A.4.2.2.5.2 Mixture Ratio Optimization 2
A.4.2.2.5.2 Mixture Ratio Optimization
An important factor in obtaining the best performance from a launch vehicle is optimizing the mixture ratio. In our analysis, we define the mixture ratio as the ratio between mass of oxidizer and fuel. Mixture ratio is also referred to as the oxidizer to fuel ratio. Maximizing the oxidizer to fuel ratio allows the maximum amount of energy release in a reaction. We ran the NASA Chemical Equilibrium with Applications (CEA) code and were able to optimize the oxidizer to fuel ratio for each propellant.
For solid propellants optimization of the oxidizer to fuel ratio is unnecessary. The CEA code was run for different oxidizer to fuel ratios with all other conditions remaining the same. The specific impulses (Isp) and characteristic velocities (c*) were recorded and charted. After this the maximum was found and we chose that value as our oxidizer to fuel ratio for the rest of the project.
Figure A.4.2.2.5.2.1 shows where the maximum oxidizer to fuel ratios occur for each of the three propellants.
Fig. Section A.4.2.2.5.2.1: Sea level Isp versus oxidizer to fuel ratio.
(Wilcox, Nicole)
In Fig. A.4.2.2.5.2.1, it is clear that the optimum oxidizer to fuel ratios is 3 for liquid oxygen and liquid hydrogen, 6 for hydrogen peroxide and RP-1, and 6 for liquid hydrogen and HTPB. These numbers are illustrated again in Fig. A.4.2.2.5.2.2 for vacuum specific impulses.
Fig. Section A.4.2.2.5.2.2: Vacuum Isp versus oxidizer to fuel ratio.
(Wilcox, Nicole)
While both specific impulse diagrams show the same optimum performance at the same oxidizer to fuel ratio, one can see in Fig. A.4.2.2.5.2.3 below that the optimum characteristic velocity occurs at a smaller oxidizer to fuel ratio.
Fig. Section A.4.2.2.5.2.3: Characteristic velocity versus oxidizer to fuel ratio.
(Wilcox, Nicole)
Due to the slight difference and our reliance on specific impulse rather than characteristic velocity for performance, we decided to choose the optimization based on specific impulses. Both variables are representative of the engine performance regardless. However, a higher specific impulse fit our needs better.
When analyzing this data another observation was made that the specific impulses between hydrogen peroxide / RP-1 and hydrogen peroxide / HTPB had little variation from one another. This small variation made us suspicious early on of the possibility that selecting a hybrid is more beneficial than selecting a storable. Our observation was later proven by the Model Analysis simulations. Mixture ratio analysis proves beneficial in understanding of the oxygen to fuel ratio, optimizing engine performance, and providing insight into future choice possibilities.
Author: Nicole Wilcox