H YPERSONIC flight began in February 1949 when a WAC Corporal rocket was ignited from a U.S.-captured V-2 rocket [1]. In the six decades since this milestone, there have been significant investments in the development of hypersonic vehicle technologies. The NASA X-15 rocket plane in the early 1960s represents early research toward this goal [2,3]. After a lull in activity, the modern era of hypersonic research started in the mid-1980s with the National Aerospace Plane (NASP) program [4], aimed at developing a single-stage-to-orbit reusable launch vehicle (RLV) that used conventional runways. However, it was canceled due mainly to design requirements that exceeded the state of the art [1,5]. A more recent RLV project, the VentureStar program, failed during structural tests, again for lack of the required technology [5]. Despite these unsuccessful programs, the continued need for a low-cost RLV, as well as the desire of the U.S. Air Force (USAF) for unmanned hypersonic vehicles, has reinvigorated hypersonic flight research. An emergence of recent and current research programs [6] demonstrate this renewed interest. Consider, for example, the NASA Hyper-X experimental vehicle program [7], the University of Queensland HyShot program [8], the NASA Fundamental Aeronautics Hypersonics Project [9], the joint U.S. Defense Advanced Research Projects Administration (DARPA)/USAF Force Application andLaunch fromContinentalUnited States (FALCON) program [10], the X-51 Single Engine Demonstrator [11,12], the joint USAF Research Laboratory (AFRL)/Australian Defence Science and Technology Organisation Hypersonic International Flight Research Experimentation project [13], and ongoing basic hypersonic research at the AFRL (e.g., [14–20]). The conditions encountered in hypersonic flows, combined with the need to design hypersonic vehicles, have motivated research in the areas of hypersonic aeroelasticity and aerothermoelasticity. It is evident from Fig. 1 that hypersonic vehicle configurations will consist of long, slender lifting body designs. In general, the body, surface panels, and aerodynamic control surfaces are flexible due to minimum-weight restrictions. Furthermore, as shown in Fig. 2, these

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Aeroelastic and Aerothermoelastic Analysis in Hypersonic Flow: Past, Present, and Future

The field of aerothermoelasticity plays an important role in the analysis and optimization of airbreathing hypersonic vehicles, impacting the design of the aerodynamic, structural, control, and propulsion systems at both the component and multi-disciplinary levels. This study aims to expand the fundamental understanding of hypersonic aerothermoelasticity by performing systematic investigations into fluid-thermal-structural coupling, and also to develop frameworks, using innovative modeling strategies, for reducing the computational effort associated with aerothermoelastic analysis. Due to the fundamental nature of this work, the analysis is limited to cylindrical bending of a simply-supported, von K arm an panel. Multiple important effects are included in the analysis, namely: 1) arbitrary, nonuniform, in-plane and through-thickness temperature distributions, 2) material property degradation at elevated temperature, and 3) the effect of elastic deformation on aerodynamic heating. It is found that including elastic deformations in the aerodynamic heating computations results in non-uniform heat flux, which produces non-uniform temperature distributions and non-uniform material property degradations. This results in reduced flight time to the onset of flutter and localized regions in which the material temperature limits may be exceeded. Additionally, the trade-off between computational cost and accuracy is evaluated for aerothermoelastic analysis based on either quasi-static or time-averaged dynamic fluid-thermal-structural coupling, as well as computational fluid dynamics based reduced-order modeling of the aerodynamic heat flux. It is determined that these approaches offer the potential for significant improvements in aerothermoelastic modeling in terms of efficiency and/or accuracy.

