THIS THREAD IS PERFECT!!! My LDRS Drag Race Special Punisher is going to be my first FG build and this already answers so many questions. Naturally, I have a couple more
1.-I see the "encapsulate with epoxy" technique a lot on these builds for the shock cord at the motor mount. Is there any advantage (or any reason not) to have a knot in the shock cord just below the mmt? So on the off chance that the gobs of epoxy don't hold it in, the knot would just butt up against the CR?
2.-I've heard so many good things about Rocketpoxy for fillets; if I recall, you let it set up for ~30 minutes until it starts getting thick, then lay it on. Do I need to add microballons or should I just lay it down for the fillets?
3.-I have both JBWeld and RocketPoxy. Is there a good reason to use one vs. the other for the MMT?
4.-Is there any reason not to do internal fillets?
5.-Can you tell me more about the Y-harness? Why not run a single piece of Kevlar from mmt to NC?
Thanks so much for this thread, it explains so much.
1. No knot needed. In 14 yrs of doing this & instructing others on "how to'' I have never seen it fail. Even on large projects with 4in motor tubes flying N motors. All the force is in 'pull', not 'peel'. shock cord is pulling up through airframe.
2. NEVER, EVER add ANYTHING to Rocketpoxy. It already has all the fillers needed in it. Adding anything else just weakens it. Timing is the key for successful use of RP. Totally time & temperature dependent, when mixed and used.
It goes through 3 basic phases.
Using 70 degrees ambient temp: first 15-20 minutes, runny enough for normal epoxy glueing of parts. 2nd 15-30-40 minutes, stiff enough to be used for external fillets, placed in V-groove, can be pulled with tool and hold fillet shape. 3rd. 40-60 minutes, can be rolled into strings or balls and used like epoxy clay.
Best mixed on cardboard. run 2 equal lines of resin & hardener along side each other. Add few drops of color if using. Pull, scrape,& mix just like using Bondo until color is same. Spread out on cardboard like cake frosting to allow air bubbles to rise and escape. [for several minutes when doing fillets] then use. Higher or lower ambient temp will result in shorter or longer cure times. At 85-90 outside, it will speed up considerably. Click on pic to see air bubbles rising. You want them gone when doing fillets or they will appear in them.
3. On small projects with 29mm motor mounts, there is not enough ''heat soak" [transfer of heat from motor to retainer] to really matter. I use RP or JB. Depends on what I have already mixed to do something else, just use left over to put retainer on. For 38mm and up size motors I use JB for it's high temp qualities. [this is my opinion only] I have never lost a retainer & I use dozens of them.
4. Once again, small project. If you do "double dip" method on fin root when installing fins, there really is no need for internal fillets, as long as you do nice externals. The rocket doesn't weigh enough to exert enough force to cause damage by knocking fin loose......unless you hit a paved road or rock in field. In that case, I would rather knock a fin loose, instead of breaking a fin. I save internal fillets for larger heavier projects. Or in some cases where a small rocket will be using the largest motor made to fly. [38's & up.]
5.Actually you can just run the shock cord on one side of MM tube. Many of us do, I do sometimes. Only on small stuff. Using a "Y"-harness keeps the fincan falling straight, not tilted to one side, which a single cord does.:smile:
PS I'm the guy who was in the red truck that pulled over to let you pass on the field at Camden last weekend. We briefly met at the flight line. I flew my Punisher Sport SS [38mm motor & Up graded NC] with a GPS installed. You are lucky to fly at one of the oldest clubs around, with some of the best fliers & builders in the country....by the way.