About BurnSim

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BurnSim is a solid rocket motor internal ballistics simulation and grain design program for experimental and amateur rocketry, with support for custom grain shapes, erosive burning simulation, and propellant characterization from real test data. You describe a motor - its grains, nozzle, and propellant - and BurnSim computes the Kn - the ratio of burning surface area to nozzle throat area - through the burn and predicts chamber pressure, thrust, total impulse, and specific impulse over time. Change a nozzle throat or a grain core diameter and the whole prediction updates immediately, so a design can be tuned on screen before any propellant is cast.

BurnSim has been in continuous development since 2000. It is written and maintained by a single author, Greg Deputy, as a personal project - not a company product. It is free to download and try, with a low-cost personal license ($39, unchanged for more than 15 years) for continued full-featured use, and free licenses for student and education (.edu) users. See the download page.

The current version is BurnSim 4, a complete cross-platform rewrite that runs on Windows, Linux, and macOS. This wiki documents BurnSim 4; BurnSim 3 documentation is archived under BurnSim 3 (Legacy).

What BurnSim Does

At its core BurnSim is a steady-state internal ballistics solver. It steps the burn forward in time, regressing every grain's burning surface, computing the burning surface area and therefore the Kn at each step, and solving for the equilibrium chamber pressure that a propellant with the given burn rate coefficients produces at that Kn. From pressure and nozzle geometry it derives thrust coefficient, thrust, total impulse, and Isp.

That means BurnSim answers the questions that actually decide whether a motor is worth building: Will the chamber pressure stay inside the case's limits? Where does the pressure peak, and how progressive or regressive is the thrust curve? Is the port-to-throat ratio large enough to avoid erosive burning? What total impulse and burn time will this motor deliver, and what class does that put it in?

What BurnSim produces is a simulation, not a measurement. Every number it reports rests on the values you feed it - burn rate coefficients, C*, density, geometry - and on models that are approximations of a genuinely complicated process. Real motors depart from their predictions, sometimes substantially, and a design that looks fine on screen can still fail on the test stand. Treat BurnSim's results as a design aid, never as evidence that a motor is safe to fire. Verify designs with instrumented static tests, conducted at a safe distance with proper protection, under the safety code of your rocketry organization and whatever your local law requires, and work up to a new design in steps rather than trusting a simulation on the first burn. Comparing measured data against the prediction is both the best check on a simulation (Test Data Tab) and the best way to improve the propellant numbers feeding it (Propellant Characterization).

Feature Overview

Grain design

  • Any number and combination of grains in a single motor, each with its own geometry and its own propellant.
  • Parametric grain types: BATES (with 0-2 inhibited ends), end burner, tablet (uninhibited BATES), moon/offset core, D grain, C-slot, finocyl, star, X-core, and tapered core.
  • Custom grains - draw your own cross-section in the built-in shape editor, or import one from a DXF, an SVG, or a raster image.
  • Radial (end-on) and axial (side-on) cross-section views, with an erosion slider that animates both so you can watch the grain regress through the burn.
  • Import an existing design from an openMotor .ric file - all eleven of openMotor's grain types, plus its nozzle and propellant.

Propellant

  • A propellant database with burn rate coefficients (a and n), density, C*, specific heat ratio, molar mass, and Isp*, stored per propellant and reusable across motors.
  • Different burn rate coefficients over different pressure ranges, for propellants whose behavior changes across the pressure regime.
  • Built-in thermochemistry - enter an ingredient recipe by weight (oxidizer, fuel, binder, metals, additives) and BurnSim computes C*, specific heat ratio, and molar mass directly, ProPEP-style, using NASA CEA thermochemical data. No external program required. The ingredient table is searchable and extensible, and BurnSim can read an existing ProPEP installation's PEPCODED.DAF so you can bring your own ingredients across.
  • Characterization from test data - fit a and n to the real pressure traces from a series of your own test burns, rather than taking numbers from a table.

Simulation and analysis

  • Live graphing of Kn, chamber pressure, thrust, and mass flux over the burn, with selectable graph lines and axes.
  • Thrust estimates derived from actual nozzle geometry, including nozzle erosion over the burn.
  • Erosive burning - an optional mode that slices each grain along its length and augments the burn rate near the nozzle end where crossflow is highest, instead of applying one rate to the whole grain. Three switchable models (Lenoir-Robillard, Mukunda-Paul, and Ma et al.), off by default.
  • Warnings for the conditions that actually bite - port-to-throat ratio, excessive mass flux, and similar.

