Steven G. Tragesser

Aerocapture with a Flexible Tether

In previous work, the authors have demonstrated that the aerobraking tether, modeled as a rigid rod, could achieve aerocapture at any atmosphere-bearing planet in the solar system for less mass than the corresponding propellant of a typical retro-rocket system. In this paper, the great promise of the aerobraking tether is further explored by developing the equations of motion for the analysis of flexible tether behavior during the maneuver. A standard Lagrangian approach is taken with the tether modeled as a chain of linked rigid rods. Since an arbitrary number of rods can be used, the flexible behavior can be approximated to an arbitrary degree of accuracy. The results indicate that the aerobraking tether concept remains feasible when flexibility effects are included in the model.

Static Formations Using Momentum Exchange Between Satellites

Feasible operations of satellite formations are dependent upon an efficient way to affect the relative motion. One possible means of formation stationkeeping is for satellites to exchange momentum in the form of radiation or mass. This approach could potentially provide a mutually repulsive force between satellites without the expenditure of propellant. This paper characterizes equilibria positions in the rotating orbital frame for an arbitrary number of satellites subjected to equal and opposite repulsive force between specifiable satellite pairs. Equilibrium configurations are found that allow arbitrary dispersion in the plane perpendicular to nadir. Stability analysis of these equilibria show stable out-of-plane motion and unstable in-plane motion. A strategy is developed to stabilize the system and to reconfigure the formation geometry. Finally, a simulation including Earth oblateness demonstrates the dynamic feasibility of this method of orbit control.

Open-Loop Spinup and Deployment Control of a Tether Sling

Atether slingholds tremendous promise as an energy-storage device, permitting space transportation systems that are capable of injecting a large number of payloadswith low incremental costs. This paper developsmotor and tether deployment controls to go from rest to a desired final tether length and end velocity. Oscillations in the tether deployment angle are minimized to avoid additional tension in the tether. Open-loop control laws are developed for both power-unlimited and power-constrained cases. For unlimited power, the time-optimal mission profile dictates deployment at large angular velocities. The opposite is true when power is constrained: spinup and deployment time are reduced by deploying at very small angular velocities.

Dynamics and Control of a Tether Sling Stationed on a Rotating Body

This paper analyzes the dynamics of a tether sling stationed on a rotating moon or asteroid. Emphasis is placed on the out-of-plane motion induced when the facility is located at a nonpolar latitude. Although significant out-of-plane oscillations are found in the open-loop behavior, the motion stabilizes about some finite amplitude. The mechanism for this stabilization is analyzed and the out-of-plane oscillation is characterized with respect to system parameters. Further stabilization of the system is shown to be possible by modulating the tether length, maintaining the reusable nature of the system.

Dynamics of a Thrusting, Spinning Satellite with Changing Center of Mass Position

2This paper considers a thrusting, spinning satellite with an asymmetric fuel draw from the tanks. As fuel is expended through the thruster, both the angular momentum of the satellite and the location of center of mass change. The results show that the amplitude of the spin-axis precession is not significantly impacted due to the changing center of mass unless fuel flow rate is impractically large. The resultant forces on solar array booms are also computed, and compared to those of a satellite with two operational, symmetric fuel tanks, in order to determine the safety of performing a thrusting maneuver on this type of satellite. The migration of the center of mass away from the solar array centerline results in a small shear force that is not present in the nominal system.

Dynamics and Control of a Tether Sling on a Rotating Body

Previous studies on the design of a tether sling have indicated that the facility must be located at the poles of the moon or asteroid so that the angular momentum vectors of the moon and tether are aligned. This paper investigates whether the motion induced by locating the facility away from the poles is controllable by only actuating the tether length. The dynamic model for a general station location on a rotating body is developed and simulated to show that the uncontrolled system does exhibit large out-of-plane oscillations. A linear quadratic regulator is developed which actuates the tether length rate to stabilize the system, preventing the payload from crashing into the surface of the moon as well as reducing tension spikes due to libration. The control authority is achievable using only electrical power (versus propellant), so the reusable nature of the system is maintained. This paper shows that it is dynamically feasible to station this simple, reusable launch system at any location on the moon.

LABORATORY DEMONSTRATION OF A TETHER SLING FOR SPACE TRANSPORTATION

A tether sling holds tremendous promise as an energy storage device, permitting space transportation systems capable of injecting a large number of payloads with low incremental costs. The Space Grant student organization at the University of Colorado is developing a scaled-down laboratory model of the tether sling to demonstrate the feasibility of the basic concept and study the engineering challenges of the system. Like the actual system, the laboratory demonstrator consists of an electric motor, a tether with the payload attached to the end of the tether and a platform that transfers the energy from the motor into the spinning tether. This paper describes the design of the bearing assembly, motor and drive train, deployment mechanism, tether angle sensor, and the digital control and data acquisition system. Data from the laboratory demonstrator is used to validate the dynamic modeling and control law development.