Alessandro Antonio Quarta

Parametric model and optimal control of solar sails with optical degradation

Solar-sail mission analysis and design is currently performed assuming constant optical and mechanical properties of the thin metalized polymer films that are projected for solar sails. More realistically, however, these properties are likely to be affected by the damaging effects of the space environment. The standard solar-sail force models can therefore not be used to investigate the consequences of these effects on mission performance. The aim of this paper is to propose a new parametric model for describing the sail films optical degradation with time. In particular, the sail films optical coefficients are assumed to depend on its environmental history, that is, the radiation dose. Using the proposed model, the optimal control laws for degrading solar sails are derived using an indirect method and the effects of different degradation behaviors are investigated for an example interplanetary mission.

Invited Article: Electric solar wind sail: Toward test missions

The electric solar wind sail (E-sail) is a space propulsion concept that uses the natural solar wind dynamic pressure for producing spacecraft thrust. In its baseline form, the E-sail consists of a number of long, thin, conducting, and centrifugally stretched tethers, which are kept in a high positive potential by an onboard electron gun. The concept gains its efficiency from the fact that the effective sail area, i.e., the potential structure of the tethers, can be millions of times larger than the physical area of the thin tethers wires, which offsets the fact that the dynamic pressure of the solar wind is very weak. Indeed, according to the most recent published estimates, an E-sail of 1 N thrust and 100 kg mass could be built in the rather near future, providing a revolutionary level of propulsive performance (specific acceleration) for travel in the solar system. Here we give a review of the ongoing technical development work of the E-sail, covering tether construction, overall mechanical design alternatives, guidance and navigation strategies, and dynamical and orbital simulations.

Electric Sail Performance Analysis

. These values render the electric sail a potentially competitive propulsion means for future mission applications. The aim of this paper is to provide a preliminary analysis of the electric sail performance and to investigate the capabilities of this propulsion system in performing interplanetary missions. To this end, the minimum-time rendezvous/transfer problem between circular and coplanar orbits is considered, and an optimal steering law is found using an indirect approach. The main differences between electric sail and solar sail performances are also emphasized.

Impact of Optical Degradation on Solar Sail Mission Performance

The optical properties of the thin metalized polymer films that are projected for solar sails are likely to be affected by the damaging effects of the space environment, but their real degradation behavior is to a great extent unknown. The standard solar sail force models that are currently used for solar sail mission analysis and design do not take these effects into account. In this paper we use a parametric model for describing the sail film’s optical degradation with its environmental history to estimate the impact of different degradation behaviors on solar sail mission performance for some example interplanetary missions: the Mercury rendezvous missions, fast missions to Neptune and to the heliopause, and artificial Lagrange-point missions.

Trajectory Design with Hybrid Low-Thrust Propulsion System

propulsion thruster is coupled with an auxiliary system providing an inverse square radial thrust. In this way the spacecraft is virtually subjected to a reduced gravitational solar force. The primary purpose of this paper is to quantify the impact of the reduced solar force on the propellant consumption for an interplanetary mission. To this end the steering law that minimizes the propellant consumption for a circle-to-circle rendezvous problem is found using an indirect approach. The hybrid system is compared with a conventional solar electric thruster in terms of payload mass fraction deliverable for a given mission. A tradeoff between payload size and trip time is established.

Electric Sail Mission Analysis for Outer Solar System Exploration

Missions towards the boundaries of the Solar System require long transfer times and advanced propulsion systems. An interesting option is offered by electric sails, a new propulsion concept that uses the solar wind dynamic pressure for generating a continuous thrust without the need for reaction mass. The aim of this paper is to investigate the performance of such a propulsion system for obtaining escape conditions from the Solar System and planning a mission to reach the heliosphere boundaries. The problem is studied in an optimal framework, by minimizing the time to reach a given solar distance or a given hyperbolic excess speed. Depending on the value of the sail characteristic acceleration, it is possible that, in an initial mission phase, the sailcraft may approach the Sun to exploit the increased available thrust due to the growing solar wind electron density. The corresponding optimal trajectory is constrained to not pass inside a heliocentric sphere whose admissible radius is established by thermal constraints. Once the escape condition is met, the sail is jettisoned and the payload alone continues its journey without any propulsion system. A medium performance electric sail is shown to have the potentialities to reach the heliosheath, at a distance of 100 AU, in about fifteen years. Finally, the Interstellar Heliopause Probe mission is used as a reference mission to further quantify the electric sail capabilities for an optimal transfer towards the heliopause nose (200 AU).

