Mixture-Ratio Sensitivity of a Hydrogen–Oxygen Rocket Engine Using NASA CEA
How Tc and Isp change with O/F for GH2/GO2 at 68 atm, restricted vs full equilibrium chemistry
Overview
This project studied how combustion chamber temperature (Tc) and specific impulse (Isp) vary with oxidizer-to-fuel ratio (O/F) for a gaseous hydrogen–oxygen rocket engine using NASA CEA (Chemical Equilibrium with Applications). The engine operated at Pc = 68 atm and expanded to sea-level conditions. Mixture ratios from O/F = 2 to 10 were evaluated to identify performance trends and the mixture ratio that maximizes Isp.
Two chemical models were compared:
- Case A (restricted products): H₂O, H₂, O₂ only
- Case B (full equilibrium): all species active (CEA default equilibrium chemistry)
Method
For each O/F value, NASA CEA was used to compute equilibrium products and performance metrics. The outputs of interest were:
- Chamber temperature, Tc
- Specific impulse, Isp (sea-level expansion)
This comparison isolates how chemistry model fidelity influences predicted performance.
Key Results
Case A (restricted chemistry)
The restricted product set suppressed both temperature and performance. The output was not physically realistic for combustion conditions:
- Tc fixed at ~298 K
- Isp peak ~736 m/s
Case B (full equilibrium chemistry)
Full equilibrium chemistry produced physically realistic rocket-combustion behavior:
- Tc increased from ~2070 K at O/F = 2 to ~3670 K at higher O/F
- Isp peaked near O/F ≈ 2 at ~1790 m/s (≈183 s)
This reflects a known trade-off: as the mixture becomes more oxidizer-rich, Tc rises but average molecular weight increases, which reduces exhaust velocity. The optimum occurs on the fuel-rich side, where lower molecular weight exhaust compensates for a slight temperature drop and increases jet momentum.
Performance Summary (from CEA outputs)
| Model | Chemistry | Pc | Expansion | O/F range | Tc trend | Isp peak |
|---|---|---|---|---|---|---|
| Case A | Restricted (H₂O, H₂, O₂) | 68 atm | Sea level | 2–10 | ~298 K (non-physical) | ~736 m/s |
| Case B | Full equilibrium (all species) | 68 atm | Sea level | 2–10 | ~2070 → ~3670 K | ~1790 m/s at O/F ≈ 2 |
Takeaways
- Chemistry assumptions strongly affect predicted rocket performance.
- Restricting products can produce misleading outputs for combustion modeling.
- For GH2/GO2, the best Isp often occurs fuel-rich because lower molecular weight exhaust increases effective exhaust velocity.
Tools
- NASA CEA for equilibrium chemistry and performance prediction
- Plotting and post-processing (MATLAB)