Pratt & Whitney JT-11
Archive Photos ¹
Pratt & Whitney JT-11 Turbojet on display at the Hill Aerospace Museum, Hill AFB, Roy, Utah (John Shupek photo copyright © 2002 Skytamer Images)
The Pratt & Whitney J58 (P&W designation JT11D) was a jet engine used on the CIA's Lockheed A-12 “Oxcart”, and subsequently on the YF-12 and SR-71 “Blackbird” aircraft. The J58 was a variable cycle engine which functioned as both a turbojet and a fan-assisted ramjet. The J58 was a single-spool turbojet engine with an afterburner. Bypass jet engines were rare at the time, but Ben Rich later described the engine as “bypass jet engine by air withdrawal”.
“Personal note from John Shupek:” This is undoubtedly my favorite engine. During the mid-1960s, I was one of Pratt & Whitney's design engineers working on the continued development and upgrades of the J58 engines. I was responsible for the design of the internal cooling of the high turbine's air-cooled blades and vanes. For a young engineer straight out of the “cookie jar”, it was a great project to work on.
Design and Development ²
The J58 was initially developed for the US Navy to power the planned version of the Martin P6M jet flying boat. Upon cancellation of this aircraft, it was selected by Convair and Lockheed for their supersonic projects. Other sources link its origin to the USAF's requirement for a powerplant for the WS-110A, the future XB-70 Valkyrie. The J-58 produced 32,000 lbf (142 kN) of thrust. It was the first engine to be able to operate on afterburner for extended periods of time, and the first engine to be flight-qualified by the United States Air Force for Mach 3. A major feature of the J58 was the conical spikes in the variable-geometry inlets, which were automatically moved fore and aft by an Air Inlet Computer. The spike altered the flow of supersonic air, ensuring subsonic airflow at the engine inlet. The conical spikes are locked in forward position below 30,000 feet. Above that altitude they are unlocked. Above Mach 1.6 airspeed they are retracted approximately 1-5/8 inch (4 cm ) per Mach 0.1, up to total of about 26 inches (66 cm).
The J58 was a variable cycle engine which functioned as both a turbojet and a fan-assisted ramjet. Bypass jet engines were rare at the time, but Ben Rich later described the engine as “bypass jet engine by air withdrawal”. At Mach 3.2, 80% of the engine's thrust came from the ramjet section, with the turbojet section providing 20%. At lower speeds, the J58 operated as a pure turbojet.
The engine was started with an AG330 starter cart, with two Buick “Wildcat” V8 internal combustion engines with a common drive shaft. The cart spun the J58 to 3,200 rpm before the turbojet cycle could start. Later, a conventional pneumatic start cart was used.
The engine's high operating speeds and temperatures required a new jet fuel, JP-7. Its reluctance to be ignited required triethylborane (TEB) to be injected into the engine to ignite it, and to ignite the afterburner in flight; above -5°C TEB spontaneously ignites in contact with air. Each engine carried a nitrogen-pressurized sealed tank with 600 cm³ (20.7 ounces) of TEB, sufficient for at least 16 starts, restarts, or afterburner lights; this number was one of the limiting factors of SR-71 endurance, as after each air refueling the afterburners had to be re-ignited. When the pilot moved the throttle from cut-off to idle position, fuel flowed into the engine, and shortly afterwards an approx. 50 cm³ (1.7 ounce) shot of TEB was injected into the combustion chamber, where it spontaneously ignited and lit the fuel with a green flash. In some conditions, however, the TEB flow was obstructed by coking deposits on the injector nozzle, hindering restart attempts. Refilling the TEB tank was a perilous task; the maintenance crew wore silver fire suits. Conversely, the JP-7 fueling was so safe that some aircraft maintenance was permitted during filling. The chemical ignition was chosen instead of a conventional igniter for reliability reasons, and to reduce mechanical complexity. The TEB tank is cooled with fuel flowing around it, and contains a disk that ruptures in case of overpressure, allowing TEB and nitrogen to discharge into the afterburner.
The fuel flowing into the engine is used as a coolant to cool the engine, hydraulic fluid, oil, TEB tank, afterburner nozzle actuator control lines, air conditioning systems, and the parts of the airframe subjected to aerodynamic heating.
The engine lubricant was a silicone-based grease. It was solid at room temperature, and was preheated prior to engine start.
Turbo-ramjet Design ²
The J58 is a hybrid jet engine: effectively, a turbojet engine inside a fan-assisted ramjet engine. This was required because turbojets are inefficient at high speeds but ramjets cannot operate at low speeds. To resolve this, the airflow path through the engine varied, depending on whether ramjet or turbojet operation was more efficient, thus the term variable cycle. To create this effect, at speeds over 2,000 mph (3,200 km/h) the nose cone of the engine was pushed about 26 inches (66 cm) rearward to improve the air flow in the ramjet cycle.
Air is initially compressed and heated by the shock wave cones, and then enters 4 stages of compressors, and then the airflow is split: some of the air enters the compressor fans (core-flow air), while the remaining flow bypasses the core to enter the afterburner. The air continuing through the compressor is further compressed before entering the combustor, where it is mixed with fuel and ignited. The flow temperature reaches its maximum in the combustor, just below the temperature where the turbine blades would soften. The air then cools as it passes through the turbine and rejoins the bypass air before entering the afterburner.
At around Mach 3, the initial shock-cone compression greatly heats the air, which means that the turbojet portion of the engine must reduce the fuel/air ratio in the combustion chamber so as not to melt the turbine blades immediately downstream. The turbojet components of the engine thus provide far less thrust, and the “Blackbird” flies with 80% of its thrust generated by the air that bypassed the majority of the turbo-machinery undergoing combustion in the afterburner portion and generating thrust as it expands out through the nozzle and from the compression of the air acting on the rear surfaces of the spikes.
Aircraft Applications ²
Specification of J58-P4 ²
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