V2 for my L3

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He has a trailer mounted 21 foot rail power tilted by a pair of very large air shocks supplied by an onboard compressor .

That would be perfect. In retrospect, rocketry related questions that start with "Does Marlin have..." Is likely a silly one.

Thanks.
 
Got a visit from Pyrobob at lunch time. Got the motor, reload and some Black Friday goodies too. The motor and reload have already been turned over for wrapping...I promised to look surprised.



I kept everything in the lab for the day. Having a bunch of rocket motors around didn't shock anyone. As is evident every year at fire extinguisher training, the entire company thinks the chemists are the pyros anyway...they're not wrong.

As a fellow chemist, I am proud of the moniker "pyro"! That's the whole reason most of us got into the business!

Seriously, great build. I'm learning a ton by reading this.
 
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As a fellow chemist, I am proud of the moniker "pyro"! That's the whole reason most of us got into the business!

Seriously, great build. I'm leaning a ton by reading this.

Glad to hear you are finding some useful info here. I must again emphasize though, I don't necessarily do everything "the right way". There is definitely some experimenting going on at all times, but being a chemist yourself I'm sure you understand that.
 
Just like college....it wouldn't be fun without a little experimentation.
 
I'm back at this one. The MM/boat tail is easily the biggest headache in this build. I've been trying to think up the best eat to do it, and I think I have figured out the order of operations. I'll post some pics after I make some headway.
 
Wow, how did I miss this thread? That is one fine looking V2 build Chris, gotta love the "stash". It just occurred to me that there may be enough time for you to get this ready to fly at Airfest, any plans yet?
 
Wow, how did I miss this thread? That is one fine looking V2 build Chris, gotta love the "stash". It just occurred to me that there may be enough time for you to get this ready to fly at Airfest, any plans yet?

Not the way I build. Revised timeline is to aim to fly this in Asa sometime this coming winter/spring. I am kinda picky on finishing, and life with two young kiddos does not afford a bunch of excess time or energy.
 
Glad your back to it, your quite the craftsmen. So you don't like the low and slow for cert.. Huh, maybe I need to rethink my L3 motor.

Thanks for the inspiration
 
As promised, the madness resumes...with CRs.

There are 4 CRs on the MM. The aft CR was fitted with an Aeropack flanged motor retainer exactly by the directions. The holes were marked, center punched, drilled on the drill press and the inserts installed. A nice easy way to get back on the horse, and everything fit when complete.



Y-harness Recovery points (SS U-bolts) were installed in the fore CR exactly how you think they would be...marked a center line, spaced the holes evenly about it, and on to the drill press.



A Y-harness was made from 7/16" (5000#) Kevlar with a 12/0 Nickel plated swivel (1500#) in the loop. I took care to make sure the Y was symmetrical, and long enough that when pulled tight it would not rub the longest case that would fit the rocket. The Y-harness was then attached, and the back of the U-bolts got some left over Aeropoxy to ensure they never loosen.

 
Ok, this is the part of the build that scared me off for the better part of 7 months (4 really, my garage renovation was responsible for 3 months lost...worth it). Anyhow, cutting the fin slots with equal spacing and perfectly straight is of course key. That being said it is hard to do in a boat tail without a router jig for the purpose (maybe even with one for that matter). Just finding even spacing is difficult when everything is curved. In the end I touched up my previously drawn slot guides by very carefully making a large set of foam board fin alignment jigs. These held a piece of scrap that I drew in new lines (in copper) to use as a guide. About 4" of the aft end of the boat tail was left unslotted, because it feels like it will be stronger if it isn't split to close to the aft end...no science here, just feels right.

Pilot holes were then drilled at each end of the slots and the rest cut out by repeated passes with a sharp utility knife. I prefer to cut fin slots by hand because it is inherently slow. You can get in trouble real fast free handing a power tool for this and bugger the whole part. This may say more about my power tool skills than the tools themselves, however. Lastly the boat tail was prepped for MM install by sanding the CR bonding areas, and wiping down with ethanol.

 
The motor mount install was a real head scratcher. I needed to get 4 CRs inside the boat tail. The fore most of which would lock inside the forward lip created by the shoulder. This would be the mechanical carrier of the thrust load. Otherwise I would be relying on wood-plastic epoxy joints all of which slant away, if you know what I mean. Definitely wanted a mechanical way to transfer literally hundreds of pounds of thrust to the airframe. I know the Fins would do this, but I want the MM completely locked in on it's own. This desire creates a conundrum, how to get the forward CR past the shoulder with the MM already installed...you can't if everything fits well. The answer was to only attach the aft most CR before the MM is installed. Then letting the other three rings float, and be epoxied to the MM and the boat tail all in one step...yeah, that sounds easy, right?

