Black Brant II: BT-50 size for 18mm motors

The Black Brant II is simply one of the coolest rocket designs ever made. I have had a fascination with it since I was young and chose an Estes BBII kit to be my first model rocket. Since getting back into rocketry last year, I was shocked to find there are no low power Black Brant II models in production at this time. I did manage to snag a couple Estes kits and built one of them, but these kits are getting harder to find, and it's still a scary thing for me to fly one anywhere but a pretty decent sized club field. So I figured if no one else was going to do it, that I'd simply do it myself and scratch-build a smaller version for 18mm! After scoring a deal on a Designers Special kit and learning it included the Estes PNC-50BB nose cone, I decided that this was as good of a time as any to start sketching. Having a freshly-built Estes kit in front of me to grab measurements from sealed the deal. Before I knew it, my jumbled thoughts were put to paper. The results are a 73% downscale of the Estes BT-55 Black Brant II.

After compiling my dimensions on paper, it was time to see how everything looked in Open Rocket. Fair warning! I did this open rocket file simply as a stability validation of the design, as I was interested in motor placement and the weight of the motor mount parts pretty far aft. Being that this is a friction-fit setup for the motor, I wanted it to stick out 3/8" so I could get a good grip on it for removal (the Estes version is the same way - motor sticks out a little ways). The other problem I had was not being able to accurately model the payload section of the PNC-50BB, so there is extra body tube length in the .ork file. Additionally, since I was mostly interested in stability, the body tube is missing the gap where the wasp waist is.

Without overriding any weight and balance info, preliminary design showed the rocket to be slightly on the marginally stable side with a heavy C6-5. I added in just 0.1 ounces of weight to the nose and that cleared things up no problem, while changing predicted altitude by just a few feet. Once the rocket it complete, I'll see where it actually balances and whether I will want to use some clay up front.

View attachment Mini Black Brant II 18mm.ork

I'll be watching this - I think the Black Brant is pretty cool, too....

To warm the build up, I started with the motor mount assembly.

The motor block was placed 2-3/8" into the motor mount tube, leaving 3/8" of the motor exposed.

You may notice three centering rings (Estes 2050 rings) on my sketch, but four centering rings in the picture below. In trial fitting the main body tube, I realized attaching it to the edge of the center ring wasn't going to give the motor mount enough stability and strength to make me happy. So I added a second ring above the center ring that the body tube will fully slide over later.

The bottom of the bottom centering ring is 9/16" from the bottom of the MM tube.

The bottom of the second centering ring is 1-3/16" from the bottom of the MM tube.

The third centering ring sits on top of the second centering ring.

I didn't get a measurement for the top centering ring. Just place it somewhere near the top.

In my sketch, you'll see I figured the nose cone would need a 1/2" extension for the payload bay. After some trial fitting, I decided the way the PNC-50BB is designed made that particular length difficult to accomplish (there would have been very little overhang for a tubing coupler to attach to). So in the end, I decided to simply add that length to the body tube and paint the top 1/2" of the tube all black. So, for my model, the main body tube length became 10.35". I think I used 10-3/8" on the ruler. 10.375 is close enough for me.

Long story short, be cool and be like Ryan. Cut the main body tube 10-3/8".

Then throw some glue in the bottom and slide the tube over the top and upper middle centering rings on the motor mount, just to where the body tube covers up the gap between the to centering rings.

*someone remind me if I forget, to detail my thoughts on how to do this slightly better next time.*

The lower body tube is .64" on my drawing. 5/8" will do just fine. So cut a BT-50 tube to 5/8".

Next, glue that tube to the bottom centering ring and the lower edge of the lower centering ring. Be sure to leave a ledge on the lower centering ring exposed so the boat tail transition has something to center up against.

The result should look pretty close to the picture below.

My .ork file gives you a nice outline to cut the paper wrap for the boat tail. Did I use it? Of course not as I didn't know at the time that OR would do that for me. So I wag'd it by using the templates in the Designer Special kit. The nice thing about the paper wrap is you have easy access to just wrap it up right on the body tube, check fit, then glue it together and glue the assembly to the body tube. If there are small gaps in the upper joint, a little filler fixes that. A little overhang on the bottom? Some sandpaper will do nicely.

