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MediaWiki:Sidebar

2026-08-10T22:10:48Z

Greg: Remove dead Characterization Motor sidebar link

BurnSim Fields

2026-08-10T21:43:20Z

Greg: Correct Settings menu item name, add Set Simulation Timestep, mention the Properties/Erosive Burning tab split and the new Results erosive-augmentation line

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]].

[[File:BurnSim4MainWindow.png|900px|thumb|center|alt=BurnSim 4 main window|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
* '''Save''' (Ctrl+S) / '''Save As...''' - save the current motor to a .bsx file
* '''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
* '''Print / Report...''' (Ctrl+P) - generate a PDF motor report

== Action Menu ==
* '''Simulate''' (F5) - run the simulation now. Usually not needed - see Auto-Simulate below.

== Settings Menu ==
* '''Auto-Simulate on Changes''' - when on (the default), BurnSim re-runs the simulation automatically as you edit grain, nozzle, or propellant fields
* '''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
* '''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|Erosive Burning Overrides]] below
* '''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 Molar Mass set. 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. Has a hard threshold: below it, there's no augmentation at all
* '''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
* '''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 ==
* '''Activate...''' - enter or check your license
* '''About...''' - version info and license terms

== Grains Grid ==
One row per grain in the motor, in burn order. Columns are Type, Length, Dia, Core, Offset, Fin len, Pts, Inhib. (number of inhibited ends, 0-2), and Propellant - which fields apply depends on the grain [[Grain Types|type]] selected. Click ''add a grain'' or use the Grain Editor below to add one; select a row to edit it.

== 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.

Below the editor, a '''Cross-Section''' preview shows the selected grain's end view (left) and the whole motor's longitudinal cutaway (right). The slider and Play button scrub through the burn, animating how the grain erodes over time using the same pixel-based eroder that drives the simulation.

== Motor and Nozzle ==
* '''Throat dia (in)''' / '''Exit dia (in)''' - nozzle throat and exit diameters
* '''Efficiency (%)''' - combustion/nozzle efficiency applied to the theoretical performance
* '''Ambient (psi)''' - ambient pressure the motor fires into
* '''Thrust method''' - how thrust is derived; ''Nozzle geometry'' computes it from the throat/exit dimensions above

== Propellant ==
* Dropdown selects which propellant in your database applies to the currently-selected grain(s); '''New''', '''Edit''', '''Save As...''' and '''Delete''' manage the database itself
* Two tabs: '''Properties''' (the fields below) and '''Erosive Burning''' (the override fields further down) - 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]] for how to derive them. Fields are read-only until you click ''New'' or ''Edit''

=== Erosive Burning Overrides ===
On the Propellant panel's '''Erosive Burning''' tab. Only relevant when ''Simulate Erosive Burning'' is on (see [[#Settings Menu|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

== 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.

== Results, Test Data and Notes Tabs ==
* '''Results''' - a text summary of the burn: designation, total impulse, burn time, peak pressure/thrust, delivered Isp, Kn max/min, web thickness, propellant mass and length, initial core volume, peak mass flux/flow - plus any warnings BurnSim generated about the design (for example, a core narrower than the nozzle throat). If erosive burning is on, a line also reports whether it actually had any effect on this particular run (peak augmentation over the nominal burn rate), not just that the option was enabled
* '''Test Data''' - import, edit and manage real motor-test data; see [[Test Data Tab]]
* '''Notes''' - free-text notes, saved with the .bsx file

[[Category:BurnSim 4]]

Erosive Burning

2026-08-10T21:43:14Z

Greg: Correct Settings menu item name, Propellant tab reference, and mention the new Results erosive-augmentation report line

Erosive burning is the increase in propellant burn rate caused by high-velocity gas flow across the burning surface, on top of the normal pressure-driven rate. It's strongest near the aft end of a grain, where mass flux is highest, and shows up as an ignition pressure spike and/or the aft end of a grain burning through before the forward end. BurnSim 4 can model this; BurnSim 3 could not.

This is opt-in and off by default - a motor with the feature off behaves exactly as before.

== Turning it on ==
Under '''Settings''':
* '''Simulate Erosive Burning (Use Axial Grain Slices)''' - the master switch. When on, each grain is divided into axial slices and burn rate is augmented per-slice by the selected model; when off (default), BurnSim uses a single lumped burn rate per grain as it always has.
* '''Set Slice Count...''' - how many axial slices each grain is divided into. More slices resolve a tapered port or a burnthrough more precisely, at the cost of simulation speed. 100 is a reasonable default, depending on available processing power.
* '''Erosive Burning Model''' - a submenu to pick which correlation the sliced simulation uses: '''Lenoir-Robillard''' or '''Mukunda-Paul'''. This is a global setting, not a per-motor one - a saved .bsx file doesn't record which model produced its numbers. Switching models re-simulates immediately if slicing is on, since the two can produce visibly different traces.

