Designed and built to launch satellites for the U.S. Air Force, the Delta II has been a highly successful project. What happened? A committee of engineers from McDonnell Douglas and The Aerospace Corporation, the Air Force contractors on the Delta II, investigate. They find that a mechanical component malfunctioned, causing a lower than intended orbit. What caused the malfunction? Analysis leads them to suspect overheating due to "backflow" from the rocket engines.
How can the engineers confirm this analysis, and in a hurry? No one wants to trust an educated guess, no matter how sophisticated, when millions of dollars are on the line for upcoming satellite launches already scheduled. Wind tunnel testing? This tried-and-true design tool is almost useless for testing rocket performance with the "plume on" -- i.e., with the rocket engines firing, and the plume contributes dramatically to the aerodynamics of a rocket in supersonic flight. Furthermore, the Delta II is a clustered or "multi-body" rocket -- a core rocket surrounded by nine boosters. This by itself creates problems for wind-tunnel testing, which even in simple cases is costly and time-consuming. Is there a way out of this bind?
To exploit the potential for improved performance on scalable, parallel systems, which team tens, hundreds or even thousands of processors to work simultaneously on a single computing task, Wang in 1991 joined forces with Stephen Taylor, head of the Scalable Concurrent Programming Laboratory at Caltech. Taylor works with a group of scientists who have pioneered basic programming technology to address important, large-scale problems in industrial and defense applications. Their work feeds directly into development of advanced software and the next generation of parallel machines.
Over the past four years, Taylor and Wang worked to implement their flow solver on a range of parallel computers, including work-station networks and shared memory parallel systems. Along with improved performance, the parallel implementation includes features that make it especially useful for rocket design, including the ability to simulate multi-body rockets.
In early 1995, Taylor and Wang put their software through its paces by simulating the Titan IV, another multi-body rocket. These runs proved that the parallel flow solver can provide the kind of data the Delta II team needs. "We can calculate the forces acting on the vehicle," says Taylor. "We can calculate aerodynamic drag and determine the strength of the vehicle's sonic boom. And we can calculate temperature contours that predict heat transfer, which allows engineers to design appropriate shielding and paint."
Part 2: The Caltech Group
In the late 1980s, Johnson Wang, a senior engineer at The Aerospace Corporation, developed a numerical scheme for simulating the aerodynamics of complex, multi-body rockets like the Delta II with high accuracy. In technical terms, his software is an efficient "flow solver" for the three-dimensional Navier-Stokes equations, the classic mathematical formulation of fluid flow.
Part 3: A Scalable, Parallel Solution
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PSC responds to the situation by making their T3D available for "dedicated" runs. All 512 T3D processors become a powerful team, working together to compute the complete "plume on" flow field of the Delta II. In a matter of days, the researchers have the results they need. The computations quantify all relevant parameters -- velocity, density, pressure -- in three dimensions.
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