SR-71 Blackbird

Flight Journal, Feb 2000 by Wainfan, Barnaby

Supersonic cruise. Although many types of military aircraft are capable of supersonic flight, very few actually fly supersonic for long. Fighters have supersonic dash capability, but they cruise at subsonic speed. Only a few airplanes have ever been designed for sustained supersonic flight. Of these, the SR-71 and its relatives are the fastest.

Heat, A dash-capable airplane does not stay at high speed long enough to heat up. Prolonged flight at supersonic speeds causes an airplane to get very hot from air friction. The skin temperature of an SR-71 can approach 900 degrees in some spots. Robert Gilliland, the chief test pilot, has said they wore many layers of clothing under their "moon suits" specifically to protect themselves in case they accidentally touched any metal in the airplane. If they put a multi-gloved hand to the windshield, the heat was felt instantly.

The high temperatures made it impossible to use aluminum in the plane's structure. The other materials available at the time were stainless steel and titanium. Weight was critical. Titanium has approximately the same strength as stainless steel, but it weighs a little more than half as much as steel. The decision was made to build the airplane of titanium.

This was an entirely new structural technology. The process of learning how to work with titanium was long and difficult. It is a very hard material; tools that work well on aluminum break when used on titanium. New machining techniques and tools had to be developed. Titanium reacts badly with chlorine, fluorine and cadmium. This proclivity caused significant problems on two fronts. Initially, the water used to wash parts for spot welding came from the Burbank municipal water supply. In the summer, this water was heavily chlorinated to prevent algae growth. Lockheed had to use distilled water for all its titanium processing.

Cadmium was also a problem; many ordinary tools (particularly wrenches) were cadmium-plated to prevent them from rusting. Tiny flakes of cadmium from tools attacked titanium bolts and caused their heads to drop off when the structure was heated. The high temperatures also caused great difficulty in the development of a camera window that could take the heat and retain the required optical properties.

The heat issue dominated the design of almost everything on the airframe. Even fluids like fuel, lubricants and hydraulic fluid had to be specially formulated to withstand high temperatures and still perform their functions. One heating-related problem was overcome in a clever, low-tech way. As the wing skin heats up, it expands. Early tests showed that this would cause the skin to buckle in the area between spars and ribs. The solution was to corrugate the skin; this allowed it to expand and contract like a bellows without buckling unpredictably. Gilliland said they were continually fine-tuning the structure in that they would fly it to see where rivets popped, then they would redesign that area.

Inlet. Another area that required extensive work (it may well have been the most difficult technical problem in the program to solve), was getting air to the engines. In order for a turbojet to run properly, air must arrive at the engine face at a relatively low, subsonic speed. Also, engines function less efficiently with a turbulent air supply, so the inlet system has to deliver smooth flow to the engine. This is why the engines are canted inward so the airflow strikes the inlets perfectly squarely.