Pratt & Whitney J58
Encyclopedia
The Pratt & Whitney
J58 (company designation JT11D) was a jet engine
used on the Lockheed A-12, 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".
. 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 inlet
s, 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 engine
s with a common driveshaft
. 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 reignited. 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.
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 2000 mile per hour the nose cone of the engine was pushed about 2 inches (5.1 cm) forward 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 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.
Pratt & Whitney
Pratt & Whitney is a U.S.-based aerospace manufacturer with global service operations. It is a subsidiary of United Technologies Corporation . Pratt & Whitney's aircraft engines are widely used in both civil aviation and military aviation. Its headquarters are in East Hartford, Connecticut, USA...
J58 (company designation JT11D) was a jet engine
Jet engine
A jet engine is a reaction engine that discharges a fast moving jet to generate thrust by jet propulsion and in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets...
used on the Lockheed A-12, and subsequently on the YF-12
Lockheed YF-12
The Lockheed YF-12 was an American prototype interceptor aircraft, which the United States Air Force evaluated as a development of the highly-secret Lockheed A-12 that also spawned the SR-71 Blackbird.-Design and development:...
and SR-71 Blackbird
SR-71 Blackbird
The Lockheed SR-71 "Blackbird" was an advanced, long-range, Mach 3+ strategic reconnaissance aircraft. It was developed as a black project from the Lockheed A-12 reconnaissance aircraft in the 1960s by the Lockheed Skunk Works. Clarence "Kelly" Johnson was responsible for many of the...
aircraft. The J58 was a variable cycle engine
Variable Cycle Engine
A variable cycle engine is an engine that is designed to operate efficiently under mixed flight conditions, such as subsonic, transonic and supersonic.The next generation of Supersonic transport may require some form of VCE...
which functioned as both a turbojet and a fan-assisted ramjet
Ramjet
A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of airbreathing jet engine using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move an aircraft from a standstill...
. The J58 was a single-spool turbojet engine with an afterburner. Bypass jet engines were rare at the time, but Ben Rich
Ben Rich
Benjamin Robert Rich was the second director of Lockheed's Skunk Works from 1975 to 1991, succeeding its founder, Kelly Johnson. Regarded as the "father of stealth," Ben Rich was responsible for leading the development of the F-117, the first production stealth aircraft...
later described the engine as "bypass jet engine
Turbofan
The turbofan is a type of airbreathing jet engine that is widely used for aircraft propulsion. A turbofan combines two types of engines, the turbo portion which is a conventional gas turbine engine, and the fan, a propeller-like ducted fan...
by air withdrawal".
Design and development
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 ValkyrieXB-70 Valkyrie
The North American Aviation XB-70 Valkyrie was the prototype version of the proposed B-70 nuclear-armed deep-penetration strategic bomber for the United States Air Force's Strategic Air Command...
. The J-58 produced 32,000 lbf (142 kN) of thrust. It was the first engine to be able to operate on afterburner
Afterburner (engine)
An afterburner is an additional component added to some jet engines, primarily those on military supersonic aircraft. Its purpose is to provide a temporary increase in thrust, both for supersonic flight and for takeoff...
for extended periods of time, and the first engine to be flight-qualified by the United States Air Force
United States Air Force
The United States Air Force is the aerial warfare service branch of the United States Armed Forces and one of the American uniformed services. Initially part of the United States Army, the USAF was formed as a separate branch of the military on September 18, 1947 under the National Security Act of...
for Mach 3. A major feature of the J58 was the conical
Cone (geometry)
A cone is an n-dimensional geometric shape that tapers smoothly from a base to a point called the apex or vertex. Formally, it is the solid figure formed by the locus of all straight line segments that join the apex to the base...
spikes in the variable-geometry inlet
Intake
An intake , or especially for aircraft inlet, is an air intake for an engine. Because the modern internal combustion engine is in essence a powerful air pump, like the exhaust system on an engine, the intake must be carefully engineered and tuned to provide the greatest efficiency and power...
s, which were automatically moved fore and aft
Aft
Aft, in naval terminology, is an adjective or adverb meaning, towards the stern of the ship, when the frame of reference is within the ship. Example: "Able Seaman Smith; lay aft!". Or; "What's happening aft?"...
by an Air Inlet Computer. The spike altered the flow of supersonic
Supersonic
Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...
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
Variable Cycle Engine
A variable cycle engine is an engine that is designed to operate efficiently under mixed flight conditions, such as subsonic, transonic and supersonic.The next generation of Supersonic transport may require some form of VCE...
which functioned as both a turbojet and a fan-assisted ramjet
Ramjet
A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of airbreathing jet engine using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move an aircraft from a standstill...
