Orbital plane changes require a considerable amount of propellant to be completed if the traditional fully propulsive maneuver is used. The cost associated with this type of maneuvers is one of its major drawbacks, and performing an inclination change in orbit would require a large part of the total mass budget of the mission. Moreover, launching such vehicles or satellites would demand the usage of a larger launcher, further contributing to the increase of the mission cost. An alternative to the fully propulsive maneuver is the aeroassisted maneuver, proposed London (1961), where the vehicle performs one or more passes through the atmosphere of the central body. This allows for the aerodynamic forces to steer and brake the vehicle, reducing its orbital speed and changing the inclination of the orbit with less propellant required. To investigate the impact of the aeroassisted maneuver in the propellant requirements, the following research question was proposed:
What is the impact of using an aeroassisted maneuver in reducing the amount of propellant needed to achieve a certain orbital inclination change?
A simulation environment was developed using the TU Delft Astrodynamics Toolbox, capable of simulating an aeroassisted trajectory from a Geostationary Earth Orbit to a Low Earth Orbit, with an associated inclination change. A simple lateral guidance algorithm, capable of tracking the orbital plane, was also developed, and a node control algorithm was implemented to chose the guidance nodes during each individual trajectory.
The performance of three vehicle configurations, with different lift-to-drag ratios, were investigated in terms of achievable inclination change. The HORUS-2B, a vehicle with a moderate to high lift-to-drag ratio, was the most suitable to perform the maneuver, as it allows for a larger inclination change while reducing the energy dissipation rate in the atmosphere. The simulator was integrated with an optimization algorithm, such that optimal trajectories could be investigated.
The performance of the aeroassisted maneuver is measured in terms of three different objectives: the offset in inclination between the final and the target orbital planes, the velocity impulse applied at atmospheric exit to target the desired apogee and the maximum heat load experienced by the vehicle during any given atmospheric pass. The NSGA-II optimizer is selected to minimize the three objectives, and several parameters are tuned to increase the probability of finding the global optimum, namely the tuning parameters of the optimizer itself, the values of the constraint and the number of guidance nodes. It was found that an orbital plane of 20 degrees could be targeted with an error of only 0.28%, with the total maneuver requiring 87.32% less Delta V needed from thrust impulses. The heat load obtained is 1315.6 kJ/m2, which is well within the allowable range for Thermal Protection Systems currently available, and the total number of atmospheric passes is five. Moreover, it is possible to improve the Delta V performance by 193.2%, if the inclination change is limited to approximately 15 degrees, resulting in a single-pass atmospheric trajectory and an apoapsis targeting impulse of merely 1.692 m/s.
This trajectory would however result in a very high heat load acting on the vehicle, specifically 3098.1 kJ/m2. A single-atmosphere pass is the reason for the high heat stress acting on the vehicle. A four-atmosphere pass trajectory with a maximum heat load of only 1307.6 kJ/m2 was also found, which still achieves an inclination change of around 15 degrees, and provides a performance improvement of 110.5% in terms of propulsive Delta V needed.
Trajectories with more atmospheric passes result in higher achievable inclination changes, but at the cost of higher apoapsis targeting Delta V. If the mission requires smaller inclination changes, and has tighter constraints in the heat maximum heat load, trajectories with many passages are also an option, and part of the Pareto fronts obtained from the optimization.