This invention relates generally to devices capable of generating substantial volumes of gas within a very brief time span, generally a few milliseconds.  Gas generators of this type are used to inflate air cushion restraints of the type used in passive vehicle restraint systems designed to insulate vehicle passengers from the harmful secondary impact effects of rapid deceleration caused by vehicle collisions.  One technique for inflating air cushion restraints is to connect a reservoir of gas in a cylinder under very high pressure to the air cushion restraint.  Upon receipt of a signal from a crash impact sensing device an explosively actuated valve opens, releasing the gas from the cylinder.  However, in order to obtain a sufficient volume of gas [approximately 10 cubic feet] for inflating the cushion restraint a relatively large reservoir of gas at pressures of 3,000 psi or more is required.  The size of such a container creates difficult engineering problems, with regard to cylinder location, safety and vehicle balance requirements.  The compressed gas cylinder technique for inflating air cushion restraints suffers from the additional disadvantage in that pressure is maximum at the commencement of the deployment of the air cushion and decreases as a function of time as the gas in the cylinder is depleted.  Further, a minor leak in the cylinder can result in loss of substantial amounts of gas by accidental discharge during the long period that the restraint system must remain in the vehicle.  The pressurized gas technique also results in substantial cooling of the gas as it expands thus reducing the effective available volume of gas.  This requires a total storage volume significantly greater than if the gases were at an elevated temperature and further adds to the above-discussed automotive engineering problems. 
Another technique for generating gas within the requisite short period of time is the use of pyrotechnic charges to inflate air cushion restraints.  In such a device the inflating gas is generated by the rapid reaction of the charge reactants upon receipt of a signal from an impact sensing device.  While pyrotechnic devices eliminate the problems heretofore discussed which are associated with compressed air systems, the results of the required exothermic reaction, heat and by-products, must be controlled within acceptable limits for safe use.  The gases inflating the air cushion restraint must not have chemical or thermal characteristics which will undermine the mechanical strength of the cushion restraint itself or injure the passenger in the case of a cushion restraint rupture.  Therefore the generated gas flow, which includes a gas phase as well as a particular phase, must be cooled and filtered within the generator before it enters into the air cushion restraint.  In addition, automobile manufacturers have further required that such gas generating units must be nonpropulsive, that is, the gas escaping through the gas exit ports into the restraint cushion must exert a net reactive force of approximately zero, to preclude what would otherwise be a dangerous condition in the event the generator becomes detached from its mounting.  In devices of this type the use of mechanical filters utilizing fine screen to filter and cool the gas prior to its exit from the gas generator has been shown to be most effective.  However, certain known devices using such mechanical filters and conforming to the nonpropulsive requirement have heretofore been provided with a filter covering the gas exit ports of the combustor chamber that is radially and axially continuous from end to end.  This is very inefficient as it requires the use of a large amount of expensive screen material.  Further, such a construction prevents the use of interchangeable filter components which may be mass produced and assembled by relatively unskilled personnel thereby further increasing the cost of each inflator unit.  One such device comforming to the nonpropulsive requirement is in the form of concentrically mounted cylinders, with the outer cylinder having gas exit ports for directing flow into an inflatable bag.  The gas exit port configuration is necessarily limited to two rows of such ports each being on an opposite side of the cylinder.  Efficient operation of an inflator having such a gas exit port configuration requires that the gas entrance ports of the inner cylinder, which functions as the combustion chamber, also be limited to two rows each being on an opposite side of the inner cylinder.  The rows are interleaved with and separate from the gas exit holes in the outer cylinder.  This configuration requires a filter which is radially and axially continuous to compensate for possible mechanical stress on the filter means resulting from such a gas entrance and exit port configuration.  If a continuous screen filter is not used in conjunction with the above structure it is difficult to seal the system adequately to prevent unwanted by-products of the exothermic gas producing reaction from bypassing the filter and reaching the restraint cushion.  Also, a noncontinuous filter would be prone to displacement possibly resulting in a pressure drop across the gas exit ports hampering proper cushion deployment.