SR-71 Blackbird engineers had a perplexing problem: why some of the aircraft’s titanium corroded?
The SR-71, the most advanced member of the Blackbird family that included the A-12 and YF-12, was designed by a team of Lockheed personnel led by Clarence “Kelly” Johnson, then vice president of Lockheed’s Advanced Development Company Projects, commonly known as the “Skunk Works” and now a part of Lockheed Martin.
The Blackbird design originated in secrecy during the late 1950s with the A-12 reconnaissance aircraft that first flew in April 1962 and remained classified until 1976. President Lyndon Johnson publicly announced the existence of the YF-12A interceptor variant on Feb. 29, 1964, more than half a year after its maiden flight. The SR-71 completed its first flight on Dec. 22, 1964.
Production of the SR-71 was done at the Skunk Works plant, in Burbank, California, as the photos in this post show [the image were taken in 1965 and show some of the first SR-71 under construction: featured in the pictures are the first B models (957 and 956) under construction with an A-model (955). My dad Richard “Butch“ Sheffield told me that he watched these airplanes under construction at the Skunk Works plant at Burbank].
The airframes were built almost entirely of titanium and other exotic alloys to withstand heat generated by sustained high-speed flight: in fact, the Blackbirds were designed to cruise at Mach 3 continuously for more than one hour at altitudes up to 85,000 feet.
Blackbird’s engineers had a perplexing problem as to why some of the titanium was corroding. However, they figured out the reason. It was chlorine in the water!
According to Wisconsin Metal Tech, the engineers of the SR-71 were among the first people in history to make real use of the material. In that process, they ended up throwing away a lot of material, some through necessity, some through error. At times the engineers were perplexed as to what was causing problems, but thankfully they documented and cataloged everything, which helped find trends in their failures.
They discovered that spot welded parts made in the summer were failing very early in their life, but those welded in winter were fine. They eventually tracked the problem to the fact that the Burbank water treatment facility was adding chlorine to the water they used to clean the parts to prevent algae blooms in summer, but took it out in winter. Chlorine reacts with titanium, so they began using distilled water from this point on.
They discovered that their cadmium plated tools were leaving trace amounts of cadmium on bolts, which would cause galvanic corrosion and cause the bolts to fail. This discovery led to all cadmium tools to be removed from the workshop.
Be sure to check out Linda Sheffield Miller (Col Richard (Butch) Sheffield’s daughter, Col. Sheffield was an SR-71 Reconnaissance Systems Officer) Facebook Page Habubrats for awesome Blackbird’s photos and stories.
Lockheed photos via Tony Landis and CIA