The 1/4 and 1/2 A motors are 13 mm in diameter and 44 mm long.
First I made a little python script to find the minimum volume Sears-Haack body that would fit a 13x44 mm cylinder. The bold black curve is the minimum volume body; it happens to have a length of twice the motor length.
Blender. Then the svg curve must be converted into a Mesh, and the
Spinmethod applied to generate the body of revolution.
Now I should be able to add some fins and export an stl from Blender for my rapid prototyping friends to play with. The design goal for this rocket will be to have positive static margin with the motor in the rocket, but neutral or negative static margin once the ejection charge pops it out the back (that way it does a tumble recovery).
Update: To turn the surface mesh generated above into a real solid that can be printed requires a couple of extra steps. First, all of the faces should be selected in Edit mode, then choose the Mesh-->Solidify, this method allows you to select a thickness for the extrusion to generate a volume grid from the surface grid. After that, the quadrilateral cells need to be converted into tetrahedrons. With all of the faces selected, this is done by simply typing CTRL-T.
For some reason the stl exported from Blender did not work. I exported a ply format file from Blender instead, and then imported it into meshlab, which then exports to a variety of formats, stl included.
Update: The fine folks at fabbr printed me one in ABS on one of their makerbots.
Well, it looks like I need to bump out the inner diameter just a bit; I over-sized it, but not quite enough.
Update: Shapeways managed to print one of the thin-wall bodies; it's a very nice part.
Update: Static fire of the little ABS Sears-Haack motor carrier. Enlarged the crack caused by the initial motor load; caused another crack on the opposite side of the fuselage; breaks introduced around the ringfin. The ejection charge successfully kicked the motor out the back.
Update: Bumped out the inner diameter to make loading the motor easier. Also made the outer diameter slightly smaller (now 20mm). The ringfin is twice as long, and cantelevered further aft to improve stability. The three support fins that connect the fuselage and the ringfin are thicker as well.
I've been playing with Blender's mesh Boolean operations to combine all the parts, but I've found that those methods usually result in really heinous meshes from a 3D-printing perspective. All sorts of overlapping faces with normals pointing everywhere. The best method I've found is just to Join all the parts into one object (without doing any Boolean ops), and then exporting the mesh in something meshlab can read (.ply). Then I use meshlab's Filters-->Cleaning and Reparing-->Remove Faces from Non Manif Edge. This usually results in a part that Shapeways is happy with (watertight geometry with consistent normals). After removing the non-manifold faces, if the normals need to be re-oriented use Filters-->Normals, Curvatures and Orientation-->Re-Orient all faces coherently in meshlab, and finally export the .stl for printing.