Société d'Etudes pour la Propulsion par Réaction

Société d'Études pour la Propulsion par Réaction (SEPR)

A SEPR 841 rocket pack for the Mirage IIIC. Yellow and green banding on the acid oxidiser pipework, and the dump port, may be seen.
Type	Research, development and manufacturing company
Industry	Aero-engine manufacture
Headquarters	37 rue des Acacias, Paris XVIIe, France
Key people	Jean Volpert
Products	liquid-fuelled and solid-fuelled rocket engines
Owner	French Government

The Société d'Études pour la Propulsion par Réaction (SEPR) (in French: Jet Propulsion Research Company) was a French research and manufacturing company which specialised in the development of liquid-fuelled rocket engines during the 1950s, 60s, 70s and 80s.

The SEPR 841 is a liquid-fuelled rocket engine used as an auxiliary power unit for the Dassault Mirage III mixed-power high-altitude interceptor aircraft of the 1960s. The engine was one of several similar developed by SEPR.

In the 1950s, there was much concern in Western Europe about attacks by fleets of high-flying bombers, such as the Tu-95 Bear. These pre-dated the development of either practical afterburners or surface-to-air missiles and so means to improve the performance of conventional aircraft were sought. Particularly in France, and to some extent in the UK,mixed-power interceptor aircraft were studied, with a combination of jet and rocket power. The rocket would be used for performance at high altitude, increasing either speed or ceiling.

French studies for mixed-power interceptors began in 1948. By 1953 the SNCASO Trident aircraft was flying. This was an unusual design, with a single three-chambered SEPR rocket engine as the main engine, assisted for take-off and low altitude flight by two Turbomeca Marboré turbojets. The Trident was difficult to handle on the low-powered turbojets alone and was thirsty for fuel on rocket power. This primary use of the rocket was not repeated in the future: later aircraft would be jet-powered, with the rocket reserved for high-speed dashes. Later rockets would also be considerably less powerful than the Trident's SEPR 48-1.

A development for the Trident II aircraft was the two chamber SEPR 631 engine. The two chambers could be fired separately. Although not throttleable, this did give a half-thrust setting.

The Mirage and its distinctive delta wing planform began with the prototype MD.550 Mystère-Delta. This bore little relation, other than its name, to the Dassault Mystère; France's swept-wing fighter of the period. The delta aircraft was smaller, around two thirds of the Mystère's weight and was powered by two small Viper turbojets and the SEPR 66 rocket. All three of these engines barely exceeded the thrust of the Mystère's ATAR 101D, although they also only weighed about half of the ATAR.