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The MiG-15Bis("second"), entered service in early 1950 with a Klimov VK-1 engine, an improved version of the RD-45/Nene, plus minor improvements and upgrades. Visible differences were a headlight in the air intake separator and horizontal upper edge airbrakes. The 23 mm cannons were placed more closely together in their undercarriage. Some "bis" aircraft also adopted under-wing hardpoints for unguided rocket launchers or 50–250 kg (110–550 lb) bombs. Fighter-bomber modifications were dubbed "IB", "SD-21", and "SD-5". About 150 aircraft were upgraded to SD-21 specification during 1953–1954. An unknown number of aircraft were modified to "IB" specification in the late 1950s.