Monday, October 31, 2016

Rocketober, Nearly Over

Rocketober 31: Halloween Theme - Spooknik!
This month, I've been posting photos on Twitter for #Rocketober. They'll appear here with slightly expanded text.

October has gone fast! While I haven't kept up with the Apogee Components #Rocketober theme of the day on Twitter, I have been building. For those following the Sky Wolf rocket build, that will continue until the rocket is finished, despite the fact that I haven't completed it within the month. But it's nearly done, and I've had a couple projects on the table, so I feel I've been productive.

I'm planning more material for beginners, which is what The Rocket N00b is meant to be about, and once these other three or four rockets are completed, I'll have some more good stuff for you.

For now, however, the night grows short - the witching hour is near at hand, and Halloween is almost gone! So, let's get back to Sky Wolf.

To catch you up, Sky Wolf is a 38mm diameter mid power rocket, sold by, and created by Sky Pyrates. It's one of a couple designs created for the 2015 National Sport Launch as a limited edition commemorative rocket. The 38mm tube is about the same outer diameter as a BT-60, so the rocket is roughly the same size as something like the Estes Big Bertha, but sturdier and much more powerful - it can fly on anything from a D to an H motor!

I made a lot of work for myself attempting to sand the fins into streamlined, airfoil shapes. Plywood is much harder than balsa, and after an hour working on one fin, I set everything aside for a few days.

When I returned to building a few days later, I realized the problem: I had been using 220 grit sandpaper.

220 grit is a medium fine grit, which is fine for shaping balsa fins, but far too fine for shaping tougher plywood. I switched to 150 grit and got back to it. Sanding fins to shape can be tedious work, so it's helpful to have a CD player in the workshop.

I also realized that the Sky Wolf fins aren't supposed to be airfoiled. Airfoiling isn't necessary on any model rocket. Some say good airfoils will help you achieve a higher altitude (some say it won't really, but we'll get to that in another post). I do it because I like the finished look of a nice airfoil shape.

But Sky Wolf was designed to whistle in flight - to get the best results, you're meant to merely bevel the leading edge of the forward fin, and leave all other edges square. The way I've shaped these fins, the rocket may not whistle.

But it will look great, and I was committed once I did the first fin. So forward I went.

Continuing to stack fins together to compare them, then sanding the uneven spots, eventually you get to the point where you've sanded enough, and it's time to stop. Now, if you look at the above photos, you can see that the airfoils are a little uneven.

Sanding decent airfoils in model rocket fins is surprisingly not terribly difficult. It takes a little practice, and it is a challenge, but it's not hard. Still, it does take patience. If you rush, you'll get sloppy airfoils.

After an hour and a half or so working on these, you can see I got a little tired, and they came out uneven.

While I do sand airfoils in the hope that it might help increase the altitude of the rocket, this probably won't be the case here. An uneven airfoil can cause the rocket to roll, or spin about its vertical axis.

Uneven airfoil over the fins can cause constant lift, an aerodynamic force, on one side of the fins, which may cause the rocket to spin rapidly in flight. That spinning takes energy, and the process increases aerodynamic drag on the rocket. Lift is a corrective force which pushes on the fins when a rocket starts to deviate from its intended path in flight, but constant lift on one side of all the fins can cause the rocket to spin.

Lift is a stabilizing force, but it also causes drag. Lift on one side of the fins can cause roll - spin around the vertical axis.

But most model rockets roll to some extent, regardless of whether the fins are streamlined, or how good the streamlining is. Building a rocket that doesn't roll at all is pretty hard. Since, at this point, I've decided I'm going for looks over performance, I don't mind. You can see how uneven the fins are in the above photo, where they're sandwiched together, but once they're on the rocket, you'll have to look pretty critically at the fins to see the flaw. The rocket will look great - and regardless of roll, will still fly very high.

Next I moved in to the aft fins. I started the beveled edge with a wide angle - about 35 to 45 degrees per side. Again, I'd drawn a guide line down the middle of the fins' edges to help keep my sanding centered.

After that, I slowly sanded the bevel angle down until I reached the guide lines I had made on the faces of the fins. (I keep referring to the flat plane of fins as "the face." I don't think that's correct, but I'm not sure what to call it. Perhaps the "span" or "chord." Someone may tell me in the comments.)

Again, I went a little too far on some of the sanding, and they came out pretty good, but not perfect.

Still, looking at each fin individually, they look good enough, cosmetically speaking.

Finally, it was time for one of my favorite parts, when the rocket actually looks like a rocket - attaching the fins.

Getting the fins to go straight up the body tube was no problem, since I had cut the fin slots. But to keep the fins perpendicular to the body tube, I used the Guillotine Fin Jig.

Attaching the first fin set. As per instructions, I used slow cure epoxy.

The fin jig not only keeps the fins perpendicular to the airframe, but in line with each other.

Finally it was time for fin fillets. For these, I again used 30 minute epoxy, mixed with a filler called microballoons. These are microscopic spheres of silica glass. They make the epoxy less dense, so the fillets will add less weight. They make the epoxy more viscous and less prone to sagging, so the fillets will hold their shape as they cure. And they make epoxy easier to sand, if necessary.

I did only one fillet to begin with. The microballoons give the epoxy its milky white appearance.

I went with a fairly small fillet radius, using a thin dowel rod dipped in rubbing alcohol to shape it. I also need the epoxy to fill in any gaps in the fin slot. Once the epoxy is cured, I'll need to check with a fingernail to be sure the gap is filled. If it isn't, I'll go over the fillet with one of a slightly larger radius.

It can be hard to see if the slot is really filled just by eye. But if it isn't, it will certainly show up when I put on primer. I'll need to check it by feel before I get to that point.

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