This chapter covers the development of cold gas microthruster, which is widely regarded as the simplest way of generating thrust in space, for nanosatellites. A brief background and principles of operation were given, followed by the introduction of nozzle theory that could be used in the preliminary estimation of microthruster performance and considerations in selecting a suitable propellant. The current state of development in cold gas microthruster (at the time of writing in 2020) was provided, from its first use in SNAP-1 in 2000 to another 12 nanosatellites as well as one technology demonstration mission in Prototype Research Instruments and Space Mission technology Advancement small satellite. The chapter ends with a discussion on future nanosatellite missions that will feature a cold gas microthruster system and the challenges, such as fabrication of micronozzle and its design optimization, to improve overall efficiency.

A major concern in the development of micro-propulsion systems, used for CubeSat or PocketQube missions, is the significantly increased viscous losses encountered in the nozzle. The viscous losses, associated with the low throat Reynolds numbers, limit the usability of micro-propulsion systems. A promising solution is the use of linear aerospike nozzles, which mitigates a portion of the viscous losses by removing the walls normal to the nozzle profile, thus reducing the flow-wall interactions. This paper presents the results of the design, numerical study, and fabrication of a novel double depth aerospike micro-nozzle. This micro-nozzle uses different depths before and after the nozzle throat, aiming to reduce over-edge expansion losses encountered in single depth aerospike nozzles. The steady state micro-nozzle performance is evaluated through three dimensional numerical simulations, using a continuum model, with nitrogen gas as the working fluid. The numerical simulations are performed over a range of spike depths between 200 and 1000 µm, and for the throat Reynolds numbers in the range of 191 to 2861. Additionally, linear micro-nozzles with varying divergence half angles (?=15°-45°), and single depth linear aerospikes truncated at 20%, 40% and 60% are investigated. Evaluation of the results shows that the double depth aerospike micro-nozzle outperforms both the linear and single depth aerospike nozzles across the entire range of investigated Reynolds numbers. That is, with performance improvements between 19.2% and 33.6% compared to the best performing linear nozzle. Also, indications will be given on the manufacturability of this novel micro-nozzle geometry using microelectromechanical systems (MEMS) fabrication techniques. Thus, with the application of the double depth aerospike nozzle geometry, doors are opened for new complex nano- and pico-satellite missions.