OK, so this half-baked plan started with sanding the entire MM, wiping down with ethanol, gluing on the aft CR, and letting that cure. Tape was placed on both sides so no fouling of the inserts would happen.



That part was easy. Next I made a CR stack with the other three "floating" CRs (The stack had to be about 2" thick to force into the tail cone at an achievable angle...most of the excess U-bolt threaded area had to be cut off to achieve this, no problem there, weight savings! So I don't lose my mind, or you, trying to describe this the CRs will be referred to by number 1-4 from aft end forward. Behold the CR stack:



Epoxy was smeared over the boat tail in the bonding areas of CRs 1-3. The MM and CR stack was forced in by angling the entire mess and shoving.



The MM was pushed in about 80% of the way, so that the CRs 3 and 4 had stopped, but neither 1 or 2 had seated. Epoxy was dripped in through the fin slots about an inch fore CR 2. When the MM was shoved all the way in, seating CR1, CR2 was pushed up through the epoxy, coating the MM, and filleting CR2. This CR had to be shoved aft about 1/4" from where it initially stopped using a screw driver so that it seated snugly at the aft edge of the fin slot. Clear as mud, huh?



A similar method was applied to CR3, but because of the fin slots (and the extra flex they provided) it was able to initially slide about 1" further aft than it should have. This exposed the MM fore of the CR. Epoxy was dripped into this area through the fin slots and smeared around with a bamboo skewer as was done for CR2. CR3 was then pushed forward through this epoxy until it aligned with the upper edge of the fin slot where it should be. You could see the MM left with a thin coat of epoxy where it had pushed through, so I am confident it is well glued also. No pics on CR three as it is basically exactly what is shown for CR2.

Lastly CR4, the forward most, recovery harness mount, thrust transfer, jack of all trades, probably most important piece of wood in the rocket. CR4 slid about 2" aft of the shoulder lip on inserting the MM...not surprising as the rocket is basically straight that first couple inches. The MM and boat tail were slathered with copious amounts of epoxy, and the CR pulled forward using the Y-harness while maintaining pressure on the MM to keep the entire MM-CR assembly from sliding forward. Once seated the entire assembly was hung by the Y-harness while the epoxy cured. Next day another round of epoxy on the top CR to make double-super-duper-ultimately sure that it would stay put was applied. Amazingly I managed to get little or no epoxy on the Kevlar harness during all this.

 
After everything cured, I sanded the boat tail flush with a palm sander (I intentionally left a couple extra mm's long, so I could square it after the CR was in. Doing a perfectly square cut on the boat tail was not going to happen, and since I wanted it perfectly flush on the aft end I left some extra when I cut the boat tail and aft CR, so I could sand flush at this point. With that done I had to test fit the retainer and motor...because why not.



I almost can't believe it worked. The one kick in the pants is that I realized I had not aligned the retainer mounting screws with the fin slots after I had CRs 1-3 in seated. I could have twisted the MM at that point, but I wasn't sure I had enough working time left on the epoxy should all the CRs become unseated in the process. Fitting the MM took well over an hour, and I was worried about some of the bigger goobers of Aeropoxy kicking. Oh well, if that turns out to be the worse flaw I will be grateful.

That brings me up to date. Next up, the fins.
 
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The motor mount install was a real head scratcher. ...

Ok, mind blown from too much exposure to this thread ! :wink: But... I... still... can't... look... away... Amazing build sequence in an insane build ! Is this what happens when you get to work all summer in air conditioned comfort ?
 
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Ok, mind blown from too much exposure to this thread ! :wink: But... I... still... can't... look... away... Amazing build sequence in an insane build ! Is this what happens when you get to work all summer in air conditioned comfort ?

Yeah, the AC definitely helps out. Hauling this size build in and out of the house to work on it would really have been prohibitive, so it is great to have a dedicated space. I do share the main workbench with the boys, hence the Legos, Perler beads and water colors in the background of some of the photos. Getting that kind of stuff off the dining room table also makes the better half happy too.
 
Thank God for the fin slots. I know it was a pain to cut them, but without them you would have been completely blind on this step.

And..We are all jealous of that AC that is flaunting itself in the background next to your launch tower.
 
Thank God for the fin slots. I know it was a pain to cut them, but without them you would have been completely blind on this step.

And..We are all jealous of that AC that is flaunting itself in the background next to your launch tower.

Yep, no fins slots and this wouldn't have worked, not just blind, but no way to add epoxy to the MM. Normally I like to put in the MM first so the CRs support what I'm cutting, but just not doable this time. The slots required a little tweeking after everything dried, but no major problem.
 
