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Radiographic Analysis of the TSS-1R Conductive Tether
by Ronald D. Beshear and Lisa Hidiger*
Yes, there is NDT after a failure. Sure, once something has failed, it is too late for NDT to prevent that failure. Keep in mind that it is not too late for NDT to assist in understanding the failure, and it is not too late to develop NDT that can prevent similar findings in the future.
Frank A. Iddings
Tutorial Projects Editor
The Tethered Satellite System Reflight (TSS-1R) experiment was interrupted when the conductive tether that attached the satellite to instrumentation in the space shuttle payload bay separated during deployment of the satellite on shuttle mission STS-75. The remaining segment of the tether was examined by the Nondestructive Evaluation Branch at Marshall Space Flight Center (MSFC) in Huntsville, Alabama.
The composition of the tether was as follows: a central core of Nomex, a served group of ten #34 AWG copper strands, extruded fluorinated ethylene propylene (FEP) insulation, Kevlar braid, and an outer Nomex braid. The overall diameter of the tether was 2.4 mm (0.095 in.).
Two radiographic techniques were developed at MSFC for analysis of the tether. A low energy technique was developed to image the FEP and Kevlar, and, due to the strong absorption of X-rays by copper at lower energies, a higher energy technique was developed to image the copper strands.
A 27 m (89 ft) segment including the failed end was analyzed first. A total of 130 radiographs were produced. Twenty various anomalies were observed in the radiographs. An image of one of the anomalies is shown in Figure 1.
With the advent of small handheld real time radiography systems, we anticipate that radiography could become a routine tool for quality assurance in the production of similar hardware.
The anomalies located with radiography were flagged for removal from the tether and analysis. The anomaly shown in Figure 1 was found to consist of several contaminants that may have been introduced during production or handling of the tether.
Figure 1 - Radiographic image of tether showing anomalous material suspended in Kevlar braid. The dimensions of the inclusion are approximately 1.5 x 1.0 mm (0.06 x 0.04in.). This radiograph was produced using the lower energy technique, which did not penetrate the copper strands.
A radiograph of the failed end is shown in Figure 2. Evidence of melting in the copper strand and the surrounding FEP insulation was evident from radiographs of the failed end. This corroborated other visible evidence that the tether failure resulted from excess arcing between the tether and the ambient environment.
Figure 2 - Radiograph of the failed end of the TSS-1R tether. The broken and melted ends of several copper strands and melted FEP insulation are visible.
Beyond the initial 27 m (89 ft) segment, approximately 1,800 m (5,900 ft) of tether remained on the payload bay reel after separation. The remaining tether was also brought to MSFC for analysis. Thirty random locations along the longer segment were selected for radiographic analysis, as well as three anomalies that were noted during visual inspection. The analysis of the 1,800 m (5,900 ft) segment showed that it contained far less particulate contamination than the failed end, which would be consistent with the conclusion that arcing had occurred at the location of the separation.
Our experience with the tether demonstrated that radiography is an important tool in failure analysis and can also be important in the quality assurance and pre-deployment assessment of conductive tethers and related systems. With the advent of small handheld real time radiography systems, we anticipate that radiography could become a routine tool for quality assurance in the production of similar hardware.
Acknowledgment
Nomex and Kevlar are trademarks of DuPont de Nemours.
*NDE Branch, Marshall Space Flight Center, Huntsville, AL 35879; (205) 544-2550; fax (205) 544-2543.
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