The SEPR 84 is a French rocket booster developed by the Société d'Études pour la Propulsion par Réaction (SEPR). It is a liquid-fueled auxiliary rocket engine designed to enhance the performance of jet fighters, particularly in high-altitude and high-speed scenarios. Introduced in the late 1950s, the SEPR 84 is notably used in the Dassault Mirage III, significantly improving its overall performance during interceptor missions. The booster operates using a combination of nitric acid and kerosene, providing an additional thrust of approximately 14.7 kN for short durations. This capability allows the Mirage III to achieve rapid ascents and higher operational ceilings, making it more effective against high-flying adversaries such as the Mikoyan-Gurevich MiG-21 (Fishbed). As part of an arms deal in the late 1950s, the Federal Army Air Corps purchased a significant amount of cutting-edge Dassault Mirage IIIs, and with them, a number of SEPR-84 rocket boosters. Many of these went missing following the Oceanykan Revolution and the ensuing bureaucratic chaos, being spotted in airframes which weren't really designed to have it mounted on.