After 135 missions in 30 years the space shuttle program has come to an end. The accomplishments were remarkable and the legacy the program has left is profound and will be long lasting. Of course it was marred by disaster more than once, including the destruction of the Columbia upon reentry due to damage of the heat shielding tiles on the leading edge of the wing.
Readers may not be aware of this fact, butafter the accident, thermal imaging was regularly used to verify the integrity of the carbon-carbon composite tiles. These unique materials were fairly delicate, a fact that became obvious when a piece of foam insulation falling from the booster fuel tank damaged the tile leaving it vulnerable to overheating upon reentry. The future safety of the mission depended upon being able to find this kind of damage that was not visible to the eye.
The inspection technique, pulse-active thermography, involved using a high-powered pulse of light from a commercial xenon flash lamp that, when absorbed, produced a “wave” of heat flowing into the tile. The heat would transfer predictably if there were not problems inside the composite material. If there was a problem—and that was not often the case—heat transfer would be disrupted and the discontinuity would show up as a warmer (or colder) spot on the surface a few seconds after the flash lamp pulsed.
At some point it also became obvious that it might be necessary to conduct an inspection in space. A commercially available, hand-held system was made space worthy and, again, The Snell Group provided training to the astronaut training group to ensure it was used properly.
A flash lamp was not deemed a viable heat source to be used in space so a technique was developed whereby the orbiter would be turned toward the sun. This caused the leading edge to be heated. During a space walk it was then shaded by the astronaut conducting the inspection and an image generated showing the distribution of surface temperatures related to heat transfer.
Amazingly, it was critical that the “thermographer” focus the imager and adjust the level and span properly! In the end the technique was practiced several times in space, but was never required to actually be used in space to detect damage.
Thermography was also used in other ways at the Kennedy Space Center. Systems were used to monitor fuel tanks and transfers for hydrogen fires which are invisible to the human eye but are visible in the longwave spectrum. The technology was also widely used in all aspects of electrical and mechanical maintenance, particularly important at controlling program costs as budgets were being subjected to cuts.
Readers may think this has little to do with our day-to-day work but the same active and pulse-active techniques used on the orbiter are widely employed in more mundane applications. Composite marine hulls, for instance, are more and more often inspected for both impact damage and moisture intrusion using a heat source and a thermal imager.
Even when inspecting homes, we often employ a variation on pulse-active thermography. The next time you are in a home where conditions are insufficient for good work, try “pulsing” the AC or the heat for about 15-20 minutes and then let the heat transfer into the walls for about the same time. Chances are good that the thermal patterns you see at that time will be sufficiently good to see framing and discrepancies in insulation due to a disruption of the heat transfer.
We might not be rocket scientists but we share a common need to locate problems in the materials with which we work and in the rather simple techniques we might employ, along with a bit of “thinking thermally,” to find any problems that exist.
John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner