The AG-330 Start Cart orginally were Buick wildcat 401 cubic inch V-8 engines developing 400 horsepower. There were two Buick engines mounted tandem side by side with automatic transmissions. They were coupled together with a steel woven drive belt to drive a vertical shaft that was inserted into the starter mechanism on the bottom of the SR-71's engine.
On engine launch there was one AG-330 Buick for each SR engine. For the first time observer of an SR-71 launch, the engine starts in themselves were almost indescribable. For one thing there was no mufflers on the Buick engines. As the Buick RPM was advanced, Flames almost three feet long erupted from the side of the start cart. It truly sounded like the beginning of the Indianapolis 500. Combine that sound with the steady increase of SR-71 RPM to engine TEB ignition at 3,200 RPM and then idle aircraft engine speed as the Blackbird comes to life.
The Mechanical Details
Each of the two Buick V-8s had eight straight exhaust pipes that exited out of the bottom of the AG330. Each engine had a Hydromatic transmission with electric shift, and the two transmissions were coupled together by a 12 inch wide, toothed Gilmer belt.
A view of the drive belt connecting the two engines
The coupled engines drove a gearbox that turned the output 90° to the vertical for the probe head. This gearbox was hydraulically controlled and had a chip detector that would trigger a warning light on the AG330 control panel if metal debris was detected in the gearbox. The control panel was located at the end of the AG330. Each engine had ampere, water temperature, oil pressure, and oil temperature gauges, a tachometer, a start switch, large chrome choke control handle, and engine oil overpressure override button. Water temperature, oil pressure, and oil temperature gauges had red limit warning lights.
The probe instruments included green indicator lights for Probe Engaged, Starter Ready, Transmission Shifted, Probe Disengaged, and Probe Brake On. Toggle switches for the probe included Engage Probe, Disengage Probe, Probe Brake release, Jet Start, and a guarded Cart Shutdown switch. A single Morse throttle controlled both start cart engines; idle was at the top of the throttle travel, moving through Run and Advance as the throttle was moved downward. Probe gauges included Jet Speed and Torque Pressure.
On the far right side of the Instrument Panel were a Cart Positioning lever, which could be used to move the whole cart left or right on the chassis frame to align the probe, a master Power On indicator light, Elapsed Time Hobbs meter, Main Power toggle switch, Main Power circuit breakers, and a Number of Jet Starts gauge (missing in the photographs). The Hobbs meter indicates this cart had 408.5 hours on the Buicks.
The probe head had two handles. The right side handle had two switches; the top switch raised the probe while the lower switch lowered the probe. Retraction was passive by venting the hydraulic pressure. The left handle had a single switch, which may have released the probe brake. The drive gear was approximately three inches across and had a surrounding fitting with holes that received the three alignment pegs on the engine drive pad to align the probe.
The probe could gimbal approx. 1/4 to 1/2 inch to account for slight misalignment from true vertical between the cart and the engine pad. There were three microswitches on the probe head that all had to close to confirm proper probe placement, and this was indicated by two green "Probe Engaged" and "Starter Ready" lights on the start cart panel. This would arm the probe.
The Buick engines were started ahead of time and warmed up in neutral. The curved starter drive access panel, approximately 15 x 30 inches and attached by CalFax fasteners, was opened, the cart positioned, and the probe raised to mate with the drive pad. Maximum vertical probe travel was about 24 inches, but a loaded airplane typically required a vertical travel of 8 to 10 inches. The starter pad is only an inch or two inside the outer surface of the aircraft.
Engine Starting Procedure
The pilot told the Crew Chief via intercom "Engage Buicks". The Buick operator pressed "Jet Start", and movement of the Buick throttle (one throttle lever controls both engines) downward engaged the transmissions. The "Transmissions Shifted" illuminated green, and the Buicks started to load up.
It was important for the Buick operator to accelerate but maintain 700 to 725 lb ft of torque during starting procedure. Too high a torque pressure would cause the probe to drop out, which prevented overstressing the gear box, but resulted in a "cut off throttle" command from the Crew Chief to the pilot. With the Buick disengaged and the jet engine unable to sustain itself, the risk of an over temperature condition was high. Since the probe could not be re-engaged to a spinning J58, it was imperative to quickly re-engage the probe once the J58 wound down to motor the jet engine and blow out any fire. Too low a torque pressure during Buick start would cause a lag in acceleration to idle speed and an over temperature condition.
The pilot would watch his onboard gauges to confirm minimum oil pressure, fuel pressure, and rising RPM, and then set the jet throttle to idle. At that point 30cc of TEB was injected into the burner cans, a characteristic green flame was emitted, the J58 lit off, accelerated and started to run on its own. The accelerating jet started to unload the Buicks, and when 3,200 rpm was reached, the pilot called "Buicks out", the Crew Chief signaled "cut", the Buick operator hit "Cart Shutdown", the probe fell free passively, and the cart throttle automatically returned to idle.
Neither the J58, the probe, nor the start cart transmission contained an overrunning clutch, so it was important that the probe fall free. If it hung up, the crew had to quickly get in and shake the handles as fast and as hard as they could. The Buicks would reach 4,800 to 4,900 rpm, the redline on the start cart, just to get the J58 to 3,200 rpm. Overspeeding of the Buicks caused by a probe hang-up contributed to the occasional thrown connecting rod and oily parts dropping out from under the cart. Idle speed for the J58 is 3,950 rpm, which could drive the Buicks to over 6,000 rpm. The potential for engine failure, as well as the Buick exhaust stream, required the crew to stand only at the ends, not aside, the start cart.
Then it all starts all over again with #1 engine.
A bit of History
The original design was conceived by two Lockheed Skunk Works engineers who had prior race car engine knowledge. In the mid-1970's, the engines of the start carts was changed to Chevrolet LS-7 454's. The Chevrolet big block engines developed 465 horsepower each. Even so, for many years they were still referred to as the Buick's.
In the '80's the AG-330 Start Carts were mothballed and in there place came a Pneumatic Air starting system for the SR-71's. It is true that the start carts were hard to maneuver around. Logistically, a recovery of a Blackbird at another base could be done easier with Pneumatic Air. Garrett Air Research installed the Pneumatic Air starting system in each of the hangers at Beale AFB to accommodate the SR-71 engine starts. The launches were never quite the same without the Buick's or the 454's.
Credit should be given to those people that maintained the AG-330's in the 9th SRW Aerospace Ground Support Section. Along with maintaining all support equipment, they took great pride in maintaining the Start Carts tuned and in tip top condition. A job well done.