Posted for Dan O'Brien <limadelta@gmail.com>:

 Most of the strength of a sandwich structure comes from the separation
 of the skins.  It doesn't matter much whether the core is foam or
 honeycomb, as long as it can bear the relatively small shear loads at
 each point along the rib.  I've vowed that if I need any more prepreg
 for do-dads I want in the interior, I'm going to fabricate my own foam
 sandwiches.  I made up some sample ribs for a presentation on
 composites I did for my 4th grader's class, and they're strong as can
 be.  Can't break em.  I imagine they're a little heavier than pre-preg
 though.