Back in space: Sandia assists with NASA Discovery return-to-flight projects
Several Sandia projects have been instrumental in helping NASA with its Space Shuttle Discovery’s return-to-flight mission (STS-114). Discovery has been in orbit on its two-week mission since its successful launch last week (July 26), temporarily ending the hiatus on space shuttle flights caused by the Columbia disaster in 2003.
Sandia’s projects range from creating an orbiter inspection sensor, to analyzing sensors placed on the orbiter’s wing leading edges, to providing peer review reports. Sandia also studied the vibrations caused during the rollout of the space vehicle and developed an ultrasonic nondestructive inspection method.
Sandia was instrumental in analyzing the cause of the accident that destroyed the Columbia during reentry on Feb. 1, 2003. A Labs-wide team effort helped confirm that the accident was caused by foam from the external tank that impacted the wing leading edge on takeoff.
Orbiter inspection sensor
Sandia provided the primary Thermal Protection System (TPS) inspection system to NASA for the mission of the space shuttle Discovery, successfully launched on July 26 after a long hiatus due to the 2003 Columbia disaster.
Bob Habbit and Bob Nellums (both 2624) led a collaborative effort of nearly 120 Sandians in creating the sensor. Many of the team members worked nights and weekends to meet NASA’s critical need to return to flight ASAP in support of the International Space Station.
Using 3-D imaging, the sensor inspected the orbiter for critical damage to alert astronauts if further investigations are needed to repair the damage. The crew used the orbiter’s robotic arm to scan the front edge of both wings for damage as little as a 0.02-inch crack.
The Sandia-patented 3-D technology uses a modulated laser illuminator coupled with a modulated receiver to image and spatially locate each point in the scene. The intensity data is used to detect damage and the geometric data to assess the damage criticality.
The sensor data was relayed back to the Mission Control Center at Johnson Space Center in Houston. A team of more than 20 Sandians working in the Mission Control Center processed and reviewed the data. The processed data were provided to the NASA Mission Management Team. The Mission Management Team used the Sandia data as well as other data to determine if it is safe for the Orbiter to re-enter.
Bob Habbit said he is proud to be part of the mission. “It’s exciting to be a contributor to the space program,” he says. “This is truly Sandia providing a service to the nation.”
Inspection hardware
NASA funded a Sandia team to develop an ultrasonic nondestructive inspection method (hardware, techniques, and standards) that led to a scientifically rigorous pre-flight shuttle certification process. The team investigated and proposed ways to improve nondestructive inspection methods for certifying the flightworthiness of orbiter wing leading edges.
The team, led by Dennis Roach and Phil Walkington (both 6252), initially evaluated and refined their inspection methods and hardware using carbon-composite samples with known defects created by the Sandia team. Later, as part of the selection process, a NASA engineer hand-carried orbiter wing samples to all the labs involved in the project and asked that each lab try to find defects known only to NASA
scientists.
The team developed the revised inspection and certification protocols, and the ultrasonic scanning system was integrated into NASA’s Shuttle Orbiter Processing Facility at Kennedy Space Center to monitor the health of the shuttle after each orbiter flight.
Sandia produced an in-situ ultrasonic inspection method while NASA Langley developed the eddy current and thermographic techniques. These groups were the primary players on the NASA In-Situ NDI Team. The NASA In-Situ NDI Team consisted of members from all of the NASA facilities and was assembled to guide NASA as it moves to increased use of advanced nondestructive testing techniques to closely monitor the health of the space shuttle.
In 10 months the Sandia team developed and assembled customized hardware to produce an ultrasonic scanner system that can meet the shuttle wing inspection requirements. Optimum combinations of custom ultrasonic probes and data analysis were merged with the inspection procedures needed to properly survey the heat shield panels. System features were introduced to minimize the potential for human factors errors in identifying and locating the flaws. A validation process, including blind inspections monitored by NASA officials, demonstrated the ability of these inspection systems to meet the accuracy, sensitivity, and reliability requirements.
Team members are Phil, Dennis, Kirk Rackow, and Dick Perry (all 6252). The NASA project manager was Ajay Koshti at Johnson Space Center.
Sensor tests
David Crawford (9116) and Kenneth Gwinn (9126) analyzed tests conducted on sensors that were placed on the leading edge of the orbiter’s wings.
