Staged Black Brant XII Build

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micro

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Having some good experience with successful airstarted staging under my belt I thought I'd document my latest build for anyone interested in following along.

The BBXII is actually a four stage sounding rocket, but only three stages have fins since the fourth stage burns after being propelled beyond the atmosphere. I will be building it as a three stage model. There are other three stage BB variations but I happen to have the Rockets of the World 2002 supplement which contains the XII, so that's what I'm building, deal with it! :)

This will be a low power model, motors will be 29mm, 24mm, and 24mm. My last project was a high power three stage, 38mm to 29mm to 29mm, but the electronics were small enough I thought gee I could use them with low power. This project should be challenging due to smaller size, varying body tube diameters, and a few other constraints. In my high power model I used Raven3's near the fins for airstarting and a Stratologger in the sustainer for dual deployment, but this one will not have room for the Raven3 to be placed near the motors. I will have to bring down wiring from Raven3's placed higher in the bodies.

I started by drawing a full scale model in OpenRocket. I just used the scale data and created all the major components to the actual dimensions, not worrying about materials or anything like that yet.

1to1drawing.jpg
 

BDB

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This, I have to see.
 

micro

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You must have a big launch area! What's the altitude this simmed to?
There are some nice fields around here, but I'm not going for any altitude record. One of my favorite parts about flying rockets is getting them back safely and in flyable condition. My "working" sim currently shows over 2,000 feet on G to D to D motors but that is a very rough guess that will keep going down as I build it out and use actual masses. I figured 29mm to 24mm to 24mm would be able to fly safely at its heaviest, but I can always put smaller motors in it. Also, the sustainer will be dual deploy, so that helps.
 

micro

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Once I had the full scale drawing I started to use the "scale" function in OpenRocket to see what things would look like as far as body tubes. Basically scaling it to an available body tube size for the largest diameter and then seeing if there were tubes available for the smaller diameters that were close. Just a "guess and check" method. I found the best fit with BT-80, ST-20, and BT-58, all available from eRockets/Semroc.

scaling.jpg

Once I knew my body tube sizes I started to build it out in OpenRocket. First I replaced the main parts with actual tubes from the catalog. For example the first booster that was to be a BT-80, so I looked at the calculated scale length, chose the preset tube from the catalog, and then just adjusted the length back to what was calculated.

bt_selection.jpg
 

MaxQ

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Nice.
Looking forward to your build.
I've been looking at this same subject and the Argo D-4 Javelin and D-8 Journeyman as a scale modeling project for awhile.

Decided on a Terrier Orion instead - as a test bed for evaluating several electronics packages for cluster/2 stage projects.
Think your conservative approach in motor size for a three stage project is very wise too.
 

micro

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With the basic body drawn up I add some motor mounts in the drawing and being piecing together the details. I get out the tubes to start to visualize things and go back and forth between the workbench and the drawing.

On this model, because I have differing sized body diameters, I will be mounting a short length of tubing matching the upper stage inside the lower stage as a coupling. For example, the sustainer is BT-58 and the middle stage is ST-20, so I will mount a section of BT-58 inside the upper portion of the ST-20 to couple the sections together. This actually doubles as a channel to bring wiring down between the concentric tubes to the motor for the airstarts, as well as adds lots of physical strength. At this point my sim looks like this:

motormounts.jpg

You'll notice I extend the sustainer's motor mount all the way up to where the altimeter bay will be, this adds strength and will provide a channel for the wiring.

Between the first and second stages you can see there is both a tail cone and a transition that meet with a smaller diameter, so these sections will be coupled using an internal BT-60.

