Abstract
The ignition delay and surface-temperature evolution of ammonia-borane-doped paraffin wax exposed to white fuming nitric acid are measured in oxidizer-drop tests. The ammonia borane granulometry is varied to investigate the limits of hypergolic ignition and its underlying mechanisms. High-speed visible, Schlieren, and midinfrared optical systems are used simultaneously to observe the phenomena occurring between the first contact and the appearance of a flame. The experiments reveal the usefulness of both Schlieren and midinfrared high-speed imaging as diagnostic tools for hybrid hypergolic experiments, specifically for the detection of preignition events not observable through visible light emission. A strong relationship is found between the granulometry of the ammonia-borane additive and the ignition delay of the fuel pellets. At 25% ammonia borane by weight, pellets made with a granulometry of 600–710 μm ignited in 15.4 ms, whereas those made with a granulometry of 53–106 μm ignited in 60.1 ms on average. Granulometry also affects preignition phenomena such as gas-release velocity and surface-temperature evolution. These results reveal that the acid drop acts as a heat sink for the hypergolic ignition of the hybrid rocket fuel and that a minimum additive particle size is needed to avoid quenching.
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