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Polecat Aerospace 4" Black Brant III

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Flyfalcons

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This will be a build thread/sorta review of the new Polecat Aerospace 4" Black Brant III! I was tempted to title this one "An Accidental Build", which I'll explain in a little bit. Anyway, I purchased this kit to go for my Level 2 certification, but also because A) I'm a big Black Brant fan, B) I wanted to do a "real" high power kit, more than just running a mid-power design on a baby H motor, and C) I've never tried or even seen a Polecat kit before, so doing something new should be fun! As an added bonus, I think the kit cost of $139 represents an excellent value for what you get, and Polecat Aerospace is probably the coolest company name in rocketry. I haven't seen many BBIIIs on the forum here or at the launches I've been to, so flying something a bit different from a typical DX3 or other sport rocket will be fun. Since this is my first real HPR rocket, I'm not doing this build as a "Here's how to do this", but more of a reference if others are interested in a Polecat kit (I haven't found a whole lot myself while searching the forum on them), and to solicit help and advice when I run across a problem I can't solve.

I will say though, that when I saw this kit on display by a vendor at the big launch I went to earlier this year, I thought I was looking at a Madcow BBII. I had the vendor ring me up for a Madcow, then he went to help me pull the kit out of the display area. "Oh hey, that's not a Madcow, those come in plastic bags. These boxes with the rocket outlines painted on are Polecats". No kidding. Hmm. I originally wanted to do a Madcow 4" BBII for my L2 project, but in speaking with the vendor about the differences between the kits in terms of construction, completeness, price, etc, I decided what the heck, let's try a Polecat! The Madcow offerings will always be there for another project (and I've since added three of their fiberglass kits to my build pile), and it's so much more fun to bring an actual kit home from the vendor display instead of ordering a kit and waiting for it to show up in the mail (he didn't have any Madcow 4" kits with him).

As you can see from the photo below, I thought I was taking a 4" Black Brant II home.

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Flyfalcons

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The High Power safety code requires that when you get a new kit, that you do a dry assembly and take cheesy photos of it. So when I got home from the event, I opened the box to check out the parts. Something odd occured, though. I couldn't find a boat tail piece, and the instructions were labeled Black Brant III, complete with a drawing of the III on the front. Wait, what? That doesn't quite match the outline paint on the front. Granted the box doesn't actually say it's a II, but it didn't say it is a III either! After checking the Polecat website to find that they don't even have a 4" BBII, and realizing that this version would be longer than the same diameter BBII, I figured what the heck, let's proceed with a BBIII build! So here we are. Safety code-required cheesy dry fit photos below.

BBIII box.jpg


BBIII dry fit.jpg


BBIII fit with Alice.jpg
 

Flyfalcons

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The kit was well-packaged and there were no damaged parts. The kit is fairly complete, including two 12' tubular nylon harnesses, avbay hardware, rail buttons, and U-bolt hardware.

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Flyfalcons

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For those unfamiliar with Polecat construction, the body tubes are cardboard, wrapped with fiberglass. This makes them light and strong, but they are not smooth out of the box. In addition to the open weave of the fabric, the seam lines are visible and there were some lifted areas on my body tubes as well. I used this as a good excuse to finally buy myself an orbital sander, and I'll be doing what I can to smooth down the imperfections of the fiberglass wrap. The weave will be taken care of partially by sanding, and the rest with a high-fill primer.

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Flyfalcons

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I have discovered some inconsistencies between the instructions and the actual kit. For starters, the instructions list the kit as including four fins and resin-molded spin motors. Of course there are only three fins, and there were no spin motors. I think this was by design and not something missing in the kit. The other inconsistency actually cost me a few unnecessary bucks. I took the instructions with me on a trip to review the construction steps and techniques. The instructions only show two centering rings used on the motor mount tube. I like having a third ring so I can install it without the aft ring, do internal fillets with the fins, then install the aft ring. So I called Polecat and ordered a centering ring (and some Aeropack goodies) for the build. Upon digging into my parts, I found that the kit does indeed include a third centering ring. So, alrighty then.