OK, so this step was done about 10 months ago, but didn't make sense to discuss until this point. These are "Full Fins" meaning that they include tabs that go the full length. Some of this would require cutting out to accommodate the boat tail unslotted area, and the motor retainer.

The fins were made by pegging 5 oversized 3/8" blanks together with dowel rods, and squaring the root edge on the belt sander.



These were then rough cut on the band saw, and match sanded on the belt sander. I don't have the right tools (or patients) to do a nice crisp angle on the trailing edge, so the curves are a bit rounded here...sport scale again.

 
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So those full length tabs obviously aren't going to work. Also, there is the issue of how far past the boat tail will the fins extend. True scale would have the inner most edge about 45 mm beyond the motor retainer (which is 30 mm itself...that sounds like crispy fins to me. Therefore, the fins will extend 30 mm beyond the boat tail, i.e. level with the retainer.

The original fin template was marked to accommodate the fin slots, boat tail contour, and motor retainer clearance. This was a very iterative process until a good fitting template was achieved.



Unfortunately, I put one of the pins right on the cut line. :facepalm: Oh well, the fins were re-pinned, this time in the cutout area, and the final shape was cut, and everything match sanded as best as possible. They were then popped apart, and four of them individually fitted to a slot and marked as such. No big changes were required, but a mm or so here and there to get good fits. These four were then rounded over on the edges...more agresively on the leading edges than trailing. This is largely so the CF conforms well...this isn't a high performance bird after all.

 
So, I made 5 fins for a reason. I wanted to make absolutely sure my balsa core strategy would be fine on one of them before cutting a whole set. If it wasn't, then I had a matching spare to step in. From my larger test fin, back on page 2 of this thread, I got the impression that I left more ply than I had too. i.e. I don't need a 1" frame all the way around.

The first step was to start out with a fresh 3/8" fin, and skeletonize it. Areas that are part of the tab have a 1/2" frame, while those out in the air have at least 3/4". This was done by drawing out the area, drilling holes at each turn to help out the scroll saw, and cutting freehand. The cuts are by no means perfect, but the area will be filled in with balsa anyway, so minor deviations are ok.

Starting Weight: 377g



Plywood Skeleton: 154g



The core was then marked on end grain balsa using the fin, and cut out with a cheap hook nosed kitchen knife. A little trimming and everything fit fine.



A problem I had with my test fin was that the balsa is slightly thicker than the ply, so I had a slight raise the entire way around the core. Not terrible, but more trouble than it is worth, so I decided to epoxy in the balsa this time, so I could sand it flush before laminating. I did this with laminating epoxy, taking care to wet between the individual blocks that make up the balsa sheet, so I would be left with a fully stable balsa sheet. You can see I chose to make the side with the scrim stand out, so that could be sanded off. The entire assembly dried overnight between two sheets of parchment with about 5 lb weight to ensure it remained flat. Later, the balsa was made flush with the ply in about 5 minutes with a palm sander (not pictured).

Fin with balsa core: 206g



Obviously, these are flimsy now, but two layers of CF cloth (3K twill, 196 GSM, layers 45 degrees rotated) on each side make these exceptionally strong. The CF was laid out and taped off for cutting with an Olaf Roller Cutter. A total of 16 piece of CF (8 in each orientation) were cut. Along with 8 layers of release film and 8 layers of breather material. I have been getting repeated uses out of the same vacuum bag. I simply cut off the tape each time, so it shrinks about 3/4" with each use, but otherwise going strong. I'm using US Composites 635 with medium hardener for the laminating, and I've found 225 mbar to be a good pressure to pull the bag down to, which gives a nicely laminated part that does not appear epoxy starved or rich.



Prep is everything with making composites. Once the clock starts it better go smoothly, so everything gets laid out as it will go on, and I do a dry run to make sure I have everything I need and my timing is good. I have found that supporting the parts on a piece of hard board (and having a second piece for flips) covered in parchment paper is the best way to flip and move parts once the epoxy is applied, and the layers go on.



Once all the layers are on it is into the bag. You can see in the pic below I do one side entirely, including the release film and breather, then flip the entire stack and do the other. As you can see by the timer I got it done with 5 min to spare (started on 20).



After curing in the bag 24 hours (I decide how long by smearing a thin layer of epoxy on the parchment paper and letting that cure to the desired point) I removed the part from the bag, and immediately trimmed most of the excess fabric off with several passes from a utility knife. Do the trimming at this point, as it gets much harder after 48 hrs.

