Boeing’s strike force

Experts develop debris impact models for Space Shuttle

BY ED MEMI

Before retuning the Space Shuttle to flight, the Columbia Accident Investigation Board concluded that NASA needed to develop, validate and maintain physics-based computer models to evaluate Thermal Protection System damage to the Space Shuttle from debris impacts.

In response, a team of Boeing employees representing Integrated Defense Systems, Commercial Airplanes and Phantom Works began to pool efforts to increase understanding and predict the effects of foam, ice and ablators (insulators that act as a heat shield and help dissipate heat) striking the orbiter.

“We are running a series of tests and performing analysis to determine the impact of debris on the orbiter,” said Scott V. Christensen, Boeing manager for Orbiter Vehicle Engineering and overall lead for the debris analysis efforts.

“Various-sized pieces of foam, ice and ablator are ‘shot’ at the TPS materials, and the resulting damage is assessed. Speed, angle of shot and size are varied in each test, and the resulting data is used to validate the damage-prediction models,” Christiansen said. “For tile debris impacts, rapid-response models will be created that can be performed on a personal computer and can generate a damage prediction within seconds.”

Another tool being used is a software program called LS-DYNA, which is used to evaluate dynamic impact events. LS-DYNA has been used by the automotive industry to perform car crash simulations. Boeing has used the tool to predict damage to aircraft from bird strikes and for missiles striking targets.

“LS-DYNA allows you to look at more detailed types of failures,” Christensen said. “In some areas, we have empirical data which provides us enough information, but a DYNA model can predict where we don’t have enough materials to test in every case, like RCC [reinforced carbon-carbon] panels. One of our testing goals is to have enough test data to correlate and validate the LS-DYNA models.”

The Commercial Airplanes lofting group is assisting in the development of data for the LS-DYNA model. Gary Werner, engineer in the master dimensions or “lofting” group at Commercial Airplanes in Renton, Wash., has been making computer models for use in LS-DYNA. “We make surfaces for things that have complicated shapes,” explained Werner. “When you look at an airplane, all the surfaces and shapes on the outside of the airplane and all those shapes that touch the airflow, are all made by us. They have to be smooth and accurate.

“We have worked on about 15 orbiter panels, the T-seals located between the panels, the nose cap and chin panels. You have to be a surfacing expert to do this accurately, and there aren’t too many surfacing experts around.”

Another tool in development is the Digital Mockup that was used to convert the original space shuttle two-dimensional drawings into an easy-to-use database of computer 3-D models. Loren Humphrey, a structures design engineer in Huntington Beach, Calif., has led a team of Computer Aided Design engineers in a project that models the components of the orbiter wing. This high-fidelity model includes the wing leading edge, RCC panels, T-seals, spars, insulators and other components.

“The big challenge is to capture this information in the database properly, with everything in its proper position in space and hooked up as it would be in the vehicle,” Humphrey said. His team of engineers began its work with these models when the Columbia accident investigation began by developing a half-scale 3-D model of the wing leading edge that has been used frequently to explain the damage that occurred to Columbia.

A team from Boeing Phantom Works in Huntington Beach, Calif., is performing progressive damage studies to better understand what happens to cracks, delaminations or holes in an RCC panel resulting from a foam strike and how that damage can grow. Cong Duong, an engineer in the Structures Technology branch of Phantom Works, is managing this effort. “After simulating some initial damage on the leading edge panels and T-seals, my job is to predict how the initial damage will progressively grow,” Duong said. “We want to determine if the initial damage size will be considered acceptable for a safe return of the shuttle.”

Although damage progression models have existed for a while, Boeing Phantom Works is adapting those models to work for a complex material like RCC.

Boeing IDS in Philadelphia is using LS-DYNA to predict damage to the leading edge from various debris strikes. Jon Gabrys, lead for advanced analysis, and his team have used LS-DYNA to assist with the accident investigation and are now supporting Return-to-Flight activities. The team is currently analyzing foam impacts and is getting ready to analyze ice impacts, he said.

These Return to Flight efforts will culminate in the launch of Space Shuttle Discovery, STS-114, scheduled for March 2005.

edmund.g.memi@boeing.com