On-board three-dimensional constrained entry flight trajectory generation

This dissertation presents a method for on-board generation of three-degree-of-freedom (3DOF) constrained entry trajectory. Given any feasible entry conditions and terminal area energy management (TAEM) interface conditions, this method generates rapidly a 3DOF trajectory featuring a single bank-reversal that satisfies all the entry corridor constraints and meets the TAEM requirements with high precision. First, the longitudinal reference profiles for altitude, velocity, flight path angle, and the corresponding controls with respect to range-to-go, are designed using the quasi-equilibrium glide condition (QEGC). Terminal backward trajectory integration and initial descent approaches are used to make the longitudinal references intrinsically flyable. Then the 3DOF entry trajectory is completed by tracking the longitudinal references with the approximate receding-horizon control method, while the bank-reversal point is searched such that the TAEM heading and distance to the Heading Alignment Circle (HAC) requirements are satisfied within specified precision. For extreme entry cases that marginally allow a single bank-reversal or no bank-reversals, a terminal reference ground path tracking method and a terminal open-loop trajectory search method are developed respectively to complement the on-board 3DOF trajectory generation method. The overall computational load needed by this method for any entry trajectory design amounts to less than integrating the 3DOF trajectory five times on average. Simulations with the X-33 and X-38 vehicle models and a broad range of entry conditions and TAEM interface requirements demonstrate the desired performance of this method. The on-board entry guidance scheme is then completed and tested by integrating this trajectory generation method with a state of art reference trajectory regulation algorithm on a high fidelity simulation software developed at NASA Marshall Space Flight Center. Instead of preloading a reference trajectory, this method generates a 3DOF entry trajectory from the current state in 1 to 2 seconds on the simulator. Then this freshly generated trajectory is used as the reference for the guidance system. The results demonstrate the great potential of this innovative entry guidance method

Investigation of the Performance Characteristics of Re-Entry Vehicles

When a non-US spacecraft reenters the Earth\u27s atmosphere, having the ability to accurately determine its performance characteristics is a primary concern. This study investigated the atmospheric re-entry profiles of a maneuverable re-entry vehicle. The re-entry vehicle was modeled as a point mass with aerodynamic properties. Equations of motion were numerically integrated, giving the time histories of position, velocity and flight path angle. The algorithm is able to generate a complete and feasible entry trajectory of a approximately 25-minute flight time in about 5 to 10 seconds on a desktop computer, given the entry conditions and values of constraint parameters. This preliminary study shows the feasibility of identifying and further exploring the technical challenges involved in using a mathematical model to simulate the performance characteristics of the maneuvering re-entry vehicle

Space shuttle flying qualities and criteria assessment

Work accomplished under a series of study tasks for the Flying Qualities and Flight Control Systems Design Criteria Experiment (OFQ) of the Shuttle Orbiter Experiments Program (OEX) is summarized. The tasks involved review of applicability of existing flying quality and flight control system specification and criteria for the Shuttle; identification of potentially crucial flying quality deficiencies; dynamic modeling of the Shuttle Orbiter pilot/vehicle system in the terminal flight phases; devising a nonintrusive experimental program for extraction and identification of vehicle dynamics, pilot control strategy, and approach and landing performance metrics, and preparation of an OEX approach to produce a data archive and optimize use of the data to develop flying qualities for future space shuttle craft in general. Analytic modeling of the Orbiter's unconventional closed-loop dynamics in landing, modeling pilot control strategies, verification of vehicle dynamics and pilot control strategy from flight data, review of various existent or proposed aircraft flying quality parameters and criteria in comparison with the unique dynamic characteristics and control aspects of the Shuttle in landing; and finally a summary of conclusions and recommendations for developing flying quality criteria and design guides for future Shuttle craft

Space Shuttle flying qualities and flight control system assessment study, phase 2

A program of flying qualities experiments as part of the Orbiter Experiments Program (OEX) is defined. Phase 1, published as CR-170391, reviewed flying qualities criteria and shuttle data. The review of applicable experimental and shuttle data to further define the OEX plan is continued. An unconventional feature of this approach is the use of pilot strategy model identification to relate flight and simulator results. Instrumentation, software, and data analysis techniques for pilot model measurements are examined. The relationship between shuttle characteristics and superaugmented aircraft is established. STS flights 1 through 4 are reviewed from the point of view of flying qualities. A preliminary plan for a coordinated program of inflight and simulator research is presented

