2.6" 54mm MMT Fire Flyer (MadCow/RW) built thread

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Sep 20, 2017
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After finishing Formula 38 building last week (still waiting on EggFinder Mini shipment to arrive, or at least get shipped after ordering it last week), and have one more kit stashed away - 2.6" RW Fire Flyier with 54mm MMT.

I ordered it mostly because I got sick of spending far too much time painting my rockets, and wanted to build a few pre-colored fiberglass kits that will only need a touch of clear coat. Also, I had never built a split fin rocket before, and they look kinda cool, and RW's FF kit has split fins that are attached at the root, making for easy alignment between forward and aft fins.

MadCow/Rocketry Warehouse kit is 100% fiberglass parts + 1 sticker (crumpled) and 0% everything else.
Today, the last batch of extra part arrived, and I finally think I have everything I will need to complete the built and take the pics below.
All parts below the sticker were either scavenged from my spare parts bin (more where that came from, including electronics sled and eye bolt that will be recycled from another rocket), or were ordered separately (54" main and 18" pilot/drouge chutes, shock cords, Aeropac 54mm retainer and 54/38mm motor adapter):

Few notes thus far:
  1. The 2.6" green tubes are beautiful, and are semi-translucent in sunlight.
  2. The motor mount centering rings are silly narrow, ~1/4" / 3.5mm wide. The minimal space between the MMT and airframe will not be enough to support epoxy flow for internal fin fillets from the aft centering ring (as per usual), so I will have to inject them through the fin slots instead.
  3. The fins come already pre-beveled, though the beveling is somewhat uneven and will need to be trued up.
  4. Eggfinder Mini GPS will go into the nose cone, and I will need to adopt my 38mm plastic Eggfinder mount tray to attach to the bulkhead of the 2.6"/66mm nose cone.
  5. Huge compliments to MC/RW after initial dry-fitting: everything fits tightly and nearly perfectly. If something didn't fit just right initially, it was because I had that part fractionally misaligned (Aeropack retainer to MMT tube, centering rings to airframe vs. MMT, etc, etc). Kudos to MadCow/RW for controlling production tolerances!

Fins are also smudged in the infamous RW's glue (that holds multiple fiberglass sheets together during cutting), so a Goo-Gone soaking bath is the first order of business.


P.S.: There is another thread with someone else building an elongated Fire Flyer (link below), but it looks like it is both still in progress and also on hiatus since mid-2015, and I did not want to hijack that thread.
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Sorry for the delayed progress update - I got distracted by the prep for L2 cert flight last weekend (passed successfully) and assembling eggFinder RX/TX/LCD modules that arrived in the mail over the weekend.

Now back to FF build.

This is my first split-fin rocket, and I do have a question about beveling and rounding the split-fins: do folks recommend rounding the leading as well as trialing edges of both sets of inline fins, or just the leading/trailing edge of the complete pair?

I've seen it done both ways with other documented builds, but most just do leading/trailing of the pair.
That is the beveling that was done by MadCow, so the question for me is whether it's desirable to round the other two edges (see pic below, with the area in question highlighted)?

Other than that, the motor-mount has the centering rings tacked on with CA (motor retainer is there only fore measurement) and, once epoxied, will go into the airframe next. Then in go the fins, once I figure out to finish them.
motor tube with center rings and retainer.jpg

Next step - build two fin guides based on the fin template from:

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If you leave the facing edges between the fins square you stand more of a chance of getting the "split fin whistle". In any case, I'd leave them as is.
I've been neglecting this thread for a while, even while making progress on the rocket and ordering various supplies over the last few weeks. There isn't really any room for ambiguity with this build, as everything fits super tightly, and tiny MMT centering rings rule out possibility of rear end fillet injection. Thus the only choice for depositing internal fillets was drilling injection holes into the fin slots.

Here is the update for today, with more to come as time allows.

First off, the kit comes with just the fiberglass parts, and optionally the chute.
Following is the list of all other bits and pieces that need to be assembled to complete the build:
1). lunch lugs (I had some lying around)
2). motor retainer (Aerotech for me)
3). avionics bay materials (ordered separately from MC, more on that later)
4). electronics and wiring to go into a-bay (either Eggtimer TRS, or Missleworks RRC for me)
5). shock cord (ordered 1" tubular nylon cord)
6). Nomex blankets to protect the chutes
7). GPS tracker (EggFinder GPS for me)
8). components to modify nose-cone for GPS tracker (ordered separately from MC)
9). green pigment for the Rocketpoxy. This is mostly cosmetic, but I hated the look for black pigmented epoxy fillets on the airframe, and how black pigmented epoxy shows through on translucent FG airframes, so I'm experimenting with body-colored epoxy pigment.

