in the engine combustor to provide any thrust from the gas generator. All the thrust-producing pressure in the jet-pipe came from ram, as with a ramjet, and none from the gas generator. Fuel for thrust could only be added in the afterburner, which became the only source of engine thrust. The speed at which the gas generator produced no thrust was raised from about Mach 2.5 to about Mach 3 by patented design changes. Beyond that speed, the gas generator would become a drag item with, at Mach 3.2, a pressure ratio of 0.9. Even minimum afterburner would not balance the drag. The effect was described qualitatively by Lockheed inlet designer David Campbell "..with minimum afterburner the engine would be dragging on the engine mounts at high Mach numbers." The second problem was caused by the compressor trying to operate at too-low a corrected speed in an area of its compressor map known as "off-design". The third was caused by the afterburner duct being cooled with too-hot turbine exhaust gas. describes the changes to the engine that extended the engine's capability to Mach 3.2. They included diverting 20% of the compressor entry air after the 4th compressor stage directly to the afterburner through six external tubes. This allowed the compressor to work properly with adequate surge margin and increased airflow into the compressor. Some of the increased flow left the compressor after the 4th stage as bypass to the afterburner, and some left the last compressor stage through the previously choked area. The increased airflow gave more thrust. The inlet guide vanes were modified with trailing-edge flaps to reduce blade flutter and prevent blade fatigue failures. The afterburner was cooled by the bleed air that was cooler than the turbine exhaust gas. Not all the oxygen in the bleed air was available for combustion, as most of the bleed air was directed into the cooling shroud before entering the afterburner cavity for reheating. The improved afterburner cooling allowed a higher flame temperature, which gave more thrust. The engine was completely redesigned, except for the compressor and turbine aerodynamic definitions, so that it would be reliable running for prolonged periods at unprecedented temperatures, not only inside the engine but also surrounding the casings where the controls, accessories, electrical wiring and fuel and oil tubes were located. Starting Two starting methods were used during the life of the A-12, YF-12 and SR-71