of elevation if the recoil spring force is constant over a given stroke length and the other variables are such that the pure foob recoil condition obtains it is theoretically possible to achieve a 75 reduction in the brake load applied to the trunnions 15 as compared to an equivalent conventional recoil system a practical foob recoil system must be designed to accommodate changes in elevation angle and in the howitzer application changes in the expected firing impulse due to different zone charges since the recoiling mass is essentially unchanging during the firing cycle provision must be made to adjust either the effective stroke length or the magnitude of the recoil spring force as in ordinary liquid springs the force level of a constant force liquid recoil spring can be changed by adjusting the amount of initial fluid precompression this is accomplished by pumping a given amount of silicone fluid into or out of the liquid recoil spring pressure vessel 31 as shown by the schematic of fig4 power to adjust the liquid recoil spring is derived from a master hydraulic accumulator 32 which also provides power to the gun feed mechanism the requisite liquid recoil spring pressure for a given elevation angle and selected zone charge is controlled by mechanical pressure regulator and relief valves 34 set by the operator from a central control panel at 22 the rear of the weapon the fluid energy extracted from the hydraulic accumulator 32 is replenished as each round is fired by.