Propellant Thermochemistry

BurnSim 4 can compute a propellant's characterization numbers directly from what it's made of, instead of requiring an external tool. Enter the ingredient recipe - oxidizer, fuel, binder, and any additives, by weight - and BurnSim runs a thermochemical equilibrium calculation and fills in C*, Specific heat ratio, and Molar mass for you. This is the same kind of "ProPEP-style" analysis ProPep3 does, but built into BurnSim itself and powered by NASA's CEA thermochemical data rather than ProPep3's own database - so results are close to, but not bit-identical with, what ProPep3 reports for the same formulation. Small differences between the two are expected and not a bug in either one.

This does not replace determining density or the burn-rate numbers a and n - density still comes from the formulation's theoretical or measured value, and a/n still require real test data. See Propellant Characterization for those.

Opening the ingredient editor

On the Propellant panel, click Ingredients... (next to Delete, on the row of propellant buttons). This opens a dialog where you build the recipe for whichever propellant is currently selected.

The Ingredients dialog after entering a 65/35 potassium nitrate/sorbitol recipe and clicking Analyze - the preview on the right shows C*, Isp, flame temperature, specific heat ratio, molar mass, density, and the combustion product breakdown, without having written anything to the propellant yet.

Ingredients are picked from a list, not typed freely, so there's no risk of a misspelled or unrecognized name silently producing a wrong analysis. Each ingredient gets an amount in parts by weight - these don't need to add up to 100. "65/35 KNO₃/sorbitol" and "300g KNO₃, 200g sorbitol" describe the same propellant and BurnSim treats them identically; only the ratio between ingredients matters, not the total. A running total of the entered amounts is shown so you can sanity-check what you've typed. Ingredients marked as condensed combustion products (for example, aluminum oxide from a metallized propellant) are flagged as such - this matters because the condensed fraction changes the flame temperature and gas properties considerably.

Two buttons drive the analysis:

• Analyze computes and previews C*, specific heat ratio, and molar mass for the current recipe without touching the propellant record - use this to experiment with a formulation before committing to it.
• OK does the same computation and then writes the results into the propellant's regular Density panel fields (C*, Specific heat ratio, Molar mass), the same fields Erosive Burning and every ordinary simulation read from. Density itself is not touched - see above.

If the propellant already has values in those fields that weren't generated by this feature (for example, ones you typed in by hand from a ProPep3 report or a test), BurnSim asks before overwriting them, since a value you measured yourself should never be silently discarded. Once a propellant's numbers do come from this feature, later re-running Ingredients... and clicking OK again updates them without asking, as expected for a value that's already computed.

Editing the ingredient table

BurnSim ships with a starter table of common ingredients (oxidizers, fuels, binders, metals, and catalysts), but it's meant to grow. Click Edit ingredients... on the recipe dialog to open a searchable list of every ingredient currently known - the built-in set plus anything you've added - with a form for adding a new one, correcting an existing entry, or reverting a correction back to the built-in value. Changes save immediately; there's no separate "commit" step and no way to lose track of a half-finished edit. The main recipe editor's own ingredient picker is a type-to-filter box, so it stays usable as the table grows past the built-in list.

The ingredient table editor - search or scroll the list on the left, and use New/Save/Revert on the right to add, correct, or restore a built-in entry.

Where the recipe is stored

The recipe and where its numbers came from (when it was generated, from what formulation) are saved as part of the propellant - both in `.bsx` motor files and in the propellant database - so re-opening a motor or propellant later shows the same recipe and lets you regenerate or edit it again. Copying a propellant (Save As...) carries the recipe along with it; editing the copied propellant's characterization fields by hand afterward, without re-running Ingredients, correctly stops BurnSim from claiming those hand-edited numbers were computed by this feature.