The rough fin has a few small wrinkles (probably because I didn't make relief cuts at the sharp edges in the CF), but is perfectly useable, and exceptionally strong. I'm not going to, but I'm sure I could stand on this one like I did the test fin with no issues.

Rough final fin: 299g



So the total weight savings from wood fin to final composite fin is 78 g. That is about 0.7 lbs across the fin set, but I believe these are stronger, and I would have laminated the ply anyway, so I think the real weight savings is around 171 g each (Fin with balsa core versus ply fin) as the lamination would have added about the same amount to both. That makes the savings 1.5 lbs across the set. And since the V2 requires an almost corresponding addition of nose weight that is more like 2.5-3 lbs savings overall. Much worth the effort I think.
 
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Brief update: the third fin went in the vacuum bag tonight, and I ordered some Super Fil to finish them up with. I'm not going to try to preserve the CF. I've made a few rockets like that lately, and I'm just not up for all the sanding cycles and epoxy wipe coats. I'm just going to smooth out the exposed section of fin with Super Fil and go with it.
 
After the successful test fin I skeletonized the remaining three fins in the same pattern. Filled them with end grain balsa, and sanded flush. They were then vacuum bagged one per night, and rough trimmed with a utility knife after 24 hrs.



The roughly trimmed fins were then cleaned up and individually fitted to the slots. I used a combination of utility knife, belt/disk sander, Dremel and sanding bar to get this done. I made sure they were a good fit to the MM by shining a REALLY BRIGHT flashlight through a fin slot while looking through the opposite fin slot while test fitting opposing fins. Anything more than a faint glow meant more trimming to fully seat the fin. Once they all fit nicely I had to do a dry fit. This also let me confirm that all fins were seated by measuring the tip to tip distance of opposing fins...equal in both cases. I then marked the boat tail intersection as I won't use the superfil on the tabs.



The Superfil arrived today, so I'll start filling the exposed fin area this evening. I'll also make the steering vane actuators that go on fins 1 and 3. I've decided to do this one as V2 flight 4...the first successful V2 test flight. I'm also going to start fiddling with how to do the ports and other surface do-dads.
 
Before gluing the fins on I want to fill the exposed area as it is easier to use a palm sander on unattached fins. I prepped each fin by wiping down with ethanol as I always do before any epoxy bonding task. In a lazy man effort to finish the fins in one coat I put a heavy coat on one side using a plastic putty knife, and let it cure over night. I also took some left of each batch and worked it into imperfections in the exposed fin edges. After about 18 hours it appeared fully cured and ready for sanding, so I went at it with the palm sander and some 80 grit being careful not to eat into the CF while trying to get as thin a layer of filler as is practical. I must say I'm happy with the results. Each fin is quite smooth, and picked up less than 3 g each.



Then the second side got a similar coat, and another round to the fin edges as needed.



For those unfamiliar with Superfil (as I was before now) it is a light weight, two-part epoxy based filler used for aircraft applications. I got it from Aircraft Spruce which is one of only a handful of suppliers on the Interwebs...I wish one of our typical rocket suppliers would stock this stuff like they do a few of the better epoxies we use. The result is a sandable (with some effort) surface that is as solid as a rock, and bonded to the substrate as strongly as you would expect for epoxy. The stuff is a little spendy, but the results are worth it in my opinion. I think this will be my standard filler for structural components going forward. I'll still use Bondo spot and glazing for tiny touch ups, but this stuff is superior in my estimation.
 
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Great craftsmanship! Do you have any experience with the Polecat body tubes? They are fiberglass-covered cardboard, and are rough out of the box. I'm trying to figure the best way to smooth it out, seems like Superfill would allow me to do so without sanding too much into the fiberglass cloth? How much weight do you think it would add to a 4" x 70"ish body tube? I am looking forward to learning more composite work so maybe some day I could build parts that look as great as yours!
 
Great craftsmanship! Do you have any experience with the Polecat body tubes? They are fiberglass-covered cardboard, and are rough out of the box. I'm trying to figure the best way to smooth it out, seems like Superfill would allow me to do so without sanding too much into the fiberglass cloth? How much weight do you think it would add to a 4" x 70"ish body tube? I am looking forward to learning more composite work so maybe some day I could build parts that look as great as yours!

Thanks for the kind words. Unfortunately, I don't have experience with the Polecat tubes...yet (that 5.5" Nike Smoke may get me one day). Superfil lists the density at 3.8 lbs per gallon (water is 8.3 lbs for perspective), so I would imagine you would save some weight over a product like Bondo or straight laminating epoxy. I've read from at least one very knowledgeable gent on here that he sometimes uses water thinned Elmer's wood filler (EWF) rolled on with a small roller and sanded for filling fiberglass fabric post cure. That would certainly be light weight, and the easiest option to sand. Of fillers I have used, I would put relative ease of sandability like this (easiest to left; no science, complete opinion): EWF > Rustoleum Filler Primer > Bondo Spot and Glazing > Superfil > Laminating Epoxy...if that helps you make a decision.

