Craig A. Steeves

Feasibility of Metallic Structural Heat Pipes as Sharp Leading Edges for Hypersonic Vehicles

Hypersonic flight with hydrocarbon-fueled airbreathing propulsion requires sharp leading edges. This generates high temperatures at the leading edge surface, which cannot be sustained by most materials. By integrating a planar heat pipe into the structure of the leading edge, the heat can be conducted to large flat surfaces from which it can be radiated out to the environment, significantly reducing the temperatures at the leading edge and making metals feasible materials. This paper describes a method by which the leading edge thermal boundary conditions can be ascertained from standard hypersonic correlations, and then uses these boundary conditions along with a set of analytical approximations to predict the behavior of a planar leading edge heat pipe. The analytical predictions of the thermostructural performance are verified by finite element calculations. Given the results of the analysis, possible heat pipe fluid systems are assessed, and their applicability to the relevant conditions determined. The results indicate that the niobium alloy Cb-752, with lithium as the working fluid, is a feasible combination for Mach 6-8 flight with a 3 mm leading edge radius.

Plasma-Enhanced Hypersonic Performance Enabled by MHD Power Extraction

This paper reviews work underway on the development of new technologies that will enhance the performance of hypersonic vehicles through plasma-related processes and the utilization of MHD to provide the large power levels that are required to drive these processes. Three technologies are discussed for plasma-enhanced hypersonic performance. These include: (1) the use of off-body plasmas for drag reduction, steering, and enhanced inlet performance; (2) the use of surface or near-surface plasmas for mitigating local heating and controlling separation; and (3) the use of electron beam and plasma processes for controlling combustion for enhanced performance inside the engine and for local heat addition applications in other regions of the flow path. For realistic scale vehicles, the energies required for these applications exceed the present capability of on-board auxiliary power units, and, therefore, will require power to be generated directly from the hypersonic air passing over the vehicle or through the engine. In the high Mach number regime characteristic of re-entry vehicles, there is sufficient heating of the air to allow MHD power extraction using equilibrium ionization of alkali vapor seed material. By replacing a portion of the vehicle surface with a hollow core truss structure containing an embedded magnet coil, hundreds of kilowatts of power can be extracted during re-entry and used for vehicle control or other applications. At lower Mach numbers, MHD power extraction can be done downstream of the engine, then the temperature of the exhaust can be high enough to allow conductivity to be achieved with alkali seeding. This MHD generated power extracted from the flow aft of the engine can be used for plasma control upstream of the engine as well as for engine performance enhancement. Some aspects of this reverse energy bypass concept are analyzed in the paper, including plasma heating of the inlet flow that would allow elimination of the isolator, snowplow surface arcs for boundary layer and separation control, and electron beam and microwaves for initiation and control of combustion.

Curved fiber paths optimization of a composite cylindrical shell via Kriging-based approach

While conventional design and manufacturing techniques of fiber-reinforced laminates keep the fiber orientation angle constant within a layer, automated tow-placement technology allows fabricating laminates with curved fibers. This offers more flexibility to tailor the mechanical properties and improve the performance of laminated structures. Exploiting this flexibility requires an efficient method for finding optimal or near-optimal fiber configurations. In this paper, laminated cylindrical shells are studied. Curvilinear variations for the fiber orientations are adopted in the circumferential and longitudinal directions. The computational burden, typical in numerical optimization of complex structures, is reduced using a Kriging model, which substitutes for direct finite element simulation. A sequential quadratic programming algorithm is employed as local optimizer, coupled with a restart strategy to search for the global optimum in the entire design space. Some numerical cases are presented: the maximization of the fundamental frequency of the shell considering different boundary conditions and the minimization of the maximum displacement with a constraint on the buckling load.

Optimal curved fibre orientations of a composite panel with cutout for improved buckling load using the Efficient Global Optimization algorithm

ABSTRACT Composite structures with cutouts (like panels with holes) are a challenge to design because discontinuities of this kind provoke stress concentrations and become critical regions. With curved fibres, the effect of these discontinuities can be decreased by choosing the fibre paths properly. In this article, fibre-path optimization to improve the buckling load of laminated composite panels with cutouts is studied. Two fibre path parameterizations are tested: the usual curvilinear Cartesian and the radial one, proposed in this article, in which the fibre orientations vary linearly with the Euclidean distance from the centre of the panel. To reduce the simulation costs associated with the optimization, the Efficient Global Optimization (EGO) algorithm is used. EGO is a technique based on a stochastic process approach (Kriging) that approximates expensive-to-evaluate functions and sequentially maximizes the expected improvement to update the surrogate at each iteration. A stiffened panel with a cutout subjected to compression and in-plane shearing loads is analysed. The results show that the buckling load when curved fibres are used is substantially higher than the buckling load for straight-fibre laminates. In addition, the optimization framework indicates a low final computational burden.

