3-D Burn-Back View
BurnSim 4 can draw your motor's grains as solid, interactive 3-D objects and colour the part of it that is on fire, then let you scrub the burn and watch that burning surface recede from the unburnt grain to burnout. It answers two questions the flat views cannot: what does this grain actually look like, and which of its surfaces are burning at any moment - the question an inhibited end or a case-bonded wall is the answer to. It is a visualisation, not a separate calculation: the shape and the burning-surface classification are the same geometry the simulation runs on.
Opening the view
The 3-D view is the "3-D" tab in the picture band, beside the Along axis tab, directly under the grain list. Click it and the motor is drawn as a solid. (There is no separate window and no menu command to open it. Earlier versions had a View Grain in 3-D... menu item, and the tab replaced it.)
The view shows the whole motor: every grain in the grid, stacked nose to tail in burn order, the same stack 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 caption under the picture gives the grain count, the overall length and the triangle count.
The case, bulkhead and nozzle
With Hardware ticked (the default) the view draws the case, a forward bulkhead and the nozzle round the grains, so the stack is seen sitting in a motor rather than floating in space. Added in 4.0.0.25.
- The nozzle is drawn from the motor's own fields: throat, exit, throat length and the two cone half-angles (see Nozzle cone geometry). An exit cone too wide for the case bore stands out of the aft end.
- The case is drawn from the Case tab when a case is defined: its outside diameter, and its wall when that is known too. The caption then says case from the Case tab. A case whose bore is narrower than a grain is drawn that way, showing the clash the Case tab warns about rather than hiding it. With no case defined, the wall is drawn in proportion to the grain diameter.
- The bulkhead has no fields in BurnSim, so it is always drawn in proportion, and the caption says not to scale.
- The grains sit behind the bulkhead and any forward ullage, and the aft ullage is left open before the nozzle.
Cut away (on by default) cuts the hardware in half on the section plane and leaves the grains whole, so they can be seen inside the case. Section cuts the hardware too, whether or not Cut away is ticked. A motor whose grains are all revolved profile grains is drawn without hardware, since its outline already comes from a drawing.
Moving the camera
The camera orbits a fixed point on the grain's axis, so the grain stays framed however you turn it.
| Do this | To |
|---|---|
| Left-drag | Orbit around the grain |
| Mouse wheel | Zoom in and out (dolly) |
| Middle-drag, or Shift + left-drag | Pan the grain in the frame |
| Home key, or the reset button | Return to the starting view |
Shift + left-drag pans as well as the middle button because a trackpad has no middle button. For the same reason there are on-screen arrow, zoom and reset buttons beside the viewport, so the whole view can be driven with no mouse at all - the arrows carry the grain the way they point, matching the drag beside them.
The cutaway
A grain's interesting geometry is on the inside - the bore, the fins, the slots - and from the outside a solid grain hides all of it. Tick Section and BurnSim slices the grains open along a plane through its axis, so the core and the fin roots are visible. A cut-angle slider rotates that plane around the axis (0-360°), and because the cut belongs to the grain rather than to the camera, you can orbit all the way around a fixed cut and look into it from any side.
The cut interior is shaded a distinct darker tint so it reads as cut material rather than as a hole through the grain.
The burning surface, in red
Every surface that is burning is drawn red; every surface that is inert keeps the grain's plain colour. That is the picture's real payload. A grain's outline tells you its shape; the colour tells you which of that shape is on fire, and nothing else in BurnSim shows it.
The two kinds of surface that do not burn are drawn plain:
- an inhibited end - a face you have restricted, which the burn does not reach; and
- the case-bonded outer wall - the cylinder the grain's diameter describes, which is up against the case and never a burning surface.
So an inhibited end you have set on a grain is visible at a glance as a plain face among red ones, which is exactly the thing a flat cross-section leaves you to work out from the numbers. The classification is read off the finished grain geometry, the same geometry the simulation uses, so the red you see is the surface the run is actually integrating - not a separate artist's impression of it.
Watching it burn back
Under the picture band is a single burn scrub slider. It drives every picture in the band at once - the axial cross-section, the 3-D view, and the playhead on the performance graph below - because they are all showing the same quantity: how far the burning front has receded.
Drag it from left to right and the grain burns back in 3-D in real time: the red surface eats inward from the bore and the fins, the fins round off and merge, and sliver geometry disappears, all the way to burnout. The left end of the slider is the unburnt grain; the right end is the end of the burn. It is the clearest way to see how a grain's burning area changes through the burn - progressive, regressive or neutral - rather than reading it off the Kn curve alone.
Giving the pictures more room
The picture band shares the window with the grain list and the performance graph. Drag the bar under the pictures to give them more or less of the window; double-click it to hand them as much as the list and the graph can spare, for a close look at a detailed grain, and double-click again to put the layout back. The size is remembered between sessions. (Before 4.0.0.24 this was a View size slider and a Fill button above the pictures.)
Which grains are supported
Every grain type is drawn in 3-D, including BATES, end and moon burners, D-grain, C-slot, X-core, star, fin-o-cyl, tapered core and tapered finocyl, and custom and imported cross-sections.
A grain that cannot be drawn is skipped, and the caption counts it, rather than leaving the whole view blank.
Most grain types are cheap to draw and orbit smoothly. The Tapered Finocyl is the exception: its burn-back is computed on a 3-D voxel grid, and a large grain with fine fins can need more memory to resolve than the default allows. If a tapered finocyl refuses to erode or looks under-resolved, raise its 3-D erosion memory limit in Settings - a 12 × 60 inch grain with 0.15 inch fins, for instance, goes from refused to simulatable at a higher limit. The setting affects no other grain type.
See also
- Grain Types - every grain BurnSim models, and the shapes the 3-D view draws
- BurnSim Fields - the motor, nozzle and propellant inputs the simulation reads
- Custom Grain Import - drawing or importing your own cross-section