Janus-type hypergolic fuels for hybrid systems using hydrogen peroxide and hydroxylammonium nitrate-based oxidizers

Development of fully green hybrid propulsion systems, in which both the oxidizer and the fuel are environmentally benign, while having a desirable performance under in-space conditions, is a formidable challenge. Herein, we present new air-stable solid propellants capable of exhibiting hypergolic ignitions with commercially available H₂O₂ (70 %) and its low-freezing formulation containing hydroxylammonium nitrate (HAN). A top performing Janus-type fuel 15 showed remarkable ignition delay time of 4 ms with HAN-H₂O₂ formulation, even at −40 ℃. The Janus-type design approach allowed modulation of the ignition delay times by varying the carbon chain-length, the type of the used transition metals and the molecular structure of the resulted solid fuels. Studies of HAN and related salts formulations with H₂O₂ allowed us to get an insight into the reasons of the stability of HAN-H₂O₂ formulations. UV–vis and EPR monitoring of selected fuels reactions with oxidizers, as well as surface wettability measurements, provided a valuable information related to the better understanding of the hypergolic reaction mechanisms, which should contribute to the design of the next generation of green hypergolic propulsion systems. We also successfully tested fuel 15 in a custom-made small-scale static hybrid motor system, injecting H₂O₂ in a sequence of pulse or in continuous flow modes.