Researchers have proposed a new design for a partial space elevator that uses an electrodynamic tether to move cargo between orbits without conventional propellant. A study published in Space: Science & Technology details the system, which leverages Earth's magnetic field to generate thrust, offering a potentially clean and cost-effective method for in-space transportation.
The proposed system, called the PSE-EDT (Partial Space Elevator with Electrodynamic Tether), aims to solve the significant energy challenges associated with moving heavy payloads over long distances in space. By using an electrically charged tether, the system can accelerate or decelerate a climbing vehicle, providing the necessary energy for orbital maneuvers while also stabilizing the entire structure.
Key Takeaways
- A new partial space elevator concept uses an electrodynamic tether (EDT) for propellant-free propulsion.
- The system generates Lorentz force by interacting with Earth's magnetic field to move a cargo climber.
- This design aims to reduce the cost and complexity of transporting materials to and from space structures.
- A nonlinear model predictive controller (NMPC) was developed to ensure stable upward and downward transport.
- Simulations confirmed the system's ability to move cargo while controlling tether oscillations.
A New Approach to In-Space Transportation
Partial space elevators are considered a promising technology for efficiently moving cargo to large space structures or between different orbits. They typically consist of a main satellite connected by a long tether, sometimes hundreds of kilometers long, to a secondary satellite or anchor mass.
A climber vehicle moves along this tether to transport payloads. However, a major challenge has been powering the climber's long journey and controlling the complex dynamics of the system. Previous models often assumed the climber could move at a desired speed without a defined power source, which is not practical for real-world applications.
What is an Electrodynamic Tether?
An electrodynamic tether is a long, conducting wire extended in space. As it moves through a planet's magnetic field, it generates a voltage along its length. By running an electric current through the tether, it experiences a Lorentz force, which can be used as a form of propellantless propulsion to either boost or lower the system's orbit.
The new PSE-EDT system addresses this problem by integrating an electrodynamic tether. In this design, the climber is connected to an end body by a conductive tether. Plasma contactors at each end allow the system to collect and emit electrons from the surrounding ionosphere, creating a continuous electric current.
How the PSE-EDT System Works
The core of the PSE-EDT's function is the generation of Lorentz force. This force is produced by the interaction between the electric current flowing through the tether and Earth's magnetic field. By carefully controlling the direction and magnitude of the current, operators can use this force as a thruster.
Operating in Two Modes
The system is designed to operate in two distinct modes for versatile cargo transport:
- Thrust Mode: For upward movement, the electric current is directed to generate a Lorentz force that pushes the climber along the tether, accelerating it to a higher orbit. This mode effectively does positive work on the climber, providing the kinetic and potential energy needed for ascent.
- Drag Mode: For downward movement, the current direction is reversed. The resulting Lorentz force acts against the system's orbital motion, creating a braking effect that lowers the climber to a lower orbit in a controlled manner.
According to the research paper, this method offers significant advantages. It is structurally simple, requires no chemical propellant, and can operate repeatedly for long-term missions, making it a sustainable and cost-effective solution.
Propellant-Free Propulsion
The main benefit of the PSE-EDT is its ability to generate thrust without consuming fuel. This eliminates the need for heavy propellant tanks and complex thruster systems on the climber, reducing the overall mass and cost of space transportation missions.
Controlling Complex Dynamics
Moving a heavy climber along a kilometer-scale tether introduces significant dynamic challenges. The Coriolis effect, caused by the climber's motion, can induce oscillations, known as librations, in the tethers. These uncontrolled movements can destabilize the entire system and threaten the mission.
While traditional tension control can manage in-plane (forward and backward) oscillations, it is ineffective against out-of-plane (sideways) movements. The electrodynamic force, however, can suppress these out-of-plane librations, providing full three-dimensional control of the system without needing additional thrusters.
The Role of the NMPC Controller
To manage these complex interactions, the researchers developed a Nonlinear Model Predictive Control (NMPC) strategy. This advanced control system continuously calculates the optimal adjustments for both the tether current and tension.
The NMPC's goal is to transport the climber to its destination efficiently while keeping the system stable. It accounts for multiple constraints, such as the maximum allowable current, tether tension limits, and the desired final position of the climber. The controller's design prioritizes maximizing the power generated by the electrodynamic force to accelerate the climber's movement.
Successful Simulation Results
The feasibility of the PSE-EDT concept and the effectiveness of the NMPC controller were tested through two detailed numerical simulations: an upward-moving case and a downward-moving case. The system parameters for the simulation included a 20-kilometer initial tether length and a total climber and end-body mass of 200 kg.
Upward Cargo Transport
In the upward-moving simulation, the goal was to move the climber from a position 90% down the tether to just 10% from the main satellite. The simulation ran for a duration corresponding to approximately 13 hours (dimensionless time τf=50).
"The simulation results manifest that the appealing clean technology is feasible to long space cargo transportations without the requirements of chemical propellant and additional thrusters," the authors state in the paper.
The results showed that the NMPC controller successfully guided the climber to its target position. Importantly, it also suppressed both pitch (in-plane) and roll (out-of-plane) angle oscillations, keeping them within safe, predefined limits. The tether current was actively managed throughout the climb and reduced to zero once the final state was reached.
Downward Cargo Transport
The downward-moving case simulated transporting cargo from a higher position (10% from the main satellite) to a lower one (90% from the main satellite). This mission was completed in a shorter duration of about 7.9 hours (dimensionless time τf=30).
Similar to the upward case, the controller effectively managed the system's dynamics. The tether lengths were adjusted smoothly until they reached their target values, and all libration angles were stabilized. This confirmed the system's capability to perform controlled de-orbiting maneuvers for cargo return missions.
Future of Space Transportation
The research presents a compelling case for the PSE-EDT system as a clean, sustainable, and low-cost technology for future in-space logistics. By combining the advantages of space elevators and electrodynamic tethers, this novel concept could revolutionize how we build and maintain large structures in orbit, such as space stations, fuel depots, or manufacturing facilities.
The authors acknowledge that the current model is based on simplified assumptions, such as massless tethers and a uniform geomagnetic field. Future work will involve creating more complex models that account for tether flexibility, satellite attitude, and a more accurate representation of Earth's magnetic field to further refine the system's design and control strategies.