Ignition delay and performance of sorbitol based hypergolic hybrid fuels

Abstract

The ignition delay of ammonia-borane-doped sorbitol and paraffin-based fuel pellets exposed to white fuming nitric acid is measured in oxidizer drop tests. Various mixtures of sorbitol, paraffin and ammonia-borane are tested to investigate the effect of sorbitol concentration on the ignition behavior of hypergolic hybrid fuels. High-speed visible, Schlieren, and mid-infrared imaging techniques are used to study the ignition behavior in a non-intrusive manner. The experiments demonstrate that the ignition flame appears faster and is larger with fuel pellets made of sorbitol compared to those made of paraffin. Sorbitol pellets containing 10 percent ammonia-borane by mass ignited in 8.20 ms on average, while paraffin pellets containing 10 percent ammonia-borane ignited in 60.25 ms. The average ignition flame intensity was measured to be approximately 6 times greater in sorbitol-based pellets than in paraffin-based ones. Ignition delay and oxygen bomb calorimeter tests are also performed using aged pellets and showed that exposure to ambient air negatively impacts the storage stability of sorbitol-based fuel samples. Theoretical performance calculations revealed that sorbitol-ammonia-borane mixtures can reach density-specific impulses similar to those of paraffin-ammonia-borane mixtures. This makes sorbitol-ammonia-borane a rare hypergolic hybrid fuel that is liquefying while also having high propulsive density and a very short ignition delay.