At this point I start building the altimeter bay for the sustainer.
 

micro

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I don't have any long coupler material for the BT-58 so what I've done in the past is just glue two couplers end to end and glue the switch ring to the outside. This makes for a strong assembly. I left about one body diameter on the aft end where the bay will slip fit into the body tube, but I didn't need as much length on the top side since I will be securing it to the upper airframe with screws.

altbay.jpg

For the bulkhead ends I like to simply double up some centering rings. I need to make one ring at each end a little smaller in diameter to fit inside the coupler. I grab my drill press drum sander kit and find a mandrel that will fit inside the rings. The mandrels have a rubber cylinder that expands as you compress it with the nut on the end so it holds onto the sanding drum. I just use this to grip my rings, load it in the drill press, and use a course grit sandpaper to turn them down just a little bit.

altbaycentrings.jpgsizingcentrings.jpg

I drill a couple holes in each bulkhead sandwich after gluing them for the posts or rails that will hold the ends together. When I drill these fiber rings or any cardboard material anyway what I like to do is drill a small pilot hole with a micro drill, add a drop of thin CA that wicks into the material, and then come back and drill a larger hole. The CA stiffens the fibers so you can drill a nicer hole with the larger bit.

endcappilots.jpg

Then I cut two lengths of plastruct hollow styrene rod and tapped the holes at the end for #4-40 screws. I'll use a rectangle of balsa for the sled and I'll cut some circles from scrap material to cap the ends. Here's what the skeleton looks like.

altbayrails.jpg
 

MaxQ

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I don't have any long coupler material for the BT-58 so what I've done in the past is just glue two couplers end to end and glue the switch ring to the outside. This makes for a strong assembly. I left about one body diameter on the aft end where the bay will slip fit into the body tube, but I didn't need as much length on the top side since I will be securing it to the upper airframe with screws.

View attachment 319831

For the bulkhead ends I like to simply double up some centering rings. I need to make one ring at each end a little smaller in diameter to fit inside the coupler. I grab my drill press drum sander kit and find a mandrel that will fit inside the rings. The mandrels have a rubber cylinder that expands as you compress it with the nut on the end so it holds onto the sanding drum. I just use this to grip my rings, load it in the drill press, and use a course grit sandpaper to turn them down just a little bit.

View attachment 319832View attachment 319833

I drill a couple holes in each bulkhead sandwich after gluing them for the posts or rails that will hold the ends together. When I drill these fiber rings or any cardboard material anyway what I like to do is drill a small pilot hole with a micro drill, add a drop of thin CA that wicks into the material, and then come back and drill a larger hole. The CA stiffens the fibers so you can drill a nicer hole with the larger bit.

View attachment 319834

Then I cut two lengths of plastruct hollow styrene rod and tapped the holes at the end for #4-40 screws. I'll use a rectangle of balsa for the sled and I'll cut some circles from scrap material to cap the ends. Here's what the skeleton looks like.

View attachment 319835
Which end of the airframe are we on ?
I think this is the wide part...? or not...the BT 58 is the third stage - forward end? With the Av bay.
 
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K'Tesh

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Oh... This looks like it'll be good. I'm in... :pop:
 

BDB

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I like the use of the plastic struts in the AV bay. Where did you find the compressible rubber mandrel? This is really interesting.
 

micro

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Which end of the airframe are we on ?
I think this is the wide part...? or not...the BT 58 is the third stage - forward end? With the Av bay.
Sorry for the confusion, the pic of turning the rings on the drill was from something else, just demonstrating what I was talking about. Same idea for the smaller rings. This AV bay is for the middle of the BT-58 sustainer (3rd stage).

I like the use of the plastic struts in the AV bay. Where did you find the compressible rubber mandrel? This is really interesting.
I had the idea of the plastic struts on an earlier model where I really needed something lighter than all thread and it's always worked really well. The sanding kit I have is something like this: https://www.woodcraft.com/products/sanding-set-12-piece It's quick and gets them rounder than I can cut them :)
 

micro

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The rocket parts delivery lady brought me another box of parts so I cut some body tubes. Here's the lineup to show where I'm at. Left to right is the BT-58 sustainer with AV bay, ST-20 second stage, and BT-80 first stage.

