**2.5 Aluminum and magnesium combustion with the CO2**

Aluminum is low cost and widely used as micron/nano sized metallic powder form. Both spherical and flake shaped aluminum can be casted easily within paraffin wax. Aluminum provides high specific impulse with the *CO*2. However, *Al=CO*<sup>2</sup> combustion produces severe slag formation results nozzle blockage due to oxide ð Þ *Al*2*O*<sup>3</sup> formation. Magnesium is can also be found broadly in the market. Mg shows rapid ignition characteristics in carbon dioxide environment due to its low ignition temperature of 1000 K. In addition, metal oxide layer of a magnesium powder sphere is not strong as in aluminum that is easily breakable during combustion. Therefore, magnesium oxide ð Þ *MgO* formation by *Mg=CO*<sup>2</sup> reaction is not as severe as in aluminum oxide. It shows less particle agglomeration and two-phase losses in the nozzle.

## **3. Propellant design for Mars ascent vehicles**

This section refers the proper propellant combination for the Mars missions. Mars hoppers, ascent vehicles or any rocket systems can use the proposed propellant combination. Both aluminum and magnesium are proposed as main additive. Paraffin wax is the main binder. In addition, hybrid rocket system uses in-situ *CO*<sup>2</sup> as the major oxidizer. However, since the pure carbondioxide combustion poses several challenges due to reduces chemical kinetics, *N*2*O=CO*<sup>2</sup> oxidizer mixture is selected for performance analysis.

#### **3.1 Fuel grain manufacturing**

Paraffin wax ð Þ *C*32*H*<sup>66</sup> is the main binder of the solid fuel grain. Hydrophobic nature of the paraffin protects the metal additives from the water vapor. It is nontoxic thus produces water and carbon dioxide as combustion product. Paraffin has low glass transition temperature �180<sup>∘</sup> ð Þ <sup>C</sup> that is quite feasible colder periods of Mars atmosphere. Also, inert feature of the paraffin wax is feasible for long duration Mars missions [2, 8].

3 micron sherical shaped aluminum is casted up to 40% by mass in the paraffin. In addition, flake shaped aluminum powder is also tested. Combination of 20% flake 40% spherical aluminum-based fuels have higher carbon dioxide concentration during the combustion. Surface area of flakes are larger than spheres thus initiate the combustion easier. Magnesium is also used in actual motor experiments. 44-micron magnesium powder has 99.99% purity level. Mg amount in paraffin is 60% by mass.

Mixing metallic powders up to 40% by mass is achieved by using Silverson L5M high shear mixer [14]. High shear mixer uses square shaped blades. Mixing metal additive with paraffin binder as follows,


The **Figure 4** shows the several fuel samples that are casted with the L5M High Shear Mixer. It is worth to note that, although high shear mixer allows homogenous mixture of paraffin and metal powders, increasing magnesium amount to 60% cannot be casted by using high shear mixer due to high viscosity. The structure of 60% magnesium 40% paraffin mixture is like mud that only be casted by handmixing in a phenolic mold.
