Aero-gravity assist for high energy missions

Abstract

Earlier investigations by the authors comparing optimal atmospheric trajectories for aerogravity assist of Venus and Mars have shown that Mars is a better candidate for aero-gravty assist missions. In this paper, Mars-aero-gravity assist for high energy missions (solar flyby and Pluto missions) is considered. It is found that for high energy missions Mars-aero-gravity assist alone will involve very high associated earth launch energy and heating rate. Therefore. the potential of an alternative technique of combining Mars-aero-gravity assist with Jupiter-gravity assist for high energy missions is examined. The analysis shows that the use of this technique could reduce earth launch energy and heating rate significantly. Some problems regarding the actual implementation of this alternativc technique are also discussed. I n t r o d u c t i o n Planetary gravity assist has been used several times for exploring the solar system successfully including the recent Galileo and IJlysses mission. The concept of gravity assist is based on the phenomenon that energy can be transferred from a planet to a spacecraft or vice versa when the spacecraft’s planetocentric trajectory is deflected due t o the gravitational field of the planet. The deflection causes rotation of the planetocentric velocity vector of the spacecraft which results in a change in the direction and magnitude of the heliocentric velocity V, of the Copyright 0 1 9 9 4 by the American Institute of Aeronautics and Astronautics, Inc. AU rights reserved. spacecraft. The change in the magnitude of V, depends on deflection angle 6 and velocity (Vm) relative t o planet at a far away distance. Large changes in V, are possible either by large 6 and V, (e.g.Jupiter gravity assist) or by multiple gravity assists (e.g. VEEGA or VEGA). However, Jupiter is a remote planet and considerably high earth launch velocity V,, (subscript E stands for Earth) is needed t o send a spacecraft from the Earth to Jupiter to obtain large V,. An earth launch energy C3 (=l’2E) of about 120 !im2/s2 is needed to send a spacecraft on a solar mission using Jupiter-gravity assist. This violates the current C3 capability, which is multiple gravity assist is unable to get sufficient V, needed for solar probe or Pluto mission at Jupiter. One technique which can be more attractive for solar probe or Pluto missions is the aero-gravity assist (AGA), proposed by Randolph and McRonaldl,Z. In AGA, the spacecraft with low aerodynamic drag and high lift to drag ratio ( L I D ) flies through the atmosphere of a terrestrial planet t o rotate around the planet using the lift force in addition to the gravitational force. This can lead to quite large AV compared to conventional gravity assist (GA) alone and can be used for high energy missions (HEM), e.g. solar flyby and Pluto mission. The basic events of an AGA mission is as follows. The spacecraft is launched from a LowEarth-Orbit (LEO) into a heliocentric elliptic transfer orbit and arrives with the planetocentric velocity V , . The spacecraft passes by the planet along a hyperbolic trajectory HT, with the periapsis inside the planetary atmosphere. . , about 81 km2/sz (V,, = 9 km/s)’. And the