Studies on Fluid-Thermal-Structural Coupling for Aerothermoelasticity in Hypersonic Flow

The current air-breathing hypersonic flight (AHF) technology programs focus on development of flight test vehicles and operational vehicle prototypes that utilize airframe-integrated scramjet engines. A key issue in making AHF feasible and efficient is the control design. The non-standard dynamic characteristics of air-breathing hypersonic flight vehicles (AHFVs) together with the aerodynamic effects of hypersonic flight make the system modeling and controller design highly challenging. Moreover the wide range of speed during operation and the lack of a broad flight dynamics database add significant plant parameter variations and uncertainties to the AHF modeling and control problem. In this paper, first, different approaches to this challenging problem presented in the literature are reviewed. Basic dynamic characteristics of AHFVs are described and various mathematical models developed for the flight dynamics of AHFVs are presented. Major nonlinearity and uncertainty sources in the AHF dynamics are explained. The theoretical and practical AHF control designs in the literature, including the control schemes in use at NASA research centers, are examined and compared. The review is supported by a brief history of the scramjet and AHF research in order to give a perspective of the AHF technology. Next, the existing gaps in AHF control and the emerging trends in the air-breathing hypersonic transportation are discussed. Potential control design directions to fill these gaps and meet the trends are addressed. The major problem in AHF control is the handling of the various coupling effects, nonlinearities, uncertainties, and plant parameter variations. As a potential solution, the use of integrated robust (adaptive) nonlinear controllers based on time varying decentralized/triangular models is proposed. This specific approach is motivated by the promise of novel techniques in control theory developed in recent years. ∗This work was supported in parts by Air Force Office of Scientific Research under Grant #F49620-01-1-0489 and by NASA under grant URC Grant #NCC4-158. †Student Member AIAA, graduate student, Electrical Engineering Department. ‡Member AIAA, professor, Mechanical Engineering Department. §Professor, Electrical Engineering Department Nomenclature The following notation is used throughout the paper, unless otherwise stated. a∞ : free stream velocity of sound ĀD : diffuser exit/inlet (area) ratio c : reference length CD : drag coefficient CL : lift coefficient Cm : pitching moment coefficient (pmc) Cm(q) : pmc due to pitch rate Cm(α) : pmc angle of attack Cmα : ∂Cm/∂α Cm(δe): pmc due to δe CT : thrust coefficient fs : stoichiometric ratio for hydrogen, 0.029 h : vehicle altitude I (In) : the (n× n) identity matrix Iyy : vehicle y-axis inertia per unit width m : vehicle mass m : vehicle mass per unit width ṁair : air mass flow rate ṁf : fuel mass flow rate M : pitching moment M∞ : vehicle flight Mach Number nx : acceleration along the vehicle x-axis nz : acceleration along the vehicle z-axis P : pressure q : pitch rate Q : generalized elastic force re : radial distance from Earth’s center Re : radius of the Earth, 20,903,500 ft S : reference area T0 : temperature across the combustor Th : thrust u : speed along the vehicle x-axis V : vehicle velocity X : force along the vehicle x-axis Z : force along the vehicle z-axis α : angle of attack γ : flight path angle (γ = θ − α) δe : pitch control surface deflection δt : throttle setting ∆τ1 : fore-body elastic mode shape ∆τ2 : after-body elastic mode shape ζ1 : damping ratio of the first vibration mode η : generalized elastic coordinate 1 American Institute of Aeronautics and Astronautics 12th AIAA International Space Planes and Hypersonic Systems and Technologies 15 19 December 2003, Norfolk, Virginia AIAA 2003-7081 Copyright © 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. ηf : fuel equivalence ratio, ṁf fsṁair θ : pitch angle μg : gravitational constant ρ : density of air ω1 : natural frequency of the first vibration mode 0n×m: the n×m zero matrix Subscripts A : due to aerodynamics E : due to external nozzle T : due to engine thrust 0 : trim condition ∞ : free stream condition

Flight Dynamics and Control of Air-Breathing Hypersonic Vehicles: Review and New Directions

