and the outside surface of the deployable element 20 in one alternative embodiment shown in fig7 a flow of gas is introduced into the volume displaced by the deployment element in one embodiment one or more vents 68 are located on the outside of the body element 12 leading directly to the volume being displaced by the deployable element in another embodiment a control orifice 70 is made through the piston at a location on the piston whereby gas from the plenum is vented into the displaced volume during deployment in still another embodiment a port 72 through the support 16 may allow gas to enter the displaced volume any of these systems may be used singly or together to control the amount of gas entering the displaced volume the flow of the gun gas or air entering the displaced volume may then be used to control the rate of deployment of the deployable element in another embodiment a gas source stored on or in the body element 12 may be triggered electrically or gas may be introduced through the use of a mechanical impact igniter such as a percussion primer hot gas from other on board systems signal transfer lines such as detonation cords deflagration transfer lines or tubing carrying pressure or hot gas or particles the gas introduction system for inflation or expansion may also be a dedicated system such as a stored gas bottle or pyrotechnic gas generator alternatively the gas introduction system may comprise gas from one or more other on board sources which operated or control events other than deployment of the boat tail device such.