Miniaturization of spacecraft has been gaining wide interest in the space industry, given its potential for reducing space missions' costs and providing a novel approach to enhancing and facilitating spaceflight. Recently, a lot of research has been successfully put into this field along with the advancements that make it more feasible, though a major obstacle to achieving the new generation of spacecraft is the technical challenge of fitting a suitable propulsion system. Useful chemical propellants are usually corrosive, flammable, and/or toxic, so alternatives need to be found. The aerospace industry is shifting towards green and nontoxic propulsion systems, so water could be used as an effective propellant, considering its relatively high mass density and low molecular mass. New microelectromechanical systems (MEMS) technologies show promising opportunities for the integration of miniaturized propulsion systems, due to their versatility and robustness. In this thesis, a comparative study of nozzle flow, heat transfer, and thermodynamics in two different thrusters is conducted. One thruster is based on MEMS, with a typically quasi-2D geometry, while the second thruster is based on more conventional technologies and manufacturing techniques, with an axially symmetrical 3D shape. After briefly introducing micropropulsion and discussing the propellant selection and nozzle fabrication along with the background theory related to micropropulsion as well as the analytical and OpenFOAM numerical (DSMC, continuum, and a hybrid approach containing both to accommodate to the variation in Knudsen number throughout the computational domain) modeling methods, the used methodology is based on using OpenFOAM's DSMC solver (dsmcFoam+) following the mesh creation using blockMesh and snappyHexMesh and developed analytical model (using MATLAB and CoolProp) along with an additional VLM ANSYS Fluent CFD model prepared in advance at TU Delft, where their (steady state as well as transient for DSMC) results (including the same conventional and MEMS nozzles) are processed and discussed. The nozzles are simulated for inlet pressures of 5 and 7 bar at inlet temperatures of 550 and 773 K for a total of four cases for each nozzle. To note, many of dsmcFoam+'s functionalities (mass flow rate measurements, inlet pressure boundary condition, axisymmetric capabilities, statistical error measurements, and dynamic load balancing) are implemented and described along with the full methodology, as Blender (with add-ons) and ParaView with a Python script to extract averaged data (along the nozzle and plume region) along with sampleDict are also used in pre and post-processing respectively and the simulations are carried out on a computer cluster. Furthermore, a quite interesting theoretical project on the side has been independently worked on in parallel. It started as a noticed idea that was decided to be explored using equations, which led to extended continuum/kinetic dimensionless numbers for diffusivity and rarefaction intensity relative to the studied object’s timescale. Ultimately, there are tradeoffs to choosing either thruster, where it is impractical to fault one nozzle for not performing better than the other, as it comes back to the desired features and nature of the mission each is undertaking, where compromises have to be made.