T. Troy McConaghy

Design and Optimization of Low-Thrust Trajectories with Gravity Assists

Missions such as Mariner 10, Voyager 1, Galileo, and Stardust all used gravity-assist flybys to achieve their mission goals efficiently. Methods to design such gravity-assist missions are fairly well developed and generally assume all major maneuvers are performed impulsively by chemical rockets. The recent success of the low-thrust Deep Space 1 mission demonstrates that low-thrust (high-efficiency) propulsion is ready to be used on future missions, potentially reducing the required propellant mass or the total time of flight. By combining both gravity-assist flybys and low-thrust propulsion, future missions could enjoy the benefits of both. To realize such missions, an effective design methodology is needed. A two-step approach to the design and optimization of low-thrust gravity-assist trajectories is described. The first step is a search through a broad range of potential trajectories. To speed up this search, a simplified shape-based trajectory model is used. The best trajectories are chosen using a heuristic cost function. The second step optimizes the most promising trajectories using an efficient parameter optimization. method. Examples of missions designed using this approach are presented, including voyages to Vesta, Tempel 1, Ceres, Jupiter, and Pluto.

Analysis of various two synodic period Earth-Mars cycler trajectories

Trajectories that regularly encounter Earth and Mars but use small or no propulsive maneuvers are known as cycler trajectories, or cyclers. For cyclers that repeat after two Earth-Mars synodic periods, several variations are possible. A detailed investigation is presented of a simple two synodic period cycler, along with several promising variations using combinations of one year and half-year phasing orbits. Analysis is included for both the circular co-planar model and with actual Earth and Mars ephemerides.

Preliminary Design of Nuclear Electric Propulsion Missions to the Outer Planets

*† * ‡ Nuclear electric propulsion is expected to open new doors in deep space exploration. We study direct rendezvous missions to the outer planets which employ a constant-thrust, constant specific-impulse engine. We also consider how gravity assist can augment the capability of nuclear electric propulsion. We present numerical examples of gravity-assist missions to the outer planets, which use an engine similar to that of the Jupiter Icy Moons Orbiter. For example, in an Earth-Venus-Earth-Jupiter-Pluto mission, the spacecraft launches with a V∞ of 2.2 km/s and rendezvous with Pluto in 10.5 years, with a propellant mass fraction of 50%. We demonstrate the benefit of using intermediate gravity-assist bodies (e.g. Venus, Earth and Mars) to decrease both mission duration and propellant cost.

Design and Optimization of Low-Thrust Gravity-Assist Trajectories to Selected Planets

Highly efficient low-thrust engines are providing new opportunities in mission design. Applying gravity assists to low-thrust trajectories can shorten mission durations and reduce propellant costs from conventional methods. In this paper, an efficient approach is applied to the design and optimization of low-thrust gravity-assist trajectories to such challenging targets as Mercury, Jupiter, and Pluto. Our results for the missions to Mercury and Pluto compare favorably with similar trajectories in the literature, while the mission to Jupiter yields a new option for solar system exploration.

Analysis of a Class of Earth-Mars Cycler Trajectories

Sun-orbiting spacecraft trajectories that repeatedly encounter Earth and Mars may play a central role in a future Earth-Mars transportation system. Such orbits are known as Earth-Mars cycler trajectories (cyclers). By using gravity-assist maneuvers at Earth or Mars, many cyclers can avoid using large amounts of propellant. The known cyclers were found using heuristics or numerical searches. We describe a new, systematic method for constructing and evaluating cyclers. Our method reveals that previously known cyclers, such as the Aldrin cycler and the Versatile International Station for Interplanetary Transport cyclers, belong to a larger family of cyclers. Our cycler construction method also reveals some previously unknown cyclers. For example, we identify a new cycler that repeats every two synodic periods and has a low V ∞ at Earth and Mars (5.65 and 3.05 km/s, respectively).

A LOW-THRUST VERSION OF THE ALDRIN CYCLER

§In this paper we seek a low-thrust version of a cycler orbit between Earth and Mars known as the Aldrin cycler. The principal goal is to design trajectories that have low flyby V at both planets while minimizing the total propellant cost. Our research is aided by several powerful software tools, including a recently developed low-thrust optimizer. With these tools, we are able to produce several promising low-thrust versions of Aldrin cyclers. Our analysis shows that such cyclers can significantly reduce the total propellant cost.

Analysis of a broad class of Earth-Mars cycler trajectories

Earth-Mars cycler trajectories (cyclers) repeatedly encounter Earth and Mars. A systematic approach to constructing and evaluating such trajectories is described.

Notable Two-Synodic-Period Earth-Mars Cycler

Earth-Mars cycler trajectories (cyclers) could play an important role in a future human transportation system to Mars. A particular cycler that repeats every two synodic periods and has one intermediate Earth encounter is very promising. In a circular-coplanar model it requires no propulsive maneuvers, has 153-day transfer times between Earth and Mars, and has arrival V ∞ magnitudes of 4.7 km/s at Earth and 5.0 km/s at Mars. A method to find an analog cycler in a more realistic model (i.e., using an accurate ephemeris for the states of Earth and Mars) is described. Two cost metrics are considered: total cycler AV and total cycler AV plus total taxi ΔV. Numerical solutions are presented for both metrics. The total required AV is very small, though not zero. If the Earth-Mars and Mars-Earth transit times are constrained, then the characteristics of the optimal cycler trajectory change. Tradeoffs between maximum transit time and other mission characteristics are analyzed for all possible launch periods.

Toward a Standard Nomenclature for Earth-Mars Cycler Trajectories

Many Earth‐Mars cycler trajectories are now known and their numbers continue to grow. Unfortunately, the literature on Earth‐Mars cycler trajectories uses many different systems for naming the various cyclers being investigated. A nomenclature is proposed as a remedy to standardize the naming system for near-ballistic Earth‐ Mars cycler trajectories. Modeling assumptions are given and the proposed nomenclature is explained. All known near-ballistic cyclers fall within the scope of the described nomenclature. Examples are presented of how several well-known cyclers are denoted. The syntax of the nomenclature is formally specified using the extended Backus‐ Naur form. Criteria for evaluating Earth‐Mars cycler trajectories are summarized.

PRELIMINARY ANALYSIS AND DESIGN OF POWERED EARTH - MARS CYCLING TRAJECTORIES

We discuss preliminary results on constructing a powered cycler from semi-cycler trajectories. We present a powered cycler with reasonable transfer times and low encounter velocities. In addition, we develop a metric for evaluating cycler designs in comparison to other mission-to-Mars scenarios. The metric suggests that as vehicle mass (with respect to propellant mass) increases, the most advantageous system progresses from a “Design Reference Mission” scenario to Semi-Cyclers to Cyclers, which is highly indicative of how a human Mars transportation system might evolve.