Aerogravity assists

Abstract

Interplanetary missions use gravitational slingshots around planetary bodies to adjust their heliocentric velocity or inclination for quite some time. The momentum exchange that can be achieved during a so-called gravity assist is limited by the mass of the planetary body. To overcome this limitation, an aerogravity assist was proposed, a maneuver where, in addition to the gravitational forces, use is made of aerodynamic forces to increase the bending angle of the velocity, hence increasing the momentum exchange. To investigate how efficient an aerogravity assist can change the interplanetary orbital inclination and velocity, a simulator was developed that is capable of simulating both the gravitational and aerodynamic forces on a vehicle during an aerogravity assist. It was determined that waverider is a type of vehicle suitable for aerogravity assists due to their large lift-to-drag ratio, which reduces the energy dissipation in the atmosphere. The aerodynamic characteristics of a number of waverider shapes were evaluated, after which the one with the largest lift-to-drag ratio was selected. Furthermore, a numerical optimization algorithm was used to develop a reference trajectory planner. Finally, a guidance algorithm based on the tracking of drag accelerations was developed and tested to investigate if the found trajectories would still be feasible under the influence of uncertainties and perturbations. The angle over which the trajectory is bent is a measure for the effectiveness of the aerogravity assist. Using the reference trajectory planner, the maximum possible atmospheric bending angle was investigated for an aerogravity assist at Mars and Jupiter for different initial velocities. From this analysis, it was concluded that extremely high velocities were involved in the aerogravity assist at Jupiter, which resulted in large mechanical and thermal loads. These loads would limit the achievable bending angle when the velocities become too large. For the entry velocities investigated, the velocity bending angle could be increased by 10% for high entry (80.0 km/s) velocities and up to 143% for a relatively low entry velocity (68.0 km/s). For an entry velocity of 80.0 km/s, the initial heat-flux peak exceeded the imposed constraints, which prevented the optimization algorithm of finding any solutions. The maximum velocity bending angle that could be achieved at Jupiter was 125.1 degrees at an entry velocity of 68.0 km/s. At Mars, although the heat loads were still larger than for an Earth entry, it is believed that thermal protection systems can be designed that could handle the heat loads. The velocity bending angle could be increased by 490% to 818% depending on the arrival velocity, with a maximum velocity bending angle of 178.5 degrees at an entry velocity of 9.0 km/s. To investigate the effect of an aerogravity assist on an actual mission, two existing missions has been selected: Rosetta for Mars and Ulysses for Jupiter. Although both spacecraft did not have an aerodynamic shape, which means an aerogravity assist could not have been performed during the actual mission, it has been assumed that these vehicles would have had the geometry of a waverider. During the investigation of Rosetta swing-by at Mars, a reference trajectory was generated to investigate the amount of velocity decrease that could have been achieved using an aerogravity assist. It was determined that the reduction in velocity could be increased by 167% with respect to a gravity assist: from 2.3 km/s for a gravity assist to 6.2 km/s for an aerogravity assist. For Jupiter, it was investigated if the orbital inclination could be changed using the aerodynamic force only. As the entry velocity exceeded 80.0 km/s, the heat flux constrained was removed from the trajectory optimization to allow the optimization algorithm to find solutions. It was possible to change the orbital inclination by 54.2 degrees, but at an extremely large heat load of 40,620 W/cm2. This reconfirms that even though orbital inclination changes are possible using aerodynamic forces, Jupiter is unsuitable for aerogravity assists due to the high velocities and large heat loads associated with an atmospheric maneuver at this planet. Finally, using the aerogravity assist trajectory found for Rosetta, which was generated generated with the reference trajectory planner, the guidance algorithm was tested. The guidance algorithm was capable of tracking a drag reference under the influence of uncertain initial flight-path angles. The maximum offset in velocity bending angle occurred for a steep entry and was 1.06 degrees while the maximum offset in hyperbolic excess velocity occurred during a shallow entry and was 1.88 m/s. Furthermore, the tracking was also successful when a more accurate atmosphere model and perturbations were taken into account. For this analysis, the maximum offset in velocity bending angle and hyperbolic excess velocity were 1.24 degrees and 2.14 m/s respectively.