Apparatus for inflating an inflatable vehicle occupant protection device includes a primary housing, a primary inflation fluid source, a secondary housing, and a secondary inflation fluid source. The primary and secondary housings are constructed from sheet metal by stamping. The primary inflation fluid source is located in the primary housing. The primary inflation fluid source is actuatable to effect a primary flow of inflation fluid out of the primary housing to inflate the inflatable device. The secondary housing is disposed within the primary housing. The secondary inflation fluid source is located in the secondary housing. The secondary inflation fluid source is actuatable to affect a secondary flow of inflation fluid to supplement the primary flow of inflation fluid. The secondary housing has a first portion that moves at least partially out of engagement with a second portion of the secondary housing when the secondary inflation fluid source is actuated.