BurnSim Fields

This is a field-by-field reference for BurnSim 4's main window. For a walkthrough of building your first motor, see Getting Started with a Basic Simulation.

BurnSim 4's main window - grains and graph on the left, motor/nozzle and propellant on the right

File Menu

• New (Ctrl+N) - start a new, empty motor
• Open... (Ctrl+O) - open a saved .bsx motor file
• Recent Files - the last few motors you've opened
• Import openMotor (.ric)... - import a motor designed in openMotor, converting its grains and nozzle into a BurnSim motor
• Save (Ctrl+S) / Save As... - save the current motor to a .bsx file
• Save Copy for Sharing... - writes a separate .bsx to send to someone else, leaving out what you choose: test data (and any embedded recordings), snapshots, notes, measured burn rate points, and the propellant's ingredient list. The design and the propellant constants it simulates with always go in, so the copy still simulates. The motor you have open, and its file, are unchanged. Added in 4.0.0.24
• Export ENG... - export the burn to a .eng file for use in flight simulators
• Export CSV... - export the full simulation trace (time, Kn, pressure, thrust, mass flux, etc.) to CSV
• Export openMotor (.ric)... - the other half of Import openMotor: writes the motor as an openMotor design. See below. Added in 4.0.0.24
• Print / Report... (Ctrl+P) - generate a PDF motor report

openMotor export

File → Export openMotor (.ric)... writes the open motor as an openMotor .ric file. Like every export it needs an activated license; a trial cannot export.

Some motors cannot be expressed in openMotor and are refused with a message saying why: a Tablet, Tapered Finocyl or Revolved grain, a Tapered Core whose core stops short of the head end, a custom shape that is not a round grain with holes in it, a motor using more than one propellant, and grains with characterization overrides.

A few things are folded in rather than carried across, because openMotor has no field for them: C* efficiency goes into each propellant tab's combustion temperature, gaps between pressure bands are filled with the Properties values, and a propellant with no molar mass is written at 23 g/mol with its temperature derived from that. Chamber ullage, the ignition condition and a flat throat erosion rate are not carried at all.

openMotor models its own losses (skin and throat aspect) and burnout thresholds differently from BurnSim, so expect the same motor, not identical numbers.

Edit Menu

• Undo (Ctrl+Z) - step back one change to the motor design
• Redo (Ctrl+Y) - re-apply a change that was undone

One step is one finished edit - a field commits when you leave it - and history lasts for the session only. Changes to the propellant database are deliberately not undoable. See Undo, Redo and Snapshots for what is and is not covered.

Action Menu

• Simulate (F5) - run the simulation now. Usually not needed - see Auto-Simulate below.
• Characterize Propellant... - fit burn rate coefficients a and n to real static test data; see Propellant Characterization
• Find Best Nozzle Exit Diameter... - sweeps exit diameter and simulates the motor at each one, to find the diameter giving the most total impulse; see below

Toolbar

A row of one-click buttons under the menu bar, restoring the toolbar BurnSim 3 had. Left to right: New, Open, Save | Export ENG, Export CSV, Print / Report | Simulate, Characterize Propellant, Find Best Nozzle Exit Diameter | Erosive burning mode. Every button is a second route to the menu item of the same name, not a separate action - hover for a tooltip naming it.

The Erosive burning combo is the one labelled control on an otherwise icon-only bar, and is a second view of the Settings menu's Simulate Erosive Burning checkbox and its model submenu rather than a third setting - the two stay in sync however you set it. None is the checkbox off; picking any model turns it on. It sits here because it is a setting people flip back and forth while comparing runs, and the menu pair is two levels deep and shows nothing about the current mode until you open it.

The × at the right-hand end hides the toolbar; Settings → Show Toolbar brings it back.

