The Pratt & Whitney J58 engine, coupled with the world’s most complex air inlet system, propelled the Blackbird aircraft seamlessly through an enormous range of speeds. Originally, she was a Navy project designed to power the Martin P6M SeaMaster flying boat. She would eventually be painstakingly adapted to work at Mach 3+ flight and see operation in something very different than a seaplane.
The Blackbird aircraft is constructed of over 90% titanium. The engines, however, used materials which could withstand even higher temperatures. Pratt & Whitney turned to exotic nickel and cobalt-based alloys, like Inconel X (which was also be used in the skin of the X-15 rocket plane, Mercury Spacecraft and Apollo F-1 Engine combustion chamber), with some of these materials experiencing operating temperatures of 1,600 °F. Fluid lines were plated with gold or silver. The exhaust ejector was coated with a thermal insulating ceramic which would reach 3,200 °F, undergoing so much heat and pressure that it would never char.
When the J58 fires its afterburner, the whole aft end of the engine glows orange like molten lava. These materials allowed the J58 to operate in afterburner indefinitely, which was required for Mach 3+ cruise. Most aircraft can not continuously operate the afterburner for more than a few minutes at a time without suffering a catastrophic failure.
During development, engineers searched high and low to find a lubricant that could operate under such a wide range of temperatures. Finally, a silicone-based grease was found, which had the consistency of thick peanut butter at room temperature. Before engine start, this grease was preheated to further liquify it. For engine start, the J58 required the assistance of two Buick V-8 or Chevy Big Block housed in a start cart on the ground.
When the Blackbird cruised at Mach 3+, the compression of the air would cause incredible heating over the entire aircraft. The fuel inside the tanks would reach 350 °F. Normal JP-4 fuel would foam and possibly combust at these temperatures, so a special JP-7 fuel was developed with a special high flash point. Because of this high flash point, the J58 had a unique starting method. When the start cart had the engine spinning, a shot of Triethylborane (TEB) was injected into the combustion chamber. When TEB touches air, it explodes, which would cause the fuel in the engine to ignite, initiating engine start. Every time the pilot moved the throttle forward from idle, a shot of TEB was introduced into the combustion chamber. Additionally, every time the throttle was moved forward from full military power, teb was fired into the exhaust section of the engine to ignite the afterburner.
One of the most amazing parts of this engine is its compressor bypass system. When the aircraft flies at more than Mach 2.2, a series of large bypass tubes allow air from the inlet to bypass the compressor section, feeding it straight into the afterburner section, creating the majority of the engine’s thrust. However, this is not a true ramjet because even with these bypass tubes operate, air still flows through the compressor and combustion sections in a traditional manor. With these two concepts working together, we call the J58 a Turboramjet.
The J58 could not do its job without an incredible inlet system. A supersonic shock wave builds up on the tip of the iconic cone that protrudes from the inlet. We call this cone a ‘spike’. Once air enters the inlet, it is forced into a system of shockwaves, diffusing the supersonic air, slowing the air to subsonic speed. This process creates a huge increase in air pressure, which can be fed into the engine, increasing its power and efficiency. This process is called pressure recovery. At Mach 1.6, the system of shockwaves inside the engine is optimized for maximum pressure recovery. When the aircraft accelerates faster than Mach 1.6, the spike has to retract into the inlet to properly shape balance the shockwaves to continue optimal pressure recovery through a range of speeds. The spike retracts 1.6’’ for each additional 0.1 Mach, and is retracted a total of 26’’ at full speed, Mach 3.2.Thanks to the wonderful Frontiers of Flight Museum in Dallas, Texas for allowing visitors to get up close and personal with this J58.