Most propulsion tools answer a first-law question: how much thrust, how much
fuel, what thermal efficiency. Useful — but they never tell you where the
useful work potential is being destroyed. For that you need the second law.
I'm an aerospace engineer, and the tools for this were either five-figure
enterprise licenses or Fortran decks older than me. So I built
ExergyJet: station-by-station exergy analysis of
turbojet and turbofan engines, entirely in the browser.
What the analyzer computes
You configure the cycle — inlet, compressor, burner, turbine, nozzle,
optional afterburner, flight condition — and for every station you get
T, P, h, mass flow and velocity. Then the second-law layer:
- Exergy destruction per component via Gouy–Stodola (Ėd = T0·Ṡgen)
- Second-law efficiency η_II alongside thermal / propulsive / overall
- A Sankey diagram: fuel exergy → thrust power → destruction + exhaust loss
- Afterburner on/off comparison with variable-nozzle handling
- An engineering-grade PDF report of the whole run
Two results that surprise first-time users
1. The combustor is usually the biggest villain. Not the nozzle, not the
compressor — combustion irreversibility dominates the exergy budget. That's
why turbine inlet temperature and pressure ratio are the levers that matter.
2. Afterburners are far worse than they look. Reheat roughly doubles to
triples fuel flow for ~50–70% more thrust. In the exergy budget the lit
afterburner instantly becomes the largest single destroyer — bigger than the
main combustor. It's a thrust-per-minute device, and the numbers make that
impossible to argue with.
Why TSFC isn't enough
TSFC is one scalar: fuel flow per unit thrust. Two engines with identical
TSFC can waste fuel exergy in completely different places — one in a hot,
lossy combustor, the other in a screaming exhaust plume. TSFC can't tell
them apart; the exergy budget can. I wrote a longer piece on this:
TSFC Explained.
Try it
The free tier runs the full turbojet workflow — no credit card:
https://exergyjet.com
If you're into propulsion or thermo, I'd genuinely love feedback —
especially on the model (real-gas corrections, component efficiencies,
net-thrust convention). What would you analyze first?
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