Aerospace Engineering Analysis: SR-71 Blackbird: Part 2 - Engines
In this edition of The Engineer's Perspective, I go into deeper detail of the engines that power what is arguably the most famous high-performance aircraft in history.
The Engineer's Perspective is a reader-supported publication. To receive new posts and support my work, consider becoming a free or paid subscriber.
Table of Contents:
Overview
Engineering of the SR-71 Engines
Conclusion
References
Overview:
One of the most famous aircraft in aerospace history, the Lockheed SR-71 “Blackbird” was a high altitude, high speed, reconnaissance aircraft. In pursuit of high speed and high-altitude reconnaissance capability, new technologies were developed to handle the never-before encountered stresses and needs of the forthcoming aircraft.
Enter the legendary Pratt & Whitney J-58 engine:
This engine’s unique capability to act both as a turbojet and a ramjet was revolutionary for the time and helped the aircraft set speed and altitude records that have not been broken since they were set in the middle of the 20th century.
Engineering of the SR-71 Engines:
The J-58 engine consisted of the Engine Nacelle and the Body:
Within the Engine Nacelle was the Inlet and Inlet spike which manipulated shockwaves to sustain combustion.
Within the Body was the J-58 Turboramjet engine and the Convergent-Divergent Ejector.
We can break down the basic principles of what makes the J-58 engine unique into two major cycles:
The Turbojet cycle
The Ramjet cycle
The Turbojet cycle operated as follows:
Air would flow through the Nacelle first as it made its way through the engine at supersonic speeds. The front opening of the engine was designed to optimize the shockwave, changing the airflow from low pressure supersonic to high pressure sub-sonic speed before entering the axial compressor:
The axial compressor then further compresses air prior to having fuel added to it by the burner for combustion:
The heated air generated from combustion would simultaneously turn the turbine/compressor while providing the thrust needed for the engine to propel itself.
The J-58 would operate on these basic principles of the turbojet engine up to Mach 2.2, when the engine would then start to transition into Ramjet mode.
The J-58 did this by two primary methods:
Adjustment of the leading shockwave at the front of the engine through mechanical adjustment of the inlet spike at the front of the nacelle.
The usage of Bypass tubes on the end stage of the axial compressor to bring more air into the combustor:
The Inlet Spike:
The inlet spike at the front of the J-58’s nacelle is one of the most recognizable characteristics of the engine. It was also one of the keys to how the SR-71 achieved its blistering speed, and it did this by adjusting the shockwaves that formed in front of it while the aircraft was moving.
As the SR-71 moved through the air at greater speeds, shockwaves formed as atmospheric air molecules compressed against the leading edge of the aircraft. The inlet spike managed these shockwaves by redirecting the supersonic airflow around the engine and reducing the impact of the leading supersonic shockwave on engine components.
By doing this, the inlet spike ensured that the engine received smoother, more uniform airflow, which is especially needed at the inlet of the engine, by creating what are called Normal Shockwaves:
Inside the aircraft’s inlet, normal shockwaves formed as the incoming air underwent instantaneous transition from low-pressure supersonic speeds to high-pressure subsonic speeds due to collisions within the inlet.
This high-pressure subsonic airflow after the normal shockwave was characterized by increased pressure, temperature, and gas density, and was critical for efficient combustion in the engine. It provided the necessary pressures and density for optimal combustion, without which, could result in the engine shutting down mid-flight.
The Bypass Tubes:
The J-58 engine’s ingenuity continued by using bypass tubes, enabling the J-58 to act like a Ramjet engine. A ramjet engine operates by forcing air into an inlet at high speeds into its combustion chamber, creating extra, passive compression that increases fuel efficiency at higher speeds:
The J-58 Engine in effect does this by taking in compressed air from the 4th stage of the compressor and bring it into the burner stage directly, adding fuel that is then ignited:
Image of the J-58 Engine with the Bypass Tubes prominently placed along the outer diameter of the engine:
The addition of this pre-compressed extra bypass air added at high speeds to the combustor allowed for extra generation of thrust.
Conclusion:
One of the most famous aircraft in aerospace history, the Lockheed SR-71 “Blackbird” was a high altitude, high speed, reconnaissance aircraft. In pursuit of high speed and high-altitude reconnaissance capability, the legendary Pratt & Whitney J-58 engine was designed and developed to use a unique capability: to act both as a turbojet and a ramjet.
The front opening of the engine would optimize the shockwave flowing through the Nacelle as it made its way through the engine before entering the axial compressor, while it used bypass tubes to force air into its combustion chamber, creating extra, compression that increased fuel efficiency at higher speeds.
These innovations, combined with cutting-edge technology helped the J-58 engine push the SR-71 to speeds never before seen in the middle of the 20th century. Speeds that have not been rivaled since.
References:
The Mighty J58 - The SR-71's Secret Powerhouse (youtube.com)
SR-71 Inlet System & Supersonic Flow - Sky's Corner (theskyscorner.com)