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Electrodynamic Tether Technology for Passive Consumable-less Deorbit Kit

Periodic Reporting for period 3 - E.T.PACK (Electrodynamic Tether Technology for Passive Consumable-less Deorbit Kit)

Période du rapport: 2021-11-01 au 2022-11-30

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 to inform scientists, policy-makers and general public about the project (information available at www.etpack.eu).

Novel Vlasov-Poisson solvers to study tape-like LWTs have been developed. A database with characteristic (I-V) curves has been computed and made public. UC3M tether flight simulator BETsMA v2.0 has been updated with electrical and dynamic models. Algorithms to make optimal mission design with different types of tethers and in active and passive modes were developed. 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. A code for the simulation of the attitude dynamics during the deployment phase was also developed and used extensively to find the requirements of the avionics.

Direct synthesis of the low work function material C12A7:e- as powder, printable pastes with C12A7:e- powder and brazeable metal fillers were prepared. The printing and firing of the functional paste on a metal substrate for contacting C12A7:e- as low work function material on a tether tape and generating the LWT were done. An intensive testing campaign of the LWT coating regarding electron emission, optical, mechanical, and thermal properties as well as under space condition was carried out. Additionally, the coating of an insulated tether segment was investigated, as well as the joint of different tether segments. A 50cm long LWT demonstrator was manufactured and its industrial production assessed.

After performing a detailed design of the deployment mechanism (DM) focussed on the future IOD of the deorbit device, the mechanical parts of the DM were manufactured, including the cold gas assembly manifolds.
Gears and motors were procured and assembled in the DM. The manufactured prototype underwent key functional tests at SENER and then it was tested at UNIPD and assembled with the Cold Gas System. The manufacturing of the DM prototype is complete of the mechanical parts, motors, motor drivers, on-board computer, and Cold Gas system.

The heaterless hollow cathode electron emitter (HCE), which is the baseline emitter for the IOD mission, and the expellant system were further optimized to meet the requirements for current emission, voltage operation (about 20 to 30 V EMF provided by tether), power consumption, lifetime, mass flow and ignition cycles. An extensive test campaign was carried out to demonstrate the fullfillment of the requirement. The electronics of the HCE was developed at a breadborad level. Different prototypes of the PETE Device were manufactured and tested.

A complete Engineering model of the DK, including the tether, the deployment mechanism, the cold gas system, the Electron Emitter elements, the control board, the attitude control and the remaining avionics elements, has been design, manufactured, integrated and extensively tested. It is a 12U and 24 kg device that can host a 500m long bare tether. The verification activity concluded that the DK has reached TRL4 at system level. TRL6 has been reached by avionics, structure and tether, whereas TRL4 was reached for the cold gas system and the hollow cathode electronics.
E.T.PACK provided a first proof of concept for a LWT demonstrator. In oparallel a Deorbit Kit based on a bare tether and a hollow cathode emitter was designed with a set of requirements that are compatible with a follow-up demonstration flight and aligned with the needs of the industry and the space agencies. The kit includes 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 deployment mechanism, and software (tether flight simulator and tether-plasma interaction models). Additionally, three non-essential elements related with the C12A7:e- were developed: an electron field emitter, a hollow cathode thruster, and a Photon Enhanced Thermionic Emission Device. An unexpected result of the project was the introduction of a bare-photovoltaic tether. The deorbit device can be the game-changing technology needed by the sector to break the vicious cycle of space debris. 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
Deorbit Kit with E.T.PACK Mascot