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BurnSim 4.0.0.21

Released 9/30/2026. What's new in this release

Windows

Linux

Mac

If you're on Apple Silicon (M1 or later, 2020 onward), use the ARM64 build - it runs natively. The Intel build also runs on Apple Silicon via Rosetta 2, but there's no reason to pick it there. The download is a real BurnSim.app bundle now - unzip it and double-click like any other Mac app (or drag it to Applications first). The build is still unsigned, so macOS will refuse to launch it as downloaded until you clear the quarantine flag with a one-time Terminal step (xattr -cr BurnSim.app, see the README included in the zip). These Mac builds are still new and haven't been broadly tested. If you're a Mac user, we'd really appreciate you trying it out and letting us know (contact us) whether it runs, and any problems you hit.

You may download BurnSim and use it at no cost, with some limitations. Until activated, BurnSim will only run 25 simulations, and you cannot save your files. A personal, non-commercial license is $49.00 and a commercial license is $149.00. Either one is for a single user, on up to five computers at a time. This fee supports the author's efforts and promotes the continued development of BurnSim. Click the registration link above for more information.

Your feedback is very welcome. Please contact us (or email burnsim@burnsim.com directly) with any bug reports, feedback, or questions - it's a big help as BurnSim 4 continues to be refined.

“BurnSim is great! Easy to use, very accurate, versatile, and just plain fun to play with.”

— Terry McCreary · Tripoli Board of Directors · author of Experimental Composite Propellant

Change History

Version 4.0.0.21 - released 9/30/2026

BurnSim is now code signed (new) - the installer, the program and the portable builds are signed as BURNSIM LLC, so Windows no longer reports an unknown publisher and Smart App Control no longer blocks the download
BurnSim tells you when a grain's dimensions cannot describe a real shape (new) - the grain is marked in the list, its cross-section is left blank instead of being drawn wrong, and a message explains what is wrong. It waits until you have finished filling the grain in, so it will not interrupt you part-way through entering one
The grain editor and the grain grid now use the same name for every dimension, so a field is called the same thing wherever you meet it. Several grid columns are renamed to match the editor, among them "Fwd core dia", "Aft core dia", "Fwd fin width", "Aft fin width", "Maj point width" and "Min point width"
BurnSim 4 now has its own program icon, and can take over .bsx files from BurnSim 3 if you let it - it asks once, and Settings > Open .bsx Files with This BurnSim is there if you change your mind later
The Motor and Nozzle panel pairs related fields two to a row, and stacks them into a single column when the window is narrow
BurnSim reopens on the section tab you were last using
The printed motor report now includes everything the Results panel shows, and adds peak mass flow
Education licenses only: BurnSim now reports anonymous usage counts - which features get used, in broad buckets - to help guide what gets built next. It never sends motor data, file names, propellant names, dimensions or anything you type
BurnSim is published by BURNSIM LLC, now named as the vendor in the license agreement and the About box. The agreement also adds clauses covering privacy and the education usage reporting above, so you will be asked to read and accept it once on this version
BurnSim checks for a new version every 7 days instead of every 14
Fixed: in a motor whose grains have different web thicknesses, the burn-depth slider on the axial and 3-D views stopped short of the deepest web
Fixed: a graph rendering timing line could appear on the graph and in the printed motor report

Version 4.0.0.20 - released 9/18/2026

Fixed: exporting an ENG file with metric units selected crashed BurnSim
Fixed: cored end burners (such as a Tapered Core grain) could show enormous single-sample pressure spikes once the core burned out
Fixed: an uncored BATES end burner stopped simulating partway through with erosive burning on, and its burn animation did not play; a grain with 0 ends inhibited now burns from the nozzle end only in the views, as the simulation already modelled
Fixed: on the 3-D tab the burn-depth slider and the drag bar below it were cut off at the bottom of the grain pane
Fixed: an X core grain whose core was larger than its offset was drawn as a small square in the 3-D view
Fixed: the Tapered Core grain's core length field was hidden in the grain editor
Fixed: pressing Enter in a grain grid cell moved the selection to the empty "add a grain" row, leaving the editor showing the grain as it was before the edit
The 3-D view now shows the whole motor as a stack of grains, the same motor the axial view draws, with joint lines between butted grains
The maximum burn time is now a setting (Settings > Set Maximum Burn Time...) instead of a fixed 25 seconds
The New Version Available prompt now lists what changed in each release newer than the one you are running
The grain editor lays its fields out in rows grouped by what they describe, with no gaps where hidden fields used to sit
The measured burn rate grid has clearer, reordered columns with tooltips, and editing the web or burn time recalculates the burn rate
The thermochemistry dialog shows the throat Isp (the figure ProPEP reports as Isp*) alongside the expanded ideal Isp, so the two can be compared directly
Re-running a simulation is much faster, since grain erosion is reused when the geometry has not changed, and 3-D voxel erosion now uses every CPU core

