[2] 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.

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,[i] mixed-power interceptor aircraft were studied, with a combination of jet and rocket power.

[ii] The Trident was difficult to handle on the low-powered turbojets alone and was thirsty for fuel on rocket power.

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.

[iii] As the delta wing considerably increased the supersonic capacity of the aircraft, rocket power was retained.

The original goal of intercepting high-flying bombers also seemed to be receding in favour of missiles, for both offence and defence.

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.

A smaller 150 litres (32 imp gal) TX2[8] Furaline[5] fuel tank was mounted in the front bay just behind the cockpit, replacing the cannon pack.

[5] 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.

Power for actuating the main valves was then obtained by a compressed air bottle or fuel pressure.

HP cocks were opened by a single hydraulic piston, driven by fuel pressure controlled through a third timed valve.