propwrap

Typed propellant thermochemistry for liquid rockets — a clean Python API over NASA CEA (via RocketCEA) with optional Cantera cross-checks.

Scope: propellants and mixture performance — not full engine design (nozzles, injectors, cooling cycles).
Numbers: ideal theoretical CEA unless you apply efficiency factors. Not flight-certified.
Units: public results in SI (Pa, K, m/s, kg/m³); Isp stays in seconds (rocketry standard) plus ve in m/s.

---

Project website

website/index.html — detailed technical site (overview, motivation, architecture, SI units, validation).
Open the file in a browser, or serve the folder:

# from repo root
python -m http.server 8080 --directory website
# then visit http://localhost:8080

For GitHub Pages, point Pages at the /website folder (or copy website/ to docs/ if you prefer the docs branch layout).

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Students & coursework

Start here	Link
Install (lab PCs)	docs/INSTALL.md
How to use (beginner)	docs/how_to_use.md
Learning path	docs/learning/
1-page cheat sheet	docs/cheat_sheet.md
Sample lab assignment	docs/lab_assignment.md
Generate lab pack	propwrap homework kerolox --name YourName

propwrap homework kerolox --name YourName
# → summary.md, assumptions.txt, CSV tables, PNG figures

Presets: Case.student_lab(), Case.booster(), Case.upper_stage().

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What this project is

propwrap is a pip-installable library for defining liquid propellant systems and computing their ideal thermochemical performance:

• Vacuum / sea-level specific impulse (shifting and frozen)
• Effective exhaust velocity (v_e = I_{sp} \cdot g_0) in m/s
• Characteristic velocity (c^*), chamber / throat / exit temperatures
• Product γ, molecular weight, transport properties (SI)
• Density impulse (ρ·Isp) and fair multi-propellant trades
• Custom fuels/oxidizers and multi-component blends
• Structured results (Pydantic), SQLite cache, CLI, dark-theme plots (opt-in)
• Built-in unit converters (convert, propwrap.units)

It is meant to be the shared propellant backbone for other tools — or a standalone desk tool for propellant selection studies.

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Why it exists

Working with CEA is powerful but awkward for modern workflows:

Pain	What propwrap does
Ad-hoc scripts and copy-paste CEA decks	One importable, typed library
English units leak (psia, Rankine, ft/s)	SI public API (Pa, K, m/s, kg/m³) + converters
Dicts / parsed strings as “results”	Pydantic models with readable print(result)
Comparing RP-1 vs CH₄ vs LH₂ at the same O/F	Trades at each pair’s own optimum O/F
“Did I already run this?”	Transparent SQLite cache
Hard to trust numbers	Documented validation suite

Why that matters: a wrong γ, Isp, or mixture ratio propagates into every downstream calculation. Propellant choices are early and high-leverage. A single tested API reduces silent mistakes across a team or project.

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Overview

You (script, CLI, or sibling tool)
        │
        ▼
   Mixture("RP-1", "LOX")          ← propellant pair
        │
        ├── evaluate / scan_of / density_impulse
        ├── compare_propellants([...])   ← fair trades
        ├── registry + blends
        ├── units.convert(...)           ← bar↔Pa, Isp↔ve, …
        │
        ├── cea_backend  → RocketCEA (NASA CEA)
        ├── cantera_backend → frozen γ cross-check
        └── cache → ~/.propwrap/cache.db

In one sentence: define a fuel/oxidizer pair, evaluate it at (O/F, Pc, ε), scan mixture ratio, and compare propellant systems with theoretical metrics in SI.

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Features

• Beginner-friendly API — Mixture.evaluate(), print(result), optional plots
• SI results — Pa, K, m/s, kg/m³; Isp in s + ve_* in m/s
• Unit converters — convert(70, "bar", "Pa"), etc.
• Convenience inputs — pc_bar=70, pc_mpa=7, or pc=7e6 (Pa)
• Legacy aliases — Propellant, performance() (bar) still work
• Workflows — characterize(), compare_propellants(), define_blend()
• Registry — densities, cryo defaults, storability, name aliases
• Blends — multi-component wt% fuels/oxs
• Density-Isp — ρ·Isp curves and rankings
• CLI — propwrap run, scan-of, characterize, compare-pairs, …
• Validation — goldens + regression + physics checks

---

Install

Requirements: Python 3.10+ (3.11/3.12 recommended), Windows / Linux / macOS.

