Competitor 3

After working on the 54mm minimum diameter KestreL kit, I was eager to build a minimum diameter 76mm bird for my 76-6000Ns motor. I looked several places and found some great kits, however, the prices seemed a bit steep. I finally decided to buy a Performance Rocketry Competitor 3 (which comes with 54mm motor mount) and convert it to minimum diameter…and by convert I mean trim the fin tabs and carbon fiber the fin can. Even though I should get some respectable altitude from this rocket it won’t be optimal for a number reasons. The first is that I’ll be using the stock ogive nose cone instead of a Von Karman. Additionally the supplied fins are a tad thick to be used with the tip-to-tip carbon reinforcement, so i’ll just add two layers to each side. These are just to name a couple, however, it should still be pretty awesome.

The first thing I did after opening the box is closely examine the components to make sure everything looked okay. A few things to note is that the parts were “decent” but seemed a little rough around the edges, so to speak. In order to make this kit minimum diameter, I needed to trim the root edge of the fin tabs that would normally bond to the 54mm motor mount. Using my dremel I cut off the excess G10 with a carbide cutting wheel. After cutting, I spent some time roughing up the surface of the fins with some 60 grit sandpaper and a razor to provide a better bonding surface for the upcoming carbon reinforcement.

To tack the fins in the airframe, I placed a long strip of packing tape over the fin slots from inside the airframe. Once the tape was in place I put a 76mm motor case inside the airframe for the fins to position and rest against. A small amount of JB weld was placed on the root edge of the fin and aligned in the airframe with tape and allowed to dry.

Before I go any further I should state that all surfaces to be bonded are first prepped by sanding with coarse grit sandpaper and wiped clean with acetone. Once all the fins were tacked into place I was ready to lay the first set of fin fillets using JB weld. Using a 1” fender washer I “scored” the joint between the fin and the airframe to mark where I taped off the fillets. I then poured epoxy in the joint and used the same 1” diameter washer to smooth out the fillet and create a .5” radius. After all the first fillets were completed, I applied a second set of fillets using the same procedure with Aeropoxy Light and a 1.5” fender washer (this step is optional). The reason I use the Aeropoxy Light is to increase the fillet radius allowing a easy transition for the carbon fiber tip-to-tip reinforcement to lay over.

UPDATE: The maiden flight of the Competitor 3 at Balls 19 was a great disappointment ending with a bang. Upon ignition the motor catoed spectacularly destroying the booster section, everything above seems salvageable.


  1. #1 by David Reese on August 19, 2010 - 3:18 pm

    Oh man, genius nose cone bulkplate attachment method. I am so stealing that.

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