Version 4.0.0.19 - released 9/14/2026

Tapered Finocyl grain type (new) - a finocyl whose core and fins both taper along the axis: the bore runs from one diameter at the head end to another at the mouth, and the fins have their own axial run and a width at each end of it. It is the first grain in BurnSim whose shape is genuinely three-dimensional - axially varying and non-axisymmetric - so it is simulated from a real 3-D voxel erosion rather than a cross-section swept along a line. Web, propellant volume, burning surface and port area all come from that erosion, and Settings > Set Voxel Memory Budget... controls how finely it is resolved
Interactive 3-D grain view (new) - a "3-D" tab beside the axial cross-section shows the grain as a solid you can orbit, zoom and pan, for every grain type. A cutaway plane slices it open through the axis at an angle you choose, the burning surface is drawn in red so a case-bonded wall reads differently from a face that burns, and a burn-depth scrub walks the grain back through its own burn - or plays it back at a speed you pick. The shape you are looking at comes from the same distance transform the simulation runs, so it is the grain BurnSim is actually burning, not an artist's impression
Every grain type now checks its own dimensions - a core wider than its grain, an X core whose arms break out through the wall, a moon or D grain whose cut leaves nothing behind, a star whose points do not fit the grain they are cut into, a finocyl fin as thick as its bore: each is now reported in plain language before it can produce a meaningless curve, instead of throwing or quietly simulating an impossible grain
Igniter charge is reported as a range - the CuO/Al figure sized on burning surface is a model floor, and field practice runs higher, so BurnSim now shows the span rather than a single number that reads more precise than it is
The port-to-throat ratio is colour-coded again the way BurnSim 3 did it - yellow when it is marginal, red when it is genuinely restrictive
The nozzle's convergent cone counts toward free volume, so the free-volume igniter mass is sized on the chamber the motor really has
The Tapered Core grain gets its own columns in the grain grid instead of borrowing two that meant something else
Grain dimensions are shown to three decimals, trailing zeros included, and the recent-files list holds ten motors instead of four
Fixed: on grains simulated with the 3-D engine, Kn min could be reported far below anything the graph actually shows, and erosive burning could report a wildly overstated peak augmentation with it. Both came from the last sliver of propellant at burnout being read as if it were a full step of burn
Fixed: finocyl and X core grains reported the wrong amount of propellant - the reported mass now comes from the same geometry the simulation burns
Fixed: typing a decimal point into the burn rate data grid no longer swallows it, and the erosion preview slider no longer freezes the window on a large grain
Fixed: the macOS downloads were missing their execute permission, so unzipping BurnSim.app and double-clicking it could report "cannot be opened" instead of launching

Version 4.0.0.18 - released 8/31/2026

Nozzle and altitude modeling reworked - ambient pressure can now be entered as an elevation instead of a pressure, and a "Climbs during the burn" option lets ambient pressure fall through the burn the way it does on a real flight. The standard atmosphere is anchored on BurnSim's own 14.7 psi sea level so a zero-elevation motor simulates exactly as it always has
Divergence angle now affects thrust - the nozzle's divergence half-angle applies a real divergence loss to Cf rather than only being drawn in the cross-section, and nozzle efficiency is applied to the momentum term where it belongs. Both defaults were reconciled so existing motors keep their numbers
"Find best nozzle exit diameter" is back - the BurnSim 3 nozzle optimizer returns, now solving the expansion ratio properly instead of rounding it to a whole number, and able to sweep across a climb rather than reporting the answer for one fixed altitude
New toolbar - the common actions (new, open, save, simulate, print, export, characterize) sit on a toolbar under the menu bar, along with the erosive burning model selector, which moved out of the panel below
Opening a motor file now simulates it straight away, instead of waiting for you to press Simulate
A characterization result can be saved back onto the propellant it was based on, and BurnSim warns you when a fit does not actually reproduce the trace it was fitted to
Characterizing against a pressure trace that never rises above atmospheric is now refused with an explanation rather than producing a meaningless fit
Deleting rows off the top of a test data set rebases the remaining times to zero, so the trace starts where you trimmed it
BurnSim 4 now keeps its own propellant database file, so running BurnSim 3 and BurnSim 4 side by side no longer risks BurnSim 3 stripping the newer fields (C*, molar mass, ingredients, measured burn rate data) out of shared entries
On Windows, an available update can be downloaded and installed from inside BurnSim instead of sending you to the download page
The graph gets its full width back on a small screen, the nozzle optimizer's result text no longer overlaps its graph, and every dialog window carries the BurnSim icon

