Upper composite (without fairing) inside the test rig
Ariane-5 consists of three different large structural constituents: the main cryogenic stage equipped with the Vulcain engine, the upper composite with one or two payload compartments, and two identical solid propellant boosters. Under contracts placed by the Industrial Architect Aerospatiale, extensive structural system tests were performed on these main constituents by IABG during the last two years. Their purpose was to adapt and verify the mathematical models developed to predict the stress distribution and the dynamic behaviour during flight, including structural loading, flight control and Pogo* aspects.
Due to its size, the main cryogenic stage had to be tested at its manufacturing site, i.e. on the premises of Cryospace in Les Mureaux, France. A number of test configurations simulated the varying conditions during flight, comprising the mechanical boundary conditions existing before and after booster separation, as well as various filling levels of the propellant tanks. Static stiffness and surflux tests were performed, applying loads in axial and lateral directions and measuring strains and displacements at several levels and particularly interesting locations such as the booster attachment fittings. Dynamic excitation was applied laterally at various locations and axially on the engine dummy in order to measure acceleration and pressure response functions and to determine the relevant global and local modes of vibration.
The upper composite could be tested at IABG in Ottobrunn, Germany, due to its transportability by parts. Not less than eleven test configurations were necessary to take into account the variability of the upper composite with respect to its single and dual-launch capability as well as the changes of its structural configuration during flight and deployment of the payloads. The test programme itself was similar in principle to that of the main stage.
The solid-propellant boosters are filled with propellant and integrated at the European Space Port CSG in Kourou, French Guiana, so that also the structural system tests had to be performed there. Because the development programme of the booster includes several real firing tests, only one additional test using artificial excitation of the whole booster was needed: the booster, filled with propellant and resting on its transport pallet, was dynamically excited in lateral directions to measure its lowest bending modes. The results obtained in this manner correspond to the ground transport and assembly configuration. The dynamic behaviour of the booster flight configuration, on the other hand, was studied during the booster firing test M3 (see Reaching for the Skies, No. 12), by realising several dedicated nozzle actuation sequences to excite the booster in its most important low-frequency modes of vibration. The resulting transient vibration responses were measured with more than 100 accelerometers and analysed afterwards to determine the relevant dynamic characteristics.
More details about this test programme will be published in the proceedings of the CNES/ESA conference on Spacecraft Structures and Mechanical Testing, held in Paris in June 1994.
*Pogo phenomena are vibrations in which the coupling between structure modes and propellant feed system modes can lead to possibly dangerously high vibration levels.
Upper composite (without fairing) inside the test rig
Reaching For The Skies Nr. 13.