Search

The main goal of this paper is to analyze different methodologies and to quantitatively compare a set of algorithms for multi-objective global optimization, as an initial step toward a multidisciplinary design optimization framework for space transportation systems. Through a comparative analysis based on mathematical benchmarks, hierarchies among several stochastic techniques are proposed. This leads to the identification of two algorithms as the most promising for multidisciplinary design optimization applications: the nondominated sorting genetic algorithm-II (NSGA-II) and an improved version of particle swarm optimization. Then a significant advantage of the latter is highlighted on the problem of trajectory optimization for expendable launch vehicles. In this applicative scenario, the paper also aims at proving the capability of global methods to correctly assess the performance of expendable launch vehicles. Environmental, dynamical, and guidance models for the optimization of ascent trajectories are therefore introduced. Results for seven performance optimization test cases show that global techniques provide comparable payload masses with respect to traditional gradient-based methods. In addition to the trajectory models, historical linear regressions simulating an expendable launch vehicle's design cycle are described. The focus is in this case on the definition of a simple model as a test bench for the verification of the effectiveness of global methods on multidisciplinary design optimization problems, rather than on the quality of the design results. Three widely different expendable launch vehicle design optimization problems are successfully solved by applying the improved particle swarm optimization algorithm. This demonstrates the suitability of the developed global optimization architecture and algorithms for search-space exploration in multidisciplinary design optimization, paving the way for more detailed modeling and more realistic applications. [ABSTRACT FROM AUTHOR]

This paper concerns optimal longitudinal trajectories and guidance strategies of a reusable launch vehicle during the terminal-area energy-management phase. During this phase, it is important to know the limits of the vehicle capabilities and the best strategy for a flight in the vertical plane. This paper describes optimal trajectories using a genetic algorithm and guidance strategies for both maximum-range and maximum-dive flights. The optimal trajectories show an increased maximum range up to 6%, compared with the trajectories using an angle of attack for maximum lift-to-drag ratio. A turning flight has an influence on the optimal trajectories and optimal turns are flown at a lower velocity. Based on the calculation of optimal trajectories and an energy-state approximation analysis, a longitudinal guidance law is designed, using robust design to approximate the optimal trajectories. Offnominal conditions, in terms of a change in initial altitude and velocity, also have an influence on the optimal trajectories. Maximum-range trajectories for different initial states define the minimum-energy boundary in the energy space. [ABSTRACT FROM AUTHOR]

This paper analyzes the performance of some global search algorithms on a number of space trajectory design problems. A rigorous testing procedure is introduced to measure the ability of an algorithm to identify the set of &epsilon-optimal solutions. From the analysis of the test results, a novel algorithm is derived. The development of the novel algorithm starts from the redefinition of some evolutionary heuristics in the form of a discrete dynamical system. The convergence properties of this discrete dynamical system are used to derive a hybrid evolutionary algorithm that displays very good performance on the particular class of problems presented in this paper. [ABSTRACT FROM AUTHOR]

In this paper, we propose a methodology for the prediction and optimization of the ascent trajectory of a stratospheric balloon to target a specified three-dimensional area. The methodology relies mainly on the Analysis Code for High-Altitude Balloons, a simulation tool for the prediction of flight trajectory and thermal behavior of high-altitude, zero-pressure balloons, and on a statistical analysis for estimating the trajectory prediction errors. The paper also describes the algorithms used for balloon parameter optimization to obtain a flight trajectory that reaches a predefined target area without any ballast drop or gas venting control. The proposed methodology was successfully used during the first Dropped Transonic Flight Test of the Flying Test Bed 1 demonstrator accomplished on 24 February 2007 by the Italian Aerospace Research Center. The reported postflight analysis of all the test campaign demonstrates that the proposed methodology for trajectory prediction and optimization guarantees very satisfactory and reliable results for both the selection of the best day to perform such a mission and the definition of the correct balloon parameters to be used for targeting a predefined three-dimensional area. [ABSTRACT FROM AUTHOR]