Version 4.0.0.17 - released 8/27/2026

Support for propellant characterization with measured burn rate data (new) - BurnSim now keeps a record of the burn rates you have actually measured, firing by firing, against the propellant itself. A new "Burn Rate Data..." button on the Propellant tab lists every stored firing with its pressure, rate, web, burn time, throat diameter and where it came from, fits a and n across them, plots them on a log-log scatter, exports the set to CSV, and can write the fitted a and n straight back into the propellant. See the wiki for the full walkthrough
"Measure and Add Point" - a new button in the Characterize window that records the one burn rate a single static firing can actually establish: web thickness divided by burn time, at the mean chamber pressure over that burn. No simulation is involved in either number. Collect one from each firing at a different throat diameter and the a/n fit becomes properly determined
Fit Burn Rate Data reworked - it no longer runs a simulation to fit against. The burn depth is integrated forward through your measured pressure trace instead, and a is solved with n held at the propellant's current value, which is the honest answer from one firing. The floating-exponent fit is still reported underneath as a sanity check, and the pressure span the exponent was measured over is shown next to R² - a fit can return an impossible exponent at an R² above 0.99, so the span and the plain-language caveat are the numbers to read
Import the whole trace - BurnSim finds the ignition rise and the tail-off itself and sets them aside before fitting, so there is no longer any reason to hand-trim a test trace before importing it. The convention used to mark the start and end of a burn is selectable under Settings > Measured Burn Time (bisected tail-off, 10% of peak, 5% of peak, or your own percentage)
Measured points travel in .bsx motor files as well as in your propellant database, and the two merge when a motor file is loaded - BurnSim tells you what moved, and re-adding the same firing is detected rather than silently duplicated
Propellant notes - the propellant Notes field finally has somewhere to type it, on a new tab beside the propellant fields. Notes already in your database from BurnSim 3 show up there
A burn rate vs pressure graph is now drawn beside the propellant fields, out to 1500 psi
The Results panel reports the port/throat and throat/port ratios, and an estimated CuO/Al thermite igniter mass
Test data rows can be deleted in bulk instead of one at a time, the graphed test data set is remembered across save and load, and the whole imported set is plotted rather than only the span the simulation covered
Fixed the trial simulation limit being reported as a licensing failure

Version 4.0.0.16 - released 8/21/2026

openMotor import (new) - open an openMotor .ric design directly in BurnSim. All eleven of openMotor's grain types are read (BATES, D, Moon, X-Core, C-Slot, Finocyl, End Burner, Conical, Rod and Tube, Star, and custom), along with the nozzle and propellant, and assembled into a motor and a propellant database entry
Ingredient table nearly doubled - the thermochemistry ingredient list goes from 40 entries to 77, adding the common binders (HTPB R-45HT, CTPB, PBAN, PMMA, epoxy, PTFE, Viton A), the sugar alcohols (xylitol, mannitol, erythritol), IDP and MDI (143L), and a wider set of metals, oxides, carbonates and refractory carbides. As before, every entry records where its numbers came from
Import ingredients from ProPEP (new) - point BurnSim at your own installation's PEPCODED.DAF and it lists every substance it can read, flagging which ones your table already has and which contradict another row in the same file, so you choose what to bring across row by row rather than all at once. Existing entries are never overwritten, and everything imported goes to your own overlay file where it can be edited or deleted later. See the wiki for a walkthrough with screenshots
Check for Updates - a Help menu item that tells you whether a newer BurnSim is available, plus a quiet check at startup
Average Kn and average thrust are now reported in the Results panel alongside the peak values
The right-hand column's three sections can be collapsed, and the left/right splitter position is remembered between runs
Save As now starts with the motor's own name instead of a blank box
Nozzle cone geometry - throat length, convergence and divergence half-angles - can be recorded and is drawn in the axial cross-section view
Fixed the Kn spike and incorrect KNMin the grain eroder reported at the very end of a burn

