|Ph.D Student||Leonel Mazal|
|Subject||Reconfigurable Orbit Design and Hierarchical Control for|
|Department||Department of Aerospace Engineering||Supervisor||Full Professor Gurfil Pinchas|
|Full Thesis text|
Large and monolithic satellites, which are tailored for specific missions, are less adaptable to unexpected circumstances than fractionated spacecraft, which are systems composed of several free-flying, physically separated, modules working cooperatively. In fractionated spacecraft, each individual module can be similar to or different from the other modules. The modularity concept renders these distributed platforms more robust and provides a higher level of responsiveness under changing (possibly unexpected) circumstances. Supported by the advent of miniaturized technologies, fractionated spacecraft seem to be an appealing concept for future space missions.
In fractionated spacecraft, the modules must fly in orbits such that any inter-module
distance is bounded by upper and lower limits. The upper bound keeps the communication cross-links operational, which is essential for the mission, and guarantees a common access time for points of interest on the Earth. On the other hand, the lower bound mitigates collision risk. These requirements are encompassed in a single concept termed cluster flight.
This work focuses on developing flight algorithms for cluster flight. The aim is to
develop methods for fuel-efficient long-term cluster flight, considering realistic astrodynamical models. First, constraints on the initial conditions of the modules yielding natural long-term distance-bounded motion are derived. Moreover, analytical
expressions for upper and lower bounds of the inter-satellite distances are obtained. These constraints are important because they enable to reduce the required control efforts for cluster-keeping, and include the effects of zonal harmonics and drag. Exploiting the aforementioned constraints, an algorithm for cluster establishment and cluster keeping is developed, assuming impulsive maneuvers. This algorithm combines the Gauss variational equations, the Lambert’s problem solution and a newly-derived l1-optimal bi-impulsive coplanar two-point orbital transfer.
In a subsequent stage, a strategy based on cooperative optimal time-continuous maneuvers is developed. The obtained maneuvers are aimed at achieving terminal
states satisfying the aforementioned constraints. Since equality of ballistic coefficients is crucial for efficient cluster flight, the adopted formulation enables the modules to intentionally dispose mass for keeping balanced the ballistic coefficients of the modules.
Finally, a closed-loop cooperative algorithm is developed for holding bounded distances between the modules of a fractionated spacecraft. This algorithm assumes that each module is equipped with a constant-magnitude thruster, exerting a on-off thrust profile.
The algorithm is composed of a logic that establishes desired mean orbital elements,
which prevent the modules from exceeding the prescribed bounds, and a Lyapunov-based controller that steers each module to the corresponding desired orbital elements.