CEA-Based Performance and Off-Design Assessment of LOX/Biofuel Rocket Propellant Candidates

Technical Summary: CEA-Based Performance and Off-Design Assessment of LOX/Biofuel Rocket Propellant Candidates

Problem Statement
The study addresses the need for a standardized, early-stage screening of liquid rocket propellants, specifically comparing traditional hydrocarbon fuels against oxygenated biofuel candidates. While RP-1 remains the practical benchmark for LOX/hydrocarbon engines due to its established engineering record and high energy density, there is interest in oxygenated fuels (such as dimethyl ether, ethanol, and methanol) for cleaner, smaller, or demonstrator-type propulsion systems. A critical gap in existing literature is the lack of comparative frameworks that evaluate both peak ideal performance and mixture-ratio tolerance (off-design behavior) under a single, consistent set of thermodynamic assumptions. Previous studies often focus on peak values or specific engine designs, making direct fuel-to-fuel comparisons difficult. This paper aims to fill that gap by screening six LOX-compatible fuels to identify which candidates warrant higher-fidelity analysis (CFD, experimental, or system-level).

Methodology
The research utilizes the NASA Chemical Equilibrium with Applications (CEA) code in equilibrium rocket mode. To ensure a fair comparison, all six fuel candidates—RP-1, dimethyl ether (DME), ethanol, methanol, acetic acid, and formic acid—were evaluated under a fixed set of ideal thermochemical conditions:

• Chamber Pressure: 20 bar
• Nozzle Expansion Ratio: 40
• Fuel Temperature: 298 K
• Oxidizer (LOX) Temperature: 90 K
• O/F Ratio Sweep: 0.5 to 8.0

The study extracted key performance metrics including vacuum specific impulse ($I_{sp,vac}$), chamber temperature ($T_c$), characteristic velocity ($c^*$), thrust coefficient ($C_f$), specific heat ratio ($\gamma$), and exhaust molecular weight ($MW$).

To assess robustness, the authors introduced an off-design degradation index ($\Delta I_{sp}$), calculated as the relative loss in specific impulse when the mixture ratio deviates from the fuel's optimal point. The mean, 95th-percentile, and maximum values of this index were used to summarize performance degradation across the sweep. The analysis explicitly excluded real-world losses such as injector pressure drops, finite-rate chemistry, heat transfer, cooling requirements, and feed-system mass.

Key Results

1. Performance Ranking: RP-1 achieved the highest sampled vacuum specific impulse of 336.24 s at an optimal O/F ratio of 2.5. DME was the closest competitor, reaching 334.61 s at O/F = 1.5, a difference of only 1.63 s. Ethanol followed with 327.31 s (O/F = 1.5), while methanol, acetic acid, and formic acid showed progressively lower performance.
2. Thermodynamic Drivers: The ranking was not determined solely by chamber temperature. Although RP-1 had the highest $T_c$ (3477.44 K), DME's performance remained competitive despite a lower $T_c$ (3322.90 K) because it produced the lightest equilibrium exhaust products ($MW$ = 21.706) among the top candidates. RP-1 had a slightly higher molecular weight (22.764).
3. Optimal Mixture Ratios: Oxygenated fuels peaked at lower O/F ratios (1.5 for DME, ethanol, and methanol) compared to RP-1 (2.5), consistent with the internal oxygen content of the fuel molecules.
4. Off-Design Behavior: RP-1 demonstrated the lowest degradation across the O/F sweep, confirming its robustness. Among the renewable candidates, ethanol exhibited slightly better mean off-design degradation than DME, despite DME having a higher peak $I_{sp}$. Methanol underperformed in both peak value and tolerance. The acid fuels (acetic and formic) were deemed non-competitive due to low heat release and heavy exhaust products.
5. Benchmarking: Comparison with published CEA-type data (Burkhardt et al.) showed a consistent performance trend (RP-1 > DME > Ethanol > Methanol), though absolute values in this study were approximately 4.3–5.0% lower, attributed to differences in pressure and input state assumptions.

Significance and Claims
The paper positions itself strictly as an early-stage screening tool rather than a final propellant selection study. Its primary significance lies in providing a unified dataset that evaluates both peak ideal performance and mixture-ratio tolerance simultaneously.

• For High-Performance Applications: The results reinforce RP-1 as the difficult-to-replace benchmark when both peak performance and operational tolerance are required.
• For DME: It is identified as the strongest oxygenated candidate for applications prioritizing peak ideal performance, as it closely approaches RP-1's specific impulse.
• For Ethanol: The study highlights ethanol as a balanced candidate. While its peak $I_{sp}$ is lower than DME's, its superior mean off-design degradation suggests it may be a more robust choice for small engines or demonstrators where mixture ratio tolerance is valued over absolute peak performance.

Limitations and Scope
The authors explicitly state that these results are ideal equilibrium calculations. The study does not account for injector losses, atomization, evaporation, ignition transients, combustion instability, wall heat transfer, regenerative cooling, density impulse, tank sizing, toxicity, or material compatibility. Consequently, the findings are intended to guide the selection of candidates for subsequent, higher-fidelity CFD, hot-fire testing, and system-level design, rather than to dictate flight propellant selection directly. The paper concludes that future work should incorporate uncertainty quantification and variable operating conditions to refine these screening results.