I would have done EWF here, but I was worried about the possibility of pin holes and getting water down to the balsa layer. If that layer tries to swell all will be lost, so I went with Superfil to smooth things, and to provide another layer of epoxy seal to the core...I am a paranoid one though.

As for composites, just jump in when you are ready, and definitely make test pieces. I started by laminating fins between wax paper with Zpoxy and fiberglass cloth from the hobby store...you don't have to go vacuum bagging and buying epoxy by the gallon straight away. Not everything will work out, but you will be shocked at how light weight you can make super strong parts when you start playing around with this stuff.
 
Great craftsmanship! Do you have any experience with the Polecat body tubes? They are fiberglass-covered cardboard, and are rough out of the box. I'm trying to figure the best way to smooth it out, seems like Superfill would allow me to do so without sanding too much into the fiberglass cloth? How much weight do you think it would add to a 4" x 70"ish body tube? I am looking forward to learning more composite work so maybe some day I could build parts that look as great as yours!

That is what I do with Polecat tubes (7.5" Thumper cluster example: here and finished: here). Very little added weight.
 
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At last it is time to start attaching fins. To get them on straight I will be using opposing sets to self align each other. To do this I will need epoxy that stays put, as I will have two fins on at once, and don't want any flow around the MM to tack the opposing free fin on. This only works because the fins friction fit tightly enough into their slots to not drop out when turned up side down.

To get epoxy that will not flow (without buying yet another adhesive), and presumably add strength, I added a very small amount of chopped CF to my Aeropoxy. Note: my chopped CF is not high grade, uniform length store bought fancy stuff...it is just some left over fabric that I chopped up with the roller cutter. It is really easy to use too much and end up with something whose spreadablity and wetting power (if that makes sense) appears compromised...just too thick and stringy to work with basically. On the second try I ended up with something I loving refer to as "demon snot". Because it is nasty looking, sticky and filled with carbon, as I assume demon nostrils would be. In any event it wets well and stays put, so I went with it.



The fins were prepped by roughing up the last 1/2" or so of the tab with sand paper, and scoring "X" patterns into the base of the fin root with the Zona saw. A Generous amount of epoxy was buttered onto the root edge, and put on the MM and both bulkheads through the fin slot.



Fin one (one with epoxy) was then put in place, and fin 3 was also dry fit into place. Full seating of both fins was verified by looking through slots 2 and 4 (imperfect as you can't see the last mm or so nearest the MM), as well as, measuring the tip-to-tip distance of fins 1 and 3 (which I had previously determined should be 424 mm when I was fitting all the fins). Being comfortable the fins were seated I clamped two pieces of aluminum angle across the fins to align them. This, combined with the fin slots should ensure they end up correctly aligned...looked like it worked after the epoxy set up, so far so good.



With fin 1 successfully attached last night, I attached fin 3 this evening. Tomorrow night I will start on the other opposing set (fins 2 and 4).
 
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After fin 1 was in place I attached fin 3 (opposing one) the next night using the same CF/epoxy mixture. It looked EACTLY the same as the first attachment so no pic. Fins 2 and 4, i.e. the other set, were attached yesterday and tonight by the same method. Again the tip-to-tip distance across the set was 424 mm, so good seating of the fins to the MM. I looked back at my original scale drawings and it should have been 423 mm. I'm pretty happy with 0.2% error myself.



Now, I mentioned various modeling do-dads that I intended to make. The first of which would be the steering vane actuators on fins 1 and 3. A trip to the scrap bin yielded a nice fragment of 3/8 ply left over from making the fins, and 3/8" is close to the ideal width for these...winner! I ripped these into 13 mm wide strips on the table saw (13 mm being the tallest point of this do-dad). Four strips were clamped together and the lengthwise taper made with the belt sander using a wide scrap of plywood to push down with to ensure even pressure across all four strips.



They were then cut to length with the miter saw, individually shaped on the belt sander, and smoothed out with some quick application of 240 grit. Only belt sanded one knuckle in process, so not bad. Lastly I placed them on the fins to get a feel for if I like them or not. Honestly, I'm on the fence as to whether I will use them, or leave them off. I have plenty of time to figure that out though.



Tomorrow night I get to stack the whole thing up for the first time...only took a year.
 
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