Pratt & Whitney J58

Turbo-ramjet design

The J58 is a hybrid jet engine: effectively, a turbojet engine inside a fan-assisted [Vous devez être inscrit et connecté pour voir ce lien] engine.

This was required because turbojets are inefficient at high speeds but ramjets cannot operate at low speeds.

To resolve this, the airflow path through the engine varied, depending on whether ramjet or turbojet operation was more efficient, thus the term variable cycle.

To create this effect, at speeds over 2,000 mph (3,200 km/h) the nose cone of the engine was pushed about 2 inches (5.1 cm) forward to improve the air flow in the ramjet cycle.

Air is initially compressed and heated by the [Vous devez être inscrit et connecté pour voir ce lien] cones, and then enters 4 stages of compressors, and then the airflow is split: some of the air enters the compressor fans (core-flow air), while the remaining flow bypasses the core to enter the afterburner.

The air continuing through the compressor is further compressed before entering the combustor, where it is mixed with fuel and ignited.

The flow temperature reaches its maximum in the combustor, just below the temperature where the turbine blades would soften.

The air then cools as it passes through the turbine and rejoins the bypass air before entering the afterburner.

At around Mach 3, the initial shock-cone compression greatly heats the air, which means that the turbojet portion of the engine must reduce the fuel/air ratio in the combustion chamber so as not to melt the turbine blades immediately downstream.

The turbojet components of the engine thus provide far less thrust, and the Blackbird flies with 80% of its thrust generated by the air that bypassed the majority of the turbomachinery undergoing combustion in the afterburner portion and
generating thrust as it expands out through the nozzle and from the compression of the air acting on the rear surfaces of the spikes.