Settings Menu

• Auto-Simulate on Changes - when on (the default), BurnSim re-runs the simulation automatically as you edit grain, nozzle, or propellant fields
• Show Toolbar - show or hide the toolbar under the menu bar
• Log Activity for Bug Reports - writes a per-run activity log; use Open Log Folder... to find it if you need to attach one to a bug report
• Open .bsx Files with This BurnSim - Windows only. Makes this copy of BurnSim 4 the program that opens .bsx files when you double-click one, for machines where BurnSim 3 claimed the file type first. A per-user setting, so it needs no administrator rights
• Edit Thermo Calc Ingredients... - the table of ingredients every propellant's thermochemistry recipe is built from. Add an ingredient BurnSim does not ship, correct a heat of formation or density, revert a correction, or import from ProPEP. Not a propellant's own mix - that is the Ingredients... button on the Propellant panel. See Editing the ingredient table
• Use Eroder for Surface Area - on (default): surface area is computed from the pixel-based grain eroder, which correctly handles complex or irregular cores. Off: use the older analytic geometry formulas instead
• Simulate Erosive Burning (Use Axial Grain Slices) - off by default. When on, each grain is divided into axial slices and burn rate is locally augmented by an erosive burning model reacting to mass flux down the port - see Erosive Burning Overrides below. This checkbox and the Erosive Burning Model choice below it are mirrored by the Erosive burning combo box next to the graph (see Graph) - the two stay in sync however you set it, and the main-window combo is the quicker way to switch while comparing runs
• Set Slice Count... - how many axial slices each grain is divided into when the above is on. More slices resolve a tapered port or burn-through more finely, at the cost of simulation speed
• Erosive Burning Model - which correlation the sliced simulation uses:
  • Lenoir-Robillard - the model motor designers traditionally fit test data with. Needs a per-propellant blowing coefficient (beta) and derives its heat-transfer coefficient from thermochemistry, so the propellant needs its Specific heat ratio and Molar mass set (C* is always required regardless of model - see Erosive Burning). Always augments to some degree, with no threshold
  • Mukunda-Paul - a universal correlation fit against a broad set of published test data, with no per-propellant constant to characterize - it needs nothing beyond a/n/density plus gas viscosity, and skips the Specific heat ratio/Molar mass requirement above. Has a hard threshold: below it, there's no augmentation at all
  • Ma (2020) - zero fitted erosive constants at all (not even universal ones like Mukunda-Paul's two) - derives a real convective heat-transfer coefficient from gas/propellant properties instead. Like Lenoir-Robillard, needs Specific heat ratio and Molar mass set. See Erosive Burning for the full picture
• Measured Burn Time - how the start and end of a burn are read off an imported pressure trace. This sets the burn time that measured burn rate points are divided by, so it is not a presentational choice: published conventions disagree by several percent on the same trace, and burn time divides straight into the reported rate. See Where to set the burn time
  • Bisected tail-off (the default) - the standard in professional SRM work and the usual definition of web burn time: the burn starts where the ignition rise passes half the pressure at its steepest point, and ends where the tail-off falls through half the pressure at its steepest point. Least sensitive to how fast a particular motor decays
  • Knees - the burn starts where the ignition rise's tangent meets the plateau and ends where the plateau meets the tail-off's tangent, covering only the steady part of the burn. Shorter than the web burn time whenever the tail lingers, so on such a trace it reads the burn rate high
  • 10% of peak pressure - first crossing up and last crossing down through 10% of peak. Cruder, but unambiguous and easy to reproduce in a spreadsheet
  • 5% of peak pressure - as above at 5% of peak, which measures a longer burn and therefore a slower rate
  • Custom percent of peak... - your own threshold, as a percentage of peak pressure
• Set Simulation Timestep... - the simulation's internal time step in seconds (0.01s default). Smaller values are more precise at the cost of simulation speed
• Set Voxel Memory Budget... - how much memory a Tapered Finocyl grain's 3-D erosion may use (64-8000 MB, 512 by default). Raise it for a large grain with fine fins, which at the default can be refused as under-resolved. Affects no other grain type
• Set Maximum Burn Time... - a safety stop: a simulation that runs past this many seconds of simulated time is ended with a warning (25 seconds by default). Raise it for long-burning motors such as end burners. The setting is saved with your other preferences and works the same on Windows, macOS and Linux - there is no registry value to edit, which is what BurnSim 3 required. Added in 4.0.0.20: earlier BurnSim 4 builds are fixed at 25 seconds, and their warning's mention of the registry is wrong - update via Help → Check for Updates...
• Theme - Dark or Light
• Units - Imperial (in, psi, lbf, lbm) or Metric (mm, kPa, N, g); Always show Total Impulse in N-sec overrides just that one Results line regardless of the Imperial/Metric choice, since motor-class designations ("H128" etc.) are always in Newton-seconds

Help Menu

• BurnSim Wiki... - opens this wiki in your browser
• Activate... - enter or check your license
• Check for Updates... - asks burnsim.com whether a newer build exists. On Windows it can download and install the update in place; on other platforms it points you at the download page. The prompt lists what changed in every release newer than the one you are running, newest first
• 'About...