Advanced flight management system for an unmanned reusable space vehicle

The innovative architecture of an advanced Flight Management System (FMS) for Unmanned Reusable Space Vehicle (URSV) applications is presented with the associated re-entry trajectory computation algorithm. The SL-12 unmanned space vehicle, developed by Cranfield University as a part of the 2012-2013 Aerospace Vehicle Design (AVD) Group Design Project (GDP) is used as the reference platform. The overall avionics architecture of the future space transportation vehicle is described. A detailed architecture is developed for the FMS and the core functions of such an FMS are described. A dedicated computation algorithm is presented for re-entry trajectory planning, which involves determination of the path of re-entry vehicle by means of angle of attack and bank angle modulation. Simulation case studies are performed in a realistic re-entry operational scenario resulting in the generation of efficient and feasible trajectories, without violating any of the defined constraints

Assessment of flying-quality criteria for air-breathing aerospacecraft

A study of flying quality requirements for air breathing aerospacecraft gives special emphasis to the unusual operational requirements and characteristics of these aircraft, including operation at hypersonic speed. The report considers distinguishing characteristics of these vehicles, including dynamic deficiencies and their implications for control. Particular emphasis is given to the interaction of the airframe and propulsion system, and the requirements for dynamic systems integration. Past operational missions are reviewed to define tasks and maneuvers to be considered for this class of aircraft. Areas of special concern with respect to vehicle dynamics and control are identified. Experience with the space shuttle orbiter is reviewed with respect to flight control system mechanization and flight experience in approach and landing flying qualities for the National Aerospace Plane (NASP)

Microstructure and mechanical properties of laser cladding repair of AISI 4340 steel

Laser cladding (LC) was used to investigate the repair of high strength steel in aircraft applications, such as landing gears. This paper reports on the microstructure and microhardness properties of the deposited AISI 4340 clad layer on AISI 4340 steel substrate. Microhardness results showed the clad layer was 30-40% harder than the base material. Stress relieving the clad allowed the clad and HAZ areas to soften 10% below the base material. High dilution provided a favorable result on the hardness at the interface

Neural Network Assisted Inverse Dynamic Guidance for Terminally Constrained Entry Flight

This paper presents a neural network assisted entry guidance law that is designed by applying Bézier approximation. It is shown that a fully constrained approximation of a reference trajectory can be made by using the Bézier curve. Applying this approximation, an inverse dynamic system for an entry flight is solved to generate guidance command. The guidance solution thus gotten ensures terminal constraints for position, flight path, and azimuth angle. In order to ensure terminal velocity constraint, a prediction of the terminal velocity is required, based on which, the approximated Bézier curve is adjusted. An artificial neural network is used for this prediction of the terminal velocity. The method enables faster implementation in achieving fully constrained entry flight. Results from simulations indicate improved performance of the neural network assisted method. The scheme is expected to have prospect for further research on automated onboard control of terminal velocity for both reentry and terminal guidance laws

Simulation and Application of GPOPS for Trajectory Optimization and Mission Planning Tool

Rapid trajectory generation is crucial to prompt global warfare. To meet the USAF’s objective of Persistent and Responsive Precision Engagement, a rapid mission planning tool is required. This research creates the framework for the mission planning tool and provides a sample optimal trajectory which is solved using the GPOPS software package. GPOPS employs a Gaussian pseudospectral method to solve the non-linear equations of motion with both end conditions and path constraints. By simultaneously solving the entire trajectory based on an initial guess and small number of nodes, this method is ideal for generating rapid solutions. The sample case is a multi-phase minimum time, optimal control problem which is used to validate the planning tool. The developed framework includes different atmospheric models, gravity models, inclusion of no-flyzones and waypoints, and the ability to create a library of sample cases. This versatile tool can be used for either trajectory generation or mission analysis. The results of this research show the complexities in solving an optimal control problem with states that change from one phase of the problem to another. At the conclusions of this research multiple phases were successfully connected and solved as a single optimal control problem. However, the entire trajectory solution from launch to impact solved simultaneously, is still an objective yet to be demonstrated. The results found should be a solid foundation for a future mission planning tool