Test data

  • Sim vs. measured on one graph - import real test data from a static test and plot it directly against the simulation, predicted and measured curves overlaid on the same axes, so you can see exactly where and by how much a motor departed from its prediction.
  • CSV and clipboard import, with imported data saved into the motor file for later.
  • Data acquisition guidance for instrumenting a static test.
  • Thrust computed from imported pressure data, and .ENG export based on measured rather than predicted data.

Saving, export, and notes

  • Motor designs, propellants, notes, and imported test data all saved together in one motor file.
  • .ENG export for RockSim, wRASP, and other flight simulators - fly the motor in a rocket sim before it exists.
  • CSV export of the full simulation output for analysis in Excel or anything else.
  • Free-form notes attached to propellants and motor designs.

Platforms

BurnSim 4 runs natively on Windows (x64 and ARM64), Linux (x64 and ARM64), and macOS (Apple Silicon and Intel). Windows builds are offered as an installer or a portable zip, Linux as a portable zip, and macOS as a double-clickable app bundle.

History

2000: KnCulator

BurnSim began in 2000 as a small utility called KnCulator - a Kn calculator, and not much more than that. The problem it solved was the tedious one every experimental motor builder had: working out the burning surface area of a grain stack at each point in the burn, dividing by throat area, and then doing it all again every time a dimension changed. KnCulator did that arithmetic.

It did not stay a calculator for long. Once the burning surface was being tracked through the whole burn, predicting chamber pressure from the propellant's burn rate coefficients was the obvious next step, and going from pressure to thrust was the step after that. Within a short time the program had outgrown its name and was renamed BurnSim - it was simulating a burn, not calculating a number.

The early years

Early BurnSim was a Windows program written in Visual Basic. It grew steadily through the 2000s as people in the experimental rocketry community put it to work and sent back feedback: more grain geometries, nozzle modeling, .ENG export so a designed motor could be flown in RockSim or wRASP, and the propellant database. The codebase was carried forward from Visual Basic 6 to VB.NET in 2007.

Development has always been feedback-driven. BurnSim has no roadmap committee - features get built because someone building real motors asked for them, or because the author hit the same limitation himself.

BurnSim 3

BurnSim 3 was the long-lived version that most users know. It brought the graphical display of propellant grains, the ability to mix grains of different propellant types in one motor, varying a, n, and Isp* across different pressure ranges, notes kept alongside propellants and motors, and - significantly - importing real test data and graphing it against the simulation for direct comparison.

Later BurnSim 3 releases added nozzle erosion, batch simulation from the command line, millimeter as well as inch units, thrust curves computed from imported pressure data, CSV test data import, and a Windows installer. Version 3.3.9 (September 2023) added free licenses for education users and pointed the Help menu at this wiki. Version 3.3.10 (November 2025) added grain erosion visualization and was the last BurnSim 3 release; it also began the .NET modernization work that made the next version possible.

BurnSim 3 remains available for download and its licensing continues to work. It is Windows-only and is no longer being developed.

BurnSim 4

BurnSim 4 was released on 22 July 2026 - a complete rewrite from the ground up. The simulation model was rebuilt in C# on modern .NET and the user interface was written from scratch, which finally broke BurnSim's long dependence on Windows: BurnSim 4 runs natively on Linux and macOS as well.

The rewrite was not just a port. Since the 4.0 release BurnSim has gained a graphical grain eroder that regresses arbitrary cross-sections frame by frame rather than relying on closed-form geometry, custom grain shapes drawn or imported by the user, an axial motor cross-section view, erosive burning simulation with three selectable models, built-in propellant thermochemistry powered by NASA CEA data, propellant characterization from test data, and openMotor .ric import.

Development continues. See the BurnSim 4 change history for what is in each release.

Author

BurnSim is written by Greg Deputy, and has been from the beginning. It is a one-person project - all of the code, the simulation model, the website, and this wiki. There is no company and no team behind it; it is supported by the personal licenses users buy.

Bug reports, feature requests, and questions are genuinely welcome, and are the main thing that steers what gets built next. Get in touch through the contact page. News about new releases goes out on the BurnSim Announcements List.

See also

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