**2.3 CO2 as a novel oxidizer**

Carbon dioxide is known as a natural combustion product from hydrocarbons or explosions. However, there are certain fuels that remove the carbon–oxygen bond in the *CO*2. That is to say metals have higher reactivity series compared to carbon thus removes the carbon oxygen bond. This reaction produces a substantial energy release. Various metals and metal hydrides have been studied in order to understand the combustion characteristics of carbon dioxide as explained in [2, 10–12]. In addition, additional information is presented related to the *Metal=CO*<sup>2</sup> combustion in Chapter 3. However, fundamental findings from [2, 10–12] are explained in this section.

Kara and Karabeyoglu [2] provides practical experiments by using *CO*<sup>2</sup> as the oxidizer in lab scale hybrid motors. *CO*<sup>2</sup> is mixed with the nitrous oxide to understand the combustion characteristics. Paraffin based fuel consist of 40 % aluminum powder by mass. Aluminum powder has 3 micron spherical shape. Al has two purity levels 98.75 % and 99.99 %. The purity level has no significant effect on carbon dioxide combustion. According to [2], succesfull combustion is achieved up to 45 % by mass in the oxidizer mixture. In his comprehensive study, Boiron [10] explains the in-situ resources utilization techniques for hybrid propulsion based Mars Ascent Vehicles. Boiron promotes high performance liquid oxygen/paraffin based hybrid rocket system. He proposes two concept missions; Mars Sample Return (36 kg payload mass) and Medium-scale (500 kg payload) Rocket. Borion explains background on in-situ propellant production techniques by using electrolysis methods. He discusses advantageous and disadvantageous of Paraffin/Aluminum/ *O*<sup>2</sup> / *CO*<sup>2</sup> propellant combination for Martian rockets. Finally, Boiron presents details Zirconia cell hardware and electrochemical mechanism. Other fundamental researches related to carbondioxide combustion are studied by Shafirovich Gokalp and Zubrin [11, 12]. Shafirovich and Gokalp presents the concept of a metal/carbon dioxide propellant for Mars Sample Return missions. They provide detail thermochemical analysis of *CO*<sup>2</sup> combustion with various of metals and metal hydrides in rockets. In addition, they provides lab scale combustion experiments with the magnesium. Shafirovich and Gokalp compares several designs for their ascent/decent vehicle such as hybrid engine, liquid monopropellant engine and bipropellant engine. Robert Zubrin who is one of the pioneer scientist in the field of Mars missions proposes diborane and silane for Mars Ascent Vehicles.

Although there are many theoretical studies on *Metal=CO*<sup>2</sup> combustion, it has not been tested in actual hybrid rocket motor. There is an experimental study on combustion characteristics of carbon dioxide with magnesium rocket engines [13]. Yue Lie uses magnesium fuel in powder form with multiple gas injection mechanisms into the combustion chamber at high pressures. Yue presented the

thermodynamic calculations for the combustion process of the multiphase flow environment in a lab scale rocket engine used in the experiments. However, this design seems impractical that gas phase oxidizer needs pressurizing system.

### **2.4 Metallic powder additives**

Metallic powders serve as excellent fuel additives due to their significant volumetric and gravimetric heat release during the combustion process [8]. Purity, size and shape of metallic powders directly affect the combustion performance as they control the ignition delay and the formation of the condensed combustion product (CCP).

Wide range of additives have been studied for *CO*<sup>2</sup> combustion in rocket motors such as lithium ð Þ *Li* , boron ð Þ *B* , berillyum ð Þ *Be* , aluminum ð Þ *Al* , magnesium ð Þ *Mg* , magnesium hydride *MgH*<sup>2</sup> , diborane ð Þ *<sup>B</sup>*2*H*<sup>6</sup> and silane ð Þ *SiH*<sup>4</sup> . Lithium is highly reactive alkali metal however shows volumetric heat release is low (means low specific impulse) additives due to its low density. Boron is commonly used as a solid propellant additive. Despite it has high combustion energy per unit mass, it delivers poor combustion efficiencies. Major concern of boron is reveals high CCP in the nozzle. Magnesium hydride increase specific impulse however not practical for longer lifetime missions due to its poor dehydrogenation kinetic and hydrogen storage. Casting and lifetime are major issue for all other metal hydrides.

**Figure 3** shows the theoratical specific impulse values with respect to oxidizer to fuel ratio. Calculations are made by [11] with 10 bars of chamber pressure.

Diborane is storable at 5 bars in Martian conditions. Diborane shows high mass fraction of CCP effects the specific impulse. Boron oxide formation in liquid form generates a significant risk for slag formation in nozzle throat. Decomposition is

**Figure 3.** *O=F ratio versus specific impulse – Various of additives.*

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

another factor for diborane. Silane has silicon oxide as the major CCP. It provides lower mass fraction of CCP than diborane. However, silane is quite toxic there is not enough reliable data for *SiH*4*=CO*<sup>2</sup> combustion [10, 12].

Therefore, analysis shows that beryllium, aluminum and magnesium are most prominent candidates for carbon dioxide combustion due to performance and safety aspects. Beryllium is extremely toxic despite its high performance. All in all, magnesium and aluminum are left over as major additives casted in the paraffin wax.
