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.