Is that an aerospike engine?
Looks like an ion motor.
The Mighty J-58
The design of the engine's thermodynamic cycle was begun in 1956, and it was flying by April, 1962. Classified missions began in 1964, and continued into the mid 90s. No SR-71s have ever been lost to enemy fire, because no weapon has ever been able to go high enough and fast enough to actually hit one.
Although the internal turbomachinary design is primitive by modern standards, the design point is certainly unique! In cruise it operates as a ramjet, reaching speeds of Mach 3 to Mach 3.5 (depending on the weather conditions above 80,000 feet).
It burns an extremely low vapor pressure fuel called JP-7, which was formulated specially for the SR-71. You can put a cigerette out in a dish of JP-7. The fuel is designed to be hard to light because the airframe skin in the vicinity of the fuel tank is 500 - 600 degrees F at cruise. Even worse, the fuel is actually used as the hydraulic fluid in the engine actuators! The idea is, a steady supply of "cool" fluid arrives from the fuel tanks and gets used in hydraulic devices around the engine. Hydraulic fluid in a closed system would soon get as hot as the parts through which it ran - - but this special fluid gets used as fuel before it gets too hot! Afterburner ignition is spectacular, when an ultra-hot flash of Tetra-Ethyl Borane ("TEB") is squirted into the flame holders.