Advanced In-Space Propulsion (AISP): Micro Electrospray Propulsion (MEP)

Propulsion technology is often critical for space missions. High-value missions could be done with very small spacecraft, even CubeSats, but these nanosatellites currently have little propulsion capability. After CubeSats are deployed, they usually just tumble or drift away from the transport spacecraft. They cannot transfer to higher value orbits, maintain their orbit, or even deorbit. Larger spacecraft would benefit from highprecision attitude-control systems to maintain the desired orbit and point in the desired direction. Existing attitude-control systems, like reaction wheels, are very complex and may have insufficient lifetimes. NASA is investing in Microfluidic Electrospray Propulsion (MEP) thrusters to provide the new propulsion capabilities to address both of these mission needs. Chemical propulsion systems are limited to the combustion energy available in the chemical bonds of the fuel and the acceleration provided by a converging-diverging nozzle. Electric propulsion uses electric power to accelerate propellant to very high exhaust velocities—up to 10 times greater than for chemical propulsion. This increases the momentum transfer efficiency or the fuel economy. The propellant efficiency of thrusters, which is proportional to the exhaust velocity, is referred to as the "specific impulse," or ISP, measured in seconds. The state of the art for CubeSats is cold gas propulsion with an ISP of 50 to 80 s. The chemical propulsion main engine for the space shuttles demonstrated an ISP of 450 s. However, the target ISP for MEP systems is greater than 1500 s—enough to transfer a 1-kg 10-cm cube from low Earth orbit to interplanetary space with only 200 g of propellant.