Fuel-optimal, power-limited rendezvous with variable thruster efficiency

The problem of minimum-fuel, time-fixed, three-dimensional rendezvous for a solar electric propulsion spacecraft is discussed. The problem is solved via an indirect approach. The formulation takes into account both a variable bounded specific impulse and a variable thruster efficiency and permits us to manage solutions with coast arcs. The thruster efficiency is assumed to vary with the specific impulse through a polynomial approximation. The optimal specific impulse control law is found to depend on the instantaneous values of the primer vector modulus, the spacecraft mass, the mass costate, and the thruster model. Optimal interplanetary trajectories toward Mars are discussed. It is shown that the inclusion of a variable efficiency thruster model has important effects on fuel consumption. In particular, the classic constant efficiency thruster model overestimates the final spacecraft mass.

Near-Optimal Solar-Sail Orbit-Raising from Low Earth Orbit

Introduction S OLAR-SAIL technology has attracted the interest of the scientific community as an advanced propulsion means capable of promoting the reduction of mission costs, the increase of payload mass fraction, and the feasibility of missions that are not practically accessible via conventional propulsion because of their large V requirements. Optimal solar-sail trajectories have long been investigated because of their importance for practical mission analysis purposes.1 Although globally optimal trajectories are especially well suited for interplanetary missions, there are many instances where different engineering constraints limit the possibility of using optimal steering laws. Accordingly, simpler maneuver strategies are required. This happens in most planet-centered problems, including orbitraising and Earth-escape trajectories. In these cases suboptimal (or locally optimal) control laws that maximize the instantaneous rate of change of a particular orbital element or another scalar function of the orbital elements are often employed. This approach not only provides simple answers to the designer, but also gives steering laws that, in many cases, are close to minimum-time solutions. For these reasons, locally optimal control laws for ideal sails have been studied in various forms with different perturbation models. In most papers available in the literature,2−7 the simplifying assumption of neglecting the aerodynamic drag is made. Although the aerodynamic drag has been sometimes taken into account in the mission analysis,8,9 no systematic study concerning its effects on the solar-sail trajectories is currently available. Air drag is known to be negligible above 1000 km, even if its effects are strongly influenced by the solar activity. Nevertheless, air drag can become important as long as particular maneuvers, such as orbit raising from low Earth orbit, are concerned. Accordingly, in this Note we analyze the effects of air drag on sail trajectories in a systematic way, deriving a locally optimal steering law that takes a suitable aerodynamic model into account. To this end, the sail is treated as a flat plate, and a hyperthermal flow model is assumed. The corresponding trajectories are near-minimum-time solutions for low characteristic accelerations. The steering law is shown to depend on the ratio between the local dynamic pressure and the solar radiation

Optimal Control Laws for Axially Symmetric Solar Sails

Acomparison between various models is extremely useful for mis-sion analysis purposes, and significant results can be establishedthrough the study of optimal interplanetary trajectories. A compre-hensive discussion of this subject for ideal and nonideal flat sailsis reported in Refs. 2 and 3. However, realistic sail models shouldinclude the nonplanar sail shape and/or the billowing effect dueto the solar radiation pressure. Currently, the only results availablefor three-dimensional geometries are the optimal steering laws forthe so-called parametric-force model.

Optimization of Biimpulsive Trajectories in the Earth-Moon Restricted Three-Body System

The problem is addressed of transferring a spacecraft from a low Earth to a low lunar orbit in a planar circular restricted three-body framework. A closed-form approximate expression for the total velocity variation is developed under the assumption of minimum ∆V biimpulsive maneuvers. This approximation quantifies the link between the transfer orbit energy and the minimum ∆V needed to complete the maneuver, but it gives no information on the corresponding mission time. This last problem is addressed in a systematic framework using an optimization process, and the total ∆V is minimized with the constraint that a maximum transfer time is not exceeded. Using a set of mission data taken from the literature, it is found that almost equivalent ∆V and transfer times (compared to a weak stability boundary approach) are obtained without the use of solar perturbations. More important, a consistent methodology is proposed to exploit fully the fundamental tradeoff between the time of flight and the required ∆V.