Panes

Every section of the window - Grains, Grain Editor, Cross-Section, Graph, Motor & Nozzle, Propellant and Results - has the same orange header bar. Click a header to fold that section away and click it again to bring it back. Drag bars between sections resize them: under the grain list (list against graph), under the pictures, and under Motor & Nozzle and Propellant, where the Results panel takes up the difference. Double-click a drag bar to return its section to its natural height. Every fold and size is remembered between sessions, as are the dividers on the Snapshots tab. Added in 4.0.0.24.

Grains Grid

One row per grain in the motor, in burn order. Columns are Type, Length, Dia, the core and fin dimensions, Inhib. (number of inhibited ends, 0-2), and Propellant - which fields apply depends on the grain type selected, and the dimension headers relabel themselves to match the selected grain's type (Maj point width/Min point width for a star, Tip core dia for a Tapered Core, # fins for a finocyl, and so on). Hover over a header for what it means for that type. Click add a grain or use the Grain Editor below to add one; select a row to edit it.

Pressing Enter in a grid cell commits the edit and stays on that row. Selecting the empty add a grain row blanks the editor, the cross-section and the 3-D view until a real grain is selected again.

Grain dimensions are shown to three decimal places, including trailing zeros - a 2" grain reads 2.000 - so a column of dimensions lines up on its decimal point. A value finer than a thousandth keeps its extra digits rather than being rounded away, so what the grid shows is always exactly what the motor holds.

Grain Editor

Below the grid, an editor for the selected grain(s):

• Add - adds a new grain with the fields below
• Remove - deletes the selected grain(s)
• Up / Down - reorders the selected grain within the motor
• Select All - selects every grain, so a field change and Apply updates them all at once
• Apply - writes the Type/Length/Diameter/Core dia/Ends inhib./Propellant fields back to the selected grain(s)

Grid cells can also be edited directly, without going through this editor.

The editor's fields are laid out in rows: Type, Propellant and Ends inhib. first, then Length and Diameter, then the selected type's own fields, grouped by what they describe (a star's point count beside both point widths; a finocyl's core, then its fins). Only the fields the type uses are shown.

Below the editor, a Cross-Section preview shows the selected grain's end view (left) and, on the right, two tabs: Along axis, the whole motor's longitudinal cutaway, and 3-D (below). The Burn depth slider and Play button scrub through the burn, and the end view, the axial cutaway, the 3-D solid and the graph's playhead all follow it, so all of them show the same instant of the same burn. The dropdown beside the slider sets how long a playback takes.

The drag bar under the pictures sets how much of the window they get; double-click it to give them everything the grain list and graph can spare, and double-click again to put them back. (Until 4.0.0.24 there was a View size slider and a Fill button above the pictures for this, and the drag bar did not actually respond to the mouse.)

3-D view

The 3-D tab shows the whole motor as a solid: every grain in the grid, stacked nose to tail in burn order - the same motor the Along axis tab draws. Grains are butted with no gap, and a dark line marks each joint so a stack of identical grains does not read as one long grain. The shape comes from the same distance transform the simulation runs, so it is the grain BurnSim is actually burning.

• Drag to orbit, mouse wheel to zoom, middle-drag (or Shift-drag on a trackpad) to pan. The +/− and arrow buttons do the same in steps, and Reset view (or the Home key) points the camera back at the whole motor.
• Section cuts the motor in half on a plane through its axis so the bore and fin roots can be seen; the cut face is drawn solid, so web thickness reads true. The slider beside it turns the cutting plane - it is fixed to the grain, so orbiting turns the cut away from you, and sweeping the slider brings it back.
• The burning surface is drawn in red, so a case-bonded or inhibited face reads differently from one that burns. Moving the Burn depth slider burns the solid back.

Motor and Nozzle

• Throat dia (in) - nozzle throat diameter. This is the number Kn is measured against, and the strongest single lever on chamber pressure.

