[Test-Pilot Report] issued 23rd August 2021
The F-4E is powered by two J79-GE-17 engines, where each engine is equipped with an afterburner. A fact that may be new to some: Afterburning engines are designed with variable-geometry exhaust nozzles!
Engines with afterburners are equipped with variable area exhaust nozzles. The purpose of the variable area nozzle is to utilize maximum thrust from a given jet-flow, and to accelerate the exhaust-jet velocity from subsonic to supersonic velocity (in afterburner mode), to fully utilize the afterburner system.
The internal geometry of the nozzle in this case must be of a convergent-divergent cross section, as convergent nozzles cannot accelerate the exhaust jet velocity into the supersonic regime (in short - as the flow exceeds the sonic regime in the nozzle, shock waves are formed in the convergent cross-section, reducing the jet velocity to subsonic right after...).
To form a convergent-divergent cross-section that allows supersonic flow, the variable area exhaust nozzle includes TWO sets of cylindrical nozzles, that are working together (per engine).
The inner nozzle controls the convergent section of the nozzle, while the outer nozzle controls the divergent section. The two nozzle sections are linked together, forming the required cross-section during engine operation.
When the engine is set to full "dry" power (without afterburner), the inner nozzle opens while the outer nozzle closes. The whole nozzle is working in this case as a convergent cross-section nozzle.
When the afterburner is ignited, the inner nozzle closes, while the outer nozzle opens widely. Now the nozzle is set with a convergent-divergent cross section (similar to a rocket nozzle).
Jet flow accelerates in the convergent section and proceeds to the divergent section, becoming supersonic. The shock waves may be visible as "rings" as the high energy, high velocity jet leaves the divergent section, behind the aircraft.
The (upcoming) VSKYLABS F-4E Phantom II is equipped with functional variable area exhaust nozzles that works with two, animated nozzle sections for each engine, forming a convergent-divergent cross-section during engine operation from idle to afterburner. To note that when on idle, the nozzles are widely opened, mainly to reduce the thrust to minimum.
The featured screenshot in this post shows the (Work In Progress) variable area nozzles in action, where the right-hand engine is set to maximum "dry" thrust (no afterburner), and the left-hand engine is set to idle.
JMH