Design of a Robust, Multifunctional Thermal Protection System Incorporating Zero Expansion Lattices

On hypersonic vehicles flying within the atmosphere, viscous drag on acreage surfaces, even far from the hot leading edges, generates high temperatures, and, consequently, high thermal stresses and strains. This leads to two significant problems in the design of hypersonic airbreathers: insulation of the cool internal volume of the vehicle from the inclement exterior conditions, and reducing geometric changes in the vehicle surface to preserve aerodynamic shapes and to make it possible to connect the aeroshell to the cool structure within. A multifunctional sandwich structure incorporating very low density insulation as the core, with the structural stiffness provided by a hot face consisting of a low (or zero) thermal expansion lattice, fulfills these requirements. Here we analyze the feasibility of such a structure and discuss design and manufacturing issues, concentrating particularly on bonded joints in the lattice and connections between the hot face and the cool internal structure.Copyright

Thermal Actuation Through Bimaterial Lattices

The goal of this study is to examine the theoretical capability of bimaterial lattices as thermally driven actuators. The lattices are composed of planar non-identical cells. Each cell consists of a skewed hexagon surrounding an irregular triangle; the skew angles of the hexagon and the ratio of the coefficients of thermal expansion (CTEs) of the two component materials determine the overall performance of the actuator. Such a cell has three tailorable CTEs along the lines connecting the points where adjoining cells are connected. Each individual cell and a lattice consisting of such cells can be strongly anisotropic in terms of thermal expansion. While these lattice cells have been used as stress-free connectors for components with differing CTEs, they have not been explored for their actuation capacity. This paper develops models for bimaterial lattices that can be used as mechanical actuators for valves, switches and differential motion. A general procedure for lattice design includes drawing of its skeleton, which identifies the points at which a lattice cell is connected to other cells or substrates; calculation of three CTEs in each cell depending upon the functionality desired; choosing lattice materials; and finding of the skew angles for each cell as solutions of three nonlinear algebraic equations. By changing materials and geometry, we can determine the change of their configuration when the temperature changes. This paper illustrates the concepts with several examples: a two-cell lattice that is connected to a substrate that functions as a lever in a switch; a three-cell lattice that serves as a valve; and a lattice that controls the maximum total deflection of two adjoining parts of a structure.Copyright

Effect of grain size on the optimal architecture of electrodeposited metal/polymer microtrusses

Nanocrystalline microtruss materials are novel cellular hybrids of metal and polymer produced by electrodepositing thin coatings of nanocrystalline metal over rapid prototyped polymer preforms. This study develops an optimisation method for the architectural design of electrodeposited metal/polymer composite microtrusses used as cores in sandwich beams. For an optimally designed structure employing conventional polycrystalline nickel, a direct substitution of nanocrystalline nickel will improve structural performance; however, it is likely that the structure will also become significantly sub-optimal. Achieving optimal design with nanocrystalline nickel entails large geometric changes from the conventional polycrystalline case. The same applies if the polymer preform is removed after electrodeposition. The strong connection between optimal architecture and grain size was therefore examined for the limiting cases of polymer-filled and hollow microtrusses. It was found that grain size reduction was more important than polymer preform removal such that grain size effects dominate over the majority of microtruss design space.

Design and Manufacture of a Morphing Structure for a Shape-Adaptive Supersonic Wind Tunnel Nozzle

Aerospace vehicles with fixed geometry are designed to operate at a predetermined flight condition. Variation of the aerodynamic environment, such as during acceleration, climbing, or turning, from the design condition reduces the efficiency of the vehicle. It would be advantageous to be able to adapt the vehicle geometry to maintain efficient flight over a range of aerodynamic conditions. Morphing sandwich structures offer sufficient strength and stiffness to serve as aerodynamic surfaces, while providing the shape-changing authority to attain a range of surface profiles without additional joints or seals. As a demonstration of the morphing concept in a supersonic environment, this paper describes the construction and testing of a morphing nozzle for a supersonic wind tunnel, which has been designed to operate isentropically over a Mach range from 2.5 to 3.8. The nozzle has been installed and operated in this Mach number range and the experimental results are presented.

A Heat Plate Leading Edge for Hypersonic Vehicles

The intense thermal flux at the leading edges of hypersonic vehicles (traveling at Mach 5 and greater) requires creative thermal management strategies to prevent damage to leading edge components. Carbon fiber composites and/or ablative coatings have been widely utilized to mitigate the effects of the impinging heat flux. This paper focuses on an alternative, metallic leading edge heat pipe concept which combines efficient structural load support and thermal management. The passive concept is based on high thermal conductance heat pipes which redistribute the high heat flux at the leading edge stagnation point through the evaporation, vapor flow, and condensation of a working fluid to a location far from the heat source. Structural efficiency is provided by a sandwich construction using an open-cell core that also allows for vapor flow. A low temperature proof-of-concept copper–water system has been investigated by experimentation. Measuring of the axial temperature profile indicates effective spreading of thermal energy, a lowering of the maximum temperature and reduced overall thermal gradient compared to a non-heat pipe leading edge. A simple transient analytical model based on lumped thermal capacitance theory is compared with the experimental results. The low-temperature prototype shows potential for higher temperature metallic leading edges that can withstand the hypersonic thermo-mechanical environment.Copyright

A Magnetohydrodynamic Power Panel for Space Reentry Vehicles

During reentry from space, a layer of high temperature air (>3000 K) is formed extending tens of centimeters from the surface of the vehicle, well out into the high speed flow regime. Magnetohydrodynamics (MHD) can then be used to generate power by projecting magnetic fields outside the vehicle into the conducting air stream and collecting the resulting current. Here, we analyze a multifunctional MHD panel which generates the requisite magnetic fields, protects the vehicle from high temperatures, and is structurally stiff and strong. The analysis shows that a magnetic system weighing approximately 110 kg can generate 0.6 MW of power for 1000 s.