Eric H. Cardiff

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.

Vacuum Pyrolysis and Related ISRU Techniques

A number of ISRU‐related techniques have been developed at NASA Goddard Space Flight Center. The focus of the team has been on development of the vacuum pyrolysis technique for the production of oxygen from the lunar regolith. However, a number of related techniques have also been developed, including solar concentration, solar heating of regolith, resistive heating of regolith, sintering, regolith boiling, and instrumentation development. An initial prototype system was developed to vaporize regolith simulants using a ∼1m2 Fresnel lens. This system was successfully used to vaporize quantities of ∼1g, and both mass spectroscopy of the gasses produced and Scanning Electron Microscopy (SEM) of the slag were done to show that oxygen was produced. Subsequent tests have demonstrated the use of a larger system with a 3.8m diameter reflective mirror to vaporize the regolith. These results and modeling of the vacuum pyrolysis reaction have indicated that the vaporization of the oxides in the regolith will occur a...