Oliver Neunzig

Earth’s residual atmosphere in the very low Earth orbit (VLEO) can be used as a fuel source for air-breathing electric propulsion (ABEP) systems, supplying satellites with a virtually unlimited amount of propellant so that mission lifetime becomes independent from fuel storage. Operating future satellites at such altitudes offers advantages, including low-latency communication, high-resolution Earth observation, the potential to reduce launch costs, and a rapidly self-cleaning orbit, thereby reducing the risk of long-term space debris. To characterize performance and reduce development time and costs for ABEP systems, the capability to conduct terrestrial tests of complete ABEP systems in vacuum facilities is crucial. Since 2021, a VLEO particle source has been under development at Institute of Aerospace Engineering (ILR) at TU Dresden for various applications, including material testing and ABEP system testing. This paper presents a concept for a suite of four combined diagnostics to determine the flux density and average velocity of the particle beam, which shall primarily consist of neutral atomic oxygen and molecular nitrogen, to precisely replicate the VLEO residual atmosphere. The selected diagnostics will be described, along with an explanation of how they work together to detect as many beam parameters as possible. The basic principles of the ABEP test facility are discussed, with binary conditions focusing on altitudes between 180 km to 250 km.

Electrospray thrusters have garnered significant attention throughout the years as an exceptional propulsion technology for nano- and picosatellites due to their efficiency and precise thrust control. They operate on the principle of electrostatically accelerating charged particles (liquid droplets, pure ions or their mixtures) from ionic liquids and other low-volatility propellants, which are extracted from a Taylor-cone formation on top of porous emitter arrays. In this work we characterized the thrust performance of electrospray thrusters with the ionic liquid 1-ethyl-3-methylimidazoliumtetrafluoroborate (EMI-BF₄) as well as an attempt with an acetate-based ionic liquid. The arrays were operated at different polarities and at elevated temperatures of up to 43 °C which led to a decrease in viscosity and enhanced current emission for EMI-BF₄ with a factor of 1.43 at equal voltage levels. Temperature related effects resulted in a thrust difference of 3% between the maximum and minimum temperature throughout the tested current range. Thrust measurements for emission currents between 10 µA and 200 µA revealed a detectable and temperature independent difference between the positive and negative mode in favor of the negative polarity, indicating different ion-regimes compared to most data found in literature. The paper presents a novel thrust measurement setup for micro-propulsion systems based on a counterbalanced double pendulum thrust balance that achieves nanonewton resolution with the option to heat several thrusters. A comprehensive overview of the test setup and calculations of obtained electrospray parameters from experimental data is presented.

While traditional Hall-effect thrusters are known for their high specific impulse and efficiency, they require additional mechanical components to achieve the precise thrust vector control required for orbital maneuvers and attitude corrections. These additional components can impact the overall system performance by increasing weight, volume, complexity, and susceptibility to failure. To address these issues, the Technische Universität Dresden (TUD) has developed an innovative Hall-effect thruster with a segmented anode, the TUD-3D-1. This design allows thrust vector control in all three spatial directions solely by manipulating the ion beam, eliminating the need for additional mechanical components. The paper explains the underlying principles, as well as potential benefits of this advanced thruster technology and presents test results of the prototype.

This work presents a process for the creation of emitter structures for liquid metal ion sources based on suspensions of iron and nickel in gallium-based room temperature liquid metal alloys. The process does not require needle etching or vacuum wetting of the emitter. Tip radii of less than 3 μm have been achieved. In addition, first measurements and observations from single emitter emission experiments are presented, including current-voltage characteristics, vacuum stability, degradation mechanisms and first thruster characteristics for the use in field emission electric propulsion. The impedance of the magnetic liquid metal emitters was found to correlate with the mass fraction of particles suspended in the liquid metal. Total impedances up to 30 MΩ have been achieved, indicating the feasibility of emitter clustering. Thrust measurements were performed with a commercial of the shelf load cell and verified in a dedicated setup for field emission electric propulsion testing, featuring a sub micro newton counterbalanced double pendulum thrust balance.

The EMDrive is a proposed propellantless propulsion concept claiming to be many orders of magnitude more efficient than classical radiation pressure forces. It is based on microwaves, which are injected into a closed tapered cavity, producing a unidirectional thrust with values of at least 1 mN/kW. This was met with high scepticism going against basic conservation laws and classical mechanics. However, several tests and theories appeared in the literature supporting this concept. Measuring a thruster with a significant thermal and mechanical load as well as high electric currents, such as those required to operate a microwave amplifier, can create numerous artefacts that produce false-positive thrust values. After many iterations, we developed an inverted counterbalanced double pendulum thrust balance, where the thruster can be mounted on a bearing below its suspension point to eliminate most thermal drift effects. In addition, the EMDrive was self-powered by a battery-pack to remove undesired interactions due to feedthroughs. We found no thrust values within a wide frequency band including several resonance frequencies and different modes. Our data limit any anomalous thrust to below the force equivalent from classical radiation for a given amount of power. This provides strong limits to all proposed theories and rules out previous test results by at least two orders of magnitude.

Since modern propulsion systems are insufficient for large-scale space exploration, a breakthrough in propulsion physics is required. Amongst different concepts, the EMDrive is a proposed device claiming to be more efficient in converting energy into propulsive forces than classical photon momentum exchange. It is based on a microwave resonator inside a tapered cavity. Recently, Taylor suggested using a laser instead of microwaves to boost thrust by many orders of magnitude due to the higher quality factor of optical resonators. His analysis was based on the theory of quantised inertia by McCulloch, who predicted that an asymmetry in mass surrounding the device and/or geometry is responsible for EMDrive-like forces. We put this concept to the test in a number of different configurations using various asymmetrical laser resonators, reflective cavities of different materials and size as well as fiber-optic loops, which were symmetrically and asymmetrically shaped. A dedicated high precision thrust balance was developed to test all these concepts with a sensitivity better than pure photon thrust, which is the force equivalent to the radiation pressure of a laser for the same power that is used to operate each individual devices. In summary, all devices showed no net thrust within our resolution at the Nanonewton range, meaning that any anomalous thrust must be below state-of-the-art propellantless propulsion. This puts strong limits on all proposed theories like quantised inertia by at least 4 orders of magnitude for the laboratory-scale geometries and power levels used with worst case assumptions for the theoretical predictions.