Coulomb Craft Test Bed

Coulomb image forces are generated along the line-of-sight vector between two spacecraft if they aquire an electrostatic charge. Spacecraft will naturally charge up to non-zero potential due to their interaction with the plasma environment and exposure to sunlight. Depending on the plasma density and temperature levels, as well as the intensity of the impinging sunlight, the craft can aquire positive or negative charge levels. The naturally occuring charging of GEO satellites is strong enough to cause micro- to milli-Newton levels of disturbance forces if the craft are dozens of meters apart. These disturbances are strong enough to cause hundreds of meters of departure motions if left uncontrolled.

In 2000 King and Parker at Michigan Tech University began to investigate how these electrostatic forces could be exploited to create innovative non-Keplerian relative orbits. With Coulomb spacecraft the electrostatic potential of a craft is continuously controlled through the active emission of either positively or negatively charged particles. This emmision process is very similar to how ion engines function. However, the mass flow is orders of magnitude smaller. Thus, the impulsive force exterted onto the craft due to the charge emmsion is neglitible. Instead, the dominant force acting on 2 neighboring charged craft is the electrostatic or Coulomb force.

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There have been numerous analytical research project undertaken, and continue to be performed, to investigate how the Coulomb force between spacecraft can be exploited. The AVS Lab is also investigating how to build a test bed to study the charged relative motion of small unmanned vehicles in a terrestial environment. The first stage is looking into building a one-dimensional non-conducting hover track. The goal is to develop a track which can float a conducting, charged craft whose electrostatic force interacts with that of a stationary charged craft next to the track. Due to the small magnitude of the Coulomb forces, the track should be as level as possible to provide a frictionless environment. The first stage will demonstrate that a charged vecicle can be suspended and actuated in this manner. The charging will occur from an external low-power, high-voltage power supply. The space-based charged emmision devices used to servo the spacecraft charge level do not survive operating in atmospheric conditions. Future stages will then be able to investigate adding on-board electronics, closing the control loop externally or internally, testing relative motion sensing methods, as well as investigating the charged relative motion of very close objects where their electrostatic field will induce a charge distribution on the neighboring craft.

Current research is also investigating fast force and torque modeling techniques that can account for the electrostatic force field about a general spacecraft shape. Called the Multi-Sphere-Method (MSM), the general body is approximated through a series of carefully placed spheres. Optimization methods are employed to determine the ideal location and size of these spheres such that the electrostatic force field is approximated even with very close proximity operations.

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