Grain Types
BATES Grain (and EndBurner)
A BATES grain is a simple cored cylinder which burns on the core and end surfaces. End surfaces can be inhibited. This is the most common propellant grain type. A BATES grain can be configured to burn progressively or regressively depending on length. If you want an End Burning grain, specify the core diameter as zero. When doing so, BurnSim will assume one end is inhibited and the other is not, thus only burning from one end.
The two dimensions a BATES grain takes, besides its length.
Tablet
A Tablet is the same shape as a BATES grain - a cylinder with an optional round core - and BurnSim uses the same annulus-times-length volume for both. What makes it a separate grain type is which surfaces burn: a Tablet's outer wall is a burning surface. A BATES grain sits against the case, so only its core and its uninhibited ends burn; a Tablet is free-standing, so it burns inward from its outside diameter as well.
A cored Tablet therefore burns on four kinds of surface at once: the core growing outward, the outer wall receding inward, and each uninhibited end. Without a core (Core dia 0) it is a solid puck burning inward from its outer wall and on any uninhibited end; its web is then whichever runs out first, the length or the diameter, rather than the length the way an uncored BATES end burner's is.
How it burns
How progressive or regressive a Tablet is depends on its proportions and on how many ends are inhibited, so the two extremes are worth knowing:
- Cored, both ends inhibited: exactly neutral. The outer wall recedes inward at the same rate the core grows outward, so their combined circumference never changes, and with both ends inhibited the grain never shortens. The burning surface is constant for the whole burn.
- Everything else: regressive, because the ends and the shortening length both shrink. How steeply depends on the shape. A long, thin grain is dominated by its walls and barely falls off; a short, fat one is dominated by its ends and falls off hard.
For example, with both ends uninhibited, a 2"-diameter grain 8" long on a 0.6" core finishes at about 84% of its starting surface area, while a 6"-diameter grain 1" long on a 1" core finishes at about 43% of its - the same grain type, behaving quite differently.
What a Tablet cannot do is burn progressively: none of its surfaces grow faster than the others shrink, so its burning surface never rises above its starting value. If you need a rising Kn, that is what a star, finocyl or moon core is for. As always, the Kn trace on the graph is the thing to read - these are tendencies, not a substitute for simulating the grain you actually have.
That outer burning surface also means an uncored Tablet with both ends inhibited still burns, from the OD inward. The identical BATES grain has no burning surface at all, and BurnSim refuses it with a message saying so.
Fields: Length, Dia, Core dia (0 for no core) and Ends inhib. There is no edge offset - like a BATES grain, a Tablet's core is centred.
C-Slot Grain
A C-Slot grain consists of a cylinder with a lengthwise slot removed from the side of the grain. The grain burns on the slot/core surface and grain ends. End surfaces can be inhibited. C-Slot grains are often used to give a longer burn time by providing a larger web thickness than is possible with a BATES grain in the same diameter. Disadvantages include increased case insulation requirements along the edge of the grain surrounding the slot.
Both slot fields are named for a round core they don't describe: Core dia is the width of the slot, and Edge offset is how far it cuts in from the outside wall.
"Core dia" is the slot width; "Edge offset" is the slot depth.
D-Grain
A D-Grain is a simple cylinder with a slice removed from the side, resulting in the end of the grain appearing in the shape of the letter D. The grain burns on the ends and flat surface of the 'D'. The ends can be inhibited. D-Grains can provide a very large web thickness in a given grain diameter, allowing for long burns. Thermal issues are more of a concern with this configuration due to the large area of the motor case that is exposed to the flame front for the entire burn.
Edge offset measures from the grain's axis out to the flat, not the amount cut away. So 0 leaves exactly half the grain, a positive value leaves more than half, and a negative value cuts past the centre.
"Edge offset" is axis-to-flat. 0 leaves exactly half the grain.
Moon Burner
A moon burner grain so named due to the shape of the grain as the propellant burns. The grain is similar to a BATES grain, with the exception of the core being offset rather than down the center. The grain burns on the end surfaces and in the core, the ends can be inhibited. The moon burner grain is another way to increase web thickness but reduces thermal concerns since the flame front is only exposed to the side of the motor case for part of the burn.
Edge offset is how far the bore centre sits off the grain axis; at 0 the grain is a BATES.
"Edge offset" moves the bore centre off the grain axis.
Star Grain
The star grain has a core in the shape of a star. The grain burns on the core surface as well as the ends, and the ends can be inhibited. A star grain can be used to get a neutral burn, and reduce the amount of heat transferred to the case.