Right-click the field for two shortcuts, both of which save doing the arithmetic yourself:

• 1/64" increments, grouped 1-16, 17-32, 33-48 and 49-64 - the sizes throats are actually drilled and reamed to. Picking one replaces only the fractional part of the value and keeps the whole inches, so a 1.25" throat becomes 1.078125" when you pick 5/64", while a value already under an inch is replaced outright.
• Loki Nozzle #'s, by motor size (29mm, 38mm, 54mm, 76mm, 98mm, 114mm). A Loki nozzle number is the throat diameter in 64ths, so #24 is 24/64" = 0.3750". For 38mm, 54mm and 76mm the entries also fill in Exit dia, because Loki publishes exit diameters for those three; 29mm, 98mm and 114mm have no published exit figures, so those entries set the throat only and leave Exit dia alone rather than inventing a number.

Both menus always read in inches, since both catalogues are inch-based, but the value written into the field is converted to whatever units are currently on screen - so the picker is just as usable in Metric mode.

• Exit dia (in) - nozzle exit diameter. With the throat above it sets the expansion ratio, and so the thrust coefficient, whenever Thrust method is Nozzle geometry. Action → Find Best Nozzle Exit Diameter... will solve for it - see below

Left blank, or no larger than the throat, it is treated as a bare throat - a real nozzle with an expansion ratio of 1, which gives a Cf of about 1.2. Changed in 4.0.0.24: earlier versions dropped the exit-plane and back-pressure terms for such a motor and simulated it at a Cf of about 0.69, so a motor with no exit diameter now reports roughly 76% more thrust and impulse than it did. That is the fix, not a regression. Motors with a real exit cone are unaffected

• Other Cf losses (%) - the lumped nozzle efficiency: the fraction of ideal momentum thrust actually delivered, covering the losses BurnSim does not model individually - two-phase flow (condensed metal oxide lagging the gas, usually the largest term on an aluminized propellant), boundary-layer friction, recombination lag, and heat lost to the walls. Divergence loss is not included - it has its own field under the nozzle cone geometry below. 95% is a reasonable starting point for a well-made hobby nozzle and is the default; 90-96% is the usual range

Changed in 4.0.0.18. This field was called Efficiency (%) and behaved differently in two ways. It scaled the whole of Cf including the ambient back-pressure term - which is negative, so a less efficient nozzle paid less of the sea-level penalty; it now multiplies the momentum term only. And its default was inconsistent: a new motor started at 85% while a .bsx file that carried no efficiency attribute loaded at 99%, a 16% thrust difference decided by nothing but the age of the file. Both now start at 95%. If you are carrying an older motor forward at 85% because that allowed for a conical nozzle, raise it back toward 95% once you enter a divergence angle, or the same loss is counted twice

• C* efficiency (%) - combustion efficiency: the share of the propellant's theoretical C* this motor actually delivers. Real motors fall short of the ideal through incomplete combustion and heat lost to the walls - more so the smaller the motor, the shorter the residence time and the lower the chamber pressure. Small amateur motors typically deliver 85-95%; 100% (the default) reproduces the ideal. It lowers chamber pressure and mass flow, and with them Isp. Fit Burn Rate Data measures it from a static test firing and offers to set it. Added in 4.0.0.24
• Ambient (psi) - back pressure the motor fires against
• ...or elevation (ft) - the same number expressed as field elevation, which is what most people actually know. Type either box and the other follows, using the US Standard Atmosphere (sea level 14.70 psi, 5,000 ft 12.23, 10,000 ft 10.11, 50,000 ft 1.68)
• Climbs during the burn - lets back pressure fall as the motor climbs instead of holding it at pad elevation for the whole burn. Ticking it reveals a Burnout elev. (ft) box; elevation is interpolated linearly between the two and pressure read off the standard atmosphere at each step. A burnout elevation below the pad is legal - a motor lit on the way down - and simply costs impulse instead of buying it. The Results tab reports the pressure range a ramped run actually covered

A straight-line climb is an approximation. A real rocket accelerates, so it spends more of the burn low down than this assumes, and the gain is therefore slightly optimistic; doing it properly needs mass, drag and a trajectory, which BurnSim does not carry. It is still far closer than assuming the whole burn happens on the pad

