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1. Research Topic:

My name is Natalia Ortiz Gómez. I am an Early Stage Researcher within the Stardust Programme and I am doing a PhD at Southampton University. My research focuses on the analysis of rotational dynamics of space debris objects and a de-tumbling method based on eddy currents. Existing active debris removal methods which require physical contact with the target object have limitations due to the tumbling of space debris objects. A de-tumbling phase prior to the capturing phase may be necessary and the de-tumbling of these metallic objects inducing eddy currents is a viable option which may help ADR processes. In addition, this area of research has been hardly unexplored and there is a need for both numerical simulations as well as experimental validation, which are the focus of my investigation at Southampton University.  

Main Results:

The first part of my research has focused on development of a new mathematical model of the eddy currents based on a new tensor called the `Magnetic Tensor'. The presented theory provides with the means to reach a direct formula to evaluate the torques due to the eddy currents and avoid solving the classical Poisson equation with Neumann boundary conditions in the integration process of Euler's equation. The existing analytical solutions in the literature on the eddy current torque for some canonical shapes are only valid for specific orientations of the angular velocity vector and magnetic field vector in the body fixed frame. Instead, the Magnetic tensor generalizes these solutions for any type of body and its relative orientation with respect to the field. 
The final formula reached for the eddy current torque induced on an object by a homogeneous and constant field has the form

`vec T = (MvecOmega) xx vec B`

where `M` is the magnetic tensor of the object, `vec B` is the magnetic field and the `vecOmega` vector indicates the source of generation of the eddy currents, which could be for instance time-varying magnetic fields, relative linear or angular velocity.

A complete description of this mathematical model can be found in [1,2].

Experimental validation of this theory has been carried out at Airbus Defence & Space in Stevenage (UK) during one of my secondments. Several aluminum metallic objects were tested with a uniaxial rotation subject to a homogeneous magnetic field. The metallic pieces were suspended from a strut with a tungsten torsion wire which allowed generating an angular velocity with a very low friction. The eddy torques decrease the angular velocity of the objects and the experimental and theoretical time of decay were compared.
The second part of my research concentrates on the systems engineering design of a de-tumbling method based on eddy currents for space debris objects called ‘Eddy Brake’. Given the fact that space debris objects contain a large amount of metallic components, this allows togenerate eddy currents. The eddy currents phenomenon is a dissipative process that transforms the kinetic energy into heat due to Joule’s law. This offers the possibility to decrease the rotational rates of the target without any mechanical contact. The de-tumbling process is carried out actively by a chaser spacecraft which has a magnetic coil on board based on high temperature superconducting (HTS) wires. This design allows placing the chaser spacecraft several meters away from the target object. Moreover, the magnetic interactions between the two objects generate forces and torques on the chaser spacecraft which need to be counteracted. Consequently, the GNC subsystem plays an important role in the de-tumbling process [2,3].
Moreover, I am involved in the systems engineering design of an active debris removal mission of an Ariane-5 rocket body (AGORA) within the Stardust program. The de-tumbling process based on eddy currents is integrated in this mission which has given me the opportunity to understand the major interactions with the rest of the subsystems of the chaser spacecraft and its design for a real object [4,5].
[1] Ortiz Gómez, N., Walker S.J., ‘Eddy Currents applied to De-tumbling of Space Debris: Analysis and Validation of Approximate Proposed Methods’, Acta Astronautica, 12th, Volume 114, Pages 34-53, April 2015.
[2] Ortiz Gómez, N., Walker S.J., ‘Earth's Gravity Gradient and Eddy Currents Effects on the Rotational Dynamics of Space Debris Objects: Envisat Case Study’, Advances in Space Research, 7th January 2015.
[3] Jankovic, M., Kumar, K., Romero Martín, J., Ortiz Gómez, N., Kirchner, F., Topputo, F., Vasile, M., Walker, S. J.I., ‘Autonomous robotic system for active debris removal: requirements, state-of-the-art and concept architecture of the rendezvous and capture (RVC) control system’, 5th CEAS Air & Space Conference (Council of European Aerospace Societies), The Netherlands, 2015.
[4] Jankovic, M., Kumar, K., Ortiz Gómez, N., Romero Martín, J.M., Kirchner, F., Topputo, F., Walker, S.J.I., Vasile, M., ‘Spacecraft concept for active de-tumbling and robotic capture of Ariane rocket bodies.’, ASTRA Conference, ESTEC The Netherlands, May 2015.
[5] Kumar, K., Ortiz Gómez, N., Jankovic, M., Romero Martín, J.M., Topputo, F., Walker S.J., Kirchner, F., Vasile, M., ‘Agora: Mission to demonstrate technologies to actively remove Ariane rocket bodies. ’, Proceedings of the International Astronautical Congress IAC-15,A6,6,1,x28851, Jerusalem, October 2015.


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