I may be forgetting something.
Here are some of the extra supplies after they arrived in the mail (Eggfinder TRS is already out of the package, and is being soldered. Spare TX is still here):

Either way, the first order of business was to prep the fiberglass by washing and cleaning it with alcohol, then marking up the motor mount and fin-can with target locations for where everything should go.
Then I ordered the missing parts (#3, 4, 7, 8), and proceeded with making fin alignment guides by printing out the pattern from the link below, and cutting out the guides from foam board

Pics below:
fin guide 0.jpgfin guide 1.jpgfin guide 2.jpgfin guide 3.jpg

P.S.: Pics are not uploading for me right now, so I will come back and edit this post later and insert them.
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Now onto the motor mount.

I test fit and CA-glued the centering rings and the motor retainer to dry fit the MMT inside the airframe, and make certain that the fins aligned and fit into the space between the centering rings. The first time around, they were about 1mm too tight, so I used CA un-cure to remove the front CR, re-positioned it, and re-fit everything again.
FF motor tube with center rings and retainer.jpgMMT 1.jpg

After I was 3x sure the fit was perfect, I marked the position of the centering ring, the motor mount, and the fin attachment points with a pencil, and sanded them with 120-grit sandpaper to rough up the surface for better epoxy bonding.

I had a choice to push the centering ring to the front of the motor tube, or leave it at 14" mark (relative to 19" long MMT tube) and use it as an additional attachment point for the front edge of the fins. I had no worries about MMT stability with 14" CR spacing vs 19" max, and additional fin sturdiness is always a good thing, so I went with option 'B' - front CR was going to be epoxied at the point where it would bond to the front edge of the fin root.

Then I used JB Weld to attach the centering rings and the Aeropack motor retainer to the motor tube.

MMT 2.jpgMMT 3.jpg

After JB Weld cured overnight, I came back to finalize the shock cord attachment point.

I usually snake it through the front centering ring, but these CRs are way too thin, and I would have broken them if I tried to make room for my 1" tubular nylon shock cord. Plan 'B' was to rough up the attachment point, and glue the shock cord to the ~5" of motor mount tube in front of the front centering ring. Initially I toyed with the idea of webbing laterally /circularly wound strands of fabric or spare kevlar line give extra strength to the attachment point. Then I realized I was being paranoid, and that 5"x1" worth of JB-Weld attachment surface area, with some JB-Weld overlay-ed on top, was going to be more than sufficient. So that's what I did:
MMT 4.jpgMMT 5.jpgMMT 6.jpg

The second picture shows the presumptive launch lugs that I was thinking of using on this rocket.
I also have some spare surface mount aerodynamic 1010 launch lugs, and fly-away rail guides are also an option.
Decision, decisions.

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While the motor mount epoxy was curing, I was off to make progress on the airframe/booster.

First of all, the internal fin fillets were going to have to be injected through the fin slots, even though I had previously always left the rear centering ring unglued, and would pull it out to inject the internal fin fillets, then glue the rear CR, followed by the motor retainer. With tiny CR's, there was no room for that, so they rear CR and motor retainer went in permanently, and now was the time to drill some fin filet access holes.

Drilling figerglass tube is tricky, the drill always catches a strand and pulls it out a bit. Drilling next to pre-cut fin slots was going to be even less fun, so I used a conical dremel grinding stone bit like this one:

This was the result, with zero drama:
Fin fillet injection holes.jpg

Then came the decision on rail buttons.
Between the traditional 1010 buttons (picture in previous post), the surface mounted rails, and the fly-away guides, I went with the traditional button. For one thing, I wanted secure attachment I knew the inner metal sleeve was going going to provide, for another, I can still remove the surface derlin part of the button and use fly-away rail with minimal air disturbance. I don't think I will ever do that, since maximum height and heavy fiber-glass airframes don't go together logically, but I could if I wanted to experiment with fly-away guides (never had, so some day I will!).

w.r.t. placement, I wanted to put the lower button as I could, after clearing the lower centering ring.
The upper one was a bit of a puzzle. One option was to put it as high as I could, or roughly 3.5" below the top edge of the 32" booster (to clear the avionics bay coupler. Another was to move it lower, to buy more rail time for the rocket, but how much lower?
I started playing with Open Rocket, and confirmed that the CG of the rocket w/o motor was exactly 3" below the top edge of the booster. The CG with G-motor (the smallest I would fly) was 7" below the top edge.
The CG with H-motor was 9" below the top edge. With "I" it was 10" below, with most "J's" it was 12" below, with "K" it was 15" below, and with "L" it stared going back up to 14" below. Had my booster been longer, a Loki M1376 would have pushed the CG further up by full five inches, but that M-motor is full 44" long and I would never be able to fly it in North East (Fire Flyer is simm-ed to hit 30+K feet on M1376!), so now I was just goofing off.