• Thrust method - how thrust is derived; Nozzle geometry computes it from the throat/exit dimensions above, Given Cf uses the thrust coefficient you type into the next box. A hand-entered Cf is scaled by Other Cf losses as a whole, having no momentum and pressure terms to separate
• Throat erosion (in/sec) - rate the nozzle throat diameter grows during the burn. 0 (default) disables throat erosion. This is nozzle material erosion, not the propellant burn-rate effect covered by Erosive Burning - the two are unrelated despite the similar name
• Erosion coeff (in/sec/psi) - pressure-linked throat erosion: the rate the throat recedes per unit of chamber pressure, so erosion is fastest at peak pressure rather than constant through the burn. 0 (default) disables it. Added in 4.0.0.23. Typical values are small - of the order of 1e-5 - so expect to see scientific notation
• Slag coeff (in*psi/sec) - molten residue accumulating in the throat and narrowing it, which raises chamber pressure. 0 (default) disables it. Added in 4.0.0.23. This is the only thing in BurnSim that can make a throat smaller than it started

Three things about these two fields are worth knowing, because none of them is visible on screen.

Slag is inversely pressure-dependent - the term is the coefficient divided by chamber pressure, so a low-pressure motor slags more. That is the opposite of erosion, and on a motor where both are set the two effects partly cancel, which can make a trace look suspiciously flat for reasons that are real.

Both rates are radial, not diametral. BurnSim doubles them internally to get the change in throat diameter. The convention matters if you are taking numbers from elsewhere: it is the same one openMotor uses, so coefficients transfer between the two programs with a units conversion and nothing more.

The throat is floored at 5% of its original diameter. That is a numerical guard to keep the pressure solve finite, not a model of a nozzle plugging up - a motor that reaches the floor has left the range where any of these numbers mean anything, and did so long before.

All three erosion and slag terms add up during the burn; the flat Throat erosion rate above is unchanged and is not replaced by the coefficients. A motor that only ever set the flat rate behaves exactly as it did before 4.0.0.23.

• Fwd ullage (in) and Aft ullage (in) - the open space ahead of the first grain (between the forward closure and its head end: an igniter well, a spacer, slack in the stack) and behind the last one (up to the start of the nozzle's convergent cone). Added in 4.0.0.24. Three things to know:

Both are lengths, not volumes, measured along the motor axis. BurnSim sweeps each around the largest grain diameter to get its volume, since there is no case-bore field.

Do not include the convergent cone. Its volume is already computed from the cone geometry, so adding it here counts it twice.

There is no field for the gaps between grains in a stack; if you want them counted, add their total to the aft figure.

Both default to 0, so a motor that leaves them alone is unchanged. They are drawn to scale in the Along axis view, and they feed the initial free volume and the new Initial L* line on the Results tab and in the report.

The Along axis view with 1.5 in of forward ullage (between the closure and the grain) and 0.75 in aft (between the grain and the convergent cone). The grey line under the drawing repeats both figures.

Nozzle cone geometry

Three fields describing the shape of the cone either side of the throat. All three are drawn to scale in the longitudinal view of the Cross-Section panel, and 0 in any of them means "not set" rather than zero - BurnSim falls back to an assumed value for drawing.

• Throat length (in) - the straight land at the throat. 0 falls back to an assumed fraction of the throat diameter. Drawing only
• Conv. half-angle (deg) - convergent cone half-angle. 0 falls back to an assumed 45 degrees. Drawing, and free volume: it sets how long the convergent cone is, and that cone is counted in the initial free volume the Results tab reports - which in turn feeds one of the two igniter estimates. It does not affect thrust or chamber pressure
• Div. half-angle (deg) - divergent (exit) cone half-angle. Drawing, and performance: it applies the standard divergence (angularity) loss of (1 + cos a) / 2 to the momentum part of Cf. A 15 degree half-angle costs about 1.7%. 0 leaves thrust uncorrected and draws an assumed 15 degrees. When an angle is set, the Results tab reports the loss it cost

The divergence loss stacks on top of Other Cf losses, which no longer folds it in - see the note on that field above before carrying an old motor forward. Throat length stays drawing-only, and the convergent angle still reaches no performance term either: every published discharge-coefficient correlation is parameterised on the throat wall radius of curvature, which BurnSim has no field for, so deriving a thrust or pressure correction from the two angles alone would be inventing a number. The convergent angle does size a real volume, though, which is why it reaches the free-volume figures and the divergence angle does not.

