Hypergolic ignition of hybrid rocket fuels in a slab burner experiment

Hypergolic ignition systems, where combustion is initiated nearly instantaneously upon contact between the oxidizer and the fuel, can improve the reliability, safety, and simplicity of hybrid rocket engines. Hypergolic performance, first and foremost the ignition delay, is of critical importance as it determines the response time of the propulsion system. To evaluate this performance metric, a novel slab burner experiment is developed to study the hypergolic behavior of hybrid rocket propellants in an engine-like configuration. The liquid oxidizer injection system and the flow conditions implemented in this slab burner are characterized in detail. Two types of experiments are presented. In the first, nitric acid is used essentially for ignition and is injected as a spray into a combustion chamber containing a paraffin-based fuel slab doped with ammonia borane and filled with gaseous oxygen. Rapid hypergolic ignition, in the order of 250 to 3000 ms, with nitric acid and sustained combustion with gaseous oxygen are observed. In the second experiment, concentrated nitric acid is injected into a combustion chamber under an inert atmosphere resulting insignificantly slower ignition. The experiments are filmed with a high-speed camera to measure the hypergolic ignition delays and the location of flames. Additionally, the reignition capability of these two systems is evaluated. Successful reignitions are observed with delays greater than the initial hypergolic event. Novelty and significance This study presents an in-depth investigation of hypergolic ignition in hybrid rocket engines. A novel hypergolic slab burner experiment is designed and presented. This experimental setup allows fora detailed analysis of hypergolic ignition delays and combustion dynamics in hybrid rocket propellants, providing valuable insights for improving hypergolic engine reliability and performance. This test configuration allows for providing insights more relevant to oxidizer injection conditions encountered in full-scale hybrid engines. In addition, hypergolic reignition performance are evaluated.