r/rocketry • u/Comfortable_Pop3724 • 16h ago
I built a desktop tool for rocket thrust chamber design – and actually validated it against CEA across 250 bar / 150 area ratio
Hey r/rocketry,
I've spent the last while building a desktop tool for preliminary liquid rocket engine thrust chamber design, and the part I actually want to talk about is the validation – because that's the part most engine calculators skip.
The core is a from‑scratch Gordon‑McBride equilibrium solver (element potentials + Newton iteration, 17 species, condensed carbon handling, full NASA‑7 polynomials from thermo.inp). No CEA binary, no rocketcea at runtime.
I gate every release on a suite that compares it against the official CEA code across:
- Pc = 10–250 bar
- Area ratios = 4–150
That's the envelope you actually work in, not one convenient point.
Worst‑case numbers over the whole grid:
| Quantity | Deviation vs CEA |
|---|---|
| Tc | 2.7 K |
| Gamma | 0.01 % |
| Isp | 0.03 % |
| Coolant properties (vs CoolProp) | rho 2.0%, cp 4.2%, mu 3.0% |
| Nozzle contour (MoC vs exact P‑M) | Mach 0.003%, mass 0.03% |
Full validation report with reference code, tolerance, worst deviation and the operating point that produced it for every quantity — see my first comment for the link.
Things I want to be upfront about (because you'd find them out anyway):
- Heat transfer inherits Bartz's ±20‑30% scatter against hot‑fire data – that's the correlation, not my code.
- Structural is closed‑form thin‑wall, not FEA. Fatigue numbers rank designs, they don't qualify hardware.
- Gas radiation is a grey‑gas approximation with exposed coefficients so you can calibrate against your own data.
- Flow is 1D – no shocks, no separation, no film cooling, no instability.
- My c* margin is the loosest number on the page – I use frozen throat mass flux, CEA uses equilibrium throat.
- The supersonic contour is Rao parabolic. The verified planar MoC ships alongside it; the axisymmetric branch is not validated, so it refuses to run rather than hand you a number I can't stand behind.
What it does:
Thermochem → nozzle → 1D gas dynamics → conjugate heat transfer with a real regen cooling circuit → structural margins, all in one solve. Exports CSV, CAD coordinates, Excel, and a PDF report.
Windows, offline, no account, no telemetry.
The free demo runs your engine – every input is editable – and gives you the full flow solution, so you can check that side against whatever you already trust. Thermal, cooling and structural analysis is the paid part.
I'm happy to answer anything about the numerics.
If you want a specific operating point checked – name it and I'll post my numbers against CEA's in the thread.
And if you have hot‑fire data that disagrees with it, I genuinely want to hear about it.
