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.

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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.

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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.

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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.@en

As a further step in the research towards miniaturization of satellite components and sub-systems, the Department of Space Systems Engineering at the Delft University of Technology has recently embarked in the end-to-end engineering of the Delfi-PQ picosatellite platform, designed according to the PocketQube size standard. This new satellite platform, inspired by the success of previous Delfi satellite projects, is seen as a great opportunity for innovativeness and offers great research challenges. Since a consolidated standard for PocketQubes has not been established yet, a significant amount of design freedom can be harnessed despite the small volume available. The miniaturization process required to integrate the core bus forces the team to think differently about space technology: it is not sufficient to simply down-scaling existing concepts used in larger satellites, and it is often necessary to develop and qualify completely new components and integration methods. The paper is about systems engineering process, technology developments, and verification and validation for the design and development of the micro-propulsion payload for PocketQubes and its integration with the core bus platform.@en

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.

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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 flight of the satellite: one based on vaporization, heating and expansion in a nozzle of pressurized liquid water (Vaporising 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.@en

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%.

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A Low-Pressure Micro-Resistojet (LPM) is under development at TU Delft with the intention to provide future nano- and pico-satellites with the necessary capability to execute formation flying maneuvers, orbit change maneuvers, and station keeping. In this particular type of electro-thermal thruster, water is a green propellant of excellent performance, which can be stored as a liquid or solid operating at very low pressure, under evaporation or sublimation conditions. The formed vapor flows to a series of hot microchannels in a heater chip. Then, the flow is heated and expanded at high Knudsen numbers to a high exhaust velocity. This concept is very promising when associated to the typical CubeSat or PocketQube requirements that demand low tank pressure, low system mass, intrinsic safety, “green” propellants – non-corrosive, non-flammable, non-toxic, with limited energetic content – and a sufficiently long operational life. This paper discusses the optimization of the LPM design applied to two different missions, one for a CubeSat mission which requires a formation flight and other for a PocketQube mission which will be used as a flight demonstration platform.@en

The aerospace industry is recently experiencing growing interest in very small spacecraft like nano- and pico-satellites. However, these very small satellites are still being developed in most cases without a dedicated propulsion system limiting their capabilities. The micro-resistojet has been recognized as a suitable propulsion system for these classes of satellites due to its scalability and performance. Additionally, it can be classified as a "green" propulsion system since it can use naturally any kind of propellant, including "green" propellants. The Low-Pressure Micro-Resistojet (LPM) is a type of micro-resistojet concept that works under very low pressure. This PhD thesis is focussed on the development of this propulsion system concept with the goal to enable very small satellites to perform manoeuvres. This improvement allows, for instance, to increase the spacecraft lifetime by active orbit keeping. Furthermore it can enable orbit change manoeuvres and formation flight. @en

Vaporizing Liquid Microthrusters (VLM) have recently received attention as promising propulsion technology for highly miniaturized spacecraft due to its high thrust levels and low power consumption. This paper presents the results of numerical optimization of the parameters for the design of the heating chamber of VLMs that use water as the propellant. The optimization is aimed to increase the heat transfer coefficient of the heating chamber in order to maximize the heat convection while minimizing the heat and pressure losses from the inlet to the nozzle as well as the size of the device. The simulations are carried out in a combined environment using Computational Fluid Dynamics (CFD) and an optimization tool to run the algorithms. The results of the optimization are compared to the results of a comprehensive experimental campaign and are intended to be used in the next design of the VLMs produced by TU Delft that will fly on-board of a PocketQube.@en

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.@en

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.@en

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.

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One of the micro-propulsion research activities currently conducted at Delft University of Technology is concerned with the development of a low-pressure micro-resistojet based on a sublimating solid propellant. Specifically the heater chip and expansion slots for this thruster have already undergone extensive research and testing and are at a very good level of maturity. In contrast, the propellant tank still requires some numerical and experimental analysis and is currently considered the main obstacle towards full implementation of this system in a spacecraft. The tank shall be designed to work with sublimating water ice, at temperature and pressure not exceeding 273 K and 611 Pa respectively, which are the triple-point conditions of water. A simple test vial, outfitted with a simple heater, as well as temperature and pressure sensors was filled with demineralized water. This was in turn frozen prior to placing the vial on a weighting scale in a vacuum chamber, which was used to measure the mass flow rate of sublimation. The experiments performed were adjusted gradually throughout the course of the project according to the data gathered up until a certain point. The final experiments clearly demonstrated the basic theory of sublimation and most importantly, how the mass flow rate due to sublimation can be manipulated by aid of the heater which accelerates the sublimation process up to a certain point. These first series of experiments will be followed by a second series, encompassing a more dedicated tank design in which pressure and the temperature at the crucial air/ice interface can be better controlled. These experiments form the basis for the design of an optimally controlled propellant tank, based on the concept of sublimation. @en

