Elsevier

Acta Astronautica

Volume 198, September 2022, Pages 707-719
Acta Astronautica

Validation of enabling technologies for deorbiting devices based on electrodynamic tethers

https://doi.org/10.1016/j.actaastro.2022.06.013Get rights and content

Highlights

  • Optimization of full-length deployment trajectory.

  • High fidelity simulations of deorbiting with electrodynamic tethers.

  • Experimental validation of technologies for tape electrodynamic tethers.

  • Close-proximity deployment testing with frictionless table and trajectory control.

  • Development and testing of passive in-line damper.

Abstract

The increasing number of man-made objects in near-earth space is becoming a serious problem for future space missions around the Earth. Among the proposed strategies to face this issue, and due to the passive and propellant-less character, electrodynamic tethers appear to be a promising option for spacecraft in low Earth orbits thanks to the limited storage mass and the minimum interface requirements to the host spacecraft.

This work presents the roadmap that the Electrodynamic Tether Technology for Passive Consumable-less Deorbit Kit (E.T.PACK) is following to develop a prototype of a deorbit device based on electrodynamic tether technology with Technology Readiness Level 4 by the end of 2022. The paper illustrates the roadmap of the activities carried out at the University of Padova, where software and hardware have been prepared to validate some of the critical elements of the deorbit device. Specifically, the software tools include: (a) the software called “DEPLOY” that allows the computation of a reference trajectory for the deployment of the tether and the completion of sensitivity analysis of the deployment trajectory to key error sources; (b) the software called “FLEXSIM” that predicts the performances of electrodynamic tethers as a function of the system configuration employed; and (c) the software called “FLEX” that includes the dynamical effects of tether flexibility and provides important information on the dynamic stability of the system during deployment and deorbiting phase. The paper describes in detail the three software tools and provides results of a simulation showing how it is possible to deorbit a 24-kg satellite from an initial orbital altitude of 600 km in less than 100 days using a 500-m long tape-like bare tether.

The team has also developed laboratory mock-ups and performed experimental activities to: (a) determine the tether mechanical properties; (b) test the functionality of mechanisms used to deploy the tether; (c) test the functionality of the attitude control assembly used during the deployment phase; and (d) validate a passive damper designed for dissipating the longitudinal oscillations of the tether and thus guarantee the stability of the system during both deployment and deorbiting phase. The paper provides a description of both the laboratory setup and the experimental activities performed to validate EDT technologies, including the damping capability of a compact passive-damping mechanism, showing how it can reduce consistently the peak forces up to about 80%.

Introduction

The issues related to the presence of man-made objects in near-Earth orbits were firstly addressed by Kessler in the 70s’ [1], which stated that without direct action (e.g. by reducing the in-orbit artificial objects net input) the number of in-orbit human-made objects and their fragments could increase exponentially with time, up to quickly exceed the meteoroid background environment. The qualitative description of the cascade “Kessler Syndrome” was later quantified by the author, which confirmed that we are now entering a time when the orbital debris environment will increasingly be determined by random collisions and that we should avoid leaving future payloads and rocket bodies in orbit after the end of their operational life [2]. In fact, the recent constant growth of the small satellite launch rate [3,4] as well as the on-going deployment of several large constellations [5,6] have further distressed the scientific community. In this context, Foreman et al. [7] used the NASA Orbital Debris Engineering Model software and a Monte Carlo analysis to examine the potential implications of two recent satellites constellations by OneWeb and SpaceX on the LEO environment. In addition, Olivieri et al. [8] developed a simple statistical tool for lost vehicles during large constellations life, analyzing the effects of the main parameters of the constellations on its vulnerability.

The sustainability of the access to several Low-Earth Orbits (LEO) is under discussion [[9], [10], [11]] and the increasing number of resident objects is enhancing the probability of in-space collisions [12]. It was estimated that in case of future events such as the Cosmos-Iridium collision [13] the generated fragments are not only limited to the altitudes involved but may contaminate neighboring orbits. In this context the current orbital population is under constant scrutiny, with the development of lists of potential objects whose fragmentation might strongly influence the access to some orbits [[14], [15], [16]], the investigation of potential critical fragmentation events [[17], [18], [19]], and the definition of strategies for the long-term sustainability of space activities [20].

The scientific community is evaluating mitigation strategies, considering both the utilization of enhanced protections [21] and the implementation of Post Mission Disposal (PMD) [22] and Active Debris Removal (ADR) [23] operations and strategies [24]. In addition, responsible conducts and self-regulations are appearing as promising solutions to address the issue of space debris [25,26]. In parallel, guidelines regarding the deorbiting of all new satellites within 25 years from mission completion, if their deployment orbit altitude is below 2000 km (i.e., in LEO), have been introduced [27] and, in a few cases, they have been absorbed into the countries legal frameworks [28].

