The SEPR 84 is a family of liquid-propellant rocket engines used as boosters for the Dassault Mirage III mixed-power high-altitude interceptor aircraft of the 1960s.

The engine was one of several similar developed by SEPR (Société d'Etudes pour la Propulsion par Réaction).

The delta aircraft was smaller, around two thirds of the Mystère's weight and was powered by two small Viper turbojets and a SEPR 66 liquid-fuelled rocket.

The Mirage's rocket was thus mounted as a removable pod which could be replaced with a 90 imperial gallons (410 L) jet fuel tank for additional range.

The 310 litres (69 imp gal) nitric acid oxidiser tank was mounted directly ahead of the rocket engine.

[3] Fuelling the rocket oxidiser was potentially somewhat hazardous and so it was carried out away from other aircraft, by groundcrew in protective clothing and with a fire crew standing by in order to flush away any spillage.