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PokerJones

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My only experience with Polecat is their 5.5" Thumper Jr that I built last year. I really like the kit but as you mentioned, the porosity of the fiberglass can be a bit of a challenge to fill and smooth properly. After 3 attempts of high build primer/sanding I still had quite a few pin holes in the surface.
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I used a thin layer of bondo over the trouble areas. It fills completely the first time, it dries quickly and it's very easy to sand. You will save a lot of time if you start with this step before priming.
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Finished

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iqsy59

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I have this one in my queue as well. It looks like a nice kit! I am looking forward to following your build.

Mark at Stickershock has a nice set of vinyl for this kit. Also, you may try painting your tubes with wood filler thinned with water to a thick paint consistency to fill the weave before priming. It works pretty well. Since it is a single wrap of glass, you don't want to sand too deeply into the weave.

Cheers,
Michael
 

dixontj93060

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I love Polecat kits. Beyond what had been suggested the two routes I have taken for fill are 1) SuperFil and, 2) (in my current Skeeter build) Bondz primer. Both work, but I believe the two thick coats of Bondz is easier. The only place I them needed to use filler was along the seam edges and to fix my not-so-perfect fillets.
 
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Flyfalcons

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Thanks for the suggestions on the filling, guys. I will definitely try to take off as little fiberglass as I can while sanding, though the high spots are going to have to go. I have already procured a vinyl set from Mark. He's an expert in getting me to give him my money. This set should turn out just as excellent as my other orders from him.

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Flyfalcons

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Continuing with the inconsistencies theme, the backplate for the top centering ring U-bolt is too large to fit between a 4" tube and 54mm motor mount. The picture in the manual is definitely from another kit, as it shows plenty of room available. Fortunately I have a band saw with metal-cutting blade, so trimming and deburring wasn't an issue for me.

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Flyfalcons

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With the backplate issue sorted out, the center and top centering rings have been added. I just need to add a fillet on the underside of the top ring and attach the harness, then I'll install the mount.

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dixontj93060

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Continuing with the inconsistencies theme, the backplate for the top centering ring U-bolt is too large to fit between a 4" tube and 54mm motor mount. The picture in the manual is definitely from another kit, as it shows plenty of room available. Fortunately I have a band saw with metal-cutting blade, so trimming and deburring wasn't an issue for me.
Wow, those u-bolt holes look big. Did they give you a 1/4"-20 or 5/16" u-bolt?
 

Flyfalcons

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Hmm, I'm actually not sure and now I'm on a six-day trip. I want to say they are 1/4" but will let you know when I get back home.
 

jd2cylman

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Tell me the wrench size and I'll tell you what nut/bolt size you have. 1/4" nuts use a 7/16" wrench and 5/16" nuts use a 1/2" wrench.

Adrian
 

dixontj93060

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Tell me the wrench size and I'll tell you what nut/bolt size you have. 1/4" nuts use a 7/16" wrench and 5/16" nuts use a 1/2" wrench.

Adrian
Is this always true? I ask because I have some #8 nuts of two sizes so I have to pull out both 3/8" and 5/16" wrenches :/.
 

Flyfalcons

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Wish I could remember but I'm on the road now so I'll let you know when I get back home. I'm pretty sure the hardware is 1/4".

If I could caption the photo below, it would be something like "Hey Dad, I see the bulkhead, but no vent band. What's up with that? The instructions have you epoxy the avbay to the upper body tube, isn't that old school and makes it tough to set a charge cap?" Excellent question, Alice. I'm not sure how I want to do the avbay. I'm pretty tempted to cut a vent band off the main body tube and glue that to the avbay coupler.

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Flyfalcons

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Oh, nearly forgot, sim file! I weighed and measured all of the components of the kit individually, as well as the Aeropack retainer, and used those to make this OR file. I can see now that I made a mistake with the nose cone bulkhead, as the kit bulkhead is much smaller and fits about halfway up the nose cone. And of course I have to guess on the avionics and parachute weights, and I'm sure there'll be some extra weight from glue and what not, but this is a good starting point for dreaming up what motors I'd like to fly. I'm sure for the most part I'll be using 38mm motors with the Aeropack adapter. Probably a J270W DMS or J350W RMS for my cert flight, nothing groundbreaking there.

