Hypergolic ignition testing of solid fuel additives with MON-3 oxidizer

A droplet test capability for evaluating hypergolic reaction behavior of solid materials with low temperature-conditioned N₂O₄-based (e.g., MON-3) liquid oxidizer was developed and used to screen a large number of candidate particulate materials. High-speed color video (≥1000 fps) was used for observation and characterization of the reaction extent and timing. Based on a broad literature review of historic and recent publications, previous experience, consultation with leading researchers in the field, and consideration of chemical characteristics, a catalog of 188 materials of interest was generated. After applying a set of application-specific selection criteria, a list of 50 leading candidates was identified; 42 of which were procured and screened at a-10 °C test condition in an inert argon environment. Seven showed a positive hypergolic ignition reaction: sodium amide, sodium cyanoborohydride, lithium amide, triaminoguanidinium azotetrazolate (TAGzT), trimethylamine borane complex, borane tert-butylamine complex, and ammonia borane all demonstrated ignition delays of less than 500 ms. Sodium amide was by far the fastest, with ignition delay of less than 0.5 ms, with the visible reaction starting immediately upon contact of the MON-3 drop as recorded at 2000 fps. High solids loading mixes (approximately 50 wt%, 25% also tested) were made with SP7 wax and materials having melting points compatible with the wax melt processing temperature of about 100 °C (sodium amide, sodium cyanoborohydride, lithium amide, TAGzT). Disc-shaped samples were cut, and tested with the same experimental procedure as used for the powders. In some cases, a small amount of additional loose powder (few mg of the same material as the additive) was added as a surface coating in an attempt to augment ignition. Unfortunately, no samples containing particulate additives in a wax matrix resulted in hypergolic ignition with MON-3. Results from this investigation indicate that the wax matrix greatly reduces the magnitude of reaction compared to the neat powder samples even at high particle loading densities in the wax. It is believed that the primary obstacle to a hypergolic solid fuel formulation (for the wax/MON system) is the effective contact barrier formed by the wax matrix (the wax may also have served as an effective heat sink through melting and vaporization).