RustPEP documentation
Understanding the results
What each number RustPEP reports means, and how far to trust it.
The headline numbers
| Result | Meaning |
|---|---|
| Specific impulse (Isp) | Seconds of thrust per unit weight of propellant, for an ideal nozzle expanding to the exit pressure. |
| Characteristic velocity (c*) | How much chamber pressure a propellant makes per unit of mass flow through the throat. This is the number BurnSim needs. |
| Chamber temperature | The adiabatic flame temperature at the chamber pressure. |
| Density | Ideal density with no voids. Cast propellant is usually 1 to 5 percent lower. |
| Gamma | The ratio of specific heats of the chamber products. |
| Optimum expansion ratio | Nozzle exit area over throat area that expands the gas exactly to the exit pressure. |
| Density impulse | Specific impulse times density. It matters when motor volume, not propellant weight, is the limit. |
Frozen and shifting Isp
RustPEP reports two specific impulse values. The shifting value assumes the gas keeps reacting as it expands through the nozzle. The frozen value assumes it does not. Real motors land between them and below both.
Performance detail and combustion products
Under the result tiles, Performance detail and combustion products opens the full breakdown: chamber and exhaust conditions, and the species present in each.
Choosing a product database
| Database | Use it when |
|---|---|
| NASA CEA (recommended) | You want the larger, more current species set. This is the default. |
| Legacy JANNAF 1986 | You want to reproduce the original PROPEP program’s results. |
Ideal versus delivered
Everything on this page is an ideal figure. Real motors deliver less than the ideal c*, commonly 90 to 97 percent. Adjust with your own test data before you depend on a prediction.