Back to my point - I wanted the upper button to be at least at or above the CG on the smallest engine, to provide maximum stability off the rod for the longest possible time. And I wanted a spot where I could still work with inserting and epoxying the mental insert for the button with minimal practical inconvenience, so I drilled the upper rail button hole 7" below the top of the booster, giving me the distnace of 24" between the two rail buttons on a 60" rocket.

I think it's good enough, but if someone has a better suggestion for the future, please chime in!

With the rail buttons drilled and metal inserts epoxied in (left to harden overnight), it was time to file the upper motor mount centering ring to allow it to be inserted past the lower rail button insert. See the pic below:
Rail button CR clearance.jpg

Once the MMT was free to go in, it was time to epoxy it in for good.
I toed with the idea of using JB-Weld yet again, but did not want to deal with a black epoxy sludge inside my translucent green rocket body. Plus, I had bought green dye for the Rocketpoxy, now was the time to put it to good use.

Attaching the MMT to the airframe steps:
1). Mix the green pigment into Rocketpoxy after the resin has been mixed with the hardener, else the color pigment makes it impossible to judge when the resin and hardener have been
2). Lay a circumference of green Rocketpoxy 1/4" below where the front MMT CR will be placed.
3). Insert the front CR past lower rail button insert and 90% into the booster
4). Apply green Rocketpoxy to the circumference of the MMT around the lower CR and the motor retainer, and push the MMT the rest of the way into the booster with a circular motion to spread the epoxy
5). Clean up the mess
6). Have a beer

I have to admit that the extra effort of mixing in the green pigment was worth it.
I still made a bit of a mess with epoxy inside the airframe, but it's less offensive then it would have been with grey or black epoxy showing through the translucent body.

The see-through pre-colored fibglass tubes from MadCow are great for not having to paint your rocket at the end, but they do expose your clumsiness with epoxy. The black CRs show through as well. I am OK with this trade-off, but it would have even better if I had fewer screw ups to hide.

We'll see how the fin fillets go down the road.

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And now I would be moving on to finishing the avionics bay and the nose cone, except for the two MadCow screw ups.

1). The Madcow's very own 2.6 avionics bay kit was shipped with 6 3/4" long #10 threaded rods that are too short for 7" long avionics bay coupler in Madcow's Fire Flyer kit
2). The Madcow order I placed earlier this week for new centering rings and 38 motor mount tube that will be the basis for my nose cone Eggfinder GPS attachment had not yet been shipped as of Saturday, never mind arrived.

Thus I am in a holding pattern until I come back from Home Depot with new threaded rods to fix MC kit inconsistencies, and then I'm switching gears to finish soldering Eggfinder TRS to finish the avionics bay.

I'm also still waiting on the JST connector plugs to arrive, so that I can gracefully plug/unplug and swap the LiPo batteries in the field (if the charge runs down, or I forget to charge them in advance). But that's a nice to have.

With some luck everything will arrive and be ready for 11/11 launch!

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Next up - tacking on fins, and fin fillets.

Attaching fins went easy, thanks to 2x buttering Rocketpoxy, after allowing it to firm up a bit for ~15 minutes.
First I polished the fins' edges a bit with 400 grit sandpaper, to remove ragged edges. Then cut zig-zag pattern into fin roots to increase surface area for epoxy adhesion.
fins attachment 1.jpg

Then I mixed up another batch of green-pigmented Rocketpoxy, let it sit for 10-15 minutes. After applying the first layer of epoxy to the fin #1 root edge, I inserted it into the fin slot, wiggled it around, then pulled the fin back out. This left some epoxy on the motor mount surface. Then I applied another layer of epoxy to the fin root, and inserted into the precut slot for good. Fit was perfect, just like during dry fitting.