Both point widths are measured right across the core, tip to opposite tip (Maj point width) and valley to opposite valley (Min point width) - not from the axis out to one point.
Both point widths go right across the core, not from the axis outward.
X-Core
An X-Core grain is similar to a star grain except the core is in the shape of an X instead of a star. Its primary advantage is ease of manufacture compared to a star grain.
Core dia and Edge offset are interchangeable here: whichever is larger spans the cross, and whichever is smaller becomes the thickness of its arms.
The larger of the two fields spans the cross; the smaller is the arm thickness.
Fin-o-cyl
A Fin-o-cyl grain consists of a grain with a cylindrical core, like a BATES grain, but then has 'fins' added. Its advantages are similar to that of the star grain but is easier to manufacture.
Fin width is how far each fin reaches out from the core wall - so the fin tip sits at half the core diameter plus the fin width, measured from the axis. This is not the same convention as Tapered Finocyl, whose fin width is measured from the axis itself.
Fin width reaches out from the core wall, not from the axis.
Tapered Core (BurnSim 4)
A Tapered Core grain is an end burner with a hole drilled part-way (or all the way) down its axis. The core can be a straight cylinder or a taper - a Base core dia at the aft (mouth) end narrowing to a smaller Tip core dia further in. If the core stops short of the forward end (a blind core), the plug of propellant ahead of the tip can't burn from the closed side, so the head end is normally inhibited; a core that runs the full length behaves like a tapered BATES grain instead. The grain burns on the core surface and the aft face (the core mouth), and the head end can optionally be left uninhibited only when the core runs the whole length.
Unlike every other grain type, a Tapered Core's cross-section genuinely changes along the axis - it's an annulus near the mouth, a smaller annulus further in if tapered, and a full disc past the core tip - so BurnSim simulates and draws it from a meridional half-section rather than the usual constant-cross-section extrusion every other type uses. Both cross-section views (end view and longitudinal cutaway) reflect this directly. Because a blind core builds up mass flow fast over a short axial run, erosive burning (see Erosive Burning) is particularly relevant to this grain type.
Core length is measured forward from the aft (mouth) face, so the core tip is the blind end.
Tapered Finocyl (BurnSim 4)
A finocyl whose core and fins both taper along the axis. The bore runs from one diameter at the head end (Fwd core dia) to another at the mouth (Aft core dia), and can stop short of the head end (Core length, measured back from the mouth; blank or 0 is a core through the whole grain, and a blind core inhibits the head end). The fins have a thickness around the bore (Fin thickness), a count (# fins), an axial run forward from the mouth (Fin length, blank or 0 for the full depth of the core), and a width at each end of that run (Fwd fin width and Aft fin width).
Each Tapered Finocyl dimension, labelled with the grain editor's field names. The grains grid uses the same names, so a dimension reads the same wherever you meet it.
One vocabulary, used throughout
Length is always axial - along the motor. Width is always radial - outward from the axis. Thickness is across the fin.
Three things that catch people out
- Fin width is measured from the axis, not from the core wall. The distance a fin actually reaches into the web is fin width minus the core radius at that station. In the diagram above, that is 1.25 − 0.60 = 0.65 at the aft face. A fin width smaller than the core radius describes a fin that never leaves the bore, and BurnSim will refuse the grain.
- Core length and fin length are both measured forward from the aft (mouth) face. A bore shorter than the grain is blind, leaving solid propellant at the head - which forces the head end inhibited.
- "Fwd fin width" is at the forward end of the fins, which is the head of the grain only when the fins run the full depth of the bore.
The diagram is drawn from Length 6.00, Dia 3.00, Fwd core dia 0.80, Aft core dia 1.20, Core length 5.00, Fin length 3.00, Fwd fin width 0.85, Aft fin width 1.25, Fin thickness 0.20, # fins 6. Type those in to compare it against the 3-D view.
It is the first grain in BurnSim whose shape varies both along the axis and around it, so it cannot be simulated from a single cross-section. BurnSim erodes it as a full 3-D voxel field instead, and web, propellant volume, burning surface and port area all come from that erosion. Settings → Set Voxel Memory Budget... controls how finely it is resolved: raise it for a large grain with thin fins, which at the default may be refused as under-resolved. A larger budget takes longer to simulate, and the setting affects no other grain type.
Custom Grains (BurnSim 4)
BurnSim 4 adds a custom grain shape editor for cores that don't fit any of the standard types above - you can sketch a shape directly, or import one from a DXF, SVG, or raster image. The custom shape is still simulated through the same pixel-based eroder used for every other grain type. See Custom Grain Import for how to build or import a custom cross-section.