Electrothermal propulsion can be seen as an intermediate concept between electrical and chemical propulsion. Propellant heating typically happens by means of a resistance (resistojet) or an electrical discharge (arcjet). For the extremely miniaturized applications that will be discussed in this chapter, resistojets are by far the most commonly used form of electrothermal thrusters.

Delft University of Technology is currently developing the pico-satellite platform Delfi-PQ, based on the PocketQube standard, in pursuit of a new generation of satellites with lower cost, flexibility and short development time. A technology demonstration payload expected to fly in one of the first Delfi-PQ satellites is a dual thruster micro-propulsion system based on the use of water as propellant. Two different micro-resistojet concepts will be demonstrated in the same satellite flight: one based on vaporization, heating and expansion in a nozzle of pressurized liquid water (Vaporizing Liquid Micro-resistojet); the other based on heating and acceleration in slots with simple geometry of molecules of vapour under transitional or free molecular flow regime (Low Pressure Micro-resistojet). The demonstrator is based on a common propellant storage for the two micro-propulsion concepts, based on the use of the capillarity properties of water in a small diameter tube connected to the two separate MEMS thruster chips with their own dedicated valves. This paper describes the requirements and design of the complete micro-propulsion demonstrator as well as its expected operational envelope for in-orbit functional testing, based on the currently validated performance characteristics of the two thrusters.

The authors regret about a mistake in Eq. (2). The correct equation is as follows. [Formula presented]The exit temperature [Formula presented] is the one used to calculate the exit velocity instead of the chamber temperature [Formula presented] as stated in the paper. We assure that the results have been correctly calculated using the correct equation. The authors would like to apologize for any inconvenience caused.

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.

There is a clear trend towards the developments of micro-propulsion system to enhance the capabilities of nano- and 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 Direct Simulation Monte Carlo 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%.

This brief presents a comprehensive approach for the modeling of micropropulsion systems based on the vaporization of a liquid. The model combines the analytical and empirical relations derived from extensive experimental analysis and fundamental physical laws. This allows modeling of key parameters, such as mass flow rate, for the entire system comprising a tank to store the liquid propellant, a valve to control the mass flow, and a microthruster that vaporizes the propellant and accelerates it generating thrust. The model is evaluated by a sensitivity analysis considering the boundaries of the modeling space, and it has been tested in a simulation loop demonstrating the attitude control of a nanosatellite using a set of four thrusters. The results of the simulation are used to test the developed model.

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.

This paper presents the results of design, manufacturing and characterization of Vaporizing Liquid Microthrusters (VLM) with integrated molybdenum 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 are used to validate the thrusters and show a performance close to the design parameters and comparable to other devices found in the literature.