Sandia assists NASA with space shuttle rollout test
Transporting the space shuttle from the assembly building to the launch pad is quite a heavy task.
The three-million-pound shuttle sits on an eight-million-pound mobile launch platform (MLP) and is carried by a six-million-pound crawler. The crawler transports the vehicle and platform four miles from the Vehicle Assembly Building at Kennedy Space Center in Florida to the launch pad.
Moving the shuttle that distance, which normally takes five to six hours at 0.9 mph, had been considered a relatively low-stress process during most of the life of the shuttle system. As the equipment ages, however, more emphasis is being given to understanding how the rollout may fatigue the transport system.
NASA contacted Sandia to assist with a series of tests to help understand the fatigue caused by vibrations during the rollout. Overall, the tests are part of NASA’s return-to-flight mission, with first flight scheduled between May 15 and June 3.
Sandia helped NASA design the test and instrumentation to measure the dynamic vibration environment of the rollout. Sandia is also providing 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.
T om Carne (9124) has 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.
"NASA requested Sandia to assist them in this project because of our expertise in planning and conducting structural dynamic tests on very large structures," Tom says, adding that the Solid Mechanics/Structural Dynamics Group has done numerous structural analysis projects on large structures including the I-40 Rio Grande bridge, numerous large wind turbines up to 110 meters tall, and the Armored Tractor with SafeGuards Transporter. One of the group’s main missions is analysis and testing of the shock and vibration requirements for weapons.
Rollout data analysis
Data were collected for rollouts of the MLP-only and the MLP with the two solid rocket boosters, at five different speeds ranging from 0.5 to 0.9 mph. For the tests more than 100 accelerometers were placed on the MLP, crawler, and solid rocket boosters. A data acquisition system installed inside the MLP for the road test measured and recorded the accelerations. The data were analyzed so that the character of the rollout environment is understood and can be analytically imposed on the shuttle using a finite element model to predict fatigue damage to critical components. Even though these stresses are much lower than those seen during the launch, the five- to six-hour duration of the transport and the low-frequency content in this environment could cause fatigue in components within the orbiter.
Tom says the rollout analysis team determined that there are two families of forcing harmonics caused by the crawler drive train that excite the MLP as a function of crawler speed, in addition to the random inputs induced by the road bed. Fortunately, he, says the harmonic forcing frequencies can be adjusted by merely changing the drive speed of the crawler, moving the inputs to less damaging frequencies.
Forcing function analysis
The team used a Sandia-developed algorithm, the Sum of Weighted Accelerations Technique (SWAT), to estimate the applied forces. Tom says the SWAT results were very beneficial in choosing a new rollout speed that will extend the fatigue life of the shuttle components that were affected by rollout.
The SWAT-generated input forces have subsequently been used as the force input for NASA’s NASTRAN structural analysis of the MLP, emulating the test conditions. The correlation between the rollout-measured data and the predictions from the NASTRAN analysis has engendered confidence in both the SWAT computed forces and the NASTRAN model of the MLP and solid rocket boosters.
"We are able to help NASA and Boeing by providing force input to their computer models to predict fatigue life," says Tom.
The analyses showed that modifying the speed of the crawler would reduce the fatigue stresses of the critical shuttle components. They showed through analysis that merely reducing the speed from 0.9 mph to 0.8 mph would significantly reduce the vibrations in the shuttle by shifting the impact 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.
Space shuttle Discovery is scheduled to roll over to the VAB this week for STS-114, the space shuttle’s return-to-flight mission. Tom will again be on-site at Kennedy Space Center to help analyze and interpret the rollout vibration data from this return-to-flight mission.