**5. Combustion boundary of the CO2**

The motor chamber pressure (adiabatic flame temperature) is the key factor to achieve a successful combustion. Combustion quenches below a certain *Tflame* level. Thus, *Tflame* profile of Al tests is shown in **Figure 10**. Aluminum mass fraction in the paraffin wax is increased up to 60% (60% Aluminum and 40% Paraffin wax). In the fuel grain, 20% of this content is casted as 3 micron flaked shaped Al, and 40% is casted as spherical shaped aluminum. Flake shaped powder has larger surface area that forms better ignition characteristics. Thus, successful combustion limit is increased up to 55% *CO*2. However, this limit is considered as "stochastic limit" since there are also ignition failures.

Adiabatic flame temperature has different profile in Mg based experiments. Thus the **Figure 11** shows *Tflame* variation due to *CO*<sup>2</sup> addition at hybrid motor experiments. Combustion quenches (fails) below a certain *Tflame*. The stable ignition occurs at 75% *CO*<sup>2</sup> level around 1700–1800 K. Ignition quenches below the 1700 K.

**Figure 10.** *Flammability limit of Al=CO*<sup>2</sup> *experiments.*

*Hybrid Propulsion System: Novel Propellant Design for Mars Ascent Vehicles DOI: http://dx.doi.org/10.5772/intechopen.96686*

**Figure 11.** *Flammability limit of Mg=CO*<sup>2</sup> *experiments.*

**Figures 10** and **11** show Mg can easily ignites up to 75% carbon dioxide by mass in the oxidizer mixture. Mg has easier ignition capability than the aluminum. For example, flame temperatures of Mg and Al at 35% carbon dioxide are 3000 K and 2400 K. Although aluminum mass fraction increases to 60%, ignition fails around 55% *CO*<sup>2</sup> level.

Combustion boundary of the *CO*<sup>2</sup> due to the flame temperature is compared with the literature [20]. In this study, Reina et al. showed the flame temperatures both aluminum and magnesium in *CO*<sup>2</sup> environment between 1 and 10 bars. Various of powder sizes in micron level are studied. Literature survey by Reina explains that micron sized aluminum has around 1800 K ignition temperature with the carbon dioxide. However, Reina uses nano sized aluminum in their study. Thus, results showed that nanoAl has ignition temperature around 1000 K. Furthermore, ignition temperature is changing between 900-1000 K micro Magnesium based powder.

Therefore, results shows that ignition fail around 1600 K using the *Paraffin=Mg=N*2*O=CO*<sup>2</sup> propellant. Also, ignition temperature of *Mg=CO*<sup>2</sup> is found as 1000 K in the literature. This means that excessive 600 K (or more) can be used to vaporize the paraffin wax during the combustion. If the heat required to vaporize paraffin is excluded, the combustion quenches. The increased internal ballistics is needed to achieve higher amount of *CO*2.

Combustion mechanism of Metal/*CO*<sup>2</sup> based propellants also discussed due to oxidizer mass flux of the rocket. Flux dependent results are significant for scale up rocket motor design. The ignition limit also stated as averaged mass flux versus adiabatic flame temperature. **Figure 12** shows that the combustion quenches at motor chamber pressure (adiabatic flame temperature) below the 1600 K. So, the yellow zone is practically impossible region due to ignition boundary. Red dots states the quenched ignitions. In addition, 1000 K is the theoretical combustion value of *Mg=CO*<sup>2</sup> due to [11, 12] Therefore, the blue zone is theoretically impossible zone for the combustion. Practically possible zone is shown as orange region. Paraffin/Mg based hybrid motor that operates in this region successfully ignites *CO*2. There is a quenched ignition point at average mass flux of 375 *kg=m*<sup>2</sup>*s* for 70 % *CO*2. Details of this analysis are explained in PhD thesis version of this book chapter by Kara.

**Figure 12.** *Oxide formation after the motor experiment.*

#### **5.1 Issues during the combustion**

Hybrid rocket experiments reveal a significant issue during the combustion. That is the slag formation due to condensed combustion products (CCPs). CCP means the oxide formation as the combustion product. Aluminum (or magnesium) combustion produces aluminum oxide (or magnesium) oxide that has no effect on the combustion performance. Because, combustion performance such as thrust and specific impulse are only formed by the gaseous products. After experiments, 8% of fuel residual is oxides. **Figure 13** shows the motor condition after the experiment. Oxide formation blocks. In addition, poor-quality fuels can be wrapped during the experiment thus blocks the nozzle and the injector.

It is worth to note that although oxide formation reduces the performance, it blocks the nozzle throat and increases the motor chamber pressure during experiment. Therefore, increasing motor chamber pressure causes high efficiency combustion.
