The scoop that I got was that on Tuesday 7-29 the scouts are supposed to bring (2) empty 2 liter bottles to their activity that afternoon. This is when they will make their Bottle Rockets that will then be launched at pack meeting on Tuesday 8-5.
Also, if you have other children going to pack meeting that may want to participate in this...that they will need to bring (to pack mtg) their rockets already made from home. I think that the scouts will be bringing home instructions on Tuesday.
I hope that all of this is right and that it helps. Any questions call you scout leader.