Study of Orbiter Wing Leading Edge Panel and Flowliner Anomalies

In this paper, two investigations involving space shuttle orbiter are described. Both
investigations demonstrate the need for identification of root causes of anomalies and the
need for careful inspections of space hardware.

The wing leading edge (WLE) of the space shuttle orbiter consists of reinforced carbon
carbon panels. The WLE panels provide the aerodynamic performance during descent
and landing and thermal protection during reentry into the Earth’s atmosphere. The
orbiter’s WLE panels are inspected routinely after the orbiter returns to Earth using
infrared (IR) flash thermography technique. On two occasions a small portion of the
silicon carbide (SiC) coating on the slip-side joggle of the panels that undergo peak
heating was liberated. On the return-to-flight mission during August 2006, an IR
indication due to a material separation defect was detected on panel 8R. The panel was
removed and a detailed inspection was performed. Although the panel did not spall
during flight its SiC coating adherence was degraded compared with other panels and
was readily removed using a dental pick. A relatively small area of the affected material
was initially observed with IR compared with the area that was removed via dental pick.
Clearly, SiC coating loss of this extent, were it to occur early enough in re-entry and in a
critical lower surface location, would result in loss of crew and vehicle. An investigation
was launched to understand the root cause(s). After a comprehensive investigation, the
likely mechanism causing spallation of SiC coating in the slip-side joggle region was
hypothesized to be consisting of three distinct phases: Phase 1: Defect formation during
fabrication, refurbishment, or subsequent heat treatments/exposures at the curved joggle
radii, Phase 2: Increased subsurface material separation at the SiC/CC interface and,
Phase 3: Oxidation of subsurface carbon fiber bridging which significantly reduces
coating adherence and ultimately results in SiC coating liberation. This paper describes
some aspects of this investigation.

The second investigation involves the liquid hydrogen (LH2) feed-line flowliners. In the
feed lines there are two flowliners, upstream and downstream. These flowliners maintain
laminar flow of the fuel in the feed lines and are designed as non-structural members. The
flowliners are thin metallic cylindrical shells that are welded to the feed line on each end
and overlap in the middle. Each flowliner has slots oriented in the direction of the flow.
Fatigue cracks were observed initiating and propagating from these slots. Soon after the
discovery of the cracks, a comprehensive investigation into this problem was undertaken.
The investigation showed that the simulation of flight environments on ground tests is
impossible and instrumenting the flowliners during flight is impractical. Due to these
difficulties, reaching flight rationale from various processes proved to be impossible.
The investigation also showed that the elimination of the fatigue cracks was the only path
to safe flight. The paper describes efforts undertaken to the elimination of cracks in
flowliners.