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160,000 Horsepower Diesel?

A closer look at the SR-71 Blackbird’s Pratt & Whitney J58 Jet Engines
Posted August 1 2008 05:26 PM by David Kennedy -Publisher 
Filed under: Editorials, Editor's Blog

SR-71 Blackbird photo from Lockheed Martin

When I was up in Clackamas, Oregon this week at Warn’s 60th anniversary media event John Cappa and I decided to hop in a rental car and drive down to McMinnville, Oregon to see the Evergreen Aviation and Space Museum.  

We went there to see Howard Hughes’ H-4 Hercules (more commonly known as the Spruce Goose- a name that Hughes hated) but ended up spending nearly two hours looking at Evergreen’s SR-71 Blackbird. 

SR-71 Blackbird’s Pratt & Whitney J58 Jet Engine afterburner
SR-71 Blackbird’s Pratt & Whitney J58 Jet Engine

Even though it’s been more than 40 years since the SR-71 jets first flew they are still amazing to see in person.  The Pratt & Whitney J58 Jet Engines (rated for 32,000 pounds of thrust each) are some of the largest jet engine ever built.  According to the SR-71 was originally designed to run a slurry of coal, but was later developed to operate on a unique blend of jet fuel called JP-7.

Are the Pratt & Whitney J58 Jet Engines in the SR-71 Blackbirds diesels?  Technically, no.  But I have no doubt that I couldn’t burn JP-7 jet fuel in our 7.3L Power Stroke… So that begs the question: what makes a diesel, a diesel? Is it the fuel it burns?
Or is it the combustion process?

I say it’s the combustion process, but that’s not going to stop me from showing you a cool engine.

From Wikipedia:
JP-7 jet fuel had a high flash point (140°F) to cope with the heat. In fact, the fuel was used as a coolant and hydraulic fluid in the SR-71 aircraft before being burned. The fuel also contained fluorocarbons to increase its lubricity, an oxidizing agent to enable it to burn in the engines, and even a cesium compound, A-50, which disguised the exhaust's radar signature.

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.

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