We address the design of a flight vehicle from the viewpoint of a system of systems and we discuss the integration of the individual technical disciplines. Then a conceptual fundamental methodology and tools required for the analysis, design, and optimization of aerospace vehicles in terms of the efficient use of on-board energy are discussed. This suggests changing the design paradigm to the optimization of a system of energy systems. We propose a foundation for system-level design with optimization based on minimum exergy destruction.

Aircrafts have evolved to a point where they are extremely complex machines posing a highly integrated design problem. A military vehicle includes many systems that are all interrelated and dependent on power (or energy) in some form. In some of the systems there is also the creation of by-products, in the form of heat energy, that have to be removed from that equipment. There obviously exist methods for the design of all these systems, based on the evolutionary nature of vehicle development. However, the more we depart from existing data bases and experience levels, the less confidence we can have that we are close to an optimal design. In addition, many of the classical techniques are based on simplifying assumptions that were used in the original derivation. If these are not considered, then there is no guide to when those classical techniques no longer give an acceptable solution.

The need exists for a methodology that can support design of the complete vehicle as a system of systems in a common framework. It must allow consideration of all aspects in terms of common metrics in order to conduct fully credible trades. The vision is to develop such a methodology that will support all required levels of design activity in a natural fashion, from conceptual comparisons through the final configuration and lead to a true system-level optimized design. In nature, if something is inefficient, then it dies out or it adapts to the environment. We claim that this “inefficiency” can be quantified in terms of useful work versus energy wasted. We also consider an aerospace system as consuming fuel and doing work in some form. Lower specific fuel consumption of the engine is one dominant factor, but when this is considered completely separated from vehicle application, then the system will probably not be truly optimized. Therefore, there is a need for tools and processes enabling the discovery of new and innovative configurations by designing for maximum efficiency and minimum energy waste at the overall optimum system design level, subject to the appropriate constraints of course [

We suggest that the first question in the design process of any flight vehicle should be whether the solution is expected (or desired) to be evolutionary or revolutionary. Evolutionary can be defined as applying to concepts for which there is significant existing data on “similar’’ configurations. There is then some confidence in the extrapolation of that data. A revolutionary concept, with no prior flight experience, requires a more rigorous analytical process. Any application within these extremes will differ in detail, but the object of this paper is to suggest that a system-level process for integrating the various technical disciplines requires a “common currency’’, which is exergy.

Consider the process for a new aircraft design. The concurrent design process relies on some form of system analysis or systems engineering [

break the system down into component parts,

understand each individual part,

determine how the parts interact,

define the contribution of each to system performance,

put the system back together,

build it when the analysis shows that the design meets requirements.

Written in this form, then Systems Engineering seems like a straightforward sequential process. So we can ask what are the problems with Systems Engineering?

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For an aircraft in cruise, lift must be equal to weight. So that, if

In one theory, that vertical force is created by giving vertical momentum to a quantity of freestream air as the vehicle passes through it. The “laws of physics’’, however, are written in the format of air flowing horizontally into the control volume and leaving with some vertical velocity. Force equals rate of change of momentum, exact physics.

The above discussion helps little in designing a flight vehicle: it is the engine “that is doing all the work” in cruise. So, the practical physics is to analyze the vertical work in terms of induced drag. This was done with assumptions to get an analytical solution followed by adjustments to match “real data”. If we simplify conventional aircraft design, the process was to optimize the various parts, then build multiple prototypes against a set of requirements and fly them to choose the winner.

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As a final comment for this section, we strongly suggest that “System Engineering’’ should not be applied as a sequential process, but as an integrated and continual iterative process.

Great many physical problems have been solved at this stage of flight vehicle evolution. As previously stated, “the laws of physics” are frequently written in a form suitable for a specific application, and that application has not been the complete system. Considering a flight vehicle as device to do work (expanding the approach by the first author, Moorhouse [

As we have discussed, consideration of a system does require the design team to address the components. A critical part of this was the development of a decomposition strategy where all the subsystem components can be optimized to a system-level metric; see Munoz and von Spakovsky [

Periannan et al. [

Effect of optimization metric on generic high-speed fighter as exergy destruction. Minimizing exergy destruction directly gives the same overall performance as minimizing take-off weight; the advantage with exergy destruction is to identify areas for improvement.