Version 4.0.0.15 - released 8/19/2026

Propellant thermochemistry (new) - enter a propellant's ingredient recipe and BurnSim computes its C*, specific heat ratio, and molar mass from first principles (ProPEP-style analysis, powered by NASA CEA thermochemical data), from a new "Ingredients..." button on the Propellant tab
A searchable ingredient table editor lets you add, correct, or revert entries without hand-editing any files; the recipe editor's own ingredient picker is now type-to-filter instead of a plain dropdown
New Tapered Core grain type - an end burner with a straight or tapered core drilled part-way or all the way down its axis, including support for erosive burning's axial-slice simulation
Added Ma et al. (2020) as a third erosive burning rate model, alongside Lenoir-Robillard and Mukunda-Paul
Simulations can now be set to end at a chosen percentage of max thrust instead of always running out the tail
macOS builds now ship as real BurnSim.app bundles - unzip and double-click like any other Mac app, instead of running from Terminal
Fixed several propellant-conflict/compare dialogs sometimes needing two clicks to register a button press
Fixed the grain grid's Propellant column sometimes staying blank after loading a file from the compare dialog
Nozzle throat erosion now correctly reduces vacuum thrust coefficient as the throat erodes over a burn
Added a crash notification dialog, and hardened narrow-window layouts against field clipping/overflow
Several smaller accuracy and export fixes: unit conversions, culture-invariant .bsx save/load, CSV export formatting, motor classification percentages

Version 4.0.0.13 - released 8/10/2026

Added erosive burning - an opt-in simulation mode (Settings > "Simulate Erosive Burning (Use Axial Grain Slices)") that slices each grain along its length and computes burn rate per slice, so the rate can increase near the nozzle end where gas velocity across the grain surface is highest, instead of using a single rate for the whole grain. Off by default - existing motors and simulations are unaffected unless you turn it on
Two switchable erosive burn-rate models (Settings > Erosive Burning Model): Lenoir-Robillard (the default) and Mukunda-Paul
The Propellant editor gained a separate Erosive Burning tab for the override fields the models use, keeping the main Properties tab uncluttered for propellants that don't need them
The Results panel now reports whether erosive burning actually had any effect on a run (peak augmentation over the nominal burn rate), not just whether the option was on; also added a Kn Eff graph series and CSV column, and Kn min, propellant length, and initial core volume readouts
Added a Settings option to set the simulation timestep
Fixed a bogus, very large "peak erosive augmentation" reading that could appear right at grain burnout
Fixed the grain grid's Remove button only deleting one grain instead of all of them when several grains were selected at once
Fixed a multi-grain delete sometimes appearing to do nothing after clicking OK on the confirmation prompt - pressing Enter to confirm was landing on "No" instead of "Yes"
Ends Inhibited is now a dropdown (0, 1, or 2) instead of free text, in both the grain editor and the grid, and defaults to 0 for new grains

Version 4.0.0.12 - released 8/5/2026

Added a Custom grain type: draw or import your own grain cross-section instead of picking a parametric shape. Import from an ASCII DXF, SVG, or raster image (PNG/JPEG/BMP), or build one from scratch in the new shape editor - an outer boundary plus repeatable holes and slots you can drag directly in the preview - and export back to DXF when you're done. This feature is brand new - raster-traced shapes carry a little geometry error and thin "neck" shapes aren't fully validated yet, so double-check anything unusual before trusting the numbers
Added an axial (side-on) motor cross-section next to the radial preview, showing the grain stack, case, and nozzle for any grain type, with the same erosion slider animating both. Case/nozzle dimensions beyond throat and exit are estimated rather than modeled
Fixed a rasterization bug inflating a D grain's reported peak Kn by roughly 11% - other grain types were unaffected
Removed the Pie Segment grain type (it's no longer offered when adding a grain); a motor file that already has one still opens and explains what happened, instead of failing with an unrecognized-type error
Grain geometry the simulator can't actually represent (e.g. an oversized finocyl fin) is now flagged directly in the grid instead of silently simulating something else
Grains left with no burning surface (e.g. an end burner with both ends inhibited) are now reported instead of crashing the simulation
Sped up simulations where several grains share the same shape by eroding each distinct shape once instead of once per grain
Fixed New, Open, and Recent Files immediately discarding an unsaved motor instead of asking whether to save it first, the way closing the program already did
Fixed an imported custom grain shape occasionally being silently discarded instead of applied to the grain
Fixed the grain editor's toolbar overlapping the grain grid when the window was narrow

