Nickel-Coated Aluminum Particles: A Promising Fuel for Mars Missions
Combustion of metals in carbon dioxide is a promising source of energy for propulsion on Mars. This approach is based on the ability of some metals (e.g. Mg, Al) to burn in CO2 atmosphere and suggests use of the Martian carbon dioxide as an oxidizer in jet or rocket engines. Analysis shows that CO2/metal propulsion will reduce significantly the mass of propellant transported from Earth for long-range mobility on Mars and sample return missions. Recent calculations for the near-term missions indicate that a 200-kg ballistic hopper with CO2/metal rocket engines and a CO2 acquisition unit can perform 10-15 flights on Mars with the total range of 10-15 km, i.e. fulfill the exploration program typically assigned for a rover. Magnesium is currently recognized as a candidate fuel for such engines owing to easy ignition and fast burning in CO2. Aluminum may be more advantageous if a method for reducing its ignition temperature is found. Coating it by nickel is one such method. It is known that a thin nickel layer of nickel on the surface of aluminum particles can prevent their agglomeration and simultaneously facilitate their ignition, thus increasing the efficiency of aluminized propellants. Combustion of single Ni-coated Al particles in different gas environments (O2, CO2, air) was studied using electrodynamic levitation and laser ignition. It was shown that the combustion mechanisms depend on the ambient atmosphere. Combustion in CO2 is characterized by the smaller size and lower brightness of flame than in O2, and by phenomena such as micro-flashes and fragment ejection. The size and brightness of flame gradually decrease as the particle burns.