We're almost done with the body tube here.

Now grab some sandpaper and do what you will to the wasp waist. Is mine a perfect scale outline, heck no, but you get the point. I started with 180 grit and ended up with 320 or 400, something like that. Some sanding sealer would be a great idea at this time, much better to do it now than think "man I should have used sanding sealer" after painting it like I totally didn't think to myself......

Cutting out the fins is next. I cut mine from 3/32" balsa. Here's the trick to the fins:

Open Rocket doesn't allow me to attach the fins to the transition. So what I did was cut out the outline given to me by OR, place the fin against the body tube in position, and add another small pieces of paper to the rear of the fin to sit against the transition and complete the shape of the fin. I then used that outline to actually cut my fins. If you look closely at the picture below, you can see what I did with the extra piece of paper.

Oh, to position the fins, make a mark 2.09" from the front of the fin at the root - that mark should sit at the top of the transition.

Here I am fitting the first fin, and I cut the next two based on that one. This is a good time to get them all sanded up and sealed blah blah you guys know what to do. Just make 'em pretty.

Ok... Estes makes a PNC-50BB nosecone, it's not a simple cone. It has a cylindrical section aft of the conic section. How are you planning on doing the nosecone?

I made up a tube marking guide (remember kids, three fins..........oh and a launch lug line), and marked fin locations. Make sure to get one of the fin lines over the boat tail overlap. That way the fin will hide that little nastiness and no overlap will be visible in the finished product.

K'Tesh said:

Ok... Estes makes a PNC-50BB nosecone, it's not a simple cone. It has a cylindrical section aft of the conic section. How are you planning on doing the nosecone?

Click to expand...

In open rocket? I'm not. That's up to our resident OR ninja, which would be you. Have fun! The body tube in my .ork file accounts for the cylindrical section of the nose cone.

Time to glue them friggen fins on. Make the business end of your rocket look something like this.

This is coming alone nicely!

I was originally going to do one launch lug, but after some contemplation on how the paint scheme was going to go with masking or decals and all that, I decided that Estes had the right idea by going with two. I like using two anyway, so two it is. My lower launch lug is 1/2" long and sits on the lower tube. The upper lug needs to be clear of the bottom 1/3rd of the body tube, so as not to interfere with the four black stripes. 1/3 of the main body tube is 3.46" long, so place the upper lug somewhere north of that, like 3-5/8".

Don't forget your fillets, kids! I found I was able to fill the area between the fins and wasp waist with wood glue. No need for small balsa filler chunks.

Time to make this hawt 3FNC look even better by getting the color on. After some thought, I decided to emulate the Estes strategy for finishing this rocket. First things first, a nice priming and painting of the base layer white (gloss, of course!).

I know that I could mask the roll pattern and do everything in paint. I decided instead, to cut the roll pattern from self-stick Monokote instead. Doing it this way will guarantee sharp edges. It might also guarantee wrinkles when I take the rocket out on a hot, sunny day, so we'll see on that. I would have preferred to use a lightweight, thin vinyl, but that required effort to find and my hobby shop had a nice, juicy sheet of self-stick Monokote ready to go. I took the easy way out. I did paint the fins and wasp waist black first (and the nose cone, duh). The nice thing is the cuts needed are mostly simple rectangles, so that made the decision pretty easy.

Also, as promised earlier, the top 1/2" of the body tube is black! (tough to see in the photo below)

Flyfalcons said:

I made up a tube marking guide (remember kids, three fins..........oh and a launch lug line), and marked fin locations. Make sure to get one of the fin lines over the boat tail overlap. That way the fin will hide that little nastiness and no overlap will be visible in the finished product.

Click to expand...

Working the .ork file... How wide is your gap between the forward body tube and the aft one?

There is a method of faking a fin attached to a transition. You create a phantom body tube behind the transition (same diameter as the back end of the transition, but nothing long) then attach the fins to that. That allows you to create a fin. Its starting and ending coordinates will be X 0.0" and Y 0.0", and everything forward of the attachment point will be a negative X value.

