Herakles: working to reduce vibrations during Ariane launches

The solid rocket motors used on space launchers generate vibrations that may cause damage to the satellites they carry. Thanks to its simulation software, combined with its ability to produce a specially adapted propellant, Herakles will be able to limit the impact of this phenomenon on the thrusters of the future Ariane 6 launcher.

For the Ariane 5 launcher – as for the future Ariane 6 – takeoff thrust is mainly provided by solid rocket propellants. "After the Ariane 5 launcher had been in operation for several years, we observed that the combustion of the solid propellant (see box) generated vibrations that, when propagated, could damage the payloads", sums up Yann Talamoni, head of Preparation for the Future and Research & Technology programs at Herakles. "We are now working to significantly reduce this phenomenon, especially with the future Ariane 6 launcher in mind."

Validating the models
Since the 1990s, Herakles has been creating computer models to understand and predict how these vibrations, referred to as pressure oscillations (PO), are generated inside the thruster. "There are many causes: the geometry of the booster first of all, notably due to the internal protections between the various segments, but also the temperature and composition of the propellant, the flows and so on. This is what makes it a particularly complex phenomenon to model", explains Yann Talamoni. After several years of theoretical research, Herakles has developed very high precision simulation tools. "To ensure that these work well, we are conducting trials with the European pressure oscillation demonstrator, the POD-X", states Yann Talamoni.

A European program
A 2/9 scale model of the Ariane 5 launcher's solid rocket motor (SRM), the POD-X is a European program developed in cooperation with several partners: the Italian engine manufacturer Avio, which supplied two of the three segments used to build the demonstrator, the third being produced by Herakles at its Saint-Médard-en-Jalles plant near Bordeaux; the European Space Agency (ESA) and the French Space Agency (CNES) are participating in the financing, along with the French Defense Procurement Agency (DGA), which conducted the trials. During the first launch, successfully completed in March on the DGA site in Saint-Jean-d'Illac, more than 300 readings were taken (pressure, vibration, temperature, etc.). These data are currently being analyzed and will be compared with the results obtained by the modeling tools. A second launch, due to take place by 2016, will serve to definitively qualify the modeling tool.

Test Engines
"Once we have verified that our computer models work well, they can be implemented to reduce the vibrations caused by our future solid rocket propellants, notably those designed for the new-generation Ariane 6 launcher", explains Yann Talamoni. "To achieve this, we are going to do simulations of various propellant compositions to obtain a virtually stable launch." This new fuel will be tested on another demonstrator, POD-Y, made to a scale of 2/11 compared to Ariane 6. Its launch, which will take place in late 2014/early 2015, will consolidate Herakles' expertise in the design of large, stable engines for the future European launcher.

SRM*, a user manual
Solid rocket motors use a solid chemical fuel, a propellant in which an oxidizer and fuel are combined. An SRM is essentially composed of a casing containing the propellant, an igniter and a nozzle. The propellant block is pierced by a longitudinal channel that serves as the combustion chamber. Once set in motion, this energy-intense process cannot be stopped. When the propellant is lighted, the surface of the propellant block starts to burn, producing high-pressure combustion gas that is expelled by the nozzle.

*Solid rocket motor