Erik A. Hogan

Relative Motion Control For Two-Spacecraft Electrostatic Orbit Corrections

The charged relative-motion dynamics and control of a two-craft system is investigated if one vehicle is performing a low-thrust orbit correction using inertial thrusters. The nominal motion is an along-track configuration where active electrostatic charge control is maintaining an attractive force between the two vehicles. In this study the charging is held fixed and the inertial thruster of the tugging vehicle is controlled to stabilize the relative motion to a nominal fixed separation distance. Using a candidate Lyapunov function, the relative orbit control law is shown to be asymptotically stable. Analysis of the control system gains is performed in order to achieve a desired settling time and damping ratio. The effects of uncertainties in the vehicle charges are also examined. Using numerical simulation, the performance of the proposed control system is investigated for a formation in geosynchronous earth orbit.

Three-Axis Attitude Control Using Redundant Reaction Wheels with Continuous Momentum Dumping

A description of an attitude control system for a three-axis stabilized spacecraft is presented. A globally stabilizing nonlinear feedback control law is derived that enables tracking of an arbitrary time-varying reference attitude. This new control incorporates integral feedback while avoiding any quadratic rate feedback components. A redundant cluster of four or more reaction wheels is used to control the spacecraft attitude, and three magnetic torque rods are used for purposes of continuous autonomous momentum dumping. The momentum dumping strategy can employ general torque rod orientations, and it is developed to take advantage of a redundant set of reaction wheels.

Characterization of the Creep Deformation and Rupture Behavior of DS GTD-111 Using the Kachanov–Rabotnov Constitutive Model

Creep deforrnation and rupture experiments are conducted on samples of the Ni-base superalloy directionally solidifies GTD-111 tested at temperatures between 649°C and 982°C and two orientations (longitudinally and transversely oriented). The secondary creep constants are analytically determined from creep deformation experiments. The classical Kachanov―Rabotnov model for tertiary creep damage is implemented in a general-purpose finite element analysis (FEA) software. The simulated annealing optimization routine is utilized in conjunction with the FEA implementation to determine the creep damage constants. A comparison of FEA and creep deformation data demonstrates high accuracy. Using regression analysis, the creep constants are characterized for temperature dependence. A rupture prediction model derived from creep damage evolution is compared with rupture experiments.

An Efficient Method for the Optimization of Viscoplastic Constitutive Model Constants

Constitutive modeling has proven useful in providing accurate predictions of material response in components subjected to a variety of operating conditions; however, the high number of experiments necessary to determine appropriate constants for a model can be prohibitive, especially for more expensive materials. Generally, up to twenty experiments simulating a range of conditions are needed to identify the material parameters for a model. In this paper, an automated process for optimizing the material constants of the Miller constitutive model for uniaxial modeling is introduced. The use of more complex stress, strain, and temperature histories than are traditionally used allows for the effects of all material parameters to be captured using significantly fewer tests. A graphical user interface known as uSHARP was created to implement the resulting method, which determines the material constants of a viscoplastic model using a minimum amount of experimental data. By carrying out successive finite element simulations and comparing the results to simulated experimental test data, both with and without random noise, the material constants were determined from 75% fewer experiments. The optimization method introduced here reduces the cost and time necessary to determine constitutive model constants through experimentation. Thus it allows for a more widespread application of advanced constitutive models in industry and for better life prediction modeling of critical components in high-temperature applications.Copyright

MODELING THE TEMPERATURE-DEPENDENCE OF TERTIARY CREEP DAMAGE OF A DIRECTIONALLY SOLIDIFIED NI-BASE SUPERALLOY

Directionally solidified (DS) Ni-base superalloys have become a commonly used material in gas turbine components. Controlled solidification during the material manufacturing process leads to a special alignment of the grain boundaries within the material. This alignment results in different material properties dependent on the orientation of the material. When used in gas turbine applications the direction of the first principle stress experienced by a component is aligned with the enhanced grain orientation leading to enhanced impact strength, high temperature creep and fatigue resistance, and improve corrosion resistance compared to off axis orientations. Of particular importance is the creep response of these DS materials. In the current study, the classical Kachanov-Rabotnov model for tertiary creep damage is implemented in a general-purpose finite element analysis (FEA) software. Creep deformation and rupture experiments are conducted on samples from a representative DS Ni-base superalloys tested at temperatures between 649 and 982°C and two orientations (longitudinally- and transversely-oriented). The secondary creep constants are analytically determined from available experimental data in literature. The simulated annealing optimization routine is utilized to determine the tertiary creep constants. Using regression analysis the creep constants are characterized for temperature and stress-dependence. A rupture time estimation model derived from the Kachanov-Rabotnov model is then parametrically exercised and compared with available experimental data.Copyright

Impacts of Hot Space Plasma and Ion Beam Emission on Electrostatic Tractor Performance

A recent proposed technique for geostationary debris mitigation is the electrostatic tractor. The tug vehicle approaches a debris object to within 20 m and emits a focused electron beam onto it. This results in a negative charge on the debris and a positive charge on the tug vehicle. Used in conjunction with low thrust, the electrostatic force is used to tow a debris object into a disposal orbit. In this paper, the impacts of geomagnetic storm activity on the charging of tug and debris are considered. The influence of electrons emitted from the debris (photoelectrons and secondary electrons) on tug charging is also considered. Both of these phenomena yield improved electrostatic tractor performance. The simultaneous emission of an electron and ion beam by the tug is also considered to improve tractor performance and enable charge transfer for scenarios where it fails when only an electron beam is used. The theoretical maximum electrostatic force that is possible with simultaneous emission is computed, and the results indicate that emitting both an electron and ion beam enables smaller tug vehicles to tow larger objects that could not otherwise be towed with only an electron beam.

Attitude Parameter Inspired Relative Motion Descriptions for Relative Orbital Motion Control

This paper describes the development of novel relative orbital motion descriptions for a two craft formation, defined by a separation distance and relative orientation parameter set. These descriptions allow for the use of separation distance as a state parameter for the relative motion, while avoiding singularity issues that plague spherical coordinate descriptions. Instead, Euler parameterlike or modified Rodrigues parameterlike coordinates are employed to describe the relative orientation. Equations of motion are developed that allow propagation of these new descriptions for the case of a circular reference orbit and small separation distances between the craft. These equations of motion are derived from the Clohessy–Wiltshire equations. Feedback control laws are developed to stabilize the relative motion between the craft. Numerical simulation is used to compare the newly developed relative motion descriptions with inertial equations of motion. The results validate these new descriptions as a practica...

General High-Altitude Orbit Corrections Using Electrostatic Tugging with Charge Control

The use of an electrostatic force to perform general orbit corrections on a passive geosynchronous space object is investigated. Using inertial thrusters, a space tug approaches and settles into a piecewise fixed relative location with respect to a deputy object that needs to be towed to a new orbital location. Once in place, an electrostatic force is created between the two bodies using noncontact charge transfer, enabling the tugging craft to perform an inertial thrusting maneuver to modify the deputy orbit without physical contact. An open-loop analytical performance study is performed where variational equations are used to predict how much general orbital elements may be changed using this electrostatic force over one orbital period for a satellite at geosynchronous altitude. In contrast to earlier work, eccentric orbits and plane changes are also considered. The thrust direction issues associated with repositioning the tug craft during orbit modifications to achieve desired tugging force are also in...