Improvement of Cryogenic Space Rocket Engine Ignition: Inert Gas Sweep Effects

Abstract

Ignition of cryogenic engines of space launchers is usually preceded by a transient phase during which feeding lines of the combustion chamber are swept by an inert gas. This sweeping flow may be prolonged for several milliseconds while propellants injection begins. Despite the fact that this transient may generate instabilities influencing the ignition process, related oscillations have not been investigated. Experiments were carried out with nitrogen flowing in a full scale injector of HMB7 engine (equipping Ariane 5 launcher) in thermo-hydrodynamic conditions of similitude with oxygen flow in HM7B (actual oxider propellant of HM7B). Two conditions were investigated: 1) with nitrogen only, 2) with helium gas injected simultaneously with nitrogen during the initial 150 milliseconds. The aim was to characterize the instabilities occurring during a pre-ignition transient of 300 ms and the effect of the inert gas sweep on these instabilities. Impact force of the jet, density and pressure at injector exit were measured by means of load cell, piezoresistive sensor and resonance cavity respectively. Mass flow was then calculated through momentum equation and slip ratio correlation. Measurements and analysis showed that the flow was inverted annular flow and mist flow, instabilities were of thermoacoustic oscillation (TAO) type, and led to suggest the thermal effect as the major effect in the production of these oscillations. This also showed that a prolonged sweep of the injector with inert gas had a stabilizing effect on the two-phase flow due to the reduction of wall heat flux and of the inlet subcooling enthalpy. This finding is of great interest also because it shows that TAO are generated in the unit engine much earlier than ignition inside the combustion chamber. Possible coupling between these TAO and those occurring after ignition during combustion (which have different source and with different sustaining factors) might bring interesting knowledge regarding engine ignition process.