Modular rockeTs Building Baffles - National Association of Rocketry

Above: Photo 6. Dry-fit test. The outerchamber is in place in the fin can,and the middle chamber/bulkheadis lined up for insertion.Right: Photo 7. All three pieces dryfitted into the fin can. Parts aresanded as needed to ensure a cleanfit before assembly.by applying a generous amount of epoxyaround inside of the coupler/stiffener recessededge and about 2" inside the stiffeneritself. I also double coated the previouslytraced face of the bulkhead and its circularedge. The bulkhead/middle chamber wasthen placed into the coupler with ease andthe epoxy was allowed to set (see Photo 8).With the upper chambers finished, I wasready to epoxy the whole coupler assemblyinto the fin can. I mixed a large batch of30-minute epoxy and filleted the inside offin can and upper centering ring. Workingfast I brushed on the remainder of the epoxyall around the inside of the body tube.I saved just a little for the recessed area ofthe coupler bottom. With the 30-minuteepoxy starting to stiffen I pushed the baffleinto the fin can down to the centering ring.The baffle was seated to the halfway markthat I had previous marked on it, so I knewit was home against the centering ring bafflebase—and none too soon, as it was immovableat that point. See Photo 9.Photo 9 also shows the last thing I didto the baffle/bulkhead. I filled the 1/16"recessed area in with 30-minute epoxy tostrengthen and protect the forward face ofthe wood bulkhead.It’s interesting to note that the completedbaffle/bulkhead unit (if you includethe 10" of extended 38mm motor tube andthe upper (third) centering ring of the finPhoto 8. A close uplooking into the upperchambers of the baffle.Note the epoxy fillets onthe bulkhead/stiffenerwalls and on the inside/outside walls of the middlechamber tube. Note thestiffener recessed a 1/4"into the bottom end of thecoupler. The stiffenerwalls are 1/8" thick.can) has a total weight of over 24 ounces.This is a dramatic increase over the baffleweights in my earlier 2.56" and 4" models.Of course, the coupler stiffener weighs inat 8 ounces by itself, and the area of theparts goes up geometrically. Still, that’s a lotof weight—but I did say I wanted a large,heavy model.Before attaching the fins I measured betweenthe fin slots and marked a spot halfway between each fin pair to have a referencefor alignment of the adjacent fin. I testfitted each fin and marked the outline of itsroot edge on the body. I mixed a very smallamount of 5-minute epoxy for each fin andcoated the bottom of the fin root and sideof the tabs. I also placed a small amount onthe body in the fin outline. Working the findown into the slot I aligned the fin with themark across from two opposite fin splits.Before the epoxy set I wiped the joint toremove any fillet material. The fin tabs protrudedinto the body by only 1/16" or so.Another upgrade was to reinforce thefins. After all three fins were set in place Icut six 3/16" dowels for fairings at the fin/body joint. These were cut and shaped tomatch the lengths of the root edges of thefins. I placed a very large epoxy fillet alongeach fin joint and pushed a dowel into thatepoxy to form a double joint. This reallystiffened up those fins. Later I went backto re-fillet those double joints and laminatethe exposed sides of the dowels. See Photo10.One of the reasons for the three centeringring system is to be able to epoxyand support the fin tabs behind the rearcentering ring. However, with the Minie-Magg fins, they only go through the body1/16". So to upgrade that I made three32 MAY/JUNE 2010 Sport Rocketry