Jordi Puig-Suari

Aerobraking tethers for the exploration of the solar system

Abstract In previous work the authors developed a model for the analysis of orbiting tethered spacecraft in an atmosphere. This model was used to demonstrate the feasibility of the aerobraking tether concept for a mission to Mars. The present work studies the possibility of using such vehicles in the exploration of the other atmosphere-bearing planets and satellites in the solar system. This includes Venus, Jupiter, Saturn, Uranus, Neptune and Titan. After establishing ground rules, a study is performed in which the propellant mass for a typical rocket propulsion system is compared to the tether mass required for the aerobraking system. In every case, the tether mass turns out to be less than the propellant mass. The results have significant implications for the design of a new class of exotic spacecraft for the exploration of the solar system.

A tether sling for lunar and interplanetary exploration

Abstract In this paper, we analyze the concept of a tether sling for lunar and planetary missions. By continuous application of torque (generated by a solar-powered electric motor), the spacecraft, attached to the end of the tether, achieves the required injection velocity. Since no propulsive maneuver is required, the tether supplies a virtually inexhaustible capacity to catapult deep space probes. It is shown that all of the engineering difficulties associated with this problem are surmountable and often have simple and elegant solutions. The tether must be tapered according to a simple formula, which allows the tether to support its own mass as well as the probes. Although chemical propulsion provides a much better mass ratio for high energy transfer, the great advantage of the tether is its simplicity and reusability.

Aerocapture with a Flexible Tether

In previous work, the authors have demonstrated that the aerobraking tether, modeled as a rigid rod, could achieve aerocapture at any atmosphere-bearing planet in the solar system for less mass than the corresponding propellant of a typical retro-rocket system. In this paper, the great promise of the aerobraking tether is further explored by developing the equations of motion for the analysis of flexible tether behavior during the maneuver. A standard Lagrangian approach is taken with the tether modeled as a chain of linked rigid rods. Since an arbitrary number of rods can be used, the flexible behavior can be approximated to an arbitrary degree of accuracy. The results indicate that the aerobraking tether concept remains feasible when flexibility effects are included in the model.

Modeling and analysis of orbiting tethers in an atmosphere

Abstract A new model for tethered satellites in low orbit, where atmospheric effects are significant, is developed. The model allows the analysis of the dynamic behavior of tethered satellites in a general orbit. The results obtained for a system in circular orbit compare favorably to previous work. The behavior of tethered systems performing aeroassisted orbital maneuvers is also simulated. In particular, the cases of elliptic orbit transfer and hyperbolic aerocapture are presented. The results in the elliptic case indicate that orbital maneuvers can be performed with small tension forces in the tether. In the hyperbolic case the behavior is not so benign, because the forces are quite large, but the utilization of tethers for aerocapture appears to be physically feasible.

Optimal mass for aerobraking tethers

Earlier work has demonstrated the feasibility of using aerobraking tethers for the exploration of the solar system. In fact, compared to chemical propulsion, the tether mass is usually much less than the required propellant mass. The basic concept involves an orbiter and a probe connected by a thin tether. The probe is deployed into the atmosphere of a planet where aerodynamic drag decelerates it. The tension on the tether provides the braking effect on the orbiter, thus eliminating the need for a propulsive maneuver. During the maneuver the orbiter travels outside the atmosphere, and does not require heat shielding. In the previous work a suboptimal solution was found where the system maintained a near vertical orientation during the fly through. In this paper we consider the minimum tether mass required for specified aerocapture conditions. As an intermediate step, we find the trajectory which provides the minimum tension on the tether. The fact that the orbiter must remain outside the atmosphere is introduced as an altitude constraint. The results are significant for future solar system exploration. In the history of the exploration of the solar system, some of the most successful and most ambitious missions have involved dual vehicle spacecraft. In such a mission (the Viking program being the best known) one vehicle (a probe or lander) is delivered to the planets surface or atmosphere, while the second one (the orbiter) remains in orbit around the planet. The Galileo spacecraft, currently on its way to Jupiter, is representative of several missions being proposed in this category. When the spacecraft arrives at the target planet, the orbiter performs a propulsive maneuver to achieve capture, while the probe relies on an aerobraking maneuver to decelerate. The aerobraking tether shown in Fig. 1 eliminates the need for the orbiter propulsive maneuver. The spacecraft consists of an orbiter and a probe that are connected by a thin tether. When the vehicle arrives at the planet, the probe flies into the atmosphere, as before, while the orbiter is decelerated by tether tension, thus eliminating the need for propellant. Note that the orbiter remains outside the atmosphere during the maneuver and does not require additional aerodynamic shielding. After capture has occurred, the tether may be severed, allowing the probe to land on the planet, or the system may remain together and additional aerobraking maneuvers can be performed to finalize the orbit.

Stability and Control of an Atmospheric Tether with a Lifting Probe

This paper explores the stability and control of an atmospheric tether system that includes a lifting probe with a moveableattachment point.The dynamics of the system with the tether modeled as a rigid rod are linearized about equilibria for circular equatorial orbits. Examination of the eigenvalues of the linearized system shows that there is always at least one unstable mode that needs to be controlled. A linear control system that uses the attachment point motion and thrust at the orbiter as inputs is shown to be suitable for the conditions considered. The system is also controllable with any single control input.

Optimal Mass Flexible Tethers for Aerobraking Maneuvers

Previous results using a rigid tether model indicate that tether mass in aerobraking maneuvers is much lower than the propellant mass required for a similar maneuver. In this work the effects of e exibility on the optimum maneuvers are studied. The results indicate that tether e exibility produces a small increase in tether mass but it remains signie cantly lower than propellant mass. The effects of e exibility are greater in maneuvers where the tether remains nearly vertical during e ythrough. However, bending on the tether in the optimum maneuvers is small and the tether aerobraking maneuver remains feasible when tether e exibility is included in the analysis.