Find Best Nozzle Exit Diameter

Action → Find Best Nozzle Exit Diameter..., or the nozzle button on the toolbar. Ported from BurnSim 3, which had the same tool. It sweeps exit diameter across a range, simulates the motor at each one, and plots total impulse against exit diameter so you can see the optimum rather than just being told it.

The shape of the curve matters as much as its peak. A flat curve means the dimension does not need holding tightly and you can machine to whatever is convenient; a sharp one means it does.

Because the optimum depends entirely on the back pressure the motor fires against, the window lets you set that independently of the motor's own Ambient field:

• Optimize for ambient / ...or for elevation - the same two-way pair as on the Motor & Nozzle panel
• Optimize across a climb instead of one altitude - sweeps against a climbing motor rather than a single altitude, revealing Pad elevation and Burnout elevation boxes. This finds the diameter that is best over the whole burn, which is not the diameter that is best at either end of it

Apply This Exit Diameter writes the winner into the motor; Close leaves the motor alone. A long sweep can be interrupted with Stop.

Propellant

• Dropdown selects which propellant in your database applies to the currently-selected grain(s); New, Save As... and Delete manage the database itself; Ingredients... opens the thermochemistry recipe editor, which can compute C*, Specific heat ratio, Molar mass and Density for you (the table of ingredients it picks from is under Settings → Edit Thermo Calc Ingredients...); Burn Rate Data... opens the measured burn rate points recorded against this propellant, and the a/n fit across them, in its own window
• Four tabs: Properties (the fields below), Pressure Bands (below), Erosive Burning (the override fields further down) and Notes - split out so the common case stays uncluttered, since the Erosive Burning tab only matters once axial slicing is turned on
• Name, Density (lb/in^3), C* (ft/sec), Specific heat ratio, a and n (the Saint-Robert's law burn rate coefficient and exponent), and Molar mass (g/mol) are the propellant's characterization numbers - see Propellant Characterization (or Propellant Thermochemistry for the built-in recipe-based route) for how to derive them. Changed in 4.0.0.24: there is no Edit/Save/Cancel step any more - a field saves to your propellant database as soon as you leave it, and a value BurnSim cannot accept pops up a message saying why rather than being silently dropped. Propellant edits are not undoable, because every motor using that propellant shares them
• Notes (its own tab) - free text about this propellant: batch numbers, mix dates, which mix gave which rate, what went wrong. Saved with the propellant in your database and carried in .bsx files. The field itself is not new - BurnSim 3 read and wrote it, there was simply never anywhere to type it, so notes already in your database show up here. Saves when you leave it, same as the fields above

Pressure Bands

Added in 4.0.0.24, available with every license. A propellant's burn rate is rarely one clean power law across every pressure, and a pressure band lets it change: each band covers a chamber-pressure range (Pc low to Pc high) and carries its own a and n, and optionally its own C*, k (specific heat ratio), Molar mass and Tf. This is BurnSim 3's Pressure Varied Properties grid returning; BurnSim 4 adds C*, k and molar mass per band.

• Add Band starts a band where the last one ends, filled with the propellant's a and n to edit from.
• Leave both a and n blank and the band keeps using the Properties a and n, following any later edit to them. Filling in only one of the two is refused.
• A blank C*, k, molar mass or Tf uses the Properties value.
• Pressures outside every band use the Properties values, and the burn rate plot beside the fields shades the bands.
• A band's k changes thrust (through Cf) as well as C*, step by step through the burn.
• If a motor's pressure would jump back and forth across a band edge, it settles on the edge (a plateau) rather than alternating between the two bands, as BurnSim 3 could.

Band edits save to the propellant database like the other fields and are not undoable. Opening a motor whose propellant's bands differ from the copy in your database raises the propellant compare dialog. The thermochemistry recipe can fill in each band's C*, k, molar mass and Tf, or create a set of bands for you.

A .bsx with bands still opens in BurnSim 3, which burns the same a and n. It ignores the per-band C*, k and molar mass, and a band that keeps the Properties a and n is written with them filled in.

Where your propellants are stored

Changed in 4.0.0.18. BurnSim 4 keeps its propellant database in burnsim4_propellants.xml in your Documents folder. BurnSim 3 used - and still uses - burnsim3_propellants.xml beside it. The first time BurnSim 4 runs without a file of its own it copies your BurnSim 3 database across, so nothing is lost and nothing needs importing; after that the two are independent.

