|M.Sc Student||Mark Rozanov|
|Subject||Design of Methods for Aeroassisted Orbit Transfer|
Trajectories Control under Variable Density
|Department||Department of Aerospace Engineering||Supervisor||Professor Emeritus Guelman Mauricio|
The main difficulty in an aerocapture mission is dealing with unpredictable variations in the atmospheric density. An aerocapture maneuver takes place in an altitude region, for which no reliable atmospheric density models exist. Therefore, for the execution of such a maneuver a closed loop trajectory control is necessary.
As a first step in the development of closed loop control methods the controllability of the nominal orbit was studied. The controllability minimum principle was used to calculate limiting orbits and reachable and controllable sets from given initial conditions were determined. Using controllable and reachable sets it is possible to get an indication about the ability of tracking control maintaining the spacecraft close to a given nominal orbit.
In this work two control methods are developed, using the specific mechanical energy as the independent variable instead of time. The first control method is based on the Variable Structure Control approach.. The closed loop controller uses only kinetic variables measurements and there is no need for density estimation. This is a significant benefit, since density estimation is a complicated process.
Taking the atmosphere density as one of the system states the spacecraft differential equations can be closely approximated by a linear non-autonomous system. Using this linear system, a second control method is developed. Using an analytical analysis, the stability of the linear system is proven, and a good approximation of the state errors during the flight is established.
The performance of the two different control methods is evaluated by Monte-Carlo simulations. The simulation results show small tracking errors and orbit parameters close to nominal.