Numerical Propulsion System Simulation

NASA and its contractor colleagues soon found another use for computers to help improve engine performance. In fact, looking back at the history

of NASA’s involvement with improving propulsion technology, a trilogy of major categories of advances can be suggested based on the develop­ment of the computer and its evolution in the role that electronic think­ers have played in our culture.

Part one of this story includes all the improvements NASA and its industry partners have made with jet engines before the computer came along. Having arrived at a basic operational design for a turbojet engine—and its relations, the turboprop and turbofan—engineers sought to improve fuel efficiency, reduce noise, decrease wear, and otherwise reduce the cost of maintaining the engines. They did this through such efforts as the Quiet Clean Short Haul Experimental Engine and Aircraft Energy Efficiency program, detailed earlier in this case study. By tinker­ing with the individual components and testing the engines on the ground and in the air for thousands of hours, incremental advances were made.[1338]

Part two of the story introduces the capabilities made available to engineers as computers became powerful enough and small enough to be incorporated into the engine design. Instead of requiring the pilot to manually make occasional adjustments to the engine operation in
flight depending on what the instruments read, a small digital computer built into the engine senses thousands of measurements per minute and caused an equal number of adjustments to be made to keep the power – plant performing at peak efficiency. With the Digital Electronic Engine Control, engines designed years before behaved as though they were fresh off the drawing boards, thanks to their increased capabilities.[1339]

Having taken engine designs about as far as it was thought possible, the need for even more fuel-efficient, quieter, and capable engines con­tinued. Unfortunately, the cost of developing a new engine from scratch, building it, and testing it in flight can cost millions of dollars and take years to accomplish. What the aerospace industry needed was a way to take advantage of the powerful computers available at the dawn of the 21st century to make the engine development process less expen­sive and timelier. The result was part three of NASA’s overarching story of engine development: the Numerical Propulsion System Simulation (NPSS) program.[1340]

Working with the aerospace industry and academia, NASA’s Glenn Research Center led the collaborative effort to create the NPSS pro­gram, which was funded and operated as part of the High Performance Computing and Communications program. The idea was to use modern simulation techniques and create a virtual engine and test stand within a virtual wind tunnel, where new designs could be tried out, adjustments made, and the refinements exercised again without costly and time-con­suming tests in the "real” world. As stated in a 1999 industry review of the program, the NPSS was built around inclusion of three main ele­ments: "Engineering models that enable multi-disciplinary analysis of large subsystems and systems at various levels of detail, a simulation environment that maximizes designer productivity and a cost-effective, high-performance computing platform.”[1341]

In explaining to the industry the potential value of the program dur­ing a 2006 American Society of Mechanical Engineers conference in

Spain, a NASA briefer from Glenn suggested that if a standard turbo­jet development program for the military—such as the F100—took 10 years, $1.5 billion, construction of 14 ground-test engines, 9 flight-test engines, and more than 11,000 hours of engine tests, the NPSS pro­gram could realize a:

• 50-percent reduction in tooling cost.

• 33-percent reduction in the average development engine cost.

• 30-percent reduction in the cost of fabricating, assembling, and testing rig hardware.

• 36-percent reduction in the number of development engines.

• 60-percent reduction in total hardware cost.[1342]

A key—and groundbreaking—feature of NPSS was its ability to inte­grate simulated tests of different engine components and features, and run them as a whole, fully modeling all aspects of a turbojet’s operation. The program did this through the use of the Common Object Request Broker Architecture (CORBA), which essentially provided a shared lan­guage among the objects and disciplines (mechanical, thermo-dynam­ics, structures, gas flow, etc.) being tested so the resulting data could be analyzed in an "apples to apples” manner. Through the creation of an NPSS developer’s kit, researchers had tools to customize the software for individual needs, share secure data, and distribute the simulations for use on multiple computer operating systems. The kit also provided for the use of CORBA to "zoom” in on the data to see specific informa­tion with higher fidelity.[1343]

Begun in 1997, the NPSS team consisted of propulsion experts and software engineers from GE, Pratt & Whitney, Boeing, Honeywell, Rolls – Royce, Williams International, Teledyne Ryan Aeronautical, Arnold Engineering Development Center, Wright-Patterson AFB, and NASA’s