Version 4.0.0.11 - released 7/30/2026

The grain grid is now a full in-place editor - every grain parameter has its own editable column (including Offset, Fin length and Points), so a motor can be built end to end in the grid without using the fields below it. Ambiguous column headers carry tooltips explaining what they mean for each grain type, and parameters a grain type doesn't use are left blank instead of showing 0
Grain Type and Propellant are now single-click dropdowns in the grid - no more double-clicking to reach the list
Typing on a selected cell now starts an edit, the way a spreadsheet does. Previously the only ways in were F2 or a double-click, so the grid looked like it wasn't editable at all
Added a "click to add a grain" row at the bottom of the grid, and an "Add the first grain" prompt on an empty grid, so grains can be added without knowing about the Insert key. Pressing Enter or Space on that row adds a grain too
Insert, Delete and Move Up/Down are now available on a right-click context menu in the grid, and Delete can remove several selected grains at once
Fixed arrow keys silently rewriting a grain's Type or Propellant while moving through the grid - arrowing down a column could change every grain's type without the dropdown ever opening. Arrow keys now move between rows, and Alt+Down, Alt+Up or F4 open the dropdown
Fixed an unescapable dialog loop: closing the program with a throat diameter of 0 stacked the "Simulation Problem" alert on top of the unsaved-changes prompt, and the two traded focus so neither could be dismissed. More generally, automatic re-simulation no longer raises error popups or moves the cursor - only the Simulate button does
Added Save As to the propellant editor - saves what's currently shown as a new propellant under a different name, leaving the original untouched. Useful for deriving a new batch from a characterized one, since the copy keeps the notes and pressure-varying data the editor has no fields for
Added Delivered Isp to the Results tab
On the graph, Thrust From Imported Pc is now genuinely calculated from the imported chamber pressure, so it can be compared against the measured thrust trace. Previously the two series drew the same line whenever the imported file supplied a thrust column
Fixed values derived from imported test data continuing to compute from the last measured pressure after the imported data ended, which drew phantom thrust in a region where the measured pressure read zero. This also corrects the test impulse for any run that outlasts its test data
Imported test data columns are back in File > Export CSV - they had been silently dropped from every export. Measured thrust and thrust calculated from imported pressure now export as two separate columns, and the imported pressure/thrust columns follow the Imperial/Metric setting with the unit shown in the header
Fixed importing test data from a CSV with a header row, a units row or a trailing note - those lines became a bogus 0,0,0 sample at t=0. Quoted fields are now handled as well
Every dialog now has Alt-key shortcuts on its buttons and puts the cursor where you'd expect on open, so text-entry dialogs can be typed into without clicking first
The divider between the grain grid and the graph now keeps its position between launches

Version 4.0.0.10 - released 7/27/2026

Grains can now be multi-selected in the grain grid (ctrl-click, shift-click, or the new Select All button); editing the fields below the grid and clicking Apply updates every selected grain at once, instead of one at a time
Added Web thickness to the Results panel
Peak mass flux and peak mass flow now show their units and follow the Imperial/Metric setting - lb/in²-sec and lb/sec in imperial, kg/m²-sec and kg/sec in metric. Previously both were shown as unlabeled numbers that stayed in imperial units regardless of the setting. This applies in the Results panel, on the graph, and in the PDF motor report
On the graph, Mass Flux and Mass Flow each get their own scale on the right-hand side, with units shown in the legend, so neither curve flattens the other

Version 4.0.0.9 - released 7/22/2026

Cross-platform release: BurnSim 4 is now available for Windows, Linux, and macOS (previously Windows-only), with a native Windows installer and Linux desktop-entry packaging
Added win-arm64, linux-arm64, and macOS (Apple Silicon) builds
Replaced the graphics engine with SkiaSharp for consistent rendering across platforms
Added an editable data grid to the Test Data tab; decluttered the tab (dropped Molar Mass from the Propellant UI, hid imported-series checkboxes when not in use)
Added a separate Imported Thrust trace, distinct from the calculated one
Added light and dark theme
Fixed the "Use in simulation" checkbox only re-plotting stale data instead of re-simulating
Fixed long thrust tailoff not resolving correctly (thrust coefficient floor issue)
Fixed the burn loop not stopping once thrust had genuinely ended
Added a warning in the Results text when nozzle efficiency is below 75%
Added Print/Report: a one-page PDF motor report, with direct printing support
Added a play/stop button to loop the grain erosion visualizer
Numeric fields are now rounded to 4 decimals when saving .bsx files and the propellant database
Added a per-field unit override to lock Total Impulse to Newton-seconds
Fixed cored Tablet grains simulating identically to BATES grains, fixed the Tablet erosion preview, and fixed a crash simulating a solid (uncored) Tablet grain
Fixed a graph rendering glitch causing runaway padding/overflow at the end of a burn
Made the Grains/Graph panes vertically resizable, added a scrollbar to the grain grid
Moved the motor Notes field to its own tab, alongside Results/Test Data
Fixed simulation results silently going stale when a field was edited mid-run
Fixed several usability gaps when starting a new motor from scratch
Added a per-run activity log to help diagnose bug reports
Fixed the installer defaulting to Program Files (x86)
Added right-click pickers on the throat diameter field (1/64" increments and Loki Research nozzle sizes), working regardless of unit mode