I forgot you hollowed them out. If you want to do something like that again, take some kraft paper (ie, paper grocery sacks) and cut it into ~3/8" wide strips. Glue them to the inside walls of the hollowed transitions extending into to attached tube, overlapping one strip with the next. This will really beef up that section without adding lots of weight. By being in strips, they won't tighten and lift when the (yellow) glue shrinks.

I made this rocket's long transition from card stock, then lined it with kraft bits. It's hard as a rock, or "quite sturdy" as they might say across the pond

http://home.flash.net/~samily/toyrocket/

Dope and tissue might be another option, but again, I'd go with strips to avoid shrinkage problems

Doug
Lessee what Bill does with that straight line