Burn rate and BurnSim export

Why burn rate is measured

Thermochemistry cannot predict how fast a propellant burns. RustPEP fits the burn-rate law from your own test data:

r = a · Pⁿ

where r is the burn rate, P is chamber pressure, a is the coefficient and n is the exponent.

Running the tests

Fire several BATES grains with different nozzle throats so they run at different pressures. Enter each firing on the Burn Rate page:

• Grain outside diameter
• Core diameter
• Burn time
• Average pressure

RustPEP computes the web thickness, which is half the difference between the grain outside diameter and the core diameter, and the burn rate, which is the web thickness divided by the burn time. Units can be inches and psi, or millimetres and MPa.

Reading the fit

Three or more firings spread over the pressure range you intend to use give a trustworthy fit. The chart shows your points against the fitted curve.

An exponent near or above 0.6 means the motor will be sensitive to small changes.

Exporting

The Export page gathers density, c*, gamma, molecular weight and your fitted burn rate into one block.

Export	Contents
BurnSim propellant summary	The values to type into BurnSim’s propellant editor. Copy it or save it as text.
CSV	The results for the recipe on the Formulation page.
PDF data sheet	One page with the formulation, key results, nozzle performance, combustion products and burn rate.
PEPCODED ingredient file	The recipe’s ingredients as PEPCODED.DAF records for BurnSim 4 and other PEP-family programs.

Real motors deliver less than the ideal c*, commonly 90 to 97 percent, so adjust with your own test data.

BurnSim is a separate program by its own author. RustPEP is not affiliated with it.