From PyPI (recommended)

python -m venv .venv
# Windows:  .venv\Scripts\activate
# macOS / Linux: source .venv/bin/activate

pip install propwrap

Required dependencies (installed automatically): rocketcea, cantera, pydantic≥2, matplotlib, numpy.

propwrap run RP-1 LOX --of 2.56 --pc-bar 70 --eps 20

From source (development)

git clone https://github.com/shanthoshkv/propwrap.git
cd propwrap
python -m venv .venv
# Windows:  .venv\Scripts\activate
# macOS / Linux: source .venv/bin/activate
pip install -e ".[dev]"
pytest -q

Full walkthrough: docs/how_to_use.md · Lab install notes: docs/INSTALL.md.

---

Quick start

from propwrap import Mixture, compare_propellants, set_defaults, convert

# --- one mixture point (pressure: use pc_bar for ease, stored as Pa) ---
m = Mixture("RP-1", "LOX")
r = m.evaluate(of=2.56, pc_bar=70, eps=20)
print(r)                              # human summary
print(r.isp_vac_shifting, "s")        # Isp
print(r.ve_vac_shifting, "m/s")       # exhaust velocity (SI)
print(r.pc_pa, "Pa", r.pc_bar, "bar")

# --- O/F scan (no plot unless you ask) ---
sw = m.scan_of((2.0, 3.2, 0.1), pc_bar=70, eps=20)
print(sw.summary())
# sw.plot(save="of_scan.png")

# --- unit conversion ---
print(convert(70, "bar", "Pa"))
print(convert(343.7, "s", "m/s"))     # Isp → ve

# --- session defaults ---
set_defaults(pc_bar=70, eps=40)
m.evaluate(of=2.56)

# --- fair multi-pair trade (each at its own best O/F) ---
trade = compare_propellants(
    ["RP-1/LOX", "CH4/LOX", "LH2/LOX", "MMH/N2O4"],
    pc_bar=70,
    eps=40,
)
print(trade)

Legacy names still work: Propellant, performance(of, pc_bar, eps) with bar.

---

CLI

Plots are off unless you pass --plot or --save.

# Pressure: prefer --pc-bar or --pc-mpa; --pc is pascals (SI)
propwrap run RP-1 LOX --of 2.56 --pc-bar 70 --eps 20
propwrap run RP-1 LOX --of 2.56 --pc-mpa 7 --eps 20
propwrap run RP-1 LOX --of 2.56 --pc 7000000 --eps 20

propwrap scan-of RP-1 LOX --pc-bar 70 --eps 20 --range 2.0 3.5 0.1
propwrap characterize CH4 LOX --of 3.0 --pc-bar 100 --eps 25
propwrap compare-pairs --combos "RP-1/LOX,LH2/LOX,CH4/LOX" --pc-bar 70 --eps 40
propwrap density-isp --fuel RP-1 --ox LOX --range 1.8 3.4 0.1 --pc-bar 70 --eps 40
propwrap list --cryogenic
propwrap clear-cache

Add --json for machine-readable output.

---

Metrics (what gets computed)

Metric	Field	Unit	Meaning
Vacuum Isp (shifting)	isp_vac_shifting	s	Ideal equilibrium expansion
Vacuum Isp (frozen)	isp_vac_frozen	s	Frozen composition
Exhaust velocity	ve_vac_shifting	m/s	(I_{sp} \cdot g_0)
Sea-level Isp	isp_sl_*	s	~1 atm ambient
Characteristic velocity	c_star	m/s	Chamber performance
Chamber / throat / exit T	tc_kelvin, …	K
Chamber / exit pressure	pc_pa, pe_pa	Pa
γ	gamma_chamber, …	—
Molecular weight	mw_chamber	kg/kmol
Bulk density	bulk_density_kg_m3	kg/m³	Liquid mixture ρ(O/F)
Density impulse	density_impulse_vac_shifting	s·kg/m³	Isp × ρ
Transport	chamber.cp_j_kg_k, mu_pa_s, …	SI

Inputs (boundary conditions for CEA):

Input	Typical kwargs	Unit
Mixture ratio	of / of_ratio	mass ox / mass fuel
Chamber pressure	pc / pc_pa / pc_bar / pc_mpa	Pa (or bar/MPa helpers)
Expansion ratio	eps	Ae/At (dimensionless)

Pc and ε are thermo boundary conditions for evaluating the mixture — not a full engine design interface.

