Characterization of the hypergolic ignition delay of ammonia borane

Abstract

Hypergolic hybrid motors have the potential to improve the safety, reliability, and versatility of rocket systems. They may also serve as a viable replacement for highly toxic liquid fuels (for example, hydrazine, monomethyl hydrazine, etcetera) conventionally used in hypergolic systems. Ammonia borane (AB)-based fuels are relatively nontoxic hydrogen-dense solids that have good theoretical performance. AB has also been found to be highly hypergolic with white fuming nitric acid (WFNA), potentially enabling it to replace existing toxic hypergolic fuels. In this work, hypergolic ignition delay tests were performed on AB synthesized with a novel water-promoted scalable process to characterize its performance as a hypergolic fuel. Typical ignition delays with WFNA were found here to be approximately 2–10 ms. Ignition delay tests performed with AB powder sieved into different particle size ranges indicated a particle size dependency for the ignition delay, with the finer AB particles (D < 45 μm) igniting after shorter delays. AB was successfully incorporated into Sylgard®-184, which is a silicone elastomer binder, and ignition delay tests were performed on high solids loading (80%) Sylgard-184-AB pellets. Mean ignition delays for the Sylgard-184-AB fuel pellets tested were less than 50 ms, which may make the formulation viable for use in hypergolic hybrid motors.