LOC Athena3 Build, 3.1" 29mm - Whistler

[kevinkal]

I enjoy watching and listening to rockets that whistle loudly after burnout. Of the 4 HPR kits I've built, none whistle. Thus, I decided to add a split fin whistler to my rocket collection. Originally, I was looking for 4" airframes known to whistle such as the 4" BSD Thor and was disappointed to learn that they are no longer in production. In reviewing youtube videos, I could not find consistent whistle results for other split fin designs such as the Frenzy or Darkstar etc. However, I did find that a local club member had excellent whistle from his LOC Athena3, which looks just like a Thor but is just 3.1" diameter. I think the Thor looks good. While I prefer the easier accessibility of 4" diameter rockets, I decided that the 3" Athena3 had what I needed and picked one up from LOC precision.

Here's what came in the bag:





Originally, I thought I would simply keep it stock with motor eject at apogee, and install my Jolly Logic Chute Release. However, I've decided to convert to Dual Deploy with a single altimeter. I've witnessed too many rockets come straight back down at high speeds lately, and told myself that I'd always build in redundancy. This would give me a backup drogue deployment charge, with the Altimeter being the primary drogue charge, and the motor ejection being the backup.

Stock Tube Coupler and 1/4" open bend eyebolt:


I traced circles and cut pairs of altimeter bay bulkheads out of 0.06" G10 Fiberglass.
They're bonded together with Rocketpoxy. The pair of 3/16" through holes spaced 1.375" apart are for the 10-24 threaded rods.


I picked up some 316 stainless stamped Eye Straps to connect the shock cords. The manufacturer didn't give a load rating for these. I suppose their load capability is highly dependent on load direction. However, I did some manual pull tests and they are very strong axially and should be more than sufficient to handle any shock load this 50 oz. rocket might throw at them. The 10-24 threaded rods were cut to 6.875" long and the burrs carefully filed smooth. I debated using 8-32 threaded rod to save some weight, but went with 10-24


I found some very light 1/8" plywood in my stash, and used gorilla glue CA and microballons to build the altimeter sled. It has 2 sides with three 1/4" plywood spacers drilled to slide over the threaded rods. I manually simulated loads in excess of 100g's and the light plywood is more than strong enough.


A screw switch and StratologgerCF mounted with 3mm nylon stand offs and screws.
I simply drilled and tapped the thin and light plywood for 3mm-50 threads and was surprised/happy with the result. Strong and light. I had expected to feel the threaded wood fail when torqueing the fine threaded screws.. but that was not the case at all. Very solid. I scrubbed my plans to put a nylon nut on the backsides of each standoff thread.


I had some nice 350mAh 1S 65-130C LiPo packs with JST connector. They're thin and light at 15g. They should easily provide the 3A of current my 1.1 Ohm ematches draw.
I bonded some 1/4 plywood battery stops forward and aft, and plan to use a Velcro strap to hold the battery down and between the stops. 100 g's on this battery will produce a 3.3 lb load on the plywood stop.. which I tested to well over 10 lbs.



Today I'm planning to glue the altimeter bay tube into the payload tube. I'll then drill Three 1/8" vent hold through the bonded payload/altimeter bay tube, where one hole will lineup with the screw switch.

 

[new2hpr]

Just keep the fin edges square on the gap between the split fins. You can round or taper the leading and trailing. This will help the whistle effect.
-Ken

 

[kevinkal]

Thanks Ken.

 

[Red Phenix]

Hi Kevin,

Thanks for the build thread. When you want to step up the size, have a chat to Mike Fisher - he has the tyrannosaur which looks, and slightly improves on the Thor - including the same component manufacturers.

https://binderdesign.com/store/page2.html

Cheers

 

[kevinkal]

While it's overkill for this 3 lb rocket, it was easy to install a 1/4" closed stainless eyebolt in the forward centering ring for the shock cord anchor point.
It's rated for 400 lb working load.. well beyond the material strength of the 1/8" plywood and cardboard that it mounts too.


Braided Kevlar and a knot below the body tube's lip such that two widths of shock cord exit the tube... I figure it may help prevent a zipper if I ever deploy at excessive speed.


I only rounded the forward fin leading edges. I will leave all other edges relatively square with the hopes of more whistle.
Take a look at the grain direction of the Fins. I'm not sure that I would cut my fins with the grain running the way it is with the stock LOC fins. It would seem to me that the fins would be stiffer if the grain on the outside plys was running perpendicular or even diagonally relative to the tube.



I installed all 3 forward fins tonight. I left the aft center ring off for access should I decide to add some small internal fillets. What do you think? Does this relatively slow flying (600 ft/sec max velocity with a small I impulse motor) need internal fillets? I do plan to apply nice external fillets.


Given that I'll be adding some weight well forward with the altimeter bay modification.. I could stand to add a little weight aft of the Cp, so that I'm not too far overstable. Open Rocket has me at 3.1 with a small G, and 2.5 with an I200W.

