a device comprising a perforated housing containing therein pellets of a chemical composition which undergoes self - sustaining combustion with generation of a combustion gas and an initiator attached to the housing to ignite the pellets . to prevent breaking of the pellets resulting from their movements in the housing , a hollow and resilient retainer member is placed in the housing such that the initiator is exposed to the interior of the retainer and that the pellets are spaced from the initiator by the retainer and held under a compressive force by the resiliency of the retainer . the retainer has openings which allow a gas to pass therethrough but do not allow the pellets to pass therethrough . a helical spring and a perforated bellows are examples of the retainer .