This is one of the coolest videos I have ever seen on the web yet. It's from Tech Adams Laboratories on YouTube, and it's a fantastic animation of the SR-71's massive Pratt & Whitney J58 engine, and how it works. If you've ever really wondered how the thing worked, watch this and it will all be so much clearer.
The J58 was less an engine than a system of aerodynamic components - of which the engine was a part - that enabled the Blackbird to cruise at Mach 3.2 efficiently. And that's the key. When you're cranking full afterburner, you're burning a lot of fuel. But for long-range reconnaissance missions, you have to fly really fast for a long time, and that means re-thinking your propulsion system. P&W and Lockheed collaborated on creating the only engine of its type in the world at the time - what the video called a turbo-ramjet, or what we might call today a combined-cycle engine. Lockheed's experience with ramjets in the X-7 project no doubt enabled them to work closely with Pratt & Whitney on the incredibly complex aerodynamics taking place in and around the J58's massive inlet spike, enabling the engine to do what it was intended to do: push the airplane to incredible speed, and more importantly, stay there.
Though the J58 was proposed for use on a number of other high-performance aircraft in the 1960s, only the Blackbirds carried it in flight. Perhaps one day, when higher performance is again the goal for an aircraft, we'll see a propulsion system that is a direct descendant of the J58. But for now, we can admire it in silence, in museums, usually parked next to one of the retired Blackbirds.
Tech Adams has a bunch of other neat videos on the Blackbird and aviation history in general on his YouTube channel, so it's definitely worth checking out. Personally, I'm jealous that he has been to the National Museum of the USAF and I have not - yet.