The next step was to install the motor mount assembly into the body tube. Now since the fin tabs on this model were fairly short, there was a lot of play as to how far into the rear of the rocket I could install the motor mount. The decision here is whether you want the motor retainer to be completely recessed into the airframe which has the single advantage of allowing the rocket to stand on its own. The disadvantage is that it leaves a large lip of un-reinforced body tube exposed to potential landing damage if the bottom impacts the ground very hard. Conversely, if you let the motor retainer stick out past the bottom of the rocket, you are better protected against hard landings but the rocket will likely not stand on its own without some kind of display stand holding it steady.
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| Figure 1 - Installing the motor mount assembly |
As figure 1 shows, I chose to let the motor mount stick out past the end of the rocket for three reasons. 1.) I'd rather build a display stand than risk cracking the bottom of the airframe on landing. 2.) It keeps the heat of the motor thrust further away from the airframe, and 3.) I just like the aesthetics of the protruding motor retainer better.
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| Figure 2 - The Nike Smoke fin |
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Figure 3 - Nike Smoke fin cross-section
(from G.Harry Stein's drawing) |
The next step in construction was the attachment of the fins. The fins on this kit were very nicely done and much closer to scale than other 4 inch Nike Smokes on the market. As figure's 2 and 3 show, Performance Rocketry did a nice job getting the bevels correct. Two additional task for this step were widening the fin slots a smidge and shortening the fin tabs by about 1/4". This allowed them to insert into the fin slots and seat nicely against the motor tube.
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| Figure 3 - Attaching the first fin |
I chose to use JB Weld epoxy again here. For one, I wanted the heat resistant properties JB offers for the glue joint on the base of the fin tab where it contacts the motor tube and secondly, although I certainly didn't need it for the fillets between the root edge of the fin and the airframe, I just didn't feel like messing with two different epoxies. Ideally, I probably could have used something like West System or some other slow cure, high strength epoxy, but I think the JB is up to the job and I'm not that concerned about the extra few ounces in weight.
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| Figure 4 - Attaching the remaining fins |
The three remaining fins were attached one at a time allowing several hours between each one for the JB Weld to set up. Once all the fins were attached, it was time to install the rail buttons.
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| Figure 5 - Rail button with standoff |
The Nike Smoke has a unique nosecone that at it's widest point is larger in diameter than the airframe. This necessitates that use of "standoffs" for the rail buttons so that the nose cone won't bind on launch rail when the rocket is on the pad. I opted to use 1/2" nylon spacers I found at Ace Hardware for the job. I also had to get longer screws so that the threads would get some "bite" into the plywood centering rings I was using as anchor points.
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| Figure 6 - Rail buttons installed |
In figure 6 above, you can see the rail buttons installed and tapped into the built up centering rings. One design concern I have here is if I have enough seperation on the rail buttons. I'd like to have had another 6" but then I would have had to use a T-Nut epoxied to the inside of the airframe and I really didn't want to do that. If this configuration does not work for some reason I can still go back and move the upper button, but we'll try this first.
Next up is the modifications I made to the Nosecone of the rocket to accomodate a small Av-Bay. This was by far the most complex and time consuming part of the build, so it deserves it's own entry.
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