Glenn Research Center. By the end of the 2000 fiscal year, the NPSS team had released Version 1.0.0 on schedule. According to a summary of the program produced that year:

(The new software) can be used as an aero-thermodynamic zero-dimensional cycle simulation tool. The capabilities include text-based input syntax, a sophisticated solver, steady- state and transient operation, report generation, a built-in object-oriented programming language for user-definable components and functions, support for distributed running of external codes via CORBA, test data reduction, interactive debug capability and customer deck generation.[1344]

Additional capabilities were added in 2001, including the ability to support development of space transportation technologies. At the same time, the initial NPSS software quickly found applications in aviation safety, ground-based power, and alternative energy devices, such as fuel cells. Moreover, project officials at the time suggested that with the fur­ther development of the software, other applications could be found for the program in the areas of nuclear power, water treatment, biomedi­cine, chemical processing, and marine propulsion. NPSS proved to be so capable and promising of future applications that NASA designated the program a cowinner of the NASA Software of the Year Award for 2001.[1345]

Work to improve the capabilities and expand the applications of the software continued, and, in 2008, NASA transferred NPSS to a consor­tium of industry partners, and, through a Space Act Agreement, it is cur­rently offered commercially by Wolverine Ventures, Inc., of Jupiter, FL. Now at Version 1.6.5, NPSS’s features include the ability to model all types of complex systems, plug-and-play interfaces for fluid properties, built-in plotting package, interface to higher fidelity legacy codes, mul­tiple model views, command language interpreter with language sen­sitive text editor, comprehensive component solver, and variable setup controls. It also can operate on Linux, Windows, and UNIX platforms.[1346]

Originally begun as a virtual tool for designing new turbojet engines, NPSS has since found uses in testing rocket engines, fuel cells, analog controls, combined cycle engines, thermal management systems, air­frame vehicles preliminary design, and commercial and military engines.[1347]

Ultra Efficient Engine Technology Program

With the NPSS tool firmly in place and some four decades of experience incrementally improving the design, operation, and maintenance of the jet engine, it was time to go for broke and assemble an ultra­bright team of engineers to come up with nothing short of the best jet

Building on the success of technology development programs such as the Quiet Clean Short Haul Experimental Engine and Energy Efficient Engine project—all of which led directly to the improvements and production of turbojet engines now propelling today’s commercial airliners—NASA approached the start of the 21st century with plans to take jet engine design to accomplish even more impressive feats. In 1999, the Aeronautics Directorate of NASA began the Ultra Efficient Engine Technology (UEET) program—a 5-year, $300-million effort— with two primary goals. The first was to find ways that would enable further improvements in engine efficiency to reduce fuel burn and, as a result, carbon dioxide emissions by yet another 15 percent. The second was to continue developing new materials and configuration schemes in the engine’s combustor to reduce emissions of nitrogen oxides (NOx) during takeoff and landings by 70 percent relative to the standards detailed in 1996 by the International Civil Aviation Organization.[1348]

NASA’s Glenn Research Center led the program, with participation from three other NASA Centers: Ames, Langley, and the Goddard Space Flight Center in Greenbelt, MD. Also involved were GE, Pratt & Whitney, Honeywell, Allison/Rolls-Royce, Williams International, Boeing, and Lockheed Martin.[1349]

