The engines of the SR-71 Blackbird, the Pratt & Whitney J58 turbojet engines, were started using a unique system known as the AG330 Start Cart, which was far from the typical electric starter used in a car. Here’s how it worked:
1. The AG330 Start Cart
The AG330 Start Cart was a ground-based starting unit equipped with two large V8 automobile engines (later models used Chevrolet big-block engines).
These engines were connected to a gearbox that transferred their combined mechanical power to the SR-71’s J58 engines via a driveshaft.
The purpose of the start cart was to spool up the SR-71’s engines to the necessary RPM for them to ignite.
2. Starting the J58 Engines
The cart’s V8 engines turned the J58 engine's compressor through the gearbox, rapidly spinning it to a starting speed of about 3,200 RPM.
Once the compressor reached the required speed, fuel was introduced, and the ignition sequence began.
The igniter plugs lit the fuel, and the J58 engine roared to life.
Each engine was started individually, so the start cart had to be used twice for a full startup.
3. Transition to Jet Power
Once the engine was running, the start cart was disconnected, and the engine could sustain itself.
The SR-71’s engines needed to run on special JP-7 fuel, which had a high flash point. To aid ignition, a chemical called triethylborane (TEB) was injected during startup. TEB ignites spontaneously upon contact with air, producing a characteristic green flash when the engine was lit.
4. Evolution of the Start Cart
Early versions of the AG330 Start Cart used V8 engines from older cars.
Later, the carts were upgraded with more powerful engines to make the process more reliable and efficient.
5. Why Not an Onboard Starter?
The J58 engines were highly specialized and required significant mechanical power to start, which was impractical to provide with an onboard system.
The use of an external start cart allowed the SR-71 to save weight and complexity, critical for its high-speed and high-altitude performance.