Testing micro-propulsion systems is often a hard and challenging phase in the characterisation process of similar technologies, which are very sensitive to environmental noise and errors due to the human operator. At the Department of Space Engineering of the Delft University of Technology, a thrust bench of the hanging pendulum type is used to assess the performance of the in-house developed micro-resistojets. In order to reduce the influence of external factors and limit the influence of the operator during the testing phase, a calibration process that uses an electromagnetic actuator to find the relation between the pendulum displacement measured by a capacitive sensor and a known exciting force has been developed. The found relations is then used to reconstruct the variation of the thrust over time. However, the numerous wires connected to hardware on the pendulum represents a disturbing effect and seems to affect the results of the measured thrust. This work presents an upgrade version of the pendulum which aims to reduce the unwanted and unpredictable disturbing effects of the wirings through a wireless Bluetooth connection system. Besides that, a new analysis model has been developed to assess the performance of the micro-thrusters, which can be used as a comparison to the calibration method mentioned above. A series of tests has been completed to validate the model. The configuration of the pendulum used in the testing phase has been optimised with respect to the position of the counter mass and displacement sensor position on the thrust stand. The results show an unexpected apparent shift of the pendulum centre of mass. This causes the model to have a higher level of inaccuracy. Further analyses are need in order to understand the causes of this phenomenon. However, it has been possible to located the apparent shift within a specific range. The model has shown promising results, but more tests are needed in order to precisely determine its inaccuracy, mainly due to this unexpected behaviour of the shift of centre of mass. @en

Microelectromechanical systems (MEMS) techniques uncovered new opportunities in satisfying the mission requirements of the growing next generation nano- and pico-satellite missions. In particular, micro-propulsion is universally recognized as one of the key enabling technologies to help this class of satellites making the next step and become credible candidates to a wide range of scientific and commercial applications. In this context, TU Delft is developing a miniaturized electro-thermal propulsion system operating with green liquid propellants, for application on a wide range of nano-satellite formats from CubeSats (10x10x10 cm units) to PocketQubes (5x5x5 cm units). A breadboard of the complete micro-propulsion system is under development at TU Delft, including the thruster, propellant tank, the valve and the driving electronics. The design of the system shall be easily adapted to both CubeSat and PocketQube standards, with particular attention to the second one since the system is scheduled for an initial flight demonstration on the first Delfi-PQ satellite. To address this need and to fill an existing gap in the state-of-the-art of micro-propulsion, two kinds of micro-thrusters are considered in this development; a Vaporizing Liquid Micro-resistojet (VLM) and a Low Pressure Micro-resistojet (LPM). A number of test results will be shown in the paper on the electrical, mechanical and functional characterization of the MEMS thrusters, fabricated in the Else Kooi Laboratory at TU Delft, and the other components of the system. Keywords: Micro-resistojet, Microthruster, MEMS, Cubesats, Pocketqubes @en

The aerospace industry is recently expressing a growing interest in green, safe and non-toxic propellants for the propulsion systems of the new generation of space vehicles, which is especially true in the case of Cubesat micro-propulsion systems. Demanding requirements are associated to the future missions and challenges offered by this class of spacecraft, where the availability of a propulsion system might open new possibilities for a wide range of applications including orbital maintenance and transfer, formation flying and attitude control. To accomplish these requirements, Delft University of Technology is currently developing two different concepts of waterpropelled micro-thrusters based on MEMS technologies: a free molecular micro-resistojet operating with sublimating solid water (ice) at low plenum gas pressure of less than 600 Pa, and a more conventional micro-resistojet operating with liquid water heated and vaporized by means of a custom designed silicon heating chamber. In this status review paper, the current design and future expected developments of the two micro-propulsion concepts is presented and discussed, together with an initial analysis of the expected @en

Heat transfer and fluid flow through different microchannel geometries in the transitional regime (rarefied flow) are analysed by means of Direct Simulation Monte Carlo simulations. Four types of three-dimensional microchannels, intended to be used as expansion slots in micro-resistojet concepts, are investigated using Nitrogen as working fluid. The main purpose is to understand the impact of the channel geometry on the exit velocity and the transmission coefficient, parameters which are well known to affect directly the thruster performance. Although this analysis can be applied in principle to several possible microfluidics scenarios, particular focus is given to its application in the field of space propulsion for micro-, nano- and pico-satellites, for which the requirements ask for low thrust levels from some μN to a few mN and moderate specific impulse, as well as a low power consumption in the order of a few W. Analysis shows that the thrust produced by one single microchannel can be increased by about 480% with a careful selection of the channel geometry, decreasing at the same time the specific impulse by just 5%, with a power consumption decrease of more than 66.7%. @en

This paper presents a dynamic system approach for the modeling of fluid flow in microchannels to be used in thrust control applications. A micro-resistojet fabricated using MEMS (Microelectromechanical Systems) technology has been selected for the analysis. The device operates by vaporizing a liquid propellant, in this case water, and expelling it as gas that is accelerated by a micro-nozzle. The pressure variation due to boiling in the chamber might lead to unwanted behavior of the feed system and the frequency analysis in this case can indicate whether or not instabilities will be present. To handle this complex problem, the incompressible Navier-Stokes equations are linearized in the steady-state flow regime and then formulated in state space form to provide the necessary means for control analysis. Controllability and observability of the system are investigated considering low values of Reynolds numbers present in micro fluidics applications. Results from the analytical treatment are compared with CFD (Computational Fluid Dynamics) simulations of the microchannel to demonstrate the validity of the approach investigated @en