Pratt & Whitney J58

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J58 on full afterburner, showing shock diamonds

The Pratt & Whitney J58 (also known as the JT11D) was the jet engine used on the Lockheed A-12 OXCART, and subsequently on the YF-12 and SR-71 "Blackbird" aircraft. It was essentially a turbojet engine^[1] with an afterburner, although it had a variable bypass ratio.

1 Overview
2 Turbo-ramjet design
3 Specification of J58-P4
4 See also
5 References
6 External links

[edit] Overview

The J58 produced 32,000 lb_f (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 U.S. 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, keeping air entering the engine at a subsonic speed.

The J58 was a variable cycle engine which functioned as both a turbojet and a fan-assisted ramjet. Bypass jet engines were unknown at the time, but Ben Rich later described the engine as "Bypass jet engine by air withdrawal".^[2] At Mach 3.2, 80% of the engine's thrust came from the ramjet section, with the turbojet section providing 20%.^[3] At lower speeds, the J58 operated as a pure turbojet.

The engine's operation was started using an AG330 engine starter cart, composed of two Buick Wildcat V8 internal combustion engines with a common driveshaft. The cart would spin up the J58 spool to 3,200 rpm before the turbojet cycle could start. Later, a conventional start cart was used.^{[citation needed]}

The engine's high operating speeds and temperatures required a new jet fuel, JP-7. Its relative unwillingness to be ignited required triethylborane (TEB) to be injected into the engine in order to light it up, and to light up 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 of TEB, an amount sufficient for at least 16 starts, restarts, or afterburner lights; this number was one of the limiting factors of SR-71 flight endurance, as after each air refueling the afterburners had to be lit up. [1] When the pilot moved the throttle from cut-off to idle position, fuel flowed into the engine, and shortly afterwards a 50 cm³ shot of TEB was injected into the combustion chamber, where it spontaneously ignited and lit up the fuel with a telltale green flash. In some conditions, however, the TEB flow was obstructed by coking deposits on the injector nozzle, hindering restart attempts. The refilling of the TEB tank was a perilous task; the maintenance crew had to wear silver fire suits.[2] Conversely, the JP-7 fueling was so safe in operational use that some aircraft maintenance was permitted during filling. The chemical ignition was chosen instead of a conventional igniter due to reliability reasons and to lower the number of mechanical parts that could fail in the extreme temperatures they would be subjected to. The TEB tank is cooled with fuel flowing around it, and contains a rupture disk that in case of an overpressure allows discharging of TEB and nitrogen into the afterburner section.

The conical spikes are locked in forward position for altitudes below 30,000 feet. Above that altitude they are unlocked. Above Mach 1.6 airspeed they are retracted by approximately 1-5/8 inch per 0.1 Mach, up to total of about 26 inches.

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 lubricant used in the engines was a silicone-based grease. It was solid at room temperature and needed to be preheated before the engine could be started.

[edit] Turbo-ramjet design

Operation of the air inlets and air flow patterns through the J58 at different Mach numbers

The J58 is a hybrid jet engine: effectively a turbojet engine inside a fan-assisted ramjet engine. This is because turbojets are inefficient at high speeds, yet ramjets cannot operate at low speeds. The airflow path through the engine varied, depending on whether ramjet or turbojet operation was more efficient, thus the term "variable cycle". Eg, at speeds over 2000 mph the nose cone of the engine is pushed about 2 inches forward to improve the air flow in the ramjet cycle.

Air is initially compressed and heated by the shockwave cones, and then enters 4 stages of compressors, and then the airflow is split:^[4] 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 turbomachinery 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.