The project focused on validating forcing functions for NASA’s Impact Penetration Sensing system (IPSS) Wing Model. The model was developed at Boeing to predict the accelerometer data collected during ascent and micrometeoroid/orbiting debris (MMOD) impacts on shuttle wing and spar leading-edge materials.
The sensors developed by NASA are significant to the return-to-flight effort. The addition of the sensors to the leading edge was in response to one of the prime objectives identified by the Columbia Accident Investigation Board.
David and Kenneth evaluated test data and were comparing it with structural models of the shuttle and assessing what the signal levels mean. Tasks included defining the forcing functions for foam, pieces of ice (from takeoff), ablator particles, and micrometeorites. Full-scale tests of foam, ice, ablator, metal particle, and MMOD impacts were performed at Southwest Research Institute in San Antonio, Texas. Tests on fiberglass and RCC (reinforced carbon composite) wing panels were conducted at the White Sands Test Facility.
Peer reviews
Members of Sandia’s Aerosciences and Compressible Fluid Mechanics Dept. 9115 contributed two peer reviews on NASA’s development of computational tools that are being used to support rapid damage assessments should anything occur during future flights.
Basil Hassan, manager of Dept. 9115, serves as an external member of NASA’s Engineering and Safety Center’s (NESC) Flight Sciences “Super Problem Resolution Team” (SPRT). NESC was formed shortly after the Columbia accident to oversee any safety issues that might arise in any of NASA’s flight programs.
Basil and two staff members, David Kuntz and Jeffrey Payne (both 9115), participated in several peer reviews as NASA prepared for return-to-flight. They were also part of a larger group of Sandia management and staff who were active in the post-accident investigation.
Two recent reviews focused on Debris Transport Review and Boundary Layer Transition Review.
Debris Transport Review focused on NASA’s development of tools to model external tank foam or ice buildup that may come off during ascent and potentially hit the orbiter. While several efforts have been under way to minimize foam and ice release from the external tank, NASA wants to predict if the released debris will impact the orbiter in critical areas. NASA has used these tools to redesign parts of the external tank so that catastrophes like the Columbia accident will not re-occur.
Boundary Layer Transition Review focused on reentry. During the reentry trajectory the airflow around the orbiter will transition from laminar to turbulent flow. When the flow becomes turbulent, the heat transfer to the vehicle can increase two to four times above the laminar heating. While the thermal protection system (TPS) is designed to absorb the heating rates generated by turbulent flow, damage to the TPS could cause the flow to become turbulent at a higher altitude. The result of this damage could mean higher localized heating rates on the TPS, and ultimately higher than normal integrated heating on the orbiter during reentry.
“Sandia’s participation on these two reviews teams is a one part of a larger effort of the Labs supporting a variety of return-to-flight activities,” Basil says. “We expect additional requests to tap into many of Sandia’s unique capabilities.”
Basil, David, and Jeffrey also reviewed NASA’s rapid damage assessment tools to help the agency ensure that the codes were being applied appropriately and that the relevant assumptions in the codes were not being violated. In general, these tools make use of data from computer codes that model the fundamental physics, wind tunnel test data, and data from previous shuttle flights. Should it be found in orbit that damage occurred during the ascent stage, NASA engineers will use these tools to decide whether the orbiter can safely return or if some in-orbit repair is needed.
Shuttle rollout
To help understand the fatigue caused by vibrations during the rollout, NASA contacted Sandia to assist with a series of tests. Sandia helped NASA design the test and instrumentation to measure the dynamic vibration environment of the rollout. Sandia also provided additional support to NASA by computing the input forces that the crawler applies to the MLP, which are being used by Boeing and NASA to compute the fatigue life for critical shuttle components.
Tom Carne (9124) assisted with a series of tests beginning in November 2003 to develop the data necessary to understand the environment and the response of the space shuttle vehicle during rollout.
Moving the shuttle from the Vehicle Assembly Building at Kennedy Space Center in Florida to the launch normally takes five to six hours at 0.9 mph. As the equipment ages, emphasis is being given to understanding how the rollout may fatigue the orbiter.
The analyses showed that modifying the speed of the crawler would reduce the fatigue stresses of the critical shuttle components. Merely reducing the speed from 0.9 mph to 0.8 mph would significantly reduce the vibrations in the shuttle by shifting the engagement frequency of the crawler treads. The shuttle’s vibration response can be much reduced when the driving frequencies are shifted away from its own resonant natural frequencies.