For Release: Oct. 16, 1997

Dom Amatore
Media Services Branch
(256) 544-0034
dom.amatore@msfc.nasa.gov

Release: 97-251

To climb quickly from the drawing board into space -- while challenged by constraints of time and money -- NASA engineers have discovered new ways to improve their historically methodical profession as they participate in development of the X-33 technology demonstrator.

The X-33 -- whose advanced technologies could be included on America’s next-generation space transportation system -- is one of the fastest concept-to-flight space technology programs ever undertaken. But to accomplish such an engineering and technical feat, engineers at NASA’s Marshall Space Flight Center in Huntsville, Ala., found many traditional concepts and practices had to be discarded.

NASA and its industry partners would have to chart a new course.

"In the past," said Joe Ruf, a Marshall fluid dynamics engineer, "we would have conducted a careful, systematic set of experiments on the X-33 test model, looking at the different effects of each test, and refining the model and test techniques as we went."

But one of the basic concepts of the X-33 -- to shave time and money off the traditional and prolonged process of design, development and flight of a new vehicle -- required a new look at old practices.

With the first test flight of the X-33 scheduled for 1999, "We couldn’t follow those practices with the X-33," said Ruf. "We don’t have the time and we don’t have the money. We had to break new ground. So we have performed much of our design, testing and analytical work in parallel. It’s a practice called ‘concurrent engineering’ -- a practice dictated by schedule and costs."

In recent months, the X-33 design has been assessed using two very different methods, representing both the traditional and the new, at the Marshall Center.

"What we’re doing here," explained Dr. Paul K. McConnaughey, chief of Marshall’s Fluid Dynamics Analysis Branch, "is attempting to design and analyze a type of configuration that we’ve never tested or flown before."

To accomplish this, he said, "We’ve deviated from tradition."

Tradition is exemplified by wind tunnel testing. Twenty different configurations of the X-33 demonstrator have undergone extensive tests in Marshall’s Trisonic Wind Tunnel -- with over 2,500 test runs completed since December 1996.

But Marshall engineers added another, newer methodology to the mix: computational fluid dynamics, which is an analytical prediction of a fluid’s behavior. For the analysis, physical and thermodynamic laws that describe the behavior of a fluid -- whether air, liquid oxygen, liquid hydrogen or other substances -- are written in computer code. The shape of the hardware to be analyzed is then described to the computational fluid dynamics code by a mesh or grid. The grid is a series of points in space on which the code predicts the fluid’s behavior.

With computational analysis, said McConnaughey, "We’ve broken new ground in the development of a launch vehicle. The tool of computational fluid dynamics has come into its own. It allows us to supplement our traditional database and adapt it specifically for the X-33 configuration."

Added Ruf: "You can do really rapid analysis with computational fluid dynamics, quickly gaining an idea of what a change in a vehicle’s configuration will do. This can be accomplished before you try to justify spending a lot more money on additional wind tunnel tests."

Computational fluid dynamics, said McConnaughey, "is a tool that is complementary to wind tunnel testing. Computational fluid dynamics enables us to do things that cannot be done in the wind tunnel -- to go beyond what the wind tunnel can do, to perform tests that would be more expensive or more dangerous in the wind tunnel."

Such as testing for the effects of the rocket engine exhaust gases (plumes) on a vehicle. "We need to know the effect of the plume on vehicle base pressure, or what the plume is doing to the vehicle’s forebody aerodynamics," said Ruf, whose branch has performed hundreds of analyses on rocket propulsion projects at Marshall.

"In wind tunnel tests on the X-33 at Marshall, no plume was used," said Ruf. "It’s simply too expensive and too dangerous to test with a hot plume that’s at about 5,000 degrees F."

An alternate method of testing, he said, is to substitute a cold plume, at about 500 degrees F, for the hot plume. "This is much less expensive and dangerous," said ruf. "However, there are several important effects that do not scale properly between a hot and cold plume. With computational fluid dynamics, we can model the hot plume gases in a computer, achieving two goals at once: lowering the cost of wind tunnel testing and eliminating the hazards associated with the 5,000-degree gases."

Computational fluid dynamics is being used by Marshall engineers, as well, to determine the aerodynamic loads or pressures on the X-33, analyzing the majority of the vehicle’s critical body components.

Airload pressure tests historically have been performed in the wind tunnel. "In a traditionally paced program," said Ruf, "one which is not as accelerated or limited by costs as the X-33 program, there are a number of analyses we would do before testing in the wind tunnel. We would perform an analysis up front to determine maximum airload, or maximum dynamic pressure. Then, we would test that particular case in the wind tunnel. But, in this program, we were into the wind tunnel before we knew the launch trajectory, and before we knew what the final configuration was."

"Now," said McConnaughey, "because of the schedule and cost issues, the program cannot run another wind tunnel test for aerodynamic loads in time to make an impact on the vehicle structural design. So we’re doing it analytically with computational fluid dynamics -- but we’re using the wind tunnel data that is available as a check on our analytical predictions."

Analytical predictions through computational fluid dynamics also were made in a variety of other areas during the X-33’s design and development at Marshall. For instance, they enabled prediction of engine performance, and an estimate of the vehicle’s sonic boom strength, one factor in environmental impact assessment.

Tony Springer, one of the Marshall X-33 test engineers who have performed over 30 weeks of wind tunnel tests since late last year, knows the value of wind tunnel testing -- as well as the newer discipline of computational fluid dynamics.

"Wind tunnel testing allows you to fly the vehicle before you actually fly it," he said. "It allows for variations in configurations for what the vehicle will look like, whether changes in tails, fins, flaps, nose or different body shapes. Through wind tunnel testing, you can see what changes will do to a vehicle’s performance."

It was through wind tunnel tests on the X-33 model at Marshall last December that instability problems were discovered in an early vehicle configuration. By testing new configurations, controls were found to stabilize the vehicle, and successful design changes adapted.

Computational fluid dynamics played a significant role in those changes, as well. "Neither computational fluid dynamics or wind tunnel testing replaces the other," said Springer. "Instead, they are complementary of one another. We all know that the final product of both computational fluid dynamics and wind tunnel testing is to provide an aerodynamic database for the vehicle for all the various flight regimes.

"In other words," he added, "to work together to provide a successful X-33 vehicle."

NOTE TO EDITORS: Photographs are available to accompany this release. To request photos, please call Dom Amatore at Marshall’s Office of Media Services at (256) 544-0034.