This paper describes a vision-based relative navigation and control strategy for inspecting an unknown, noncooperative, and possibly spinning object in space using a visual-inertial system that is designed to minimize the computational requirements while maintaining a safe relative distance. The proposed spacecraft inspection system relies solely on a calibrated stereo camera and a three-axis gyroscope to maintain a safe inspection distance while following a circular trajectory around the object. The navigation system is based on image processing algorithms, which extract the relative position and velocity between the inspector and the object, and a simple control approach is used to ensure that the desired range and bearing are maintained throughout the inspection maneuver. The hardware implementation details of the system are provided. Computer simulation results and experiments conducted aboard the International Space Station during Expedition 34 are reported to demonstrate the performance and applicability of the proposed hardware, and related navigation and control systems to inspect an unknown spacecraft. [ABSTRACT FROM AUTHOR]

The particle swarm optimization technique is a population-based stochastic method developed in recent years and successfully applied in several fields of research. The particle swarm optimization methodology aims at taking advantage of the mechanism of information sharing that affects the overall behavior of a swarm, with the intent of determining the optimal values of the unknown parameters of the problem under consideration. This research applies the technique to determining optimal continuous-thrust rendezvous trajectories in a rotating Euler-Hill frame. Five distinct applications, both in two dimensions and in three dimensions, are considered. Hamiltonian methods are employed to translate the related optimal control problems into parameter optimization problems. The transversality condition, which is an analytical condition that arises from the calculus of variations, is proven to be ignorable for these problems, and this property greatly simplifies the solution process. For each of the five applications considered in the paper, despite its simplicity, the swarming algorithm successfully finds the optimal control law corresponding to the minimum-time trajectory with great accuracy. [ABSTRACT FROM AUTHOR]

The space community has recognized the problem of sustaining current space operations and has responded by supporting research and development in technologies to reduce cost and schedule without sacrificing performance. One solution is maneuverable satellites. There are very few studies on maneuvering satellites in low-Earth orbit from a ground-track perspective. Operational responsiveness is achievable by changing the ground track and thereby a geographical target location. The existing paradigm on maneuvering is that it is cost-prohibitive, thus orbit-changing maneuvers are done sparingly. This paper presents methodology to quantify reachability of a satellite with chemical and electric propulsion based on initial orbit, A V, and available maneuvering time. Initial orbit parameters are examined to determine which would yield the greatest benefit from maneuvering and the analysis shows that initial orbit orientation has no effect. These maneuvers are highly predictable and equations are formulated to become part of an algorithm that allows overflight of user-specified ground targets. In sacrificing timeliness with electric propulsion the system gains repeatability over chemical propulsion. Existing technology could maneuver a satellite significantly to change its ground track in a relatively short period of time and within standard propellant budgets. [ABSTRACT FROM AUTHOR]

The present paper reports on the interaction of a hot side jet in a supersonic freestream and compares the effects with results from cold-gas jet investigations. With high-speed schlieren videos, a significant enlargement of the separation area has been ascertained upstream from the hot-gas jet and quantitatively confirmed with wall pressure measurements. The schlieren videos also show a strong oscillation of the separation and bow shock. By using an algorithm that analyzes the similarity of neighboring images, the oscillation frequency of the separation and bow shock has been measured to be 11.3 kHz. As a consequence of the dominant shocks in the area in front of the jet, the hot jet plume is screened off strongly from the freestream, thus resulting in an upstream inclination. Finally, high-speed videos have revealed large-scale structures in the hot-gas jet and their convection velocities have been analyzed. According to the shocks, the large-scale structures oscillate at 11.3 kHz. Jet oscillation is thus excited by the interaction of the hot-gas jet with the freestream and not by resonances from the combustion chamber. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to describe and demonstrate new capabilities in the two-dimensional implicit thermal response and ablation program. These expanded capabilities include grid options for flight and arcjet geometries, a sizing algorithm for the flight-type geometry, and an orthotropic thermal conductivity model: Applications to analysis of an orthotropic low-density carbon-phenolic material in arcjet and flight environments relevant to the Orion crew module are presented. For the arcjet environment, multidimensional conduction effects strongly influence the in-depth thermal response. For a lunar return flight environment, in the shoulder region of the crew module (where the radius of curvature is smallest), the thermal response is influenced by multidimensional conduction and by the orientation of the orthotropic material. [ABSTRACT FROM AUTHOR]

