PERFORMANCE CHARACTERIZATION OF THE VAPORIZING LIQUID MICRO-THRUSTER (VLM)

A Vaporizing Liquid Micro-Thruster (MLM) microfabricated thruster was tested on water propellant on a thrust stand and performance data obtained. Thrust values between 50- 280 pN, thrust-to-power ratios of up to 200 pN/W at power levels of 1- 15 W were measured. Mass flow rates ranges between 100 to just under 300 pgls. Dividing measured thrust values by measured mass flow rates yielded specific impulse values around 100 sec. SpeciTc impulse values have to be viewed with caution, however, as they are based on very difficult low mass flow measurements at low feed pressures. Different thruster operating modes were observed, based on whether the VLM chip ingested vapor, liquid water, or two-phase flow. Two-phase flow ingestion led to pulsing of the thruster, while all vapor or liquid ingestion led to a steady, quiescent thrusting mode. The liquid ingestion mode led to higher thruster performances due to enhanced heat transfer from the chip into the liquid. Even the quiescent thruster operating mode for liquid propellant ingestion was diflicult to attain and maintain, however, and future VLM and feed system modifications will be required. Several design changes were easily identifiable. I