A Survey of Aeroassisted Orbit Transfer

Introduction I N 1961, Howard London presented a paper at the 29th Annual Meeting of the Institute of the Aeronautical Sciences in which he demonstated the possibility of using aerodynamic forces to produce an orbital plane change with an expenditure of energy significantly smaller than that associated with an extra-atmospheric propulsive maneuver. Since that time, numerous studies of aeroassisted orbit transfer have been carried out for a wide variety of Earthorbital and planetary missions. Almost any mission that involves changes in orbital altitude and/or inclination in the vicinity of an atmosphere-bearing planet is a candidate for aeroassist. For ease of discussion these many possible mission applications may be categorized as 1) synergetic plane change, 2) planetary mission applications, and 3) orbital transfer vehicle applications. The term, synergetic plane change, is commonly used to denote a maneuver in which a change in orbit inclination is accomplished through a combination of aerodynamic and propulsive forces rather than through propulsion alone. While the maneuver is applicable to any planet with a suitable atmosphere, practically all studies to date have dealt with low Earth orbit missions. As illustrated in Fig. 1, a retrorocket burn causes the vehicle to leave its initial low Earth orbit (LEO) and dip into the atmosphere. During the atmospheric pass, the vehicle is banked so that the lift vector is in a lateral direction, thus producing an aerodynamic turn. As will be discussed subsequently, this turn is often accomplished with continuous thrusting sufficient to balance the aerodynamic drag. Following the aerodynamic turn, the vehicle is reboosted to the desired orbital altitude and a rocket burn is effected to circularize the orbit (shown by the dashed low Earth orbit in Fig. 1). The typical orbital transfer vehicle (OTV) maneuver is in many ways similar to the synergetic plane change. In the OTV case, however, the vehicle is initially in a high Earth orbit (HEO)—geosynchronous in most studies—and must undergo a sizeable velocity decrement in its transfer to LEO as illustrated in Fig. 1. This velocity decrement is more efficiently achieved through a combination of aerodynamic drag and propulsion than through propulsion alone. The OTV mission usually involves some change in orbital plane—for instance, the return from geosynchronous orbit to a typical Shuttle orbit involves a plane change of 28.5 deg. If the OTV is a lifting configuration, some of the required plane change can be achieved by banking the vehicle as in the synergetic plane change maneuver. In planetary mission applications, aeroassist is again used to produce a velocity decrement through aerodynamic drag. For these missions two distinctly different applications of aeroassist have been studied: 1) aerocapture in which the velocity decrement achieved in a single deep atmospheric pass is sufficient to transfer the vehicle from its hyperbolic approach trajectory to a target orbit about the planet (i.e., the vehicle is captured by the planet's gravitational field); and 2) multipass aerobraking in which the vehicle is transferred from its approach trajectory to a highly elliptic orbit about the planet by a rocket burn and then the orbit is circularized by many high-altitude atmospheric passes, each followed by small corrective rocket burns at apoapsis to maintain the periapsis altitude low enough to produce the desired drag decrement but high enough to avoid excessive aerodynamic heating. The first type of planetary application, as illustrated in Fig. 1, is similar to the OTV maneuver except that it usually has a larger velocity excess and no plane change. The second type of planetary application is illustrated in Fig. 2. As is also illustrated in Fig. 2, this type of multipass orbit lowering has also been studied for the GEO-to-LEO OTV mission. In the intervening years since London presented his paper in 1961, many studies have been carried out and a rich literature has developed. In the process of researching the present paper, an extensive bibliography was compiled. It lists the relevant publications in chronological order for each of the three vehicle classes considered. While certainly incomplete (new and additional references keep turning up), this collection of papers is believed to present an accurate overall picture of the evolution of aeroassist technology. Examination of this bibliography shows that the various aeroassist mission applications received their most intensive study in different time periods. As was mentioned previously, the serious study of synergetic plane change maneuvers began with London's paper in 1961. Actually, there were studies published prior to 1961 (e.g., Ref. 1) but London's paper appears to be the first to convincingly demonstrate a significant performance gain. During the 1960s, synergetic plane change was studied extensively with the peak activity occurring around 1967. The initial