Anthony G. Cofer

Improved Design and Characterization of MicroNewton Torsional Balance Thrust Stand

Torsional balance thrust stands are in common use for performance measurement of small thrusters in the microNewton to milliNewton thrust ranges. Due to their mechanical nature these stands must be calibrated regularly to minimize drift caused by external factors such as ambient temperature change and internal variations such as inertial differences in test articles. Common calibration methods include electrostatics, linear induction motors, and piezoelectric impulse comparison, all of which in turn must be initially calibrated for force. The thrust stand in the High Vacuum Lab at Purdue University utilizes an electrostatic fin assembly originally calibrated with repeatability errors less than 3% at forces over 50 μN. The need for more accurate measurements, in particular, for a MEMS microthruster array characterization and sensing of Knudsen thermal force at low pressures, prompted a campaign of recalibration and reconfiguration to achieve better performance to values less than 10 μN. The main goals of this paper are to present the improved design of the microNewton thrust stand and characterize its performance for measurement of Knudsen thermal force and thrust performance of a microfabricated thruster array.

Film-evaporation microthruster for cubesats

This paper describes a novel MEMS thermal valving system which exploits surface tension as a control mechanism to produce thrust in the sub-millinewton range at less than 1 Watt power at 2 to 5 Volts and using pure liquid water as a green propellant. Over 120 functional field-evaporation devices of different nozzle throat geometries were microfabricated and tested. The throat width was around 8 microns to initiate evaporation at about 50°C with length varying from about 15 to 60 microns to realize different capillary aspect ratios. The thermal and thrust measurements revealed two distinct performance modes for short and long capillaries. The short capillary with a throat aspect ratio up to 2 results in high rates of bulk water cooling beneficial for thermal control. For aspect ratios > 4, the film-evaporation device gives a stable and lower mass flow rate with higher propulsion performance. The measured specific impulse (Isp) exceeds most cold gas micropropulsion systems, due to low atomic mass, and requires no high pressure propellant containment nor massive and power exhaustive conventional valves. Total dry system mass including propellant tank can be as low as 1 ½ grams to hold 1 gram of propellant and occupy less than 2 cm3 volume.

Film-Evaporation MEMS Tunable Array for Low-Mass SmallSat Propulsion: Design Improvements and Thrust Characterization

The Film-Evaporation MEMS Tunable Array (FEMTA) concept for propulsion and thermal control of picosats exploits microscale surface tension effect in conjunction with temperature dependent vapor pressure to realize a thermal valving system. The local vapor pressure is increased by resistive film heating until it exceeds meniscus strength in the nozzle inducing vacuum boiling which provides a stagnation pressure close to vapor pressure at that point which is used for propulsion. The heat of vaporization is drawn from the bulk fluid and is replaced by either an integrated heater or waste heat from the vehicle. The paper reports on the design and characterization of second and third generation FEMTA devices and initial thrust stand testing.

Film-Evaporation MEMS Tunable Array: Theory of Operation and Proof of Concept

Chemical micropropulsion options for small satellite systems (i.e. cube-sats, nano-sats, pico-sats) are currently limited by feed system complexity and viscous effects which dominate low Reynolds number flows, inhibiting efficient operation at low thrust levels. Electric propulsion offers high Isp but with high power/thrust demands and require power supplies which are bulky, complex, and expensive. The proposed Film-Evaporation MEMS Tunable Array (FEMTA), exploits the small scale surface tension effect in conjunction with temperature dependent vapor pressure to realize a thermal valving system for effective propulsion in the sub-milliNewton range with a thermal management option. The local vapor pressure is increased by resistive film heating until it exceeds meniscus surface tension strength in the nozzle inducing vacuum boiling which provides a stagnation pressure equal to vapor pressure at that point which is used for propulsion. The heat of vaporization is drawn from the bulk fluid and is replaced by either an integrated heater or waste heat from the vehicle providing a thermal control capability.

Amplification and reversal of Knudsen force by thermoelectric heating

We show that the Knudsen thermal force generated by a thermally-induced flow over a heated beam near a colder wall could be amplified significantly by thermoelectric heating. Bidirectional actuation is achieved by switching the polarity of the thermoelectric device bias voltage. The measurements of the resulting thermal forces at different rarefaction regimes, realized by changing geometry and gas pressure, are done using torsional microbalance. The repulsive or attractive forces between a thermoelectrically heated or cooled plate and a substrate are shown to be up to an order of magnitude larger than for previously studied configurations and heating methods due to favorable coupling of two thermal gradients. The amplification and reversal of the Knudsen force is confirmed by numerical solution of the Boltzmann-ESBGK kinetic model equation. Because of the favorable scaling with decreasing system size, the Knudsen force with thermoelectric heating offers a novel actuation and sensing mechanism for nano/microsystems.

Microspike Based Chemical/Electric Thruster Concept for Versatile Nanosat Propulsion

††, * Small spacecraft that can be categorized as microsats, nanosats, and picosats has a strong potential for wide applications in communication, scientific experiments, and space exploration. In order to combine the advantages of both electric and chemical propulsion thrusters, a dual-mode microspike based thruster concept is proposed. For a fixed input power, it can operate in either a high-Isp mode or a high-thrust mode depending on the propulsive maneuver requirements. The hybrid thruster consists of a plug-annular cold or heated gas thruster in the chemical mode and a field emission thruster housed within the plug operating in the electric mode using a metallic propellant. The direct simulation Monte Carlo (DSMC) technique and the particle-in-cell technique are used to model the two different modes to estimate performance parameters of the thruster. The DSMC simulations show that the microspike nozzle can provide an improved specific impulse (Isp) at low Reynolds numbers when compared to a straight orifice or converging-diverging nozzle. The PIC simulations for the field emission thruster are shown to compute the current density, ion beam density, ion beam velocities and, hence, specific impulse and thrust accurately for conditions corresponding to earlier published experiments.

Dynamic Modeling and Experimental Validation of Thrust-stand for Micropropulsion Testing

With the current expansion of microthruster research there is an accompanying need to accurately measure the performance of possible thrusters. A torsion pendulum thrust stand is a common method of measuring thrust in the microNewton range. Because of the oscillatory nature of the pendulum, the system is often in some dynamic state where measurement of thrust is difficult. The need for accurate measurements of unsteady performance in developing MEMS microthrusters led to the application of this dynamic method. Previous endeavors have focused only on steady state measurements. Proof of concept is achieved by applying electrostatic forces of known value and applything the dynamic thust extraction method. The main goals of this paper are to present the development and application of the thrust stand dynamic model and its application to unsteady force extraction.