Hypergolic ignition delay studies of solidified ethanol fuel with hydrogen peroxide for hybrid rockets

Abstract

The hypergolic interaction and ignition delay times of catalytically promoted solidified ethanol (CPSE) formulated using organic gellant, namely, methylcellulose (MC) and hydroxypropyl methylcellulose (HPMC), has been investigated using rocket grade hydrogen peroxide (90% RGHP; H2O2) as an oxidizer, for the use in hybrid rocket engines. Using iso-conversional Friedman method, the apparent activation energy (Ea) is found to be in the range of 9.06–14.01 kJ/mol for unloaded solidified ethanol samples. The hypergolic ignition delay (ID) of the CPSE fuels with H2O2 are investigated using manganese (III) acetylacetonate (Mnacac) as a catalyst using the drop-test method and found that the ID lies in the range of 49–307 ms. Five-stage events are identified based on the signals from the high-speed camera, photodiode, and microphone obtained from the drop-test. Stage I: prior to of H2O2 drop impact; Stage II: inertial spreading of H2O2 drop over CPSE pellet; Stage III: onset of exothermic decomposition reaction H2O2 droplet by Mnacac particles which leads to the formation of aerosol; Stage IV: local explosion of aerosol and ignition of CPSE pellet occur results in two types of aerosol-CPSE fuel interaction; Stage V: a weak self-sustained flame from CPSE fuel. A key finding of the study reveals that a minimum catalyst concentration ([C]Mn3+,L) is required to ignite CPSE fuel using H2O2 and ID decreases with an increase in the [C]Mn3+. Similarly, there exists an upper catalyst loading ([C]Mn3+,U) above which ID is increased due to the increase in Mnacac particle size by agglomeration. [C]Mn3+,L and [C]Mn3+,U depends on the type and concentration of the gellant and the availability of active hydroxyl (–OH) group in the gellant. Finally, SEM-EDS analysis of the residue indicates the formation of oxides of manganese with a small amount of carbon (~1.66 to 2.53 wt%), hinting complete combustion of the gellant particles.