Varnier J., Prévot P., Dunet G., Barat M., Mazin B.

At the ignition of a solid-propellant rocket engine, a brutal unsteady phenomenon called “blast wave” often occurs. This phenomenon is caused by the sudden irruption of the gaseous combustion products in the surrounding atmosphere. On a launch pad, the blast wave of the boosters characterized by a quick overpressure-underpressure alternation causes important stresses on the structure and on the payload of the space launcher. This is why studies about this phenomenon with a view to its reduction have been undertaken in the Ariane 5 framework supported by CNES. Let us note that the so-called blast wave designates in fact two distinct phenomena: the air movement due to the brutal expansion of the combustion gases, and the overpressure effect, a shock front which propagates at a long distance. By convenience, here we call “blast wave” this overpressure-underpressure alternation. In the studies carried out by the NASA, it is generally admitted that the blast wave intensity is more or less correlated with the slope of the chamber pressure rise occurring at the powder grain ignition. Yet the random character of the phenomenon suggests that thermochemical reactions may also act, as it is the case for the guns (“muzzle wave”): the unburned combustion products (hydrogen, carbon monoxide...) react in an explosive manner with the ambient air, which is called “afterburning” of “re-ignition”. In order to confirm the existence (or not) of afterburning at the time of the ignition, tests with static solid-propellant rocket engines have been carried out by the Propulsion Laboratory and by other Departments of the ONERA. The idea was to bring together thermodynamic, acoustic and optic measurements in order to prove a possible correlation between the heat, sound and luminous phenomena. More particularly, quick infra-red and optic cameras have allowed to obtain revealing pictures of the gaseous cloud in expansion.