Now I started inserting the fin guides, when I realized the fin was sitting in there really good and wasn't moving much. And I had a pool of perfectly usable Rocketpoxy that was about to go to waste, so I doubled back and glued in the other two fins at the same time. All three went it perfectly, then I strapped them in for drying with two sets of fin guides, and some rubber bands (courtesy of USPS deliveries) to hold them in place. Left them sitting overnight like this:
fins attachment 2.jpgfins attachment 3.jpg

This is my first split-fin rocket, but courtesy of the two fins being held together at the root, aligning them was not necessary.
I could move the tips of the fins around a bit, due to leverage, but they were perfectly aligned at the root (by design).
After initially holding the pairs inline together with small clamps, I removed the clamps since they were not doing anything.

They glued in perfectly, and came out looking like this:
fins attachment 4.jpg

This went really smoothly, and quicker than I planned after I figured out I could do all three at the same time.

Internal fin fillets were more trouble.
More on that later (gotta run).
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This was my first attempt at internal fin fillets by way of injection (minimal room between motor mount and airframe limited access through the rear MMT centering ring). There were some challenges - let me know what you think I can do better the next time.

First of all, I had cut two injection holes per fin slot, about 3/8" size each.
I wasn't sure if they will be large enough, but wasn't settled on the external fin fillet diameter, so played it safe.
In retrospect, I don't think the injection hole size was a constraint.
Fin fillet injection holes.jpg

I used green-pigmented Rocketpoxy, injected with a 60ml plastic syringe I got off either eBay or Amazon a while back. It worked fine, and having the injection nozzle to the side of the syringe (vs the usual center) was a definite plus.

The problem manifested itself quickly by having the Rocketpoxy puddle around the injection hole, and drip down very slowly.
I think the root cause it the fact that I only had 3-4mm or ~1/8" of space to work with between the MMT and the airframe, and the relatively high viscosity Rocketpoxy was clinging to both surface areas too much.

If you carefully look at the picture below, you will see darker patches of green Rocketpoxy puddling around the injection point.
fin fillet 1.jpg

After ~30 minutes of injecting epoxy into all eight holes, I got all 50 grams of it in, with ~10ml left in the syringe that was now refusing to budget. I could squeeze out a trickle, but the Rocketpoxy was hardening into molasses, and this was as good as it was going to get.

With the benefit of translucent body and back-lighting, I could tell exactly how the epoxy was spreading.
In the end, I got it to drip down to the root of the fins on all three sides, though not exactly by way of following the fin slot contour.
My fillets ended up looking like dental tooth X-rays:
fin fillet 2.jpgfin fillet 3.jpgfin fillet 4.jpg

The next step was going to be to flip the airframe over and inject the second batch of epoxy to allow it to flow to the top of the fin can.
But before I do that, does anyone have any suggestions on what I could be doing better to get a more ideal internal fillet distribution around the fin roots?

Thanks in advance,
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Congrats on the L2 cert!

Thank you!

If you leave the facing edges between the fins square you stand more of a chance of getting the "split fin whistle". In any case, I'd leave them as is.

OK, thanks.
I polished them up a bit, but left he inner fin cuts perfectly square.

Is "split fin whistle" an aerodynamic concept, or does it refer to an aural experience produced by split fins?
I guess I will find out about the latter in the near future!

It's an aural experience. You actually hear the whistle. In my experience split fins whistle more often than single fins, but not always. I've heard single fins whistle, too, but not as often. I haven't been able to figure out for sure what causes it.
qquake2k is right about split fins not always whistling. I scratch built a split fin just to get a rocket that would whistle. It's now named Dog Whistler because you don't hear it. If I put a really big motor in it and it's not too high at burn out, you can hear a little whistle then, but that's about it.
Time to finish the write-up and upload a few remaining pics, since the rocket is effectively build.
I just need to figure out what, if anything, I want to do with the nose cone (vs. just closing it up and epoxying the bulk plate).

Internal fillet effort ran into some challenges, with remedies are documented here, if anyone is interested:

The entire idea of injecting internal fillets was an overkill for this rocket and the forces it will experience.
It would have been far easier to either skip internal fillets altogether, or just foam the fin can. But I'm a bit of a stubborn bastard, and wanted to figure out the internal fillet technique for future reference, so here is what I did:
1). Switched to lower viscosity epoxy (30 minute Great Plains) and warmed it up in hot water bath before use. Now it flowed like melted butter on a hot toast.
2). Drilled few more access holes in the airframe, and injected fillets while laying the rocket horizontally. This slowed the progress into three passes (for 3 finned rocket) with alcohol and paper towel syringe cleaning cycles in the middle, but allowed for more controlled epoxy distribution. Changing the angle of the airframe helped spread the epoxy that had a tendency to puddle near the injection point.
3). Switched to using a plastic funnel for the syringe nose, instead of a thick needle (see pics below)