The program was comprised of seven major projects, each of which addressed particular technology needs and exploitation opportunities.[1350] The Propulsion Systems Integration and Assessment project examined overall component technology issues relevant to the UEET program to help furnish overall program guidance and identify technology short­falls.[1351] The Emissions Reduction project sought to significantly reduce NOx and other emissions, using new combustor concepts and tech­nologies such as lean burning combustors with advanced controls and high-temperature ceramic matrix composite materials.[1352] The Highly Loaded Turbomachinery project sought to design lighter-weight, reduced – stage cores, low-pressure spools and propulsors for more efficient and environmentally friendly engines, and advanced fan concepts for qui­eter, lighter, and more efficient fans.[1353] The Materials and Structures for High Performance project sought to develop and demonstrate high – temperature material concepts such as ceramic matrix composite combustor liners and turbine vanes, advanced disk alloys, turbine air­foil material systems, high-temperature polymer matrix composites, and innovative lightweight materials and structures for static engine struc – tures.[1354] The Propulsion-Airframe Integration project studied propul­sion systems and engine locations that could furnish improved engine and environmental benefits without compromising the aerodynamic performance of the airplane; lowering aircraft drag itself constituted a highly desirable means of reducing fuel burn, and, hence, CO2 emis­sions will develop advanced technologies to yield lower drag propulsion system integration with the airframe for a wide range of vehicle classes. Decreasing drag improves air vehicle performance and efficiency, which
reduces fuel burn to accomplish a particular mission, thereby reducing the CO2 emissions.[1355] The Intelligent Propulsion Controls Project sought to capitalize upon breakthroughs in electronic control technology to improve propulsion system life and enhance flight safety via integrating informa­tion, propulsion, and integrated flight propulsion control technologies.[1356] Finally, the Integrated Component Technology Demonstrations project sought to evaluate the benefits of off-the-shelf propulsion systems inte­gration on NASA, Department of Defense, and aeropropulsion industry partnership efforts, including both the UEET and the military’s Integrated High Performance Turbine Engine Technology (IHPTET) programs.[1357]

By 2003, the 7 project areas had come up with 10 specific technol­ogy areas that UEET would investigate and incorporate into an engine that would meet the program’s goals for reducing pollution and increas­ing fuel burn efficiency. The technology goals included:

1. Advanced low-NOx combustor design that would feature a lean burning concept.

2. A highly loaded compressor that would lower system weight, improve overall performance, and result in lower fuel burn and carbon dioxide emissions.

3. A highly loaded, high-pressure turbine that could allow a reduction in the number of high-pressure stages, parts count, and cooling requirements, all of which could improve fuel burn and lower carbon dioxide emissions.

4. A highly loaded, low-pressure turbine and aggressive tran­sition duct that would use flow control techniques that would reduce the number of low-pressure stages within the engine.

5. Use of a ceramic matrix composite turbine vane that would allow high-pressure vanes to operate at a higher
inlet temperature, which would reduce the amount of engine cooling necessary and result in lower carbon diox­ide emissions.

6. The same ceramic matrix composite material would be used to line the combustor walls so it could operate at a higher temperature and reduce NOx emissions.

7. Coat the turbine airfoils with a ceramic thermal barrier material to allow the turbines to operate at a higher tem­perature and thus reduce carbon dioxide emissions.

8. Use advanced materials in the construction of the tur­bine airfoil and disk. Specifically, use a lightweight single crystal superalloy to allow the turbine blades and vanes to operate at a higher temperature and reduce carbon dioxide emissions, as well as a dual microstructure nickel – base superalloy to manufacture turbine disks tailored to meet the demands of the higher-temperature environment.

9. Determine advanced materials and structural concepts for an improved, lighter-weight impact damage tolerance and noise-reducing fan containment case.

10. Develop active tip clearance control technology for use in the fan, compressor, and turbine to improve each compo­nent’s efficiency and reduce carbon dioxide emissions.[1358]

In 2003, the UEET program was integrated into NASA’s Vehicle Systems program to enable the enginework to be coordinated with research into improving other areas of overall aircraft technology. But in the wake of policy changes associated with the 2004 decision to redi­rect NASA’s space program to retire the Space Shuttle and return humans to the Moon, the Agency was forced to redirect some of its funding to Exploration, forcing the Aeronautics Directorate to give up the $21.6 mil­lion budgeted for UEET in fiscal year 2005, effectively canceling the big­gest and most complicate jet engine research program ever attempted. At the same time, NASA was directed to realign its jet engine research to concentrate on further reducing noise.[1359]

Nevertheless, results from tests of UEET hardware showed prom­ise that a large, subsonic aircraft equipped with some of the technologies detailed above would have a "very high probability” of achieving the pro­gram goals laid out for reducing emissions of carbon dioxide and other pollutants. The data remain for application to future aircraft and engine schemes.72