One of the developments that will be necessary for Europe's Next Generation Launcher (NGL) is a new design of main engine. The Vulcain engine that powers Ariane 5 is approaching the limits of its design in terms of both thrust and overall performance.

One objective of the Future Launchers Preparatory Programme (FLPP) is to enable informed decisions about NGL engine technologies to be taken later in the programme.

Choice of technology
Liquid-fuelled rocket engines can be classified as either open- or closed-cycle, according to how they derive the gases that power the turbine-driven pumps used to force the fuel and oxidant into the combustion chamber.

Europe has experience with open-cycle engines - Vulcain is an example of this type - and with closed cycle expander engines - the Vinci engine uses this design and has accumulated more than 3000 seconds of hot firing tests.

Part of the work currently being undertaken in the propulsion area of the FLPP is the development of technologies and competencies for the next step: closed-cycle, staged-combustion engines. Why staged-combustion?

The overall efficiency of a rocket engine is measured by its 'specific impulse' - the impulse (the change of momentum - or, put simply, the speed increase of the launch vehicle) that it can achieve per unit quantity of fuel. This is somewhat like the 'miles-per-gallon' or 'litres-per-100km' figures that are quoted for cars.

The specific impulse of an open-cycle gas generator engine such as Vulcain is inherently limited, while for the closed-cycle, staged-combustion design, it increases with the combustion pressure. This provides opportunities for developing engines that need less propellant to perform a given task, reducing the propellant load and, as a consequence, the size and mass of the launch vehicle.

New fuels
The energy content and storage properties of the selected fuel have a major influence on the overall 'efficiency' of a launch vehicle. Hydrogen, the fuel used in the engines mentioned above, is now in competition with hydrocarbons such as methane.

Although hydrocarbons contain less energy per kilogram than liquid hydrogen, their higher density and less demanding storage requirements (they remain liquid at much higher temperatures than hydrogen), leads to reduced tank size and mass.

Early success
The recent test at Lampoldshausen has successfully shown the ignition and ramp-up of a hydrogen powered, staged-combustion cycle demonstrator. This work is continuing and is being expanded to demonstrate the use of methane as a fuel.

ESA FLPP main stage propulsion development is being carried out by a consortium and co-contractor comprised of Snecma (France), Avio (Italy), and Astrium (Germany), supported by a team of nine other European companies and research bodies.

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