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The Korolev cross is a visual phenomenon observed in the smoke plumes of the R-7 series rockets during separation of the four liquid-fueled booster rockets attached to the core stage. As the boosters fall away from the rocket, they pitch over symmetrically due to aerodynamic forces acting upon them, forming a cross-like shape behind the rocket. The effect is named after Sergey Korolev; the designer of the R-7 rocket. When the rocket is launched into clear skies, the effect can be seen from the ground at the launch site.