From the Encyclopaedia Britannica: The Soviets pioneered the “cold-launch” method, in which the missile is expelled by gas and the rocket engine ignited after t

From the Encyclopaedia Britannica: The Soviets pioneered the “cold-launch” method, in which the missile is expelled by gas and the rocket engine ignited after the missile clears the silo. This method, essentially the same system used with SLBMs [submarine-launched ballistic missiles], allows silos to be reused after minor repair. That barrel-like object is, I expect, the unit that generates the gas that expels the rocket.