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Electrodynamic Tether Technology for Passive Consumable-less Deorbit Kit

Periodic Reporting for period 1 - E.T.PACK (Electrodynamic Tether Technology for Passive Consumable-less Deorbit Kit)

Reporting period: 2019-03-01 to 2020-02-29

Current in-space propulsion technologies, such as chemical and electrical thrusters, work under the physical law of action-reaction. Consequently, the payload mass and mission lifetime are penalized by the need of propellant. E.T.PACK envisages a new era enabled by a free-of-wet-mass device named Low Work function Tether (LWT) that operates under a different principle. Instead of carrying propellant, LWTs produce a propulsive force by taking advantage of the natural space environment, which includes the geomagnetic field, the ambient plasma and the solar radiation. A LWT is a long tape of a conductor, for instance aluminium or titanium, coated with a thin layer of a special material that emits electrons when illuminated by the Sun. The typical length, width and thickness of the tether are few kilometres, a couple of centimetres and tens of microns. The LWT is packaged in a reel onboard the spacecraft and, once activated, it is deployed along the local vertical where the gravity gradient keeps it taut. Thanks to a passive electrodynamic effect, a tether segment captures electrons from the ambient plasma and the complementary segment emit them back through the thermionic and photoelectric effects, thus yielding to a steady electric current and, consequently, a Lorentz force. In Low Earth Orbit (LEO), the Lorentz force is a drag that produces the re-entry (deorbit) of the spacecraft while giving power for on-board use. This mode of operation is called generator mode. Neither propellant nor power supply are needed, and the operation of the tether is fully passive. If a power supply is used to reverse the natural direction of the electric current, then the tether operates under the so-called thruster mode and the spacecraft is re-boosted. E.T.PACK’s overall goal is to develop a Deorbit Kit based on LWT technology with TRL 4.

The potential impact for society is supported by a fundamental characteristic of LWTs: they are reversible devices that convert orbital energy into electrical energy and vice versa without using any consumable. Such a property is key for several space applications. For instance, a LWT in generator mode can be used for deorbiting spacecraft at the end of life, thus contributing to solve the space debris problem that is one of the most important space challenges for the next decades. LWTs in thruster mode can provide indefinite station keeping, which is of particular importance for satellites orbiting at very low orbit and the International Space Station that requires about 10 tons of propellant per year. The opening of new horizons for science and technology is also envisaged because LWTs can be used as scientific instruments and in missions to planets with magnetospheres as Jupiter.
Abundant dissemination and communication activities were carried out, including the organization of the Sixth International Conference on Tethers in Space.

A novel Vlasov-Poisson solver to study tape-like LWTs has been developed and the computational cost of UC3M Vlasov-Poisson solver for cylindrical LWTs has been reduced a factor 80. The preparation of a database with characteristic (I-V) curves is currently in-progress. UC3M tether flight simulator BETsMA v2.0 has been updated with new models to simulate LWTs (electrical and thermal), include more perturbation forces, and reduce its computational cost. A novel software was developed to simulate tether deployment. An intensive simulation campaign to determine the performance and the requirements of LWTs was carried out using BETsMAv2.0 and FLEX.

Direct synthesis of C12A7 powder was done. The second batch of the powder synthesis was used to prepare a screen printable paste, which was printed on the metal/contacting layer on the Titanium substrates. The LWT will be realised by screen printing of a contacting layer at first and directly above the C12A7 layer on the thin Ti substrate. The development of a metal layer made by Ti-based alloys for contacting was started with two different composition. The characterisation of these two screen printed, contacting layers had very good adhesion on the Ti substrates. The C12A7 paste was printed on the brazing layer and co-fired in one heating step.

Three different deployer configurations were designed at the conceptual level. The preliminary tradeoff analysis identified two configurations with different designs that were developed in parallel and analysed for their modal frequencies and response to static and dynamic loads. Two options for the separation system were also traded off before settling on a cold-gas system to provide the initial separation velocity. The two deployer configurations have two radically different geometries with different footprints and, consequently, they could satisfy different accommodation requirements on potential host spacecraft. Bread boards of the two deployer configurations were also built and preliminary functional tests were carried out.

The heaterless hollow cathode with a C12A7 electride insert has been tested over a wide range of parameters with different types of gases and up to 5 A have been emitted. The hollow cathode thruster was designed and it is currently being manufactured. Several designs of electron field emitters based on carbon nanotubes were developed and tested. The latest version was successfully operated for more than 500 hours without degradation. A semi-analytical calculation model was developed to design a thermionic emitter, which is currently being manufactured. C12A7:e- electride thermal conductivity and thermal expansion coefficients have been determined for the PETE device, which has been designed. Electride deposition with Pulsed Laser Deposition technique was successfully proven.

Work has been focused on the definition of DK Target Requirements. Initial requirements gathered from ESA and European primes prior to the beginning of the project has been iterated, refined and consolidated during the preliminary design of the system. Target requirements have been delivered within deliverable D6.1. The preliminary DK system design has been prepared and will be used as starting point for the DK subsystems specifications.
E.T.PACK will provide the first proof of concept for LWTs. The Deorbit Kit will be designed with a set of requirements that are compatible with a follow-up demonstration flight and aligned the needs of the industry and the space agencies. The kit will include an onboard computer, hardware elements for telemetry and telecommand, and Attitude and Orbital Control System (AOCS) and involves breakthroughs in essential hardware elements, like the LWT and the deployment mechanism, and software (tether flight simulator and tether-plasma interaction models). Additionally, three non-essential elements related with the C12A7:e- will be developed: A hollow cathode emitter, a hollow cathode thruster, and a Photon Enhanced Thermionic Emission Device to harness Sun’s light and heat to generate electricity. The LWT can be the game-changing technology needed by the sector to break the vicious cycle of space debris. Since the operation of a LWT in generator mode does not need wet mass and power, and it is fully passive, a deorbit kit based on LWT would be light, cheap, and easy to implement. Legislative authorities could change the deorbit guidelines to being mandatory without eroding the competitiveness of their own industry, thus opening a new market on deorbit technologies.
Deorbit Kit