---

Units (SI public API)

Quantity	SI in results	Notes
Pressure	Pa	.pc_bar convenience view; input via pc_bar= / pc_mpa=
Temperature	K
Velocity / c*	m/s
Density	kg/m³
Isp	s	plus ve_* in m/s
Density-Isp	s·kg/m³
cp	J/(kg·K)
Viscosity	Pa·s

from propwrap import convert, units

convert(70, "bar", "Pa")
convert(7, "MPa", "bar")
convert(300, "s", "m/s")           # Isp → ve
convert(0.81, "g/cm3", "kg/m3")
units.bar_to_pa(70)
units.isp_s_to_ve_m_s(300)

Module reference: src/propwrap/units.py.

---

Validation

Asset	Description
docs/validation.md	Full report: NASA RP-1311, RocketCEA goldens, SI identities
tests/data/validation_catalog.json	Machine-readable anchors + primary sources
tests/test_physics_identities.py	Hard physics: (v_e=I_{sp}g_0), (C_f=v_e/c^*), CEA bit-level match
tests/test_validation_suite.py	Catalog-driven multi-propellant suite

Secured against:

• NASA CEA methodology (RP-1311) via RocketCEA
• RocketCEA published LOX/LH₂ Isp table (readthedocs QuickStart) — machine-precision match
• BIPM standard (g_0 = 9.80665,\mathrm{m/s^2})
• SI bar = (10^5) Pa exactly
• Multi-pair regression + ideal-gas chamber density check

pytest tests/test_physics_identities.py tests/test_validation_suite.py -v
pytest -q

Remember: catalog Isp values are theoretical CEA, not flight Merlin/RS-25 delivered performance.

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Documentation map

Document	Audience
docs/how_to_use.md	Beginners — start here
README.md	Overview (this file)
docs/api_reference.md	API field/method list
docs/validation.md	Validation report
examples/	Runnable scripts
LICENSE	MIT
pyproject.toml	Package metadata

---

Project layout

propwrap/
├── README.md
├── LICENSE
├── pyproject.toml
├── docs/
│   ├── how_to_use.md          ← beginner tutorial
│   ├── api_reference.md
│   └── validation.md
├── examples/
├── src/propwrap/              ← library
└── tests/

Key modules

Module	Role
propellant.py	Mixture main API
units.py	SI converters
workflows.py	characterize / compare / blend
trades.py	optimum-O/F trades, density-Isp
registry.py	propellant identity
cea_backend.py	RocketCEA + English→SI
cli.py	command line

---

Examples

python examples/basic_performance.py
python examples/propellant_trade.py
python examples/lox_ch4_case.py

Script	Topic
examples/basic_performance.py	Single point
examples/of_sweep_lox_rp1.py	O/F sweep
examples/propellant_trade.py	Multi-pair trade
examples/lox_ch4_case.py	Methalox
examples/custom_propellants.py	Custom cards
examples/gamma_profile_for_nozzle.py	γ(ε) data shape

---

Stable API (0.1.x)

Treat these as stable across 0.1.x patch releases:

Surface	Examples
Core	Mixture, Propellant, evaluate, scan_of, pc_bar / pc_mpa / pc
Workflows	compare_propellants, characterize, define_blend, density_isp_curve
Units	convert, propwrap.units
Results	PerformanceResult fields in docs/api_reference.md
CLI	propwrap run, homework, scan-of, characterize, compare-pairs

May still change: Cantera cross-check details, plot styling, η efficiency knobs, cache internals. See CHANGELOG.md.

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Limitations

1. Ideal 1-D CEA — not kinetics, multiphase losses, or flight performance.
2. Shifting vs frozen — reality is between the two.
3. Theoretical ≠ delivered — optional efficiency=(ηc*, ηCf) is rough only.
4. Custom cards — validated for structure, not thermodynamic realism.
5. Cantera γ — approximate major-species frozen compare (Cantera is a required dependency).
6. RocketCEA is GPL-family; this repo’s own code is MIT — review licenses for proprietary products.
7. Not flight-certified.

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Development

pip install -e ".[dev]"
pytest -q
python -m build          # sdist + wheel → dist/
twine check dist/*

CI runs on Ubuntu and Windows for Python 3.10–3.12 (see .github/workflows/ci.yml).

Release steps: docs/RELEASING.md.

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Acknowledgments

• NASA CEA — Gordon & McBride; NASA RP-1311
• RocketCEA — Charlie Taylor / Applied Python
• Cantera

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License

MIT for propwrap source — see LICENSE.
RocketCEA (GPL-family) and other third-party packages keep their own licenses. Installing this stack is not “all MIT.”