The scientific tourist #63 — Pratt & Whitney J58 jet engine

This week’s image comes to you from the Museum of Flight in Seattle, Washington:

P&W J58

This is a photograph of a Pratt & Whitney J58 jet engine — originally developed for a U.S. Navy program (which never seems to have gone anywhere), but used to great effect as the power plant for the SR-71 reconnaissance aircraft. The J58 was the first engine to be flight qualified by the U.S. Air Force for Mach 3 flight, as well as the first engine able to operate on afterburner for extended periods of time

The J58 had an interesting hybrid design — it functioned as a pure turbojet at low speeds, then gradually transitioned over to a mixed turbojet / ramjet mode as the aircraft it was in gained speed. By Mach 3.2, 80% of its thrust came from the ramjet section of the engine.

Since high speed flight leads to high temperatures on the aircraft, the J58 had to use a special fuel as well — JP-7. JP-7 was specially made to tolerate high temperatures well, and as a result was very difficult to ignite. Just like diesel and other low-volatile fuels, you can toss a lit match into a bucket of JP-7 without risk of lighting it on fire (it will douse the flame on the match, actually). JP-7 was so good at its job that it was used as a heat sink to help cool the otherwise very hot airframe of the SR-71, as well as being used as hydraulic fluid within the engine. Of course, this also meant that the engine was hard to start — rather than use an electrical or mechanical ignition system, a chemical one was used. After spinning up the J58 to 3,200 RPM, the fuel flow was started, and 50 cm3 of triethylborane (TEB) was injected into the engine to ignite it (TEB spontaneously ignites in contact with air, making it notoriously hard to handle).

If you’d like to know more about this exotic engine, you can check out its Wikipedia entry, as well as its fact sheet at the USAF Museum website, and an interesting set of diagrams / photographs on a site dedicated to the SR-71.

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