Nice to see so much interest in such a remarkable aircraft, definitely ahead of its time. I learned a good deal researching it a few minutes ago, I never realized that its performance was "anemic" until it got to higher speeds. The engines are a work of art.
http://en.wikipedia.org/wiki/SR-71_Blackbird#Engines
The Pratt & Whitney J58-P4 engines used in the Blackbird were the only military engines ever designed to operate continuously on afterburner, and actually became more efficient as the aircraft went faster. Each J58 engine could produce 32,500 lbf (145 kN) of static thrust. Conventional jet engines cannot operate continuously on afterburner and lose efficiency as airspeed increases.
The J58 was unique in that it was a hybrid jet engine. It could operate as a regular turbojet at low speeds, but at high speeds it became a ramjet. The engine can be thought of as a turbojet engine inside a ramjet engine. At lower speeds, the turbojet provided most of the compression and most of the energy from fuel combustion. At higher speeds, the turbojet throttled back and just sat in the middle of the engine as air bypassed around it, having been compressed by the shock cones and only burning fuel in the afterburner.
In detail, air was initially compressed (and thus also heated) by the shock cones, which generated shock waves that slowed the air down to subsonic speeds relative to the engine. The air then passed through four compressor stages and was split by moveable vanes: some of the air entered the compressor fans ("core-flow" air), while the rest of the air went straight to the afterburner (via six bypass tubes). The air traveling on through the turbojet was further compressed (and thus further heated), and then fuel was added to it in the combustion chamber: it then reached the maximum temperature anywhere in the Blackbird, just under the temperature where the turbine blades would start to soften. After passing through the turbine (and thus being cooled somewhat), the core-flow air went through the afterburner and met with any bypass air.
At around Mach 3, the increased heating from the shock cone compression, plus the heating from the compressor fans, was already enough to get the core air to high temperatures, and little fuel could be added in the combustion chamber without the turbine blades melting. This meant the whole compressor-combustor-turbine set-up in the core of the engine provided less power, and the Blackbird flew predominantly on air bypassed straight to the afterburners, forming a large ramjet effect. No other aircraft does this. (This shows how the temperature tolerance of the turbine blades in a jet engine determine how much fuel can be burned, and thus to a great extent determine how much thrust a jet engine can provide.)[19]
Performance at low speeds was anemic. Even passing the speed of sound required the aircraft to dive. The reason was that the size of the turbojets was traded to reduce weight but to still allow the SR-71 to reach speeds where the ramjet effect became prominent and efficient; then, the airplane came alive, so to speak, and rapidly accelerated to Mach 3.2. The efficiency was then good due to high compression and low drag through the engine, and this permitted large distances to be covered at high speed.
Originally, the Blackbird's engines started up with the assistance of an external "start cart", a cart containing two Buick Wildcat V8 engines which was rolled underneath the aircraft. The two Buick engines powered a single, vertical driveshaft connecting to a single J58 engine. Once one engine was started, the cart was wheeled over to the other side of the aircraft to start the other engine. The operation was deafening. Eventually, a quieter, pneumatic start system was developed for use at Blackbird main operating bases, but the start carts remained in the inventory to support recovery team Blackbird starts at diversion landing sites not equipped to start J-58 engines.
(just an interesting note... something I didn't realize... they used LIVE BEARS to test the ejection system!