This all lives in [[BurnSim Fields#Settings Menu|the Settings menu]] alongside the other simulation-accuracy toggles.

== The two models ==
Both models estimate an augmentation factor on top of the normal Saint-Robert rate (r = a*P^n), evaluated per slice from that slice's local mass flux. They differ in what they assume and what they need from you.

=== Lenoir-Robillard (default) ===
The model most motor designers actually use. It's an empirical, "data-fitting" correlation, not a first-principles prediction - the literature is explicit that it requires experimental data for each propellant formulation to be exact. BurnSim's default numbers are a reasonable starting point, not a guarantee.

* Needs a propellant with valid '''C*''', '''Specific heat ratio''' and '''Molar mass''' set (see [[BurnSim Fields#Propellant|the Propellant panel]]) - these feed a derived flame temperature, which the model needs. A propellant missing these is refused with a warning rather than silently simulated wrong; all three are ordinary ProPep/CEA outputs, so this is rarely a real blocker.
* Its erosive sensitivity constant, '''beta''', has a sensible derived default (based on the propellant's own a/n - slower propellants are inherently more crossflow-sensitive) but can be overridden if you've characterized your own propellant against crossflow.
* Its length scale is distance along the grain port from the head end, not a hydraulic diameter - not something you need to enter, but worth knowing if you're comparing against a value from another source.

=== Mukunda-Paul ===
A newer correlation whose selling point is being universal - not fitted per propellant. It's threshold-based: below a certain non-dimensional mass flux there is no augmentation at all, and above it augmentation follows a fixed exponent that doesn't vary by formulation.

* Needs only ordinary a/n/density data, plus (optionally) the combustion-gas viscosity override below - no C*/specific heat ratio/molar mass required, so it will run on propellants Lenoir-Robillard can't.
* No propellant-specific constant to supply or tune - the model's two constants are treated as universal and are not exposed as overrides.
* Because it has a hard threshold, a motor that shows a taper under Lenoir-Robillard can legitimately show none at all under Mukunda-Paul at the same conditions - that's a real difference between the two correlations, not a bug.

Neither model is "more correct" in general - they're offered so you can pick or compare, not because one is a fallback for missing data.

== Erosive burning overrides ==
On the '''Propellant''' panel's dedicated '''Erosive Burning''' tab (split out from the main Properties tab, since these only matter once slicing is on) is a section of six advanced tri-state fields: '''Beta''', '''Viscosity (mu)''', '''Prandtl''', '''Surface temp''', '''Solid heat capacity (cs)''', and '''Combustion temp'''. Each works the same way:
* '''Blank''' - BurnSim derives a value automatically (from the propellant's own a/n, C*, specific heat ratio and molar mass, or from a nominal composite-propellant constant, depending on the field). This is correct for most users and most fields.
* '''A number''' - use exactly that value instead. Useful once you have test data or a thermochemistry report (ProPep, CEA) with a directly measured or reported number you'd rather pin exactly.

The small grey text under each box always shows what value is actually in effect, whether derived or overridden. These overrides travel with the propellant in a saved .bsx file and in the shared propellant database, the same way C* and molar mass do. They only matter once axial slicing is turned on - Mukunda-Paul only actually uses the Viscosity override; the other five feed Lenoir-Robillard.

== Reading the results ==
* A '''Kn Eff''' checkbox appears next to the ordinary Kn checkbox above the graph, plotting the erosive-burning-effective Kn as its own series. It reads a flat zero unless slicing is on - BurnSim always reports geometric Kn separately rather than silently redefining it.
* The grain cross-section/axial view draws a taper once slicing is on and a simulation has run, showing the port opening up faster toward the aft end where erosive burning is strongest - the visible signature of the effect.
* The Results tab's usual summary (total impulse, peak thrust, Isp, etc.) reflects the augmented burn automatically, and also adds a dedicated line reporting whether erosive burning actually had any effect on this run - off, on with a peak augmentation percentage, or on but never triggered (the port/throat crossflow never got restrictive enough) - so you're not left guessing whether the option did anything.

== Things worth knowing ==
* This is a calibrated correlation, not predictive physics - both models are fits to test data, and published literature notes real gaps (for example, Lenoir-Robillard predicts a temperature dependence that measurements don't actually show). Treat results as informative, not exact, especially with default/derived constants rather than your own measured ones.
* The master switch and model choice are both global settings, not saved per motor - if you need to know how a particular .bsx's numbers were produced, that's state you have to track yourself.
* Turning slicing on costs simulation speed (more work per timestep, times the slice count) - most motors aren't anywhere near the flux regime where it changes the answer, which is why it's off by default.

[[Category:BurnSim 4]]