Rocket Engines and
Thrust Chambers

LV Propulsion
Aestus
Vulcain
Vulcain 2
HM 7B
Vinci
RS 72
300 N cryo. Engine

Aestus Rocket Engine

Bipropellant upper stage engine for the orbital insertion of heavy payloads

The Aestus rocket engine powers the Ariane 5 bipropellant upper stage for the insertion of payloads into LEO, SSO and GTO. Using its re-ignition capability, Aestus is also used on the ES-ATV version of Ariane 5 for the placement of ESA's 21 tonne Automated Transfer Vehicle (ATV) into a low Earth orbit.

Aestus is a pressure fed engine that consumes up to 10 tonnes of the bipropellant combination MMH/N2O4.

Aestus was developed at the Ottobrunn Space Propulsion Centre during the period 1988 - 1995. The first operational flight of Aestus was on Ariane 5 flight 502, launched on 30 October 1997.

Together with the Aestus engine, EADS Astrium is responsible for the complete Ariane 5 upper stage, under contract to the French Space Agency CNES.

Major Sub-Assemblies
Aestus comprises the following main sub-assemblies:

	Gimbal joint mounted at the top of the injector dome.
	Coaxial injectors for the mixing of propellants.
	Combustion chamber regeneratively cooled by MMH fuel.
	Nozzle extension, radiatively cooled.
	Propellant valves for fuel and oxidiser, pneumatically operated by pilot valves.
	Electromechanical gimbal actuators for pitch and yaw engine control.

Some subassemblies have been subcontracted to partners including the gimbal joint to Raufoss Aerospace, the nozzle extension to Franke AG, the propellant valves to MOOG GmbH and the flexible propellant lines to Witzenmann.

Operation
The Aestus thrust chamber design is based on the regenerative cooling principle. Prior to combustion, MMH fuel is pressurised into a distribution manifold causing the fuel to flow through narrow, closely arranged channels in the combustion chamber wall, configured to cause a highly efficient cooling. The MMH then enters an injector head which assures a uniform fuel flow rate distribution to 132 coaxial injector elements.

Aestus / Ariane 5 Upper Stage

MMH enters about the outer wall of the injector element through radially shaped slots. Simultaneously, N2O4 enters axially through the centre of the element. The unique design of the injector element causes swirl mixing and atomisation of the propellants enabling combustion efficiencies in excess of 98% to be realised during the remaining process in the combustion chamber.

Upon leaving the injector elements and entering the combustion chamber, the hypergolic propellants spontaneously ignite and are burned and accelerated up to sonic conditions. The combustion temperature in the combustion chamber reaches about 3000 K at a combustion pressure of 11 bar.

Controlling the MMH fuel temperature (20°C to 125°C) against the thin hot wall of the combustion chamber was one of the most challenging tasks that had to be overcome during the development phase. A further challenging task was the development of a new injector element for MMH /N2O4, using the same, highly efficient, coaxial injection principle used on all of our cryogenic thrust chambers.

After leaving the combustion chamber, the final acceleration of hot gases up to supersonic velocities achieved by gas expansion in the radiatively cooled nozzle extension, thereby increasing thrust.

Proven Design and Performance Flexibility
The Aestus rocket engine has proven to be a robust and flexible design, evolving harmoniously with the evolution of Ariane 5 and its various missions, as shown in the Aestus Development History below.

In addition, by varying the number of coaxial injector elements, the basic Aestus design can be used for higher, or lower thrust applications.

A turbopump version of Aestus, Known as the RS 72 has also been developed and hot-fire tested in collaboration with Boeing Rocketdyne (Pratt & Whitney). More information on this Aestus version can be found on the RS72 rocket engine page.

Aestus Development History
The Aestus rocket engine was developed at the Ottobrunn Space Propulsion Centre during the period 1988 - 1995. The first operational flight of Aestus was on Ariane 5 flight 502, launched on 30 October 1997.

In the frame of performance improvements to the complete upper stage, a delta-qualification programme was performed in 2000 - 2002. Here, the propellant mixture ratio of Aestus was adjusted from 2.05 to 1.9. Subsequently, the first operational flight of the performance enhanced Aestus was on Ariane 5 flight 518, launched on 26 February 2004.

During the period 2003 - 2006, the Aestus engine underwent a re-ignition qualification programme in readiness for the first launch of the Automated Transfer Vehicle. The in-orbit re-ignition capability of Aestus was subsequently demonstrated during the first launch of ATV aboard Ariane 5 flight 528, launched on 9 March 2008.

In ATV mode, the first Aestus ignition occurs immediately after separation of the upper stage/ATV composite from the cryogenic main stage. At the end of the first burn, a ballistic phase commences for about 45 minutes. A second ignition then provides a short duration burn for injecting the ATV into its target Low Earth Orbit after separation from the upper stage. A third and final ignition is then used to de-orbit the depleted upper stage into a safe re-entry trajectory for burn-up in the upper atmosphere.

With its proven flexibility and multiple re-ignition capabilities, the Aestus engine enables a range of mission specific profiles for the Ariane 5 launcher.

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