In 1964, Sukhoi started work on "S-58M". The aircraft was supposed to represent a modification of the Sukhoi Su-15 interceptor (factory designation "S-58"). In the meantime, revised Soviet Air Force requirements called for a low-altitude strike aircraft with STOL capability. A key feature was the ability to cruise at supersonic speeds at low altitude for extended periods of time in order to traverse enemy air defenses. To achieve this, the design included two Tumansky R-27 afterburning turbojets for cruise and four Rybinsk RD-36-35 turbojets for STOL performance. Side-by-side seating for the crew was implemented since the large "Orion" radar antennas required a large frontal cross-section. To test the six-engine scheme, the first Su-15 prototype was converted into "S-58VD" flying laboratory which operated in 1966–1969.