How Do You Start A Pratt & Whitney J58? (SR71 Engine)

In 1957, the CIA tasked aerospace company Lockheed with the design and construction of a spy plane to replace the outdated U2. Dubbed Project Archangel, work began in 1958 which resulted in the ‘A-10’ and the later ‘A-12’ which boasted a 90 percent reduction in radar cross-section than its forerunner.

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Lockheed A-12.

The A-12 evolved into the SR-71, a Mach 3 capable reconnaissance plane that was brimming with revolutionary technology. At the heart of the SR-71 were two Pratt and Whitney J58 after-burning turbojets, monsters that had to be turned on in an interesting way.

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Lockheed SR-71.
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For the J58 to fire up, it first had to reach a minimum of 3,200 rpm. To achieve this, engineers created the AG330 Start Cart. The AG330 consisted of two Buick Wildcat V8’s, each mated to an auto box which in turn met at another gearbox.

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Buick ‘Super Wildcat’.

The V8 engines chosen for the Start Cart were a pair of Buick 401’s, also known as ‘Nailheads’. The 401 refers to the displacement in cubic inches, this translates to 6.5 litres.

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The 401 Nailhead V8 with aftermarket air filter. Author: Mr.choppers CC BY-SA

The 90-degree V8 featured several distinctive characteristics, each bank only had one overhead camshaft and one exhaust and inlet valve per cylinder. This was not an oversight on Buick’s behalf, the engineers preferred to play with the camshaft lobes rather than just add more valves.

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The engineers at Buick had one thing in mind when building the Nailhead – torque. The engineers were so happy with the amount of torque they could push out of the engine, the figure adorned the roof of the engine in decals. The V8 produced 465 Foot-Pounds of torque, which translates to 630 Newton-Metres, amazingly, the Nailhead was manufacturing its max torque at incredibly low revs, 2,800 to be precise.

Needless to say, the engine became a car modifiers dream due to the neck-breaking levels of torque and the speedy delivery of it. It was probably these two features that caught Lockheed’s eye when looking for an engine to put in their start cart.

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AG330 Start Cart. Author: Jaydec CC BY-SA 3.0

When the engines were mated together they delivered 600-650 horsepower. The two engines met at a gearbox which drove a vertical drive shaft that connected to the underside of the J58.

The nailheads would start hitting their redline, at around 4,800 rpm, at this point, 30 cc of Triethylborane, a fuel catalyst, would be shot into the jet which pushed the J58 to its 3,200 rpm sweet spot.

When the J58 hit 3,200 rpm, the Buick operator would hit a switch for the drive shaft to fall free from the jet. In the event that the shaft failed to fall free, the Buick’s rpm would continue to climb until the J58 hit its idle speed, meaning the Buicks could be running at over 6,000 rpm. This usually resulted in an engine failure.

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Later in the SR-71 programme, the Buick Nailheads were replaced with Chevy big-block engines. Eventually the entire start cart system itself was replaced when airbases had large compressed air tanks installed. The air tanks had hoses that would connect to the J58, pumping in 60 psi of compressed air, starting the engine.

There are some start carts still around today, there are examples in museums and there are said to be some still in Air Force storage. Not much is known about the start carts but there is something inherently cool about one of the greatest planes ever to fly being fired up by a pair of V8’s.

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