Operational Sortie

The Decel

The SR-71's decel was a maneuver that left little room for error. Unlike most jet aircraft, Habus didn't have the latitude to change throttle settings at random or add and subtract drag devices to modify their rate of descent. Once the throttles were brought out of AB at 78,000 feet, the bottom-out point of our decel was basically set. The SR-71's engines and inlets had to be managed during the decel in precise configuration to preclude unstarts, compressor stalls, and engine flameouts.

Prior to starting down, the RSO established ranging with the tanker to make sure he was there waiting for us. When the aircraft arrived at the computed start-descent point, the pilot slowly retarded both throttles to min AB, paused momentarily, and pulled them back further to drop into full military position. As the KEAS began to decrease, the pilot slowly rolled the pitch wheel forward, and intercepted a speed between 350-365 KEAS to reduce the probability of an "unstart," engine stall, or flameouts. As the airspeed approached 365 KEAS, the "KEAS HOLD" function was engaged. At Mach 2.5 the throttles were pulled back slowly and set to 6900 rpm. Below Mach 1.3, the inlet controls were checked in their proper positions, the spike full forward, and the bypass doors closed. After Mach 1.3, the throttles could be placed anywhere to adjust the descent profile as necessary.

What is an "Unstart"?

Anytime the inlet pressure became unacceptable to the AIC (Air Inlet Computer), a phenomenon know as and Aerodynamic Disturbance, or "unstart" took place. An inlet unstart can only occur when the aircraft is supersonic and after and inlet has been "started"; that is, supersonic flow is established inside the inlet. As supersonic speed increases, the supersonic air flow inside the inlet slowly moves aft, to an exhaust port called the "shock trap bleed". An unstart occurs when the Compressor Inlet Pressure becomes too great and has no place to go inside the inlet, causing a shockwave expulsion to relieve the excess pressure build up. An unstart usually caused a very unpredictable and violent maneuver of the aircraft.

Once the aircraft was subsonic, we maintained 0.9 Mach, disengaged KEAS HOLD, and leveled off with either the pitch wheel or the control stick. The descent was fairly steep by now and required about a 2,000 foot lead point to level off at the air refueling altitude.

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