"Stress Is Where You Find It", Input/Output, ME Departments, September 2005

input/output

by Paul Sharke

Stress Is Where You Find It

You'd think that the slow-roll jaunt the Space Shuttle Discovery made from the assembly building out to the launch pad in July would be the least stressful portion of its mission. And, you'd be right.

The shuttle, its external tank, and solid rocket boosters sat on a mobile launch platform that inched its way toward the launch pad on a huge machine called a crawler transporter, which ASME has dubbed a National Historic Mechanical Engineering Landmark. The vehicle is designed to take plenty of time to make the four-mile trip at the Kennedy Space Center, to keep stress on the shuttle at a minimum. Its speed is measured in fractions of a mile an hour.

But, low stress does not necessarily mean no stress. Every mechanical engineer understands that repetitive stress cycles can induce fatigue in metals. Whereas the peak, high-frequency vibrations at launch last about 100 seconds, the low-level, low-frequency vibration during the shift from assembly building to pad can go four hours or more.

The shuttle's crawler has been named an ASME National Historic Mechanical Engineering Landmark.

That's the reason NASA and Boeing decided to investigate the transport environment to determine whether the stresses of this brief journey were anything worth worrying about. Sandia National Laboratories was requested to assist because measuring vibrations on very large structures is a lab forte, according to Thomas Carne of Sandia's technical staff, who helped design the tests.

NASA instrumented the crawler, the mobile launch platform, and two solid rocket boosters with about 100 accelerometers. The boosters, at 1.3 million pounds apiece, and the mobile launch platform, make up the bulk of the load on the transporter, an important concern when measuring vibration response. The orbiter and empty external tank—which is filled on the pad—add a smaller percentage to the shuttle's weight. Neither orbiter nor tank was present during the tests.

NASA measured the stresses during the four-mile trip of the crawler transporter at Kennedy Space Center.

Engineers collected data during several rollouts with and without the boosters and at speeds that they varied from 0.5 mph to 0.9 (the crawler's usual pace). The acceleration data was then processed by a program that Sandia developed— the sum weighted accelerations technique—to determine what input forces the crawler applied to the mobile launch platform. NASA then put these forces through a Nastran structural analysis program to emulate test conditions. The predicted and measured results correlated closely.

Jene Richart of the Space Shuttle Program Systems Engineering & Integration Office was the Johnson Space Center's test team lead during the tests at Kennedy.

George James, of the JSC Engineering Directorate and co-chair of the Space Shuttle Program Loads and Dynamics Panel, worked the technical requirements for the test and defined the analysis that would be performed using the test data. He also led the technical review of the test data and analysis results.

Analyses discovered two sets of forcing frequencies emanating from the crawler drivetrain. The roadway also generated random forces. By reducing crawler speed by a tenth of a mile per hour, to 0.8 mph, the forcing vibrations from tread engagement can be shifted away from the shuttle's resonant frequencies.

More data were recorded during the two trips to the launch pad this past July. Depending on the future of the shuttle program, more testing, including a full-up test involving the external tank and the orbiter, may be forthcoming in the months ahead.