by Nikko James and Joeseph Figura
AeroAstro 16.62x, Experimental Projects Lab, is a class where students plan and execute an experiment in aerospace engineering, performing a deep dive into a topic of their interest. As seniors in this class, we decided to tackle the problem of space debris. We conducted an experiment that used magnetic fields to de-spin a rotating aluminum cylinder, demonstrating a technique that could be used by a spacecraft to simplify docking with a target.
Certain orbits around Earth are littered with defunct satellites, spent rocket bodies, and other uncontrolled objects. These uncontrolled objects pose a collision hazard to other objects. The risk of an impact at 18,000 miles per hour poses a hazard to active missions: the collision may puncture pressurized tanks or compartments, or may damage mission critical systems. The threat is so great that the International Space Station conducts a debris avoidance maneuver if the probability of collision is greater than 1 in 100,000. Even collisions between inactive objects pose a risk; breakup of debris creates more debris.
The huge growth of the commercial space industry, especially in the domain of small satellites, has spurred research into how to avoid, mitigate, and clean up space debris. This research often focuses on spent rocket bodies: the large, robust upper stages of rockets. The prevailing strategy proposed to deal with rocket bodies is to dock with the body, attach a deorbiting thruster, and use the thruster to move it to a lower orbit. The lower orbit will degrade more quickly, so the body will burn up in the atmosphere. However, docking with the rocket body can be problematic because these objects can be rotating at up to half a revolution per second, especially in high altitude orbits. Also, it generally requires the use of a robotic arm to detumble the body, which introduces complexity and also risks over-torqueing the arm and disabling the servicing spacecraft. Docking with spinning debris requires complicated trajectories to match the debris’ spin, and is inherently risky. A collision during a docking attempt could create even more debris, defeating the purpose of the attempt.
We investigated a new approach to handling orbital debris: What if we could stop the object from tumbling without touching it, and then safely dock with it when rotation has ceased? We found a way to do just that, using magnetic fields. A conductor rotating in a magnetic field experiences induced currents, called eddy currents. The resulting electromagnetic drag slows down the spin of an object tumbling in a magnetic field. A debris-removal spacecraft could use a magnetic field generator to induce eddy current torques in a rocket body, stopping its spin. This spacecraft could then safely capture the rocket body and deorbit it.
Previous research has identified the possibility of using eddy current torques to detumble stationary space debris, but the method hadn’t been demonstrated with a freely-spinning object. Additionally, equations of the eddy current effect had been created, but previous experimental assessments of those equations were inconclusive. With this previous work in mind, we designed an experiment to demonstrate using magnetic fields to detumble an object, and to assess a model of the eddy current torque against experimental results.
Our experiments aimed to demonstrate eddy current detumbling in a laboratory environment. Much of the 2016-2017 academic year was spent preparing the experiment. We began by building an aluminum cylinder in AeroAstro’s Gelb Laboratory student workshop and MIT’s Central Machine Shop. This would represent a rocket body. We took this cylinder to department’s Space Systems Lab where it was set spinning on a frictionless air bearing. The air bearing sat inside a Helmholtz cage, a magnetic field generator that uses three wire coils to generate a controllable three-axis magnetic field. We wrote a MATLAB script to control the cage, sending signals to three power amplifiers that generated the rotating magnetic field. Lastly, we designed and built a suite of laser “tripwire” sensors to measure the rotation of the cylinder.
We then turned on a magnetic field that had constant strength, but rotated in the opposite direction of the cylinder, and we measured the cylinder’s rotation rate over time. The spinning magnetic field exerted a torque on the target object, eventually rendering it stationary. We also created a model of the cylinder’s rotation rate, based on previous work modeling eddy current torques and experimental measurements of drag and friction. The project aimed to discover whether the theoretical model was in line with the results of the experiment, thereby assessing the eddy current torque model’s accuracy.
After completing our calibration trials, we conducted experimental runs. We were successful in using the magnetic field to control the motion of the cylinder and bring it to rest, providing a proof-of-concept of electromagnetic detumbling. Furthermore, we found that an available prediction model did not accurately predict the experimental results, indicating the possibility that the equation for eddy current torque being examined was wrong. To respond to this, we iterated through a few rounds modifications to the model to investigate where the error came from. Parameters of the governing equations were varied to attempt to fit the model of the cylinders motion to the experimental results. The goal was investigating what parameters of the model were incorrect, and by how much, and to gain insight into why the results didn’t match the predictions. Although further analysis is necessary, our results seemed to indicate that the eddy current torque constant was inaccurate.
This research serves as a step in the development of a novel method to deal with the problem of orbital debris – a method that, one day, may be used to clean up Earth’s orbit.
Diagram depicting how a space vehicle (“Chaser”) would use a magnetic field to induce torques on tumbling “space junk” to render it stationary so de-orbiting thrusters could be attached. (Natalia Gomez/Scott Walker, University of Southampton image)
Nikko James (’17) is an AeroAstro master’s candidate, focusing on space systems engineering, following which he will commission into the U.S. Air Force and report to pilot training. He may be reached at email@example.com. Joseph Figura (’17) is interested in satellites, space systems engineering, and astronomy. He now works for the communications company OneWeb. He may be reached at firstname.lastname@example.org