YES2 Optimal Trajectories in Presence of Eccentricity and Aerodynamic Drag
- Paper number
IAC-06-D2.3.04
- Author
Dr. Paul Williams, Australia
- Coauthor
Mr. Andrew Hyslop, Delta-Utec SRC, The Netherlands
- Coauthor
Mr. Michiel Kruijff, Delta-Utec, The Netherlands
- Year
2006
- Abstract
Tethered satellite systems have been flown successfully in space on a number of occasions. The potential for tethered satellite systems to provide exciting alternatives to conventional rocket propulsion has seen a large amount of research undertaken to develop the technology. It is unfortunate that the initial TSS-1 flight was not deployed correctly and this “failure” has caused concern over the reliability of tethers for future applications. However, the SEDS-I, SEDS-II, and TiPS missions, for example, were all flown successfully. In particular, SEDS-II demonstrated that a tethered system can be controlled reasonably accurately by combining an open-loop optimal trajectory with a linear feedback controller. The YES2 tethered satellite mission is planned for early 2007. This mission is intended to demonstrate the ability to deploy a payload via a tether that will return the payload to the Earth using momentum transfer. By deploying the tether in an appropriate manner, the tether can gain sufficient swing velocity so that when the tether passes through the local vertical it can be severed. This effectively removes momentum from the payload, allowing it to re-enter the atmosphere. This paper will present a control strategy being considered for implementation in this mission. First, the open-loop optimal trajectories are determined using direct transcription methods. The open-loop trajectory consists of two phases. The objective of the first phase is to deploy the tether to a length of 3.5 km along the local vertical. The tether will be held in this position. Some time later, the second phase will be initiated which is designed to deploy the tether to a length of 30 km such that the swing velocity at the local vertical is sufficient to cause deorbit of the payload. Like the SEDS deployer, the YES2 deployer cannot reel the tether in. Thus, all control is achieved by controlling the number of brake turns on the friction (“barberpole”) pole used to control the deployment speed. The effects of orbit eccentricity and atmospheric drag are included into the calculations sequentially, and their impact is analyzed. Aerodynamic drag is found to be the largest perturbing force on the system due to the low orbital altitude. Second, feedback controllers are designed using a receding horizon control approach that tracks the optimal trajectory. The time-varying gains are determined efficiently using a new methodology based on Gauss-Lobatto integration techniques. This approach does not require any explicit integration or use of the Riccati equation. The controller is tested for robustness in a flexible tether system model under different environmental conditions and parameter uncertainties. Particular importance is placed on the behavior of the tether lateral dynamics during the second phase. Finally, a closed-loop hardware test using the actual deployer is used to complete the testing.
- Abstract document
- Manuscript document
IAC-06-D2.3.04.pdf (🔒 authorized access only).
To get the manuscript, please contact IAF Secretariat.