They were separated because sharing one file was silently lossy in one direction. The format has grown four things BurnSim 3 knows nothing about - C*, Molar mass, ingredient recipes (Propellant Thermochemistry) and stored burn rate data (burn rate data sets) - and BurnSim 3 drops all four when it saves, for every propellant in the file rather than just the one being edited. A propellant left with no C* cannot produce thrust, so the damage showed up as a simulation quietly returning nothing.

If you still run both programs, this is the fix: they no longer tread on each other. The cost is that they no longer share either - a propellant added in one will not appear in the other, so keep whichever one you actually characterize propellants in as the master.

Erosive Burning Overrides

On the Propellant panel's Erosive Burning tab - per-propellant overrides for the Erosive Burning models. Only relevant when Simulate Erosive Burning is on (see Settings Menu above). Each box is optional - leave it blank and BurnSim derives a value from the propellant's other properties (the small gray text under each box always shows what value is actually in effect, derived or overridden):

• Beta - blowing coefficient (Lenoir-Robillard only)
• Viscosity (mu) - combustion gas viscosity
• Prandtl - Prandtl number of the combustion gas
• Surface temp (R) - propellant surface temperature
• Solid heat capacity (cs) - propellant solid-phase heat capacity
• Combustion temp (R) - adiabatic flame/combustion temperature
• Surface Roughness (in) - propellant surface roughness height (Ma (2020) only, feeds its friction-factor calculation); 0 (hydraulically smooth) is the default, since no propellant in BurnSim has measured roughness data

Graph

Checkboxes above the graph toggle which traces are plotted against time: Kn, Kn Eff, Pressure, Thrust, Mass Flux, Mass Flow. When test data has been imported and marked "Use in simulation" (see Test Data Tab), three more become available: Imported Pressure, Imported Thrust, and Thrust From Imported Pc (thrust back-calculated from the imported pressure trace via the nozzle) - letting you overlay real test data against the simulated curves.

Two more controls sit in the same row, to the right of the checkboxes:

• End sim at ___ % of max thrust - stops the simulation once thrust decays to this percentage of the highest thrust seen so far, trimming a long low-thrust tail off the graph, burn time and total impulse. Defaults to 1%; 0 disables it and burns to completion. When a run is stopped early this way, the Results tab (and the PDF report) note that the burn was truncated
• Bleed-off after burnout - on by default. After the last propellant is gone, the chamber empties through the nozzle instead of pressure dropping from its last burning value to zero in one step: an isentropic blowdown of the chamber's free volume, so ullage matters. A small motor empties in a few milliseconds; a long measured tail is propellant (slivers, insulation, a hot casting tube) still burning, not this. End sim at trims it like any other tail. Untick it to compare with BurnSim 3, which never had it. Added in 4.0.0.24

The Erosive burning mode selector used to sit here too. It now lives on the toolbar, next to the Run button, where it is one click away while comparing runs instead of competing for space with nine series checkboxes.

Results, Test Data and Notes Tabs

The bottom-right panel carries five tabs, sharing a panel but little else (see Results, Test Data and Notes for a pointer page covering all four):

• Results - a text summary of the burn: designation, total impulse, burn time, peak pressure, peak and average thrust, delivered Isp, Kn max/min/avg, web thickness, port/throat ratio, propellant mass and length, initial free volume and L*, two igniter mass estimates, and peak mass flux/flow, with exhaust products at the bottom on a commercial license - plus any messages BurnSim generated about the design. Every line it can print is described on its own page: Results Tab
• Test Data - import, edit and manage real motor-test data; see Test Data Tab
• Notes - free-text notes about the motor, saved inside the .bsx file
• Ignition (experimental) - commercial license only. Models the chamber filling from ambient up to operating pressure, separately from the main simulation, so it changes nothing about the thrust curve, burn time or impulse
• Snapshots - named copies of the whole motor, saved inside the .bsx file. Save Snapshot... records the motor as it is now; Restore (or double-click) replaces the current motor with the selected one, and is undoable; Rename... changes a snapshot's name or notes; Delete removes one after asking, and cannot be undone. The box beside the list holds the selected snapshot's notes. See Undo, Redo and Snapshots

The Results tab's individual field descriptions - port/throat ratio, initial free volume and igniter mass - were on this page until September 2026. They moved to Results Tab, where the rest of that tab's lines are documented alongside them.