Ariane 5 Storable Propellant Stage (EPS)
The Ariane 5 Generic third stage is known as the EPS from its title in French, Etage à Propergols Stockables. It was 3.35 m high with a diameter ranging from 3.94 m at the bottom to 2.62 m at the top. Its dry weight was 1.275 Kg and roughly 11 tonnes when fully loaded.
The mission of the EPS was to provide the necessary complementary energy to inject payloads from the EPC separation up to the target orbit.
The EPS was essentially composed of a supporting structure, two pairs of propellant tanks and an engine (the Aestus rocket motor) with its associated actuators. In a standard geostationary tranfer orbit (GTO) double launch mission, the propellant loads onboard the EPS were:
- Mono-Methyl-Hydrazine (MMH): 3.2 tonnes
- Nitrogen Peroxide (N2O4): 6.6 tonnes
The pressure-fed Aestus engine was attached to the platform structure by a gimbals joint, allowing it to swivel along two axes through a maximum angle of 16° and by two electrical actuators. Ignition was accomplished by controlling the opening of the N2O4 main valve and then the MMH main valve. The engine was shut down through the same procedure in reverse order.
The vacuum thrust of the Aestus engine was 2.7 tonnes and for a standard GTO mission it operated for more than 1000 s. The engine weight was 115 kg.