[1] Looking inside the "black box" shows that the thrust results from all the unbalanced momentum and pressure forces created within the engine itself.
[3] "Flight" gives examples of these internal forces for two early jet engines, the Rolls-Royce Avon Ra.14[4] and the de Havilland Goblin.
Distortion of the engine structure has to be controlled with suitable mount locations to maintain acceptable rotor and seal clearances and prevent rubbing.
A well-publicized example of excessive structural deformation occurred with the original Pratt & Whitney JT9D engine installation in the Boeing 747 aircraft.
[6] The engine mounting arrangement had to be revised with the addition of an extra thrust frame to reduce the casing deflections to an acceptable amount.
Although the aerodynamic loads on the compressor and turbine blades contribute to the rotor thrust they are small compared to cavity loads inside the rotor which result from the secondary air system pressures and sealing diameters on discs, etc.
To keep the load within the bearing specification seal diameters are chosen accordingly as, many years ago, on the backface of the impeller[9] in the de Havilland Ghost engine.
[13] Possibly the most extreme requirement needing rocket assistance, and which was short-lived, was zero-length launching.
[17] The 'Skyburner' McDonnell Douglas F-4 Phantom II set a world speed record using water injection in front of the engine.
The afterburner also has to make up for the pressure loss across the turbomachine which is a drag item at higher speeds where the epr will be less than 1.0.
[19][20] Thrust augmentation of existing afterburning engine installations for special short-duration tasks has been the subject of studies for launching small payloads into low earth orbits using aircraft such as McDonnell Douglas F-4 Phantom II, McDonnell Douglas F-15 Eagle, Dassault Rafale and Mikoyan MiG-31,[21] and also for carrying experimental packages to high altitudes using a Lockheed SR-71.