Design and fabrication of a thrust balance to measure a pulsed plasma thruster thrust force. This permits to verify the operation of the developed PPT thruster and identify potential improvements.

Author: Andrés Cabello López
University Carlos III Madrid (Spain)

A design of a torsional thrust stand has been developed to test a µPPT at Carlos III University facilities. The proposed design is able to measure single impulses in the range of 5 − 100 µNs and thrust forces in repetitive shot operation of 1 − 100 µN with a sensitivity of 0.776 µm/s/µNs and 6.365 µm/µN respectively. The resolution is estimated in 400 nNs and 100 nN. Two approaches have been faced to model the response of the balance: a simple pendulum analysis, and a flexible beam study. The contribution of both models allows covering the effects induced on the deflection by damping, the different punctual masses and the harmonic normal modes that arise due to the excitation of the thruster.

After an analysis of the requirements imposed and considering the different alternatives for all the subsystems and sensors needed, a complete design has been provided, searching for a convenient, simple, accurate and economical solution. The asymmetrical swing-gate configuration proposed employs a pair of flexural pivots and fits in the prescribed vacuum chamber. The linear displacement response is measured with a confocal chromatic optical pen and the damping is introduced with an eddy current system that can be manually regulated. The calibration method proposed is performed with a parallel plate capacitor to generate electrostatic forces and impulses in the range of the ones exerted by the PPT. A levelling system and alternatives as coaxial liquid connectors or batteries to
decouple the power supply lines from the exterior are also described to improve the performance of the stand and reduce the overall error. Finally, a complete CAD model has been developed.

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