Revolved Profile Grain

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Revolved Profile is a grain type for a grain whose shape is drawn as a half-section and revolved about the motor axis - a domed head end, a long bore, a flared mouth, or any other axisymmetric outline that the standard types cannot describe. You draw the outline in CAD, import it, and BurnSim simulates and draws the revolved grain.

Commercial license only. Revolved Profile is available with a commercial license (added in 4.0.0.25). Personal, educational and trial licenses do not list the type, and BurnSim refuses to open a file that contains one on those licenses, naming the missing feature. See the FAQ for what a commercial license adds.

What it is for

Every other grain type is a constant cross-section extruded along the motor, apart from Tapered Core and Tapered Finocyl, which taper along the axis in fixed ways. A Revolved Profile grain has no fixed recipe: its outline is whatever you draw, so the bore can widen, narrow, dome or flare along the grain, and the grain can have any shape of head end.

It is for axisymmetric grains only - the same shape all the way round the axis. A grain with fins or slots that are not the same all the way round is not this type; see Custom Grain Import for cross-section shapes, and Tapered Finocyl for fins.

How it burns

  • Only the bore burns. The outer wall is treated as bonded to the case and does not burn.
  • The two end faces burn or not according to Ends inhib., exactly as for any other grain.
  • The burn is simulated on the half-section itself - each station along the grain regresses from the bore outward - by the same method BurnSim uses for a Tapered Core, so the burning surface, port area and Kn follow the shape you drew rather than an approximation of it.
  • A bore that closes on the centreline (a plugged or blind bore) is allowed, but only the cavity that connects to the mouth lights; BurnSim warns when it sees this.

Preparing the drawing

Draw the grain's half-section in a CAD program and save it as an ASCII DXF:

  • One closed outline - the shape of the grain from the axis out to the case wall, including the two end faces.
  • The centreline the shape is measured against, drawn as a line.
  • Draw the axis vertically on the left with the nozzle end at the bottom, or turned a quarter turn so the axis is horizontal with the nozzle on the right. If your drawing is the other way round, the import window lets you rotate it.
  • The outline must be on one side of the axis. An outline that crosses the axis, as a full section does, is refused rather than mirrored.
  • Exactly one closed outline. A border, title block or second loop in the same drawing makes the import refuse, with the count, rather than guessing which one is the grain.
  • The outline must not cross itself.

Importing

  1. In the Grain Editor, set Type to Revolved Profile. (If the type is not in the list, your license does not include it, or you activated while BurnSim was running - restart BurnSim.)
  2. Click Import profile DXF... and pick the file.
  3. The Import Revolved Profile from DXF window opens for every import, even when the drawing is unambiguous, because both of the things it asks can be wrong without any error showing:
    • Axis of revolution - which line is the centreline. Measuring from the wrong line gives a believable grain of the wrong diameter.
    • Rotate the drawing - turn the drawing in quarter turns until the preview shows the head end at the left and the nozzle end at the right. A grain with its flare at the wrong end looks plausible too.
    • Distance from the axis - where the shape sits. This is a property of the motor, not of the drawing: changing it moves the whole outline in or out, so every wall thickness is kept and only the bore changes.
  4. The window shows the grain's length, outside diameter, bore at the nozzle face, volume and a picture in BurnSim's own orientation. Check them against what you expect, then click Import.

View profile... reopens the imported half-section later, to check or change the axis, the orientation and the distance from the axis.

Dimensions

After import, Length and Dia are ordinary editable fields, and each one stretches the profile: change the length and the whole outline is lengthened by the same factor; change the diameter and it is scaled in diameter. The shape you drew stays the same shape. Axis offset is the distance from the axis described above.

What you see

  • The end view shows the plain round end view of the grain.
  • The Along axis tab sections the grain, with the burnt region following the Burn depth slider.
  • The 3-D tab shows the revolved grain as a solid you can orbit and scrub through the burn. See 3-D view.

Limits

  • The outline comes from a DXF import. There is no drawing tool inside BurnSim for this type yet, and no way to export the profile again.
  • One propellant per grain, as for every type, and the usual rule about a grain with a bore too small to pass the gas ahead of it applies (see above).
  • openMotor cannot represent the type, so File → Export openMotor refuses a motor containing one.
  • The importer has been exercised on sample drawings. As with any imported shape, read the numbers in the import window before trusting the result - a drawing measured from the wrong line, or turned the wrong way, still imports.

See also

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