The 25 years guideline for spacecraft disposal is currently leading to the development of deorbiting strategies (i.e., by saving propellant for re-entry maneuvers) or to the installation of dedicated systems on board the spacecraft by most of the major providers. Among them, dedicated electrical [29] and chemical l [30] propulsion systems were suggested for disposal, as well as drag enhancement devices [31,32] and Electrodynamic Tethers (EDTs) [[33], [34], [35]]. In a relevant way, EDTs are a promising option that may overcome the limitations of traditional chemical and electrical propulsion and can potentially be lighter [36] and less prone to debris impact risk [37,38] than other devices, such as neutral drag sails. In this context, the H2020 project E.T.PACK (Electrodynamic Tether Technology for Passive Consumable-less Deorbit Kit) aims at developing a deorbit kit prototype based on EDT technology [39].

The E.T.PACK consortium is formed by a mix of academia, research centers and companies specialized in the fields required to prove the feasibility of EDT technologies: Universidad Carlo III de Madrid (UC3M)(Spain), University of Padova (UniPD)(Italy), Technische Universitat Dresden (TUD)(Germany), Fraunhofer-Institut für Keramische Technologien und Systeme (IKTS)(Germany), Advanced Thermal Devises (ATD)(Spain) and SENER Aerospacial (Spain). The consortium has knowledge on Low Work function Tether (LWTs) and plasmas (UC3M), thermionic materials (IKTS), tethers and mechanisms (UniPD), electric propulsion (TUD), solid state devices (ATD) and space product development (SENER). The European Commission invited the E.T.PACK consortium to sign the Grant Agreement of a subsequent EIC Transition project named E.T.PACK-F (Flight), that will increase the TRL from 4 (for the current E.T.PACK) up to 8. A mission demonstrating the kit functionality (DKD – Deorbiting Kit Demonstration) is planned for 2025 [40], just after E.T.PACK-F will be completed, within the framework of a launch agreement signed during IAC 2021 in Dubai between SENER Aeroespacial and Rocket Factory Augsburg.

The University of Padova (UniPD) is one of the partners of the E.T.PACK and E.T.PACK-F projects consortia, both coordinated by the Universidad Carlos III de Madrid. The team at UniPD was tasked with the validation of a certain number of enabling technologies for the deorbit kit including the development of software tools to study the EDT performance and dynamical behaviour and the realization and test of hardware mock-ups and prototypes. In the remainder of this paper, the EDT technology and the E.T.PACK project are presented in Section 2. The roadmap of the University of Padova activities to validate the enabling technologies for the DKD mission is introduced in Section 3. Lastly, the software development and experimental activities are presented in Sections 4 Software tools for EDT, 5 Experimental activities.

Section snippets

Electrodynamic tethers and the E.T.PACK project

A modern EDT consists of a long conductive bare tether connecting two spacecraft, usually a host spacecraft and a tip mass. Referring to Fig. 1, electrons are passively captured by the bare tether from the ionospheric plasma and are reemitted back into the plasma by using either an active electron emitter (cathode) or a segment of the tether made of a low-work-function material using thermionic emission and photoelectric effects [41,42]. The electric current that flows in the tether interacts

University of Padova roadmap to validate EDT technologies

The contributions of the University of Padova (UniPD) to the E.T.PACK project are several and include both software development for EDT applications, hardware development and laboratory testing. Fig. 2 shows the roadmap of the UniPD activities to validate the technologies on which the DKD prototype is based. In the next sections, the main steps of the roadmap are described in detail.

Deployment trajectory design tools

The deployment dynamics of a tethered system is highly nonlinear [[60], [61], [62]] and the Coriolis accelerations acting on the tip masses leads to a configuration of the tethered system with libration amplitudes that are typically in the range 40°–50° if uncontrolled [63]. Consequently, the first step in the design of a deployment trajectory is to compute tether length and velocity time profiles that bring the system from given initial conditions to a small libration amplitude at the end of

Experimental activities

To validate the main technologies involved in the E.T.PACK project, a series of experiments was performed at the University of Padova. The test activity was performed in the SPARTANS facility laboratory [76,77], consisting of a 2-m x 3-m zero-friction testing table with a free floating module (Fig. 13). The Module is composed of a translational module and a mock-up of a tether deployer and its motion is tracked by a set of IR cameras pointing to spherical IR markers placed on the module. The

Conclusions

This article explains the roadmap of the activities conducted at the University of Padova to validate the technologies on which the Deorbit Kit Demonstrator (DKD) prototype of the E.T.PACK project is based. The contributions include software development for Electrodynamic Tether (EDT) applications, hardware development and laboratory testing that validate experimentally the technologies required in the phases of deployment and deorbiting of an EDT system.

Concerning software development, two

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by European Union's Horizon 2020 Research and Innovation Programme under Grant Agreement No. 828902 (3 M€ E.T.PACK Project).

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