivity of the stand is modelled to be flexible. The balance utilizes pivot bearings between the joints of the plates and the pendulum arms providing a frictionless one-dimensional motion of the top plate. The thrust measurements are performed in two methods: 1) displacement mode and 2) zero-displacement mode. During the displacement mode, the change in position of the top plate due to thrust will be sensed by a displacement sensor. During the zero-displacement mode, a known mechanical force is applied by a controlled actuator against the thrust to neutralize the displacement of the top plate, and a load cell attached to this mechanism senses and determines the applied force. The stand is levelled on three fine-adjustment screws that are adjusted by individual stepper motors based on the inclination data obtained from a biaxial inclinometer. Before and after every test, the stand is calibrated by three known weights that imitate dummy thrust forces. The propellant and cooling water are supplied by two individual pipes (Stainless Steel 316 for propellant and Polyurethane for water). Thermocouples will be attached to the thermally sensitive areas nearby the sensors and pivot bearings to actively monitor their conditions. Finally, the designed thrust stand can support experiments with a variety of thrusters with different thrust levels within the specified range in the work