Abstract: An electrothermal-chemical propulsion apparatus and method for propelling a projectile through a gun tube. The apparatus comprises a vaporizable energy coupling device between a projectile and at least one chemical propellant module. Vaporization of the coupling device creates a plasma which augments the gas expansion behind the accelerating projectile at a predetermined time after ignition of the chemical propellant. The coupling device is preferably a pair of concentric extensible sleeves telescopically folded and collapsed behind the projectile, each having one terminal end connected to the rear of the projectile and the other terminal emd connected through the propellant module to electrodes in the breech of the gun and in turn to an electrical power supply.