I gave a small allowance for the silver stripe on the wasp waist, then measured what was left of the white portion of the body tube and divided by 3. That gave me a stripe section height of 83.3mm. So I cut a section of Monokote trim just slightly larger than that. The lower stripes are 3/8" wide, the two middle black stripes are 3/4" wide*, and the top stripe is 1-1/2" wide. The cutouts in the top stripe start 1/4" from the top, go in 1/4", down 1/2", and back out 1/4". This will allow the silver black brant logos to be placed later.

My best advice for doing the roll pattern is to go to Madcow Rocketry's website and see their diagram on how it is applied. It is a complicated scheme but certainly not impossible if you just take it one step at a time.

Start with the lower stripes. You'll need a master stripe that sits midway between two fins. In order to ensure the upper launch lug stays out of the way, try to arrange the stripes the way I did - if you set the rocket upright and look straight down on it, rotate it so the launch lug is on the top. Your master black stripe should sit in the area on the lower left. You can see in the pics below that I am showing the master stripe, with the launch lug sitting up (clockwise looking down the rocket).

After the master stripe is laid down, I recommend attaching the stripe on the opposite side. This stripe will need to be trimmed to fit around the fin. Take your time on this one.

Next, add the remaining two stripes. On my rocket, they each sat against the fins to get even spacing on all stripes. So that worked out well.

You can see how the second stripe attaches. Here's where the * comes in from earlier. Madcow's diagram shows two different widths on the middle stripes, and I cut mine the same width. My Estes kit had them the same width, and positioning them was super easy, so I just went with that. If someone gives you crap over the width of your middle stripes, you have my permission to use the BBII's conical nose cone in mean-spirited and nefarious ways against the perp.

The top stripe is last. Line up its left edge with the left edge of the lower stripe. I thought I got mine down well, but looking at the pics below, it may actually be off just a bit. So when I get home from my trip I'll have another look at it and decide whether to cut and place another one. That's the beauty of these monokote stripes - you can do them over until you're happy.

So that's how she sits now. When I get back from my trip I'll get the silver on and figure something out for the CARDE lettering. I cannot tell you how absolutely pumped I am to have such a bad ass little Black Brant II in my stable! I'll be sure to get more pics in as the finishing touches are placed, and will definitely get some flight video. Unlike my OOP Estes kit, I think this one's going to be seeing lots of flights! Exactly what I wanted - a stress-free Black Brant II that I made all by myself, and could easily do again if needed!

K'Tesh said:

Working the .ork file... How wide is your gap between the forward body tube and the aft one?

Click to expand...

My sketch called for .18", but I think it came out closer to 1/8", maybe just a hair more. Maybe split the difference and call it .15". The pic in post #7 has a ruler up against the body tube gap.

I'm not about to tackle the roll pattern, but here's what I was able to do with the information I got from you Flyfalcons






I decided to stick with the 1/8" measurement for the wasp waist.

To simulate the wasp waist I used a pair of transitions (solid) to create the gap. I also added a phantom body tube to attach a filler for the gap between the fins and the wasp waist.

I managed to "surface mount" the fins to the rocket. To fill the gap between the tail cone and the fin, I attached the fins by using a phantom body tube in the manner I mentioned before. The first and last points of the fin are at the coordinates 0,0. Everything forward of that point is a negative X value, and past it is a positive X value.

View attachment Mini Black Brant II 18mm (mod).ork

Very nice work! Thanks for cleaning up the file and making it more presentable!

Saw this model fly about two and a half hours ago - very nice. Very nice indeed.

And Ryan was able to rescue it from the rocket-hiding blackberries at the north margin of our field...after a bit of a search....

That turned out AWESOME!!!!

:clap:

Outstanding job! Great looking Black Brant II.

Did you get a Video?!

Got video! Haven't had a chance to review it yet but will try to get it up tonight. Embarassingly, I haven't put the silver trim on yet. I did pick up a sheet of stick-on silver Monokote on the way home, so I'll be able to truly finish it soon. And yes, the top roll stripe did end up at a slight angle, enough that I'll be pulling it and cutting a new one. No biggie. Open Rocket calls for around 600 feet on a B6-4 and I believe my rocket saw every bit of that. Very impressive performer!

Got the silver on. Really helps to bring the whole look together. Also redid the top black stripe. Still need to figure out something for the CARDE lettering.