lineup.jpg

I flowed the balsa nose cone with thin CA to harden it up a little and sanded the base of it to fit into the body tube. It's hard to get a good perpendicular edge all the way up to the shoulder so it fit on the tube like this:

nosefit.jpg

Shoulder and cheek cuts (perpendicular and parallel to the shoulder) with the X-Acto remedy that.

shoulder.jpg

So the Nose fits nice and flush now.

nosefit2.jpg

After that I started on the motor mount for the sustainer. Here is how I build low power motor mounts. I like to beef them up so they last forever.

motormount.jpg

First I sand the glassine off the tube for good glue adhesion. The centering rings and TTW fins will all be glued to it. I put the engine hook in leaving the usual 1/4" of motor overhang. Underneath the hook I glue a rectangle of cardstock. The two layers of cardboard sandwiched with carpenter's glue provides a well reinforced area for the top of the hook to push against so it doesn't try to tear out of the motor tube. Then I cut a notch out of the aft centering ring so it slides up over the hook without deforming the tube or the ring. Sometimes I will cut a profile out of the forward ring to install it over the very top of the motor hook to keep it from trying to pop out of the motor tube, but in this case that forward ring will be further up to accomadate the TTW fins, so I add a little strap made of cardboard and glue sandwich to hold it in place. Yes I like my motors secure ;) After I glue the aft ring in place I will make another strap like this about 1" from the end of the motor tube to keep the hook sprung tightly to the motor tube.

I like a solid thrust ring too so I cut a small notch out of a motor thrust ring so the forward end of the motor casing will rest squarely on it when the ring is glued in place.

block.jpg

The motor hooks are usually a bit longer than a motor and I don't like that gap.

motorhookgap.jpg

The motor casing will accelerate backward at ejection and hit that motor hook trying to dislodge it; making it snug will all but eliminate that additional impulse. The notch I cut in the motor block is deep enough to allow me to push it down from the top with a motor casing when gluing it in place while another motor casing I have as a placeholder is snug against the end of the motor hook. You gotta do this with the right amount of glue and do it quickly and remove the motor casings before they become permanent :p

motorhooknogap.jpg

Now the loaded motor will be pushing on the motor block and not the hook during thrust and will be snug at ejection. I realize these things may seem a bit anal but since doing this I have experienced 0 motor ejections and 0 loose motor hooks after many flights, so I'm sticking to it.

In order to do this and be able to add a little fillet of glue on top of the thrust ring the motor tube can't be that long. But in this case the motor tube will extend up the body all the way to the AV bay, so I just cut it short and will reassemble it later with a coupler.
 

micro

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There are a lot of gotchas with stagers, lots of things I visualize in my head but need to be measured out as I go along, so nothing is glued together yet except the AV bay. I'm just making parts and fitting things together so far.

I made some centering rings for the BT-58 extension in the ST-20 of the second booster using my drill press mandrel method but they turned out off center. Oops!

oblongrings.jpg

I was turning them from BT-58/BT-80 rings and sanding off that much material didn't work so well. So I printed a template from https://www.payloadbay.com/page-Tools.html and cut them the old fashioned way. Unfortunately there aren't always laser cut rings available for what you need!

From the same awesome tools page I printed the transition pieces on cardstock. Here is the aft end of the sustainer with the tail cone and motor mount dry fit. I had also cut a ring to support the bottom of the tail cone.

tailcone.jpg

Now you can start to see what I'm going for. The stages will couple on the BT-58, so I need a inner BT-58 in the top end of the 2nd booster. Just for a visual:

couplingexample.jpg

But I actually need an AV bay built into that coupler to house the electronics the handle the 2nd booster airstart and chute deployment. Only when I started laying it out did I realize it also needs vent holes on the outside of the body, so I would need a kind of fancy AV bay. I was going to use two couplers end to end similar to the first AV bay but this one needed one end open. I cut a bulkhead from 1/8" basswood and glued it in between the two couplers, then glued a ring to the outside similar to the first AV bay I built before.

