A Little Oil / Eric Frank Russell
For four years the crew of the ship, in which the best of the best are selected, is performing the Earth mission. Only one person causes bewilderment - the psychologist Bertelli, an amazing stupid and simpleton. Only after returning home, Captain Kinrad will find out who this man was and why such a "ballast" was included in his team ...
A shortened version of the story in R. Rybkin's translation is published in the journal "Young Technician" No. 02, 1973 pp. 40-46