. Bypass jet engines were rare at the time, but Ben Rich
Ben Rich
Benjamin Robert Rich was the second director of Lockheed's Skunk Works from 1975 to 1991, succeeding its founder, Kelly Johnson. Regarded as the "father of stealth," Ben Rich was responsible for leading the development of the F-117, the first production stealth aircraft...
later described the engine as "bypass jet engine
Turbofan
The turbofan is a type of airbreathing jet engine that is widely used for aircraft propulsion. A turbofan combines two types of engines, the turbo portion which is a conventional gas turbine engine, and the fan, a propeller-like ducted fan...
by air withdrawal". At Mach 3.2, 80% of the engine's thrust
Thrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....
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
Buick
Buick is a premium brand of General Motors . Buick models are sold in the United States, Canada, Mexico, China, Taiwan, and Israel, with China being its largest market. Buick holds the distinction as the oldest active American make...
Wildcat
Buick Wildcat
In 1966 a one-year-only Wildcat "Gran Sport Performance Group" package could be ordered by selecting the "A8/Y48" option. Two engine choices were available. The single carb 425 CID/340 hp V8 was included in the base package price but a dual-carb set-up was also available at extra cost...
V8 internal combustion engine
Reciprocating engine
A reciprocating engine, also often known as a piston engine, is a heat engine that uses one or more reciprocating pistons to convert pressure into a rotating motion. This article describes the common features of all types...
s with a common driveshaft
Driveshaft
A drive shaft, driveshaft, driving shaft, propeller shaft, or Cardan shaft is a mechanical component for transmitting torque and rotation, usually used to connect other components of a drive train that cannot be connected directly because of distance or the need to allow for relative movement...
. 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
Jet fuel
Jet fuel is a type of aviation fuel designed for use in aircraft powered by gas-turbine engines. It is clear to straw-colored in appearance. The most commonly used fuels for commercial aviation are Jet A and Jet A-1 which are produced to a standardized international specification...
, JP-7
JP-7
JP-7 is a jet fuel developed by the U.S. Air Force for use in supersonic aircraft because of its high flash point and thermal stability. It is the fuel used in the Pratt & Whitney J58 engines, used in the Lockheed SR-71 Blackbird. The air compression of Mach 3+ cruising flight generates very high...
. Its reluctance to be ignited required triethylborane
Triethylborane
Triethylborane , also called triethylborine and triethylboron, is an organoborane , a near-colorless to yellowish transparent liquid with pungent ether-like odor. Its chemical formula can be written as C6H15B, or 3B, or 3B, or Et3B.Triethylborane is strongly pyrophoric, igniting spontaneously in...
(TEB) to be injected into the engine to ignite it, and to ignite the afterburner
AfterBurner
The AfterBurner is a lighting solution for the Game Boy Advance system that was created by Triton-Labs.Originally, portablemonopoly.net was a website created to petition Nintendo to put some kind of light in their Game Boy Advance system...
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 reignited. 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
Coolant
A coolant is a fluid which flows through a device to prevent its overheating, transferring the heat produced by the device to other devices that use or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, and chemically inert, neither causing nor...
to cool the engine, hydraulic fluid
Hydraulic fluid
Hydraulic fluids, also called hydraulic liquids, are the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water...
, oil
Oil
An oil is any substance that is liquid at ambient temperatures and does not mix with water but may mix with other oils and organic solvents. This general definition includes vegetable oils, volatile essential oils, petrochemical oils, and synthetic oils....
, TEB tank, afterburner nozzle actuator control lines, air conditioning systems, and the parts of the airframe subjected to aerodynamic heating.
The engine lubricant
Lubricant
A lubricant is a substance introduced to reduce friction between moving surfaces. It may also have the function of transporting foreign particles and of distributing heat...
was a silicone
Silicone
Silicones are inert, synthetic compounds with a variety of forms and uses. Typically heat-resistant and rubber-like, they are used in sealants, adhesives, lubricants, medical applications , cookware, and insulation....
-based grease. It was solid at room temperature, and was preheated prior to engine start.
Turbo-ramjet design
Ramjet
A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of airbreathing jet engine using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move an aircraft from a standstill...
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 2000 mile per hour the nose cone of the engine was pushed about 2 inches (5.1 cm) forward to improve the air flow in the ramjet cycle.
Air is initially compressed and heated by the shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...
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 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.