Getting Started with a Basic Simulation
This walks through building a simple three-grain finocyl motor from scratch. For what each field on screen does, see BurnSim Fields.
Start a New Motor
Open BurnSim and choose File > New (or just start with the window BurnSim opens with). You'll see an empty Grains grid, a Motor & Nozzle panel with zeroed fields, and an empty graph:
Add the Grains
In the Grain Editor (below the grid), set Type to Finocyl and pick a propellant from the Propellant dropdown - this example uses "Viper". Enter:
- Length (in): 4
- Diameter (in): 2
- Core dia (in): 0.6
- Fin width (in): 0.15
- Fin length (in): 0.3
- # fins: 6
- Ends inhib.: 0
Click Add three times to add three identical grains, stacked end to end - 12" of propellant overall. The Cross-Section preview updates to show the finned core and the longitudinal cutaway of all three grains.
Set the Nozzle
In Motor & Nozzle, enter:
- Throat dia (in): 0.5
- Exit dia (in): 1
- Other Cf losses (%): 95 (the default)
- Ambient (psi): 14.7 - or type 0 into ...or elevation (ft), which is the same thing said the easier way
- Thrust method: Nozzle geometry
Two shortcuts worth knowing on the throat: right-click the Throat dia field and you can pick a 1/64" increment directly, or pick a Loki Nozzle # - a Loki nozzle number is the throat in 64ths, and for 38mm, 54mm and 76mm motors it fills in the exit diameter as well. See BurnSim Fields.
As soon as a valid throat diameter is set, BurnSim simulates automatically (this is the Auto-Simulate on Changes setting, on by default - see Settings Menu). You don't need to click anything to run it.
Read the Results
The graph (bottom left) plots Kn, pressure and thrust against time. The Results tab (bottom right) summarizes the burn - for this motor, a K-1116 designation, roughly 1685 N-sec total impulse, a 1.51 second burn, peak chamber pressure around 1407 psi, peak thrust around 407 lbf, and a delivered Isp around 210 seconds - plus Kn max/min/avg (420 / 61 / 302 here), web thickness, the port/throat ratio (2.83, comfortably above the 2 that is the usual design target), propellant mass and length, the initial free volume (grain ports plus the nozzle convergent cone), and two igniter mass estimates. These figures reflect the default End sim at 1% of max thrust cutoff (see the checkboxes-row description under "From Here" below) trimming the burn tail; the Results panel says so in a note at the top whenever this cutoff has shortened a run. Every line the tab can print is documented on Results Tab.
If a design has a physical issue worth flagging - for example a grain core narrower than the effective nozzle throat - BurnSim adds a warning above the results explaining it, rather than silently simulating something questionable. Worth reading whenever one appears.
From Here
- Add more grains, change grain types (see Grain Types), or try different propellants and nozzle sizes, watching the results update live
- Toggle which traces are plotted with the checkboxes above the graph, which also holds the End sim at __% of max thrust cutoff box; the Erosive burning mode selector now sits on the toolbar (see BurnSim Fields and Erosive Burning)
- Try Action → Find Best Nozzle Exit Diameter... on this motor - it sweeps exit diameter and shows which one gives the most impulse, and how sharply it matters (see BurnSim Fields)
- File > Save (Ctrl+S) to keep the motor as a .bsx file
- Once you have real test data for this motor, see Test Data Tab to bring it in and compare against the simulation