Effect of optimization metric on a generic high-speed fighter as fuel weight. Minimizing exergy destruction directly yields a design with improved fuel consumption (less fuel weight).

Riggins et al. [

Entropy generation rate: blunt body at flight Mach of 10. The contribution of wake entropy production is significant. However, with strategic use of energy deposition to weaken the shock, total entropy production, and subsequent drag, is reduced.

Alabi et al. [

The development of advanced systems concepts such as a hypersonic transatmospheric vehicle described will require many key elements to be in place before even a feasible design is possible. For an optimized design, the need to understand the proper theoretical context for the generalized second-law principle becomes critical. The proper context [

The Euler (or Navier-Stokes) equations can be written in compact notation

The total specific energy is

Setting the constant reference state

The balance of entropy emerges from (

Work in this direction couples the solution of the governing equations with the satisfaction of the second law of thermofluid dynamics as an additional constraint. A step towards this goal is to utilize the second law as an

A numerical method applied to the solution of the governing equations aims at calculating the updated variables from the presently known distribution at a previous time step. The space and time integration of the balance equations can be separated so that the resulting finite-volume, semidiscrete formula

Entropy production rates computed with high-fidelity CFD code can be usefully identify regions of numerical error and where the second-law is violated. For system design, the computation would be a useful way of identifying regions of irreversible waste.

For the Euler equations, which represent the flow of an inviscid, adiabatic fluid, the entropy balance equation yields zero net entropy generation. However, under flow conditions where shocks are expected, entropy generation must be positive. This is why numerical “shock capturing” methods in CFD require a finite amount of numerical dissipation both for numerical stability and also to guarantee that only compressive shocks show up in the solution. For the Navier-Stokes equations, where viscosity and heat conduction are accounted for, the second law of thermodynamics yields two expressions for the entropy generation rate, one that serves as a constitutive constraint on the sign of the viscosity and heat conduction coefficient, and another which represents the transport of entropy with fluid flow, as shown in (

We seek to develop the computational capability to analyze systems in terms of the second law of thermodynamics. Since existing methods for modeling, simulating, analyzing, and designing aerospace vehicles rely solely on the conservation of energy, they fail to capture the subtle effects of the second law. More importantly, if we satisfy the first law of thermodynamics as a conservation of energy principle, we do not preclude the possibility of generating a design that violates the second law by creating a perpetual motion machine of the second kind. Including the second law of thermodynamics will then make it possible to perform numerical experiments for the synthesis and design of novel aerospace systems optimized according to a more complete physical theory. A generalized form of the second law beyond classical thermodynamics will thus provide the essential guidelines for developing the correct optimization principles for minimizing entropy-generation/exergy-destruction.

A very important factor in system development is the current emphasis on modeling and simulation-based acquisition. There has to be an assumption that the models will be “sufficiently accurate”. This subjective requirement means that the operational system is satisfactory. First, we can consider the possibilities of reduced emphasis on ground testing in favor of more emphasis on modeling and simulation in the development of any new system. Second is the desire to reduce the number of flight tests. The work in [

Modeling is a fundament of design; methods that enable the design and optimization of

The major purpose of this overall work has been to develop, understand, and apply entropy production as the criterion to evaluate vehicle performance and performance losses consistently. This forms the basis for the use of a “common currency’’ in the system-level vehicle design process—the use of energy concepts is one example of where entropic analysis may be of great use. Such capability may allow the development of new and innovative concepts that do not “just’’ marginally improve performance but may enable the realization of entire new regimes of performance and operability, especially for high-speed aerospace vehicles. The overall vision for this research is to prove that a complete integration of exergy-based methods promises to facilitate a breakthrough in optimization of aerospace vehicles based on a system of energy systems. The current evolutionary designs have achieved a high degree of efficiency building on previous experiences. Evolutionary methods only work, however, when the configuration under consideration is close to the existing experience and database. We have proposed a foundation for system-level design with optimization based on minimum exergy destruction. The potential of these new methods will be realized when we attempt the next revolution for which there is no previous flight experience.

The authors gratefully acknowledge the support of the US Air Force Office of Scientific Research under the program management of Dr. Victor Giurgiutiu (Structural Mechanics) and the USAF Small Business Innovative Research (SBIR) Program for funding to our industrial partners.