Safe, precise, and efficient lunar landings must detect and avoid hazardous boulders, craters, and slopes while identifying predeployed assets and managing fuel consumption. This paper describes an algorithm to designate landing points during lunar descent. Inputs to the algorithm are the lander footprint, divert A V costs, detected hazards, and identified points of interest. The algorithm is designed to accept any number of input cost functions and constraints, ensuring flexibility to a variety of missions. The input cost functions are weighted and input into a search algorithm that returns all feasible landing points on the map, prioritized by the weighted cost function. The output of the algorithm is a set of landing aim points. The algorithm provides onboard crew or autonomous robotic vehicles with landing point designation capabilities. [ABSTRACT FROM AUTHOR]

This paper investigates trajectories for spaceborne observatories with long times of flight high above the ecliptic plane. A large normal displacement is necessary for mitigating the effect of noise generated by the interplanetary (zodiacal) dust cloud, thus increasing resolution and reducing the sizes of the telescopes. Flyby paths using multiple-gravity- assist maneuvers near Earth and Venus are used to reach Jupiter, where the inclination is increased and the final orbit is produced. The trajectory design is performed using a memetic algorithm, which is a combination of global and local optimizers. The global search for optimal trajectories with minimal energy requirements and short transfer times to the highly inclined destination orbit is performed using a niching genetic algorithm improved by a gradient-based local optimization. The optimization yielded three candidate paths, each accompanied by, at most, one deep-space maneuver. The resulting energy-efficient orbits are characterized by large displacements normal to the ecliptic plane, thereby providing prolonged observation times in the interplanetary-dust-noise-free celestial sphere. [ABSTRACT FROM AUTHOR]

The increasing satellite congestion at the geostationary altitude requires positioning a few satellites in the same geostationary slot, a technique known as collocation. In this paper, we develop a geostationary orbit satellite collocation algorithm using relative orbital element corrections, which represent the differences between the impulsive orbital element corrections of any two spacecraft in a geosynchronous slot. The main idea is that formulating the problem of collocation in terms of relative orbital element corrections leaves some of the final values of the orbital elements unconstrained. The freedom rendered by this modeling can be used to find impulsive maneuvers minimizing a given performance index. The minimum distance between satellites that guarantees collision-free motion is incorporated into the design process to find necessary and sufficient conditions for the relative eccentricity and inclination vectors, guaranteeing safe collocation. The proposed collocation algorithm is illustrated in a simulation. [ABSTRACT FROM AUTHOR]

Hypersonic vehicle development, particularly for hypersonic airbreathers, will require robust-design optimization to achieve performance targets. Vehicle shape optimization will play an important role. This paper presents the first application to hypersonic vehicle shape optimization of a reduced parametric section shape representation coupled with a surrogate-assisted evolutionary-algorithm optimization approach. The particular case considered is the minimization of total drag of the nose cone of a hypersonic flight experiment. The computational fluid dynamics solver used here is ANSYS CFX. Single-point optimization at Mach 3 and Mach 8 is performed at altitudes relevant to those Mach numbers for a typical hypersonic flight experiment ascent trajectory. Without surrogate assistance, the optimized shapes for each Mach number were both found to result in significant drag-force reductions (1.39% at Mach 3 and 1.96% at Mach 8) when compared with the baseline blunted standard ogive nose-cone shape. When the surrogate assistance (which is a radial-basis-function network approximation to the drag dependence on the shape parameters) was introduced, the optimized shapes yielded nearly the same drag reduction as without surrogate assistance, but also resulted in significant savings in the computational cost. Finally, the performance of the optimum shape derived at Mach 3 is evaluated at Mach 8 and vice versa to illustrate the robustness of the nose-cone shapes derived using such an approach. [ABSTRACT FROM AUTHOR]

This paper presents the modeling, algorithm development, and laboratory testing of a two-axis digital sun sensor. The basis for our modeling is the Sinclair Interplanetary SS-330, a sensor that employs a single linear array to provide two-axis measurements of the sun vector. The detector illumination simulation includes consideration of Fresnel diffraction, finite pixel size, the extended nature of the sun, and a detector-noise model to synthesize artificial images. Simulated images agree with actual sensor images to within about 3.2% (root mean square). Algorithm studies contrast the performance of existing peak-detection routines (e.g., peak detections and centroiding) with more sophisticated parametric algorithms. Laboratory studies confirm that these modified algorithms can provide a threefold performance improvement over the baseline sensor processing. These results also evaluate the simulation's ability to provide accurate estimates of sensor performance. Although the simulations qualitatively replicate many behaviors, they are not yet accurate enough to be used in place of hardware validation. [ABSTRACT FROM AUTHOR]