Over 2500 active satellites are in orbit as of October 2020, with an increase of ~1000 smallsats in the past two years. Since 2012, over 1700 smallsats have been launched into orbit. It is projected that by 2025, there will be 1000 smallsats launched per year. Currently, these satellites do not have sufficient delta v capabilities for missions beyond Earth orbit. They are confined to their pre-selected orbit and in most cases, they cannot avoid collisions. Propulsion systems on smallsats provide orbital manoeuvring, station keeping, collision avoidance and safer de-orbit strategies. In return, this enables longer duration, higher functionality missions beyond Earth orbit. This article has reviewed electrostatic, electrothermal and electromagnetic propulsion methods based on state of the art research and the current knowledge base. Performance metrics by which these space propulsion systems can be evaluated are presented. The article outlines some of the existing limitations and shortcomings of current electric propulsion thruster systems and technologies. Moreover, the discussion contributes to the discourse by identifying potential research avenues to improve and advance electric propulsion systems for smallsats. The article has placed emphasis on space propulsion systems that are electric and enable interplanetary missions, while alternative approaches to propulsion have also received attention in the text, including light sails and nuclear electric propulsion amongst others.

This article presents the full operational experimental capabilities of the plasma wind tunnel facilities at the Institute of Space Systems at the University of Stuttgart. The simulation of the aerothermodynamic environment experienced by vehicles entering the atmosphere of Earth is attempted using three different facilities. Utilizing the three different facilities, the recent improvements enable a unique range of flow conditions in relation to other known facilities. Recent performance optimisations are highlighted in this article. Based on the experimental conditions demonstrated a corresponding flight scenario is derived using a ground-to-flight extrapolation approach based on local mass-specific enthalpy, total pressure and boundary layer edge velocity gradient. This shows that the three facilities cover the challenging parts of the aerothermodynamics along the entry trajectory from Low Earth Orbit. Furthermore, the more challenging conditions arising during interplanetary return at altitudes above 70 km are as well covered.

The development of the inductively driven plasma wind tunnel PWK3, which enables the electrodeless generation of high-enthalpy plasmas for the development of heat shield materials required for space vehicles performing entry manoeuvres in the atmospheres of Venus, Earth and Mars, is described. The facility with its modular inductive plasma generators allows operation with gases such as carbon dioxide, air, oxygen and nitrogen and was qualified for thermal plasma powers up to 60 kW. Previously developed models for determining plasma properties and plasma source related characteristics enable a maximum plasma power in combination with long operational periods using different operational gases and gas mixtures. This is achieved by an optimization using the optimum operational frequency, a minimization of field losses using very thin plasma tube wall thicknesses and the successful application of MHD effects. Based on the solved cylinder problem for ICPs, a one-dimensional model for radial Lorentz forces and magnetic pressure has been developed. Here, a synthesis of previously published data and works is made where the new algebraic model for the calculation of Lorentz forces and magnetic pressures in an ICP was used and applied to experimental data. In addition, results from the model using the experimental data are shown to be consistent and, in addition, a comparison with a simpler model based on the well-known exponential approach for ICPs showed that the simpler model is covered without fail by the new model. The new model also states that there is a maximum of the Lorentz forces over the damping parameter d/δ (plasma diameter divided by skin depth) which almost corresponds with the position of the maximum plasma power of the cylindric model for ICPs. For the magnetic pressure the position of the maximum pressure is identical to the value for d/δ for the maximum plasma power.