Sandia tests wing sensors that will be used on NASA’s next shuttle mission

Two members of a team that helped determine the cause of the space shuttle Columbia accident (Lab News, Sept. 5, 2003) are now helping NASA with its return-to-flight mission.

David Crawford (9116) and Ken Gwinn (9126) have been analyzing tests conducted on sensors that will be placed on the orbiter’s wing-leading edges.

The project is to develop impact models for NASA’s Impact Penetration Sensing system (IPSS) Wing Model. The model is being developed at Boeing to predict the accelerometer data to be collected during ascent and micrometeoroid/orbiting debris (MMOD) impacts on shuttle wing and spar leading-edge materials.

The project comes nearly two years after the shuttle fleet was grounded due to the space shuttle Columbia accident in February 2003. NASA has been working toward the final processing of hardware for the STS-114 Return to Flight mission. The space shuttle Discovery is scheduled to launch in early May.

The sensors developed by NASA are significant to the return-to-flight effort. The addition of the sensors to the leading edge meets one of the prime objectives identified by the Columbia Accident Investigation Board.

"If significant damage to the leading edge has occurred, the sensors will send a signal back to the command center and the request for an inspection can be made," says Ken.

David and Ken are evaluating test data and are comparing it with structural models of the shuttle and assessing what the signal levels mean.

Sandia’s tasks include defining the forcing functions for foam, pieces of ice (from takeoff), ablator particles, and micrometeorites.

The tests evaluate different sizes of possible debris in the range of 20 cubic inches (the Columbia debris foam impactor was in the range of 2,000 cubic inches). All debris is studied by determining the velocity and angle of the impact.

"Lots of stuff can hit the shuttle," Ken says. "Liquid hydrogen and liquid oxygen develop frost prior to launch."

Ken says background noise from aero and acoustic loads also affects the sensors. "It is our job to discriminate significant impacts from the normal loadings of the shuttle," he says.

Full-scale tests of foam, ice, ablator, metal particle, and MMOD impacts are being performed at Southwest Research Institute (SwRI) in San Antonio, Texas. Tests on fiberglass and RCC (reinforced carbon composite) wing panels are being conducted at the White Sands Test Facility (WSTF).

The forcing functions will be individually and directly validated where possible against SWRI and WSTF test data. Integrated validation of the forcing function and IPSS Wing Model will be performed in collaboration with the effort of Boeing.

"We worked with the SWRI, WSTF, and NASA engineers to design tests to validate the impact models," Ken says. "This includes various velocity ranges, various impactors, and many locations on the panels to capture as many impact scenarios as possible. We also coordinated with the test engineers to place instruments where they’ll be most effective for both analysis correlation and sensor demonstration."

Ken is analyzing the impact on the front of the wing, then providing impulse definitions to another team that determines how that impact affects the shuttle’s structure and the response at the sensor box.

David analyzes the in-orbit data. He helps coordinate an experimental program at WSTF having to do with measuring and understanding the signals expected to be seen on the IPSS sensors from the impact of micrometeoroids or orbiting debris on the RCC leading-edge materials.

David primarily runs the shock physics code, CTH. He has been in daily contact with the experimenters at White Sands. His role is to develop a theoretical model of these signals to apply to the system model that Boeing is putting together.

He also provides a theoretical model for ice impacts and writes software that will distill all of the understanding of the various impactors —ice, foam, ablator, orbital debris— and provides it to the Boeing system model.

David says the general finding is that the signals expected to occur from damaging orbital impacts are large enough to be detectable with the sensor system.

"The IPSS project generally is very important as it is considered a crucial aspect of the space shuttle return to flight. Everything I’ve seen suggests that the IPSS should function as required."

Further analysis will extend forcing functions beyond the level that is accessible to tests being conducted at WSTF and SwRI. Every effort will be made to give Boeing the ability to construct new forcing functions as may be required for future missions.