**3.2 The oxidizer selection**

Hybrid Mars Motor experiments uses *CO*2*=N*2*O* oxidizer mixture in blowdown mode. Self-pressurizing capability of both oxidizers makes Martian operations quite practical. In addition, the mixture displays several advantageous such as (i) improved *Isp* performance compared to pure *CO*2, (ii) decreased two-phase losses due to reduced mass fraction of condensed phase products (*CO*<sup>2</sup> allows additional burning with the condensed phase species), (iii) low freezing point of the oxidizer mixture is ideal for Martian environment (iv) both agendst has self-pressurizing feature that not require any additional pressurizing system in the rocket, (v) low cost, less complicated and lighter compared to liquid bipropellant engines.

**Figure 4.** *Fuel samples and Silverson L5M high shear mixer [?].*

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

Uniform mixture of *N*2*O* and *CO*<sup>2</sup> is achieved due to similar fluidic characteristics of agents. **Table 1** summarizes physical characteristics due to NIST database [15].

Mixing two self-pressurizing saturated liquids requires careful process. Actual motor experiments uses 10 liters aluminum scuba tank with maximum operating pressure of 200 bars is the main oxidizer tank. The oxidizer compound that has higher mass fraction first filled in the scuba tank. Then the tank is vented to cool the oxidizer and reduce the tank pressure around 30 bars. The reduced tank pressure allows second oxidizer (source tank that has higher pressure) compound to add the scuba tank. Therefore, the second oxidizer compound is then added in main tank.

Oxidizer mixture density is the major parameter for the performance analysis of the ignition. Specific volume of the mixed oxidizer is found by using the specific volumes of the components and the mass fraction ð Þ *χ* of *N*2*O* in the mixture.

$$
\overline{\boldsymbol{\upsilon}}\_{liquid} = \boldsymbol{\upsilon}\_{N\_2O} \boldsymbol{\chi} + \boldsymbol{\upsilon}\_{CO\_2} (\mathbf{1} - \boldsymbol{\chi}) \tag{1}
$$

The overall oxidizer density is calculated, by assuming an ideal mixture, using simple formula,

$$
\overline{\rho}\_{liquid} = \mathbf{1} / \overline{\upsilon}\_{liquid} \tag{2}
$$

Liquid *N*2*O* and *CO*<sup>2</sup> mixture operates in blow-down mode during experiments. The oxidizer mixture has two-phase flow characteristics in the feed system through the injector. The oxidizer flow is choked at the injector thus downstream pressure (motor chamber pressure) has no effect in the flow rate. A blow-down oxidizer mixture needs an advanced approach by using two phase physics. Because, selfpressurizing *N*2*O* or *CO*<sup>2</sup> in saturated liquid state cannot be modeled by using fundamental ideal gas, compressible or incompressible flow assumptions. Two phase flow approaches by using Homogeneous Equilibrium Model (HEM) is needed for more precise calculations on flow rate and discharge coefficient.

#### *3.2.1 Two phase flow background*

Typical rocket applications require fluid dynamics calculations of the injector and propellant feed system. Most of the liquid propellants such as hydrogen peroxide ð Þ *H*2*O*<sup>2</sup> , ethanol ð Þ *C*2*H*5*OH* , RP-1 ð Þ *C*12*H*<sup>24</sup> and nitrogen tetraoxide ð Þ *N*2*O*<sup>4</sup> can be modeled accurately by using classical incompressible fluid dynamics features. In addition, gaseous propellants such as gaseous oxygen, hydrogen and methane use ideal gas law and compressible fluid assumptions. Liquid oxygen ð Þ *LOX* can also be


#### **Table 1.**

*Saturation properties of N*2*O and CO*2*.*

modeled accurately. Although *LOX* is in a saturated state at cryogenic temperatures, it uses single-phase incompressible flow assumptions. Compressibility ð Þ *Z* factor of liquid oxygen at 1 atm pressure is 0.004, and 0.97 for saturated cryogenic oxygen vapor. Both values are very close to ideal values 0.0 and 1.0 [16, 17].

Nitrous oxide however has a liquid *Z* factor of 0.13 and saturated vapor compressibility factor of 0.53 at the room temperature. Therefore, incompressible liquid or ideal gas assumptions become inaccurate for modeling nitrous oxide. *N*2*O* is handled as a two-phase mixture for fluid flow modeling. Two-phase flow modeling means that fluid flows as a mixture of liquid phase and a vapor phase at the same time. A fluid quality factor is required for the fluid flow. Fluid quality is the ratio of the vapor mass fraction divided by the total fluid mass.

Injector modeling of self-pressurizing agents such as nitrous oxide is a complex process [17]. Because, fluid quality factor changes during tank evacuation; liquid phase boils into vapor phase. Therefore, internal tank pressure and fluid density changes during the evacuation. Tank pressure and fluid density directly effects the injector mass flow rate calculations. And changing the mass flow rate directly effects the combustion stability, motor pressure and thrust.
