Simultaneous Temperature and Species Measurements during Self-Oscillating Burning of HMX

The near-surface species and surface temperature were simultaneously measured during self-oscillatory burning of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). A CO₂ laser was used to heat the propellant surface at atmospheric pressure in argon. A microprobe/triple quadrupole mass spectrometer system was used to measure species profiles, and fine-wire thermocouples were used to measure surface temperature. Oscillations of species, temperature, and burning rate were observed with an average surface temperature of ∼633 K and frequency of 4 ± 0.2 Hz. The mole fraction of NO₂, HCN, and triazine oscillated in phase with temperature, whereas the mole fractions of N₂O, CH₂O, and the species at mass 28 were 180 deg out of phase with temperature. NO₂ and CH₂O were the most abundant species followed by HCN, N₂O, H₂O, the species at mass 28, and other species. The production of NO₂ and HCN was favored with an increase in temperature and burning rate, whereas the production of N₂O and CH₂O became more important with a decrease in temperature and burning rate. This was qualitatively in line with the accepted global reaction branches in the condensed phase of HMX. From these data, variations of the mole fractions of NO₂, N₂O, CH₂O, and HCN could be directly related to surface temperature. Analysis of the data indicates that the observed oscillatory burning might be related to multiple-step reactions in the condensed phase.