[Test-Pilot Notes] issued 1st November 2023
Zoom climb is a maneuver where the aircraft is climbing with excessive pitch-up attitude following a low-g/low alpha pushover to exceed high altitude without stalling. When the pull-up is done at a certain altitude and speed, some aircraft can exceed altitude which is higher than the highest 'normal' cruise altitude.
'Zoom climbs' were not within the flying practices of the SR-71, and it is not part of the official POH. It did not serve the purpose of the SR-71 nor its mission profiles, where a sustained ~80,000 feet / Mach 3.2 was the intended design and mission profile pattern. The SR-71 sustained level flight at the peak altitude.
In a zoom-climb, the aircraft is going through a kind of a powered-ballistic trajectory. At the highest point, the airspeed is below the 1-g stall speed, and the aircraft must then dive back excessively to get back into the operational envelope. The aircraft 'touches' or 'pass through' the altitude record, but could sustain level flight.
In some (or most) cases, the top-of-climb in a record-breaking zoom climb is beyond the air-breathing jet engines operational envelopes, and above a certain altitude, they flame out (or going through other stuff). Going back to the operational envelope of the engines is also a challenge in a true zoom climb. Again, not within the SR-71 intended and directed operating methods.
There are other implications/complications which may eliminate a proposed, theoretically record breaking zoom climb in the SR-71. For example, fuel-tanks pressurization, which may not fall in line with the excessive changes during the initial high-speed climb, and then, during the initial 1/3 part of the trajectory on the way down...
There are some documentation (NASA) which implied the theoretical possibility of zooming-up from the sustained ~80,000+ feet / 3.2 Mach to exceed the highest possible ceiling. Theoretically it may exceed ~90,000 - 95,000 feet, then, going back to ~80,000 - 85,000 feet cruise. But this was not a full-scale zoom climb. More of a short zoom or 'hump' to a lower airspeed/higher altitude. This is not the *traditional* zoom climb whatsoever.
*In the chart above - note the ZOOM (MAX ALTITUDE Extension) in the SR-71 Flight Envelope.
JMH