BBIII Open Rocket.jpg


View attachment Polecat Black Brant III.ork
 

Tim51

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Thanks for the engaging build thread and commentary on the kit itself. I've been curious about Polecat kits for a while so I appreciate the detail.

Good luck with your cert flight.
 

dixontj93060

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Wish I could remember but I'm on the road now so I'll let you know when I get back home. I'm pretty sure the hardware is 1/4".

If I could caption the photo below, it would be something like "Hey Dad, I see the bulkhead, but no vent band. What's up with that? The instructions have you epoxy the avbay to the upper body tube, isn't that old school and makes it tough to set a charge cap?" Excellent question, Alice. I'm not sure how I want to do the avbay. I'm pretty tempted to cut a vent band off the main body tube and glue that to the avbay coupler.
The coupler is not very long so adding a vent band is going to shorten the coupling distance even more. On my builds, I have glued the coupler in, but left the fore bulkhead unattached. This allows you to prep charges outside of the rocket. You then assemble by sliding the fore bulkhead (with all-thread sections attached) down from the top and it fits snugly into the coupler section. Any vents, switches, or switch access holes are simply positioned in the glued area of the payload bay.
 

jd2cylman

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Is this always true? I ask because I have some #8 nuts of two sizes so I have to pull out both 3/8" and 5/16" wrenches :/.
Well, machine screws do tend to mess that up. Bolts 1/4" and up hold true UNLESS you have heavy nuts (or cheap hardware). Heavy nuts are the same internal thread, but are larger outside dimensions for more surface area. Most #8 nuts are 11/32" I think. And to confuse things even more, some brands of 3/4" nyloc lock nuts use a 1 1/16" wrench instead of an 1 1/8" wrench. But not all. Clear as mud, right??? Now you know why I have $120,000.00 in tools. Of course if I wanted to sell said tools, I'd be lucky to get $50,000.00. Such is life.

And now back to your regularly scheduled build thread....

Adrian
 

Flyfalcons

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Back home and will hopefully make some progress in the next few days. I checked the hardware - the U-bolts and nuts are 1/4-20. The holes drilled in the wood (at least of the remaining bulkheads I have) are 1/4", and the metal backplates are drilled to 5/16".

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Flyfalcons

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Last night I added the lower fillet on the upper centering ring and tied the nylon harness on. I just did a test fit to make sure the fins fit properly with the mount in place and all looks good. I am setting up the motor tube to be flush at the bottom of the body tube because I purchased a flanged retainer. If you get a standard retainer, you'll want to figure out how much the motor tube needs to overhang before gluing in the centering rings.

I had to give the fins slots a pass with my sanding block for a perfect fit, which I'd rather do than have loose-fitting fins from the factory.

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Flyfalcons

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The nose cone bulkhead needs some sanding to get the best fit inside the nose cone, where the fiberglass layers add high spots here and there. I used an alignment mark to make sure I was fitting the bulkhead in the same position every time I sanded it.

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Flyfalcons

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A giant epoxy fillet seems to hold the nose bulkhead in well enough.

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Flyfalcons

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I beveled the leading edges of the fins a bit. They don't go to a point; there is about a 3/32" thick, square leading edge on the fins to help in the durability department during ground handling. I also lightly broke all the other squared-off edges to help prevent paint cracking.

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Flyfalcons

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I'm attaching the fins as per the instructions, which follow the time-honored method of 1) Glue fin in place. 2) Spend the next 30 minutes second-guessing yourself and tweaking it by .008 degrees at a time until the glue sets and you can't move it anymore. 3) Once the glue is set, never sight down the fin again for fear of noticing it being off by .003 degrees.

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Flyfalcons

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I did use the "double butter" method of gluing the fin, which results in a decent internal fillet. The instructions say an external fillet from the fin to body tube is unnecessary, but I'll still be doing those as well.

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Flyfalcons

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I had to take a picture of my 40% scale BBIII next to my 43% scale Edge 540.

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Flyfalcons

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Laying some Bondo down while the first set of fillets are curing. Looks like the Bondo will do a great job of filling the fabric weave while not being too difficult to sand down. So thanks to Jeff for the suggestion on that! Fillets are plain BSI 30-minute epoxy.

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