Experimental Assessment of the Pulse Mode Propulsive Performance of Small Chemical Thrusters

This paper addresses the experimental assessment of the propulsive performance, with special emphasis on the specific impulse, of micro thrusters in pulse mode operation designed for CubeSat applications. In particular, it investigates the effectiveness of a thrust balance capable of measuring thrust in the order of hundreds of millinewtons internally designed by the University of Pisa in the frame of the CHIPS project. The developed thrust balance can integrate an entire 3U inside its cuddle in measuring the performance given by small chemical propulsion systems with the required accuracy and frequency. The main design-driving factors in the realization of the test bench were the possibility to insert the overall propulsion system directly inside the thrust balance, the capability to detect low thrust levels, and the high-frequency response to record the thrust generated during the pulsed firings. In particular, the need to measure the pulsed thrust has led to the design of the thrust balance capable of ensuring a high resonant mode that does not interfere with the operating frequency range of the thruster, thus allowing the proper assessment of the thrust during pulse mode. The thrust balance was also required to be able to host various sensors to fully characterize the propulsion system performance. In particular, the fundamental ones for the experimental characterization of the specific impulse and thrust were the load cell and the mass flowmeter, both successfully integrated into the thrust balance. The force signal obtained by the load cell during the experimental campaign showed the effectiveness of the developed thrust balance in following high-frequency impulses. At the same time, a combustion instability that arose during the continuous firing was successfully recorded by the load cell and the combustion chamber pressure transducer highlighting the thrust balance's capability of following the oscillations generated by this propulsive instability and providing a consistent output signal. This behavior of the balance was fundamental to achieving all the objectives of the experimental campaign. In particular, it managed to precisely record the thrust during pulsed firings, which varied from an opening time of 900 ms to 50 ms. Finally, an attempt was made to characterize the dynamic response of the flowmeter and thrust balance to make the test bench even more performing in characterizing the pulse mode operations of chemical propulsion systems.