The Aerogravity Assist

Abstract

Currently one of the major drawbacks of interplanetary travel is the cost of these types of missions. Mission to extra-terrestrial bodies require a large amount of propellant to complete, which often takes up a large part of the mass budget of the spacecraft. This requires larger launchers to be used to get the spacecraft into the required orbit, which increases the cost of these missions. An often used method to decrease the propellant used during an interplanetary mission is the gravity assist. The gravity assist uses the gravitational influence of a planetary body to increase its heliocentric velocity. This is done by exchanging some of the planets momentum as it orbits the Sun to the spacecraft. The increase in velocity is limited by several factors: the gravitational influence of the body, the closest distance possible between the body and the spacecraft, and the orientation of the planetary bodies relative to each other. A new method has been proposed by McRonald (1990) called the aerogravity assist, which improves upon the gravity assist by increasing the angle over which the spacecraft can turn using the aerodynamic forces it experiences while travelling through the atmosphere of the planetary body. This increase in the velocity bending angle increases the heliocentric velocity compared to the gravity assist, and also allows for changes in inclination of the interplanetary orbit using less propellant. This report investigates both the atmospheric and interplanetary trajectory of the aerogravity assist, and combines them to understand the benefit of the aerogravity assist for interplanetary trajectories.

A modular simulation environment is designed using the TU Delft Astrodynamics Toolbox to simulate the atmospheric trajectory of the aerogravity assist. This simulator allows the selection of different types of acceleration and environment models. The waverider is selected to be the vehicle used during the aerogravity assist, as it is able to obtain high lift-over-drag ratios which reduce the drag losses during the atmospheric section of the maneuver. The specific aerodynamic characteristics and heating models of the selected vehicle are evaluated and incorporated into the simulator. From this simulation environment, the optimal atmospheric trajectories were investigated using an optimization algorithm. It was found that this specific problem is extremely sensitive to the selected optimization algorithm, the tuning parameters of the optimization algorithm, and the objecive and decision variables that are used. The MOEA/D algorithm was selected to find the optimal atmospheric trajectories for Mars, Earth, and Venus, where the objective variables were chosen to be the atmospheric bending angle and the incoming and out-going inclination difference. It was found that using the angle-of-attack and bank angle as control variables, an atmospheric bending angle of 120.5 degrees and an inclination difference of 0.2 degrees could be obtained at Mars. Using the control action instead of the inclination difference as objective variable at Mars showed that the atmospheric bending could be increased to 151.2 degrees while maintaining an inclination difference of below 3 degrees, and smoothed the trajectory in the process. For Earth and Venus the same optimization scheme was employed. They were able to obtain atmospheric bending angles between 70 and 50 degrees, while obtaining inclination differences below 1.5 degrees. For Venus, the heat load was also optimized instead of the inclination difference and it was found that the peak heat flux decreased by around 1000 W/cm$^2$, while decreasing the atmospheric bending angle by around 10 degrees.

A mission planner was designed to be able to investigate the effect of the aerogravity assist on interplanetary trajectories. To increase the accuracy of this mission planner, the numerically found atmospheric trajectories are used to determine the possible interplanetary trajectories. Pareto fronts were created for Mars, Earth, and Venus for a range of incoming velocities that showed the possible velocity bending angles and out-going velocities that can be achieved using an aerogravity assist. These Pareto fronts were then implemented in the mission planner to determine the influence of the AGA on an interplanetary trajectory. Several different interplanetary trajectories were investigated using both gravity and aerogravity assists. It was found that due to the fact that for Earth and Venus higher velocities are needed to travel through the atmosphere, the aerogravity assist would lose too much velocity and thus not improve upon the gravity assist. However, for Mars an increase was found of 4 km/s compared to the gravity assist for a trajectory to Saturn using fly-by's at Mars and Jupiter. Taking into account the extra mass of the thermal protection system, the decrease of mass was found to be around 60 percent, which could be used to lower the cost of the launcher or increase the scientific output of the mission by adding instruments to the payload.