Site 41 was a complex of three launch pads at the Baikonur Cosmodrome originally built for flight testing of Intercontinental ballistic missile (ICBMs) using storable propellant. 

The Nedelin catastrophe or Nedelin disaster was a launch pad accident that occurred on 24 October 1960 at Baikonur test range (of which Baikonur Cosmodrome is a part), during the development of the Soviet ICBM R-16. As a prototype of the missile was being prepared for a test flight, an explosion occurred when second stage engines ignited accidentally, killing many military and technical personnel working on the preparations. The disaster is named after Chief Marshal of Artillery Mitrofan Ivanovich Nedelin, who was killed in the explosion. As commanding officer of the Soviet Union's Strategic Rocket Forces, Nedelin was head of the R-16 development program.

Source: Wikipedia