(Photo above: Yuriy Gagarin (in the orange spacesuit) on his way to his booster rocket on the morning of April 12, 1961. Among the people who are identifiable are: Chief Designer Vladimir Barmin (in the back center left in black coat), Chief Designer Sergey Korolev (in foreground center left in black hat), Major General Nikolay Kamanin (in sunglasses, obscured in foreground left), Commander-in-Chief of the Strategic Rocket Forces Marshal Kirill Moskalenko (in military cap with red stripe behind Korolev), and Vostok 'lead designer' Oleg Ivanovskiy (in blue on the right walking ahead of Gagarin).
Vostok Crew Assignments
Three to four cosmonauts usually trained for each specific Vostok mission. Usually, the Air Force leadership (specifically N. P. Kamanin) in collaboration with OKB-1 representatives (specifically S. P. Korolev) informally agreed upon a primary candidate weeks before the flight. The pilot and his/her backup ('double') were formally assigned some days before the launch, and officially named about two days before launch. Sometimes, a third person ('reserve') was also assigned.
A Vostok training group was established in May 1960 and included:
Gagarin
Kartashov
Nikolayev
Popovich
Titov
Varlamov
Kartashov and Varlamov were replaced (due to injury) in the training group by Nelyubov and Bykovskiy respectively in June 1960.
On October 11, 1960, by order of the Air Force, the 'group of six' was formally established to train for the Vostok program:
Bykovskiy
Gagarin
Nelyubov
Nikolayev
Popovich
Titov