Search

Air-breathing electric propulsion (ABEP) allows for lowering the altitude of spacecraft operations below 250 km, in the so-called Very Low Earth Orbits (VLEOs). Operations in VLEOs will give radical advantages in terms of orbit accessibility, payload performance, protection from radiations, and end-of-life disposal. ABEP combines an intake to collect the residual atmosphere in front of the spacecraft and an electric thruster to ionize and accelerate the atmospheric particles. Such residual gas can be exploited as a renewable resource not only to keep the spacecraft on a VLEO, but also to remove the main limiting factor of spacecraft lifetime, i.e., the amount of stored propellant. Several realizations of the ABEP concept have been proposed, but the few end-to-end experimental campaigns highlighted the need to improve the concept functional design and the representativeness of simulated atmospheric flows. The difficulty in recreating the VLEO environment in a laboratory limits the data available to validate scaling laws and modelling efforts. This paper presents a comprehensive review of the main research and development efforts on the ABEP technology.

Test and qualification (T&Q) phases take a significant portion of the time to market for critical products in the space industry, especially when introducing new technologies. Since T&Q are treated as standard procedures, they tend to be independent of the architectural design phases and kept away from design decisions. However, when introducing new technologies, qualification procedures may differ from those established in regular design scenarios, and the estimation of qualification costs and duration is problematic. There is a lack of design for qualification methods capable of modelling these activities in early phases and use those models to support the architecture design of products with affordable test and qualification phases. In this article, a computer-assisted, model-based design method to model T&Q activities concerning early product architecture designs is proposed. Product architecture alternatives, test schedules and cost are connected through the quantification of T&Q drivers and driver rates. The design method is demonstrated using a case study about electric propulsion for satellites. The method is applicable for design situations where the choice of technology has a strong dependence on the qualification procedure.

In Hall propulsion systems, a direct and non-isolated connection between the thruster discharge and the solar array implies a significant change in terms of spacecraft grounding, thruster operation and control. In this so-called direct-drive architecture, since discharge voltage and current are no more fixed by a power processing unit but are constrained by the solar array characteristic, which changes during a space mission, specific control laws need to be developed and tested to assess the feasibility of thruster operation. In this paper, in-depth investigations performed in SITAEL on thruster performance and operation controllability of a direct-drive Hall thruster system are presented. The HT5k, a 5 kW-class Hall thruster, was connected to a solar array simulator (SAS) and tested at different operating conditions. Three different thruster control laws were implemented and were proved effective in controlling the thruster behavior with respect to on-orbit-representative variations in the SAS characteristics. The results also highlighted minimal difference between thruster performance parameters in direct-drive and with a conventional power supply. Finally, the impact of grounding on thruster plume interaction with the spacecraft was analyzed.

One of the main oscillatory modes found ubiquitously in Hall thrusters is the so-called breathing mode. This is recognized as a relatively low-frequency (10–30 kHz), longitudinal oscillation of the discharge current and plasma parameters. In this paper, we present a synergic experimental and numerical investigation of the breathing mode in a 5 kW-class Hall thruster. To this aim, we propose the use of an informed 1D fully-fluid model to provide augmented data with respect to available experimental measurements. The experimental data consists of two datasets, i.e., the discharge current signal and the local near-plume plasma properties measured at high-frequency with a fast-diving triple Langmuir probe. The model is calibrated on the discharge current signal and its accuracy is assessed by comparing predictions against the available measurements of the near-plume plasma properties. It is shown that the model can be calibrated using the discharge current signal, which is easy to measure, and that, once calibrated, it can predict with reasonable accuracy the spatio-temporal distributions of the plasma properties, which would be difficult to measure or estimate otherwise. Finally, we describe how the augmented data obtained through the combination of experiments and calibrated model can provide insight into the breathing mode oscillations and the evolution of plasma properties.

As part of on-going efforts in advancing air-breathing electric propulsion, the HT5k Hall thruster was characterized in six operating conditions, ranging from 5 mg s−1 to 7 mg s−1 of 0.56N2 + 0.44O2 mass flow rate and 225 V to 300 V of discharge voltage. The cathode was operated with N2 at mass flow rates between 0.5 mg s−1 and 0.7 mg s−1. This paper presents a 0D-hybrid model for atmospheric propellant fed Hall thruster discharges. Verified performance ranged between 30 mN to 120 mN in thrust, 1.2 kW to 5.2 kW in discharge power, and 8% to 18% in anodic efficiency. Calibrated model comparison against experimental data resulted in a mean absolute error of 3.7% in thrust and 7.6% in discharge power.

We performed a test campaign in order to fill the gaps in the experimental data and understanding of the performance of the Applied Field Magnetoplasmadynamic Thruster in low current, high mass flow rate regime. Thrust and discharge voltage were measured for two anode geometry configurations with different mass flow rate distribution, discharge current and external magnetic field strength. The general trends in thruster behavior using argon as propellant are analyzed in this paper and an approach for thruster optimization is investigated. We find an unexpectedly weak dependence of thrust efficiency on discharge current in low current, high mass flow rate regimes. Using a cylindrical anode configuration, we managed to achieve thrust efficiency of 21%, which is significantly higher than that reported in literature for AF-MPDTs operating in this regime.

The different flow regimes occurring in T-mixers are investigated by means of direct numerical simulations. Three different values of the aspect ratio of the inlet channels, κi, that is their width to height ratio are considered, namely κi = 0.75, 1 and 2. For the configurations with κi = 0.75 and 1, the same behavior as previously described in the literature, is found. In particular, as the Reynolds number is increased, the flow evolves from vortical to engulfment steady regimes, then to unsteady asymmetric and symmetric periodic regimes, until, finally, it becomes chaotic. All the critical values of the Reynolds number, at which the transitions between the different regimes occur, are found to be very similar for κi = 0.75 and 1, while some differences are highlighted in the vorticity dynamics and characteristic frequencies of the unsteady regimes. The observed scenario is completely different for κi = 2. Indeed, in this case, the flow evolves directly from the vortical regime to an unsteady symmetric behavior, with a vorticity dynamics that is significantly different from those observed for the other aspect ratios.