Burning Behavior of ADN-Based Propellants Loaded with Al-Mg Mechanically Activated Powders

In recent years the awareness of the environmental risks associated with the use of ammonium perchlorate in solid propellants has led the research community to find an environmentally friendly alternative. Ammonium dinitramide (ADN) is by far the most promising choice and the present work aims at investigating the burning behavior of this oxidizer in combination with the two binders HTPB and GAP, hydroxyl-terminated polybutadiene and glycidyl azide polymer respectively. In order to highlight the differences between the two binders coupled with ADN, specular formulations were produced, loaded with a number of metal powders. The study also intends to assess the influence of mechanically activated powders on the agglomeration process, one of the major drawbacks related to the use of aluminum. With this regard, the latter fuel was combined with magnesium and two batches of activated powder were produced, employing different intensities of the treatment. The powders were comprehensively characterized in their morphology, size, reactivity. The burning rate and the flame temperatures were measured from combustion tests performed in the pressure range from 2 to 13 MPa, by using a high-speed camera and an emission spectrometer. The agglomeration phenomena were investigated in dedicated tests carried out at 0.5, 1 and 2 MPa. The results have pointed out several substantial differences in the behavior of ADN with the inert (HTPB) and the active binder (GAP). All the HTPB/ADNbased propellants exhibit a relatively high ballistic exponent (0.7 - 0.9), above the common standard adopted for civil applications. On the other hand, the GAP/ADN formulations feature a weak pressure dependence (ca. 0.3), with a superior synergy between the components, but also a high burning rate that makes them difficult to apply in civil rockets. Computed tomographies and SEM images of samples extinguished by rapid depressurization evidenced some issues concerning the coupling of ADN with HTPB, also observed in the combustion videos. A poor interaction between the ingredients is acknowledged, underlined by the fact that the oxidizer decomposes quickly and leaves the binder behind, which pyrolyzes at a later stage in the flame zone. Furthermore, such propellants featured very low flame temperatures, far below the adiabatic values. For what may concern the aggregation phenomenon, both the addition of magnesium and the activation treatment proved to be extremely effective in reducing the size of the agglomerates. The study is part of the GRAIL project, which has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 638719.