Atmospheric Effects on the Chemical Delay Time for Hypergolic Bipropellants

Abstract

Ignition delay time (IDT) for hypergolic bipropellants is classically defined as the time from initial contact of the reactants until the appearance of flame. Within the ignition delay time is embedded the chemical delay time (CDT) which is defined as the time from gas evolution after contact until the appearance of flame. The chemical delay time permits a more direct comparison of chemical performance permitting the study of combustion details typically not seen using other methods. A laser based diagnostic technique was used that measures CDT of classical hypergolic bipropellants. Unsymmetrical dimethyl- hydrazine (UDMH), hydrazine and mono- methylhydrazine (MMH) were individually reacted with red fuming nitric acid (RFNA) under atmospheric air and argon to determine the contributions atmospheric air has on the measured chemical delay time. Results showed that oxygen from air contributes significantly (23%) to the MMH/RFNA; however, hydrazine and UDMH reacted with RFNA showed only a slight influence. The importance of using molar ratios for reporting drop tests results is presented and compared to a volume ratio technique. The presence of a vortex ring was proposed that in essence creates a micro reactor within the oxidizer pool at the free surface between the reactants. The presence of a vortex ring may ultimately limit the molar flux to the reaction zone limiting the kinetics by decreasing combustion efficiency. Single frames taken from two high-speed videos (3000 fps) showed vortex rings clearly visible within the highly reactive MMH/RFNA system.