The SR-71 was a marvel, a super plane that marked an era. The technology it had on it is still unmatched. One of the most impressive things about the jet was its engines. Not only the sheer power of them but their placement of them. Two Pratt & Whitney J58 axial-flow turbojet engines powered the SR-71. The J58 was a significant technological advancement at the time, capable of delivering a static thrust of 32,500 IBF (145 kN). Around Mach 3.2, the Blackbird’s average cruising speed, the engine was most efficient. Normally, fighter jets have a single or dual engine that is mounted closely together. But the engines mounted on the SR-71 were quite far apart. The SR-71’s primary concept is that when it speeds up, it switches from turbojet to primarily ramjet operation. The engine’s inlet must be extremely efficient for the ramjet-like function to work. That is to say, it does more than simply slow the flow of air from supersonic to subsonic. That could be done with a simple shock.
Instead, the SR-71 intake tries to create as little entropy as possible by sending Mach 3.4 air through a series of oblique shocks that progressively slow it down to subsonic speed while compressing it to 40 times its ambient pressure. The turbojet can’t burn much fuel at this temperature, so it provides more drag than thrust. However, the majority of that pressure remains at the afterburner’s end, which is sufficient to considerably expand the exhaust through the nozzle and efficiently generate a lot of thrusts. Kelly Johnson was a pragmatist when it came to engineering. He’d already created a Mach 2 interceptor, the F-104 Starfighter. That plane has a fixed-geometry intake that is almost radially symmetrical. At most speeds, it was wasteful, and it wouldn’t function at all for a Mach 3.4 plane.
The oblique shock waves from the inlet spike, in particular, do not behave properly at the surface against the fuselage. A variable geometry intake would be used on the next plane. To get the geometry appropriate, the entrance spike that generates the oblique shocks that accomplish efficient compression would have to migrate aft at higher speeds. Because the shocks must be well behaved, the inlet must be totally radially symmetrical and have minimal interaction with the fuselage. At low Mach values, however, the shock from the aircraft’s nose is not strongly angled. Putting the engine inlet at the nose, like on the MiG-21 or Lightning, is one technique to avoid the engine interfering with the shock. Kelly, on the other hand, would require an intake that was larger than the fuselage for the high-speed, high-altitude Blackbird. There was no way the nose could be big enough. Furthermore, if he placed the intake there, the body would be mostly duct, leaving no room for fuel. As a result, the SR-71 engine intakes reside behind the shock wave produced by the plane’s nose but have no additional interaction with the fuselage. Because the nose oblique shock wave redirects the airflow outward a little, the cones actually point together a little bit if you look closely. Because the fuselage works as a lifting body in cruise and has a short angle of attack, the cones also point down.
As a result, the location of the engine intakes is confined twice. It must be far enough away from the fuselage to avoid interfering with the fuselage’s disrupted airflow. It must avoid the vortex that emerges from the fuselage chine and travels across the top of the wing. It must, however, be near enough to the fuselage to remain under the nose shock even when traveling at maximum speed. If they were any further out, the nose shock would speedily cross the inlet, resulting in an inlet unstart and an engine flameout.
The image above depicts a shock simulation at Mach 1.25. I couldn’t find a picture showing the shocks at higher speeds. The shock angle pulls more back as the Mach number climbs until the nose shock is just in front of the engine inlet cones at Mach 3.4.