Encapsulated nitrogen-rich energetic additives in AP/HTPB propellants

The present study reports, for the first time, the combustion characteristics of an AP/HTPB composite solid propellants containing nitropyrazole-based energetic materials. Three insensitive/low sensitive energetic compounds based on nitropyrazole, 4-hydroxy-3,5-dinitropyrazole (OHDNP), 4-amino-3,5-dinitropyrazole (LLM-116), and ammonium 4-amino-3,5-dinitropyrazol-1-ide (ADNP) were used as energetic additives for the preparation of three propellant formulations (EAP-1, EAP-2, and EAP-3). Each formulation consisted of 77% ammonium perchlorate (AP), 16% HTPB-based fuel/binder mixture with 4% aluminum, and 3% nitropyrazole derivative. The prepared propellants were systematically characterized in terms of their morphology, thermal decomposition, sensitivity, and combustion performance. The thermal and mechanical stability tests reveal that the overall stability of all propellants is good with decomposition temperatures above 260 °C and low impact and friction sensitivity (IS > 40 J and FS > 360 N). The burning rates of all the propellants were evaluated over a pressure range of 2–7 MPa, with values ranging from 10.32 −11.22 mm s⁻ 1 at 7 MPa. The developed formulations exhibited higher burning rates than the corresponding RDX-containing reference propellant. The static motor test of the LLM-116-based propellant (EAP-2) was conducted at a chamber pressure of 3.1 MPa, and a thrust of 50.47 N was measured. The combustion and nozzle efficiencies for EAP-2 were obtained 99% and 95.4%, respectively. Overall, the results suggest that nitropyrazole-based energetic derivatives are promising candidates as alternatives to conventional energetic additives for the development of high-performance, relatively insensitive AP/HTPB composite propellants.