At the end of the e rst century of manned, powered e ight, it is worthwhile to look backward to understand how innovation in airplane design required developments in aeroelasticity and how aeroelasticity has played a role in shaping the e rst 100 years of aircraft design. The insights gained will help to predict how and where aeroelasticity and aeroservoelasticity will ine uence the future development of efe cient, more capable, innovative air vehicles, and dee nethe needs fortechnology and tools to enable this future. By dee nition, all new aircraft begin as unconventional to a certain extent. Designs that never see universal use remain curiosities, but still help our questforbettervehiclesandguidethedevelopmentofanalysis, design, and testing tools. Innovative, nontraditional designsaffected by aeroelasticconsiderations haveincluded obliquewing aircraft, forward-swept wing aircraft,Xwings,e ying wings, andlargejoined wings. Designsthat wereunusually innovativeat thetimeoftheirintroduction but later became widespread include the swept-back wing jet, the T-tail, and the e y-by-wire control cone gured vehicle. Control and exploitation of aeroelasticity depends on the continued development of new materials, new structuralandaerodynamicconcepts,sensors,actuators,andactivecontroltechniques.Suchdevelopmentsmustbe accompanied by properintegratedanalysis/designtools,and,most importantly, by thesamehumaninquisitiveness and creativity that has driven aircraft design for over a century. This paper uses the history of nonconventional airplane cone gurations to review some of the steps taken during the past century to establish aeroelastic effects as integrated design features that must be anticipated, controlled, and exploited. The paper goes on to discuss the potential impact of past lessons on emerging airplane cone gurations currently in various stages of study and development.

Aeroelasticity of Nonconventional Airplane Configurations-Past and Future

In this paper, we provide an overview of scramjet-powered hypersonic vehicle modeling and control challenges. Such vehicles are characterized by unstable non-minimum phase dynamics with significant coupling and low thrust or FER (normalized fuel equivalency ratio) margins. Recent trends in hypersonic vehicle research is summarized. To illustrate control system design issues and tradeoffs, a generic nonlinear 3DOF longitudinal dynamics model capturing aero-elastic-propulsive interactions for wedge-shaped vehicle is used. The model is analyzed over a broad range of hypersonic flight conditions (i.e. operating points). The paper highlights how vehicle level-flight static (trim) and dynamic properties change over the trimmable air-breathing corridor (∼ Mach 4.75-12.6, 70-115 kft). Particular attention is paid to thermal choking constraints imposed on control system design as a direct consequence of having a finite FER margin. The dependence of FER margin on altitude, Mach, and the bow flow turning angle is discussed. The latter depends on Mach, altitude, angle-of-attack (AOA), and forebody flexing. It is (briefly) discussed how FER margin can be estimated on the basis of Mach, altitude, and AOA if a flexing upper bound is assumed. The implication of this state-dependent nonlinear FER margin constraint as well as that of the right half plane (RHP) zero, associated with the elevator-flight path angle (FPA) map, on control system bandwidth (BW) and FPA tracking are discussed. It is argued that while the non-minimum phase zero limits the achievable closed loop FPA BW, FER coupling into FPA can be used to address this. This, however, is limited by FER margin limits and may impose constraints on the size of the FPA (and velocity) commands that can be followed. This is particularly important because the vehicle is inherently unstable which implies a closed loop system (with a finite downward gain margin) that can become destabilized if driven sufficiently deep into control saturation. A consequence of this is that designers must take note of the fact that FPA commands which are sufficiently large and/or rapid may be impossible to follow with the desired level of fidelity. This is quantified within the paper. Speed command following issues are also discussed.

Modeling and Control of Scramjet-Powered Hypersonic Vehicles: Challenges, Trends, & Tradeoffs

The testing of aeroelastically and aerothermoelastically scaled wind-tunnel models in hypersonic flow is not feasible; thus, computational aeroelasticity and aerothermoelasticity are essential to the development of hypersonic vehicles. Several fundamental issues in this area are examined by performing a systematic computational study of the hypersonic aeroelastic and aerothermoelastic behavior of a three-dimensional configuration. Specifically, the flutter boundary of a low-aspect-ratio wing, representative of a fin or control surface on a hypersonic vehicle, is studied over a range of altitudes using third-order piston theory and Euler and Navier-Stokes aerodynamics. The sensitivity of the computational-fluid-dynamics-based aeroelastic analysis to grid resolution and parameters governing temporal accuracy are considered. In general, good agreement at moderate-to-high altitudes was observed for the three aerodynamic models. However, the wing flutters at unrealistic Mach numbers in the absence of aerodynamic heating. Therefore, because aerodynamic heating is an inherent feature of hypersonic flight and the aeroelastic behavior of a vehicle is sensitive to structural variations caused by heating, an aerothermoelastic methodology is developed that incorporates the heat transfer between the fluid and structure based on computational-fluid-dynamics-generated aerodynamic heating. The aerothermoelastic solution procedure is then applied to the low-aspect-ratio wing operating on a representative hypersonic trajectory. In the latter study, the sensitivity of the flutter margin to perturbations in trajectory angle of attack and Mach number is considered. Significant reductions in the flutter boundary of the heated wing are observed. The wing is also found to be susceptible to thermal buckling.