That worked really well, and I got perfect coverage of all previously observed voids.
Then I laid external fillets.
Since I had green pigment on hand and will not be painting this rocket (beyond the clear coat), I added it to Rocketpoxy to see how that would come out.
I used 1 1/4 PVC pipe to spread, smooth, and "pull" the external fillets.
One interesting complication came to light, as a side effect of internal fillets - one of the injection holes did not get fully plugged with epoxy, and Rocketpoxy "drained into" that hole after I was done pulling near-perfect external fillets. It only "caved in" after I walked away for a beer brake, so another batch of Epoxy was mixed to cover this hole. A round of sanding was required afterwards to return the fillet shape back to near-perfect concavity.

Then, onto the avionics bay.

Avionics bay went fast and easy.
I built it around Eggtimer TRS board, and the only complication was that the board + antenna were longer than the 6" e-Bay.
Copper pipe end-caps for well charges.
Everything bolted and held together by brass knurled thumb nuts. off eBay.
Plastic wire connection terminals also off eBay.
800 mAh LiPo battery cross zip tied into place.
I am not sure where I got the sled, but wood will do:
IMG_20180101_155044.jpg IMG_20180101_155134.jpg IMG_20180104_220748.jpg IMG_20180114_014458.jpg

Ignition wires are connected to the altimeter via quick disconnect JST cables, for ease of disassembly.

Then I added an antenna hole to the FWD bulkhead, and epoxied a leftover motor igniter packing tube to protect the antenna from deployment charges.
Then covered the entire tube in epoxy for extra strength and durability.
Lastly, drilled some air sampling holes, and for the plastic sheer pins to hold the entire rocket assembly together securely until the BP charges fire in flight.
IMG_20180114_015049.jpg IMG_20180114_015039.jpg

At this point, all I need to do to finish this project is to close up the nose cone (epoxy coupler, bulkhead, and the eyebolt), quick-link the harnesses in place, and ground test the BP ejection charges. Then off to the launchpad.

I may take pictures of whatever it is I decide to do with the nose cone.

Right now, I'm thinking nutserts in an epoxied centering ring inside the nose cone, to which either a bulkhead with eyebolt, or a GPS sled with an eyeblot could be attached. This would buy me an option to reuse the Eggfinder TRS in another rocket in the future, and go with a cheap deployment altimeter in this one, coupled with an Eggfinder GPS in the nose cone.

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Rocket is done, and I was ready to fly it at March club launch, but it was far too windy and far too cold, so it sat in the car.
Here is what it looks like now:


FireFlyer came in at the final all-in weight of 2005 grams, or 4.42 pounds.
Madcow advertises it to be "just over 2 pounds":

I definitely overbuilt it, and my weight includes the nav-bay, electronics, laundry, and extra hardware in the nose cone.
Which brings me to the final bit I wanted to document before closing this thread out. But the last few times I tried to upload the nose cone construction pics, the browser hangs and I loose the draft. Lets see if it works this time around.

I wanted to add modular option to fly the rocket with LabRatRocketry Eggtimer GPS sled in the nose cone, or just the plane old bulkhead with an eye-bolt.

To do that, the following parts where put to work:
+ LabRatRocketry Eggtimer sled.
+ Extra FG centering rings: 1 to house the nutserts and to be epoxied into the nose cone; another to serve as the "washer" through which the bolts will attach the Eggtimer "housing" to the nose cone.
+ One more thick ply centering ring to help epoxy the FG ring exactly perpendicular to the Eggtimer housing.
+ 2" of FG coupling tube that mates perfectly with the LabRatRocketry sled.
+ 1 1/4" of coupler tubing I had lying around that goes over the FG coupling to bulk up the diameter to fit rightly with the centering rings.


Here is the final result - the nutserts and centering ring epoxied to the nose cone:

Bulkhead attached to the nutserts:

Since I have Eggtimer TRS module (includes GPS tracker) in the nav-bay right now, I do not need the GPS tracker on board for the test flight. Afterwards, I can always downgrade to RRC2 or RRC3 altimeters, and optionally fly Eggtimer GPS in the nose cone, when needed.

That's it, folks.
The rocket will fly in April, weather permitting.

Thanks for looking.