The "Right Stuff" at the Right Time
07.25.11
A confluence of skills, resources, and timing put NAS modeling and simulation experts at the forefront of safety analysis for shuttle missions over the last eight years, including the final Atlantis STS-135 mission.
The movie "The Right Stuff" portrays the real-life adventures of NASA's 1959 Mercury team of hotshot test pilots who prove they have the know-how and grit to become astronauts, facing the risks that come with human spaceflight. In the same spirit, to help reduce the risks that NASA's present-day astronauts must face, a team of experts in the NASA Advanced Supercomputing (NAS) Division has just ended an eight-year journey to improve the safety and reliability of NASA's Space Shuttle missions by performing advanced modeling and simulation of potential debris and damage risks to the vehicle and crew.
The team's work has entailed ensuring safe flying of the Space Shuttle until the last mission returned home successfully on July 22, 2011. These experts have provided unique tools and methods to obtain increasingly accurate predictions of debris strikes and other potential risks for shuttle launches and in-orbit flights. Their efforts began immediately following an event that shook the nation—the 2003 loss of Space Shuttle Columbia.
"We wanted to do everything possible to make sure this never happened again," said Stuart Rogers, computational fluid dynamics (CFD) expert in the NAS Division's Applied Modeling and Simulation group. After the Columbia accident, Rogers' team of aeronautics and computational fluid dynamics researchers was immediately brought in, and became crucial to the investigation team's final determination of what happened.
As the world soon learned, the catastrophic accident was caused by damage sustained when a piece of foam insulation falling from the vehicle's bi-pod ramp on the external tank struck the leading edge of Columbia's left wing, piercing a reinforced carbon-carbon tile of the thermal protection system.
Ready to Go
"We were ready to go," Rogers said. "The NAS Division already had in place the expertise and the supercomputing capability and capacity that allowed us to do the critical debris simulations and aerodynamic analyses needed for the investigation right at that time." Their main task was to determine the types of debris the shuttle could encounter on any given mission, how fast that debris is traveling, all the possible places it could strike, and the capability of vehicles to withstand these hits.
Fortuitously, NAS researcher Dennis Jespersen had already gained experience porting the OVERFLOW CFD software to NAS' then recently installed SGI Altix 3000-the world's first 512-processor single-system image supercomputer-which enabled the simulations needed for the Columbia accident investigation. NAS team members Michael Aftosmis and Scott Murman developed the debris aerodynamic models, while Rogers and engineers at Johnson Space Center in Houston developed the next generation of debris-transport and impact-analysis software. Together, these analysis programs became the Space Shuttle Program's primary engineering tool for studying all potential debris sources, including insulation foam, ice, ablator, caulk, putty, and other types of debris.
"What's hard to get across is that the shuttle is traveling so fast to get out of the atmosphere-but until it gets there, any debris slows down and the vehicle actually flies into it," Rogers explained. "Back then, no one had any idea that foam could cause so much damage to the leading edge, so a huge effort was needed to attack the problem."
Debris Transport Assessment
Bringing their many years of expertise in CFD together with the knowledge of engineers in the Applied Aerosciences and CFD Branch at Johnson, the team developed a set of unique debris transport analysis and CFD methods using NASA's supercomputing resources. Their approach revealed that the problem was indeed much more complex that anyone had previously understood, and proved critical to the safety of crew and launch vehicles for missions starting with the first Return to Flight mission (STS-114) in July 2005.
From then on, the NAS team, led by the efforts of Edward Tejnil, Science Technology Corp., computed and delivered hundreds of CFD simulations of the Space Shuttle Launch Vehicle (SSLV) during ascent, using the highest fidelity SSLV model grid system ever produced. Team members stayed onsite at Houston's Mission Control Center during each shuttle launch, producing quick-turnaround debris simulations that were used to analyze numerous debris events (see "Behind the Scenes: Real-Time Debris Analysis"). In each case, shuttle program managers utilized the team's simulation data to determine whether it was safe to launch.
The NAS CFD methods were also extremely cost-effective for NASA, allowing highly accurate computational simulations to produce millions of debris trajectory scenarios within hours, minimizing costly wind tunnel experiments. In the beginning, though, these experiments, along with ballistics tests, verified the accuracy of the NAS teams' simulations, Rogers points out.
Results Give New Understanding
As a result of the Ames-Johnson teams' work, the space industry now has a much greater understanding of the physics involved when debris strikes a vehicle. Because of this increased knowledge, NASA was able to take measures to ensure that large formations of debris from foam and ice didn't occur. For example, in July 2006, a major redesign to the external tank was critical to the safe launch of Space Shuttle Discovery (STS-121). This redesign included removal of the foam protuberance-airload ramps, which had produced a large debris event during the ascent of STS-114.
"We learned a tremendous amount from doing CFD simulations of the shuttle, which in turn drove development of our codes to make it easier to add complex geometries," Rogers said.
Both Rogers and Ray Gomez, senior aerospace engineer at NASA Johnson, agree that the increasingly powerful supercomputing resources available have been key to allowing much higher fidelity simulations to understand the launch environment. Their software, combined with high-end computing technologies, has been in the critical path for the shuttle's safe return to flight, and will be an indispensable element for supporting future missions to space.
"A lot of the work we've done has also been used to model the Orion spacecraft, in particular, our design analysis of the crew capsule and launch abort vehicle," Rogers said.
Back to the Future
Now that the Space Shuttle Program's inspiring 30-year history of human space flight has come to an end, NAS Division experts are continuing their work over the last several years to help NASA prepare for the future of U.S. space exploration, performing a wide-range of CFD analyses for the Space Launch System (SLS, formerly the Ares V heavy lift launch vehicle) and the Multi-Purpose Crew Vehicle (MPCV, formerly Orion). The SLS will be capable of carrying humans to the moon, asteroids, and Mars. NAS codes are being used to model new launch vehicle designs and perform advanced simulations to help improve vehicle performance and speed up design cycles (see "Streamlining Development of Next-Generation Launch Vehicles").
While NASA's vision for a new space program is still evolving, as the space agency moves forward with commercial partners to make travel to low Earth orbit and beyond more accessible and more affordable, NAS experts and supercomputers will continue to play an important role in ensuring the safety of future missions and astronauts.
– Jill Dunbar