Aeroelastic and Aerothermoelastic Behavior in Hypersonic Flow

Aeroelastic analysis of hypersonic vehicles

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Aeroelasticity of a Generic Hypersonic Vehicle

An aeroelastic and aerothermoelastic analysis of a three-dimensional low aspect ratio wing, representative of a fin on hypersonic vehicles, is carried out using piston theory, and Euler aerodynamics. Studies on grid convergence are used to determine the appropriate computational domain and resolution for this wing in hypersonic flow, using both Euler and Navier-Stokes aerodynamics. Hypersonic computational aeroelastic responses are then generated, using Euler aerodynamics in order to obtain frequency and damping characteristics for comparison with those from first and third order piston theory solutions. Results indicate that the aeroelastic behavior is comparable when using Euler and third order piston theory aerodynamics. The transonic aeroelastic behavior of the wing is also analyzed using Euler aerodynamics. The aerothermoelastic behavior of the wing, using piston theory aerodynamics, is studied by incorporating material property degradation and thermal stresses due to non-uniform temperature distributions. Results indicate that aerodynamic heating can substantially reduce aeroelastic stability. Finally, hypersonic aeroelastic behavior of a generic vehicle resembling a reusable launch vehicle is performed using piston theory. The results presented serve as a partial validation of the CFL3D code for the hypersonic flight regime.

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Hypersonic Aerothermoelastic Studies for Reusable Launch Vehicles

The aeroelastic and aerothermoelastic behavior of three-dimensional configurations in hypersonic flow regime are studied. The aeroelastic behavior of a low aspect ratio wing, representative of a fin or control surface on a generic hypersonic vehicle, is examined using third order piston theory, Euler and Navier-Stokes aerodynamics. The sensitivity of the aeroelastic behavior generated using Euler and Navier-Stokes aerodynamics to parameters governing temporal accuracy is also examined. Also, a refined aerothermoelastic model, which incorporates the heat transfer between the fluid and structure using CFD generated aerodynamic heating, is used to examine the aerothermoelastic behavior of the low aspect ratio wing in the hypersonic regime. Finally, the hypersonic aeroelastic behavior of a generic hypersonic vehicle with a lifting-body type fuselage and canted fins is studied using piston theory and Euler aerodynamics for the range of 2.5 M 28, at altitudes ranging from 10,000 feet to 80,000 feet. This analysis includes a study on optimal mesh selection for use with Euler aerodynamics. In addition to the aeroelastic and aerothermoelastic results presented, three time domain flutter identification techniques are compared, namely the moving block approach, the least squares curve fitting method, and a system identification technique using an Auto-Regressive model of the aeroelastic system. In general, the three methods agree well. The system identification technique, however, provided quick damping and frequency estimations with minimal response record length, and therefore oers significant reductions in computational cost. In the present case, the computational cost was reduced by 75%. The aeroelastic and aerothermoelastic results presented illustrate the applicability of the CFL3D code for the hypersonic flight regime.

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B. J. Thuruthimattam

Papers

Aeroelastic and Aerothermoelastic Behavior in Hypersonic Flow

Aeroelastic analysis of hypersonic vehicles

Aeroelasticity of a Generic Hypersonic Vehicle

Hypersonic Aerothermoelastic Studies for Reusable Launch Vehicles

Three-Dimensional Aeroelastic and Aerothermoelastic Behavior in Hypersonic Flow