CubeSats have been extensively used in the past decade as scientific tools, technology demonstrators and for education. Recently, PocketQubes have emerged as an interesting and even smaller alternative to CubeSats. However, both satellite types often lack some key capabilities, such as micropropulsion, in order to further extend the range of applications of these small satellites. This paper reviews the current development status of micropropulsion systems fabricated with MEMS (micro electro-mechanical systems) and silicon technology intended to be used in CubeSat or PocketQube missions and compares different technologies with respect to performance parameters such as thrust, specific impulse, and power as well as in terms of operational complexity. More than 30 different devices are analyzed and divided into 7 main categories according to the working principle. A specific outcome of the research is the identification of the current status of MEMS technologies for micropropulsion including key opportunities and challenges.

This paper presents the results of design, manufacturing and characterization of Vaporizing Liquid Microthrusters (VLM) with integrated heaters and temperature sensing. The thrusters use water as the propellant and are designed for use in CubeSats and PocketQubes. The devices are manufactured using silicon based MEMS (Micro Electro Mechanical Systems) technology and include resistive heaters to vaporize the propellant. The measurements of the heaters' resistances are used to estimate the temperature in the vaporizing chamber. The manufacturing process is described as well as the characterization of the thrusters' structural and electrical elements. In total 12 devices with different combinations of heaters and nozzles have been assessed and four of them have been used to demonstrate the successful operation of the thrusters. Results show a performance close to the design parameters and are used to validate the thrusters.

The number of launches of nano- and pico-satellites has significantly increased over the past decade. Miniaturized subsystems, such as micropropulsion, for these classes of spacecraft are rapidly evolving and, in particular, micro-resistojets have shown great potential of applicability. One of the key points to address in the development of such devices is the propellants selection, since it directly influences the performance. This paper presents a methodology for the selection and characterization of fluids that are suitable for use as propellants in two micro-resistojet concepts: vaporizing liquid micro-resistojet (VLM) and the low-pressure micro-resistojet (LPM). In these concepts, the propellant is heated by a nonchemical energy source, in this case an electrical resistance. In total 95 fluids have been investigated including conventional and unconventional propellants. A feasibility assessment step is carried out following a trade-off using a combination of the analytical hierarchy process (AHP) and the Pugh matrix. A final list of nine best-scoring candidates has been analyzed in depth with respect to the thermal characteristics involved in the process, performance parameters, and safety issues. For both concepts, water has been recognized as a very promising candidate along with other substances such as ammonia and methanol.

Three Low Pressure Micro-resistojets (LPM) with integrated heater and temperature measurement were designed, manufactured and characterized at Delft University of Technology. The devices were manufactured using Silicon-based Micro Electro Mechanical Systems (MEMS) technology including a heater made of Molybdenum for better operations at high temperature. The resistace of the heaters is used to estimate the chip temperature giving them a double function as heater and sensor simultaneously. The manufacturing steps are described in detail. A special interface was manufactured to hold the MEMS device considering the mechanical and electrical aspects. The MEMS devices are characterized for three different aspects: mechanical, electrical and propulsion. The three designed devices were tested mechanically and electrically, and one design was tested in terms of propulsion performance in a near-operational condition. The tests are promising and open the path to design a flight demonstration model.

There is a clear trend towards the developments of micro-propulsion system to enhance the capabilities of nanoand pico-satellites. A promising propulsion option to meet the strict requirements of these small satellites is the Low-Pressure Micro-Resistojet (LPM) which works under rarefied gas dynamic regime. To simplify the engineering design of this propulsion system an analytical model has been developed using the fundamental physical models. This analytical model is based on the Kinetic theory of gases and the Maxwell-Boltzmann distribution of molecular velocities to describe the macroscopic flow parameters such as mass flow rate, velocity and pressure, and then to estimate the thruster performance. The equations are well known, but they are applied in this case using a particular approach in order to describe the physics behind this micro-propulsion system. Comparisons between numerical simulations using the DSMC method and the results of the analytical model, as well as experimental results, have been carried out. The analytical model using an accurate estimation of the transmission coefficient compared to the numerical simulation presents a maximum difference of 3%.