altbay2.jpg

Then with a couple centering rings and a ring of ST-20 body tube I end up with this contraption:

altbay2ring.jpg

Believe it or not the Raven3 and battery fit fine in the lower half of that coupler, one Semroc BT-58 coupler section. In theory I'll be able to squeeze a switch in there with them.

raven3size.jpg

Now with the interstage coupler roughed out I can get a better idea of where stuff will be in the sustainer. It's a little different than what I first thought, that's exactly why things aren't glued together yet. I place the motor mount in the top of the coupler with the motor hook touching the bulkhead, then move it up about 1/8" for clearance and igniter wires. Now the aft centering ring will have to be up a bit higher than I thought to clear the coupler, you can see my original pencil mark. That's great since the more coupler the better for keeping the sustainer straight, but I'll have to trim a little off the fin tabs.

interstagelayout.jpg

Now with the fin tab length set I'll be able to start slotting the sustainer's body tube!
 

micro

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These fins have a long taper for the leading airfoil shape and I wanted to see how that would look for the fin slots before cutting. Here is what I used. A flat piece of wood with an old hacksaw blade screwed to the side with its back edge raised to a measured height above the wood surface. The straight steel edge gives me an ending point for the width of the fin edge, while a steel ruler is placed at the wide edge as a stop for my sanding block. Hopefully that makes sense.

taperingjig.jpg

Here are my fins with leading and trailing edges tapered.

taperedfins.jpg

I start marking the tube using a printed fin guide from the tool at www.payloadbay.com. I told it I had 8 fins because I want to mark an additional line between two fins for the rail guides. This stage won't actually have guides but I will use the line later for aligning sections.

markingfincenters.jpg

I mark where the forward centering ring will be at the front of the fin tabs, and mark half a coupler length ahead of that. I will cut the tube at that mark and put it back together with the coupler later. This shorter length allows me work more easily with the fin can and be able to add fillets on the forward centering ring.

markingfincan.jpg

Now I mark the slots, just using a ruler to mark the fin width centered on the center lines for the fins that I extended from the bottom. I use a piece of angle iron to mark the slot edges.

markingslots.jpg

Then I just cut the slots with a sharp knife using the same angle iron as a straightedge. I make a few passes, pretty much just scoring the surface with the first pass, then two more light cuts and I'm through. The taper for the fin roots is quite minimal, I simply rotated the blade just a bit at the appropriate point to cut just inside the line at the forward edge.

cuttingslots.jpg

It's much easier to do this and more accurate with centering rings to keep the tube round while working. I also had centering rings in the body tube previously when marking it. Here is a finished slot.

cutslots.jpg

And a dry fit for the fins. Hey it's starting to look like a rocket!

dryfit.jpg
 

micro

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I start the glue up by gluing in the forward ring. I spread a ring of glue with a bamboo skewer and push the forward ring into place with a coupler (so it's nice and perpendicular to the tube) until it is flush with the ends of the fin slots. I pay attention to align the holes for my wires between fins opposite the rail guide line.

gluering.jpggluedforwardring.jpg

Here is the aft centering ring with four wire holes. I only need three connections from the AV bay- positive, igniter, and separation charge, but I will split the positive wire into a separate wire for the igniter and separation charge in the fin can area. You can see I have my wires opposite the motor hook so things don't get in the way. I will have the motor hook aligned with the rail guide line.

aftring.jpg

Now I glue my motor tube to the forward centering ring. I've marked how deep I need it to be for the motor clip and wires to clear the interstage coupler. I use the aft centering ring to center the motor tube while gluing but I don't want it permanent yet so that I can add internal filleting around the fin tabs. So I put a couple tabs of tamiya tape on it so I can slip it in dry now but easily remove it later.

aftringdry.jpg

Then I glue the fins in place with glue only on the root edges and the front of the tab where it meets the forward ring. This model won't have external fillets so I'll be gluing the fins to the body tube via internal fillets once they are dried to the motor tube.

finsglued.jpg
 
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