I'll probably put in some internal fillets tomorrow night with some Bob Smith 15 or 30 minute epoxy. The darn 15 min epoxy has been curing too fast for me... seems like it's too thick to apply after just 5 minutes.

 

[kevinkal]

I ordered a wide range of potential drogue chutes this morning. I picked up and 9", 12" and 15" Top Flight parachutes in the neon pink color. I'm not sure which size would give the best component separation and orientation. I plan to run about 18' of shock cord between the booster and palylod, then about 12' of cord between the payload/altimeter bay and the nose cone.

 

[AlfaBrewer]

I have this rocket set up for dual deploy like you did. One change I made was to a 38mm MMT. It has whistled on every flight so far. Well, except for the one I had a forward closure failure (operator error) which required a bit of new body tube.

I originally used a 9" chute for the drogue, but have switched to a Top Flight 6" x 80" Ultra-streamer. Everything orients fine, and the streamer is a heckuva lot easier to spot than the tiny chute.

 

[kevinkal]

I have this rocket set up for dual deploy like you did. One change I made was to a 38mm MMT. It has whistled on every flight so far. Well, except for the one I had a forward closure failure (operator error) which required a bit of new body tube.

I originally used a 9" chute for the drogue, but have switched to a Top Flight 6" x 80" Ultra-streamer. Everything orients fine, and the streamer is a heckuva lot easier to spot than the tiny chute.

I would not have started with the 9" drogue. Hmm, I'll have to rethink. how well did the parts orient when falling under the 9" drogue? I probably won't have as hard of a time spotting this one since it's just a 29mm. Streamer sounds like a good idea though.

When I first received this kit, I found that the fins weren't cut right for the motor mount.. and after messaging LOC, I learned that I had been given a set of fins that were meant for a 38mm mmt. Perhaps I'll have to order a slotted body tube (if LOC has the 6 slotted bodies for sale separately) and some centering rings to build up a 38mm booster. I just need to get some slight warping out of those fins.. they were a bit warped.

 

[AlfaBrewer]

Give Barry a call, he'll send out a booster tube.

Everything oriented fine under the 9" drogue. It drifted a surprisingly long way even with that small of a drogue.

 

[kevinkal]

It looks like I'm heading for a stability number around 4 cal. I'll have to be careful with winds, or use hard hitting motors.

What stability numbers are other Athena3 owners flying with?


Here's my Open Rocket model of the Athena3 using single motor deploy on an AeroTech RMS 29/180 H238T. The parts are stock except for the eyebolt added to the forward centering ring and a Madcow 29mm motor retainer. This is overtstable at 3.31 cal.



Here's my Dual Deploy Open Rocket Model on an AeroTech RMS 29/180 H238T. It's overstable at 3.99 cal.

 

[DrewD]

I'm just finishing my own LOC Athena build.build hoping for a loud whistle. As far as stability goes, I'm going to be shooting for 2 cal. I should be able to check my CG tomorrow.

 

[kevinkal]

I'm just finishing my own LOC Athena build.build hoping for a loud whistle. As far as stability goes, I'm going to be shooting for 2 cal. I should be able to check my CG tomorrow.

Thanks, I'd be interested to know where you ended up. Is your build single motor deploy or other?
I just check the Stock Motor Deploy Open Rocket Model with an I200W / H268R load in the back, and the stability is showing 2.88 cal. Those are about the biggest 29mm motors I can see putting in, they used the RMS 29/360 hardware.

 

[kevinkal]

I switched over to 30 minute BSI epoxy (which I trusted) to finish installing the fins and add careful internal fillets, mixing just 4g Epoxy per fin.
I'm happy with the result.. very strong and light. Probably overkill strong given that this rocket will not see more than an I200W and about 600 ft/sec velocity.

Not shown, I first epoxied the forward fins, then with the forward fins solidly in place I installed the aft fins.
One clamp and two straight pcs of plywood were perfect to make alignment slip-in slots for the aft fins coated with epoxy at the interface locations.


I used a 3/16" diameter wooden dowel to carefully apply internal epoxy fillets to both the motor tube interface and the internal body tube interface.
Close up of fillets.. it was hard to get the light and camera to line up for a decent shot.




The 1010 rail buttons used the nice fat weld nuts from Apogee Components.


Nice internal fillets, with little wasted epoxy.. kept it light and strong.




I also like to use freshly mixed "thin" 30 minute epoxy to saturate the cardboard tubes near their edges.
I hope it's better than the typical CA soaking that is commonly done. The tubes seem to be stiffer and less subject to "fuzzing up" and tearing.


I plan to use G5000 Rocketpoxy for the external fillets. Hopefully I'll finish that this week. But first, I'll detail how the installation of a Madcow 29mm motor retainer went.

 

[kevinkal]

I noticed last night that the grain on all my fins matches.. front grain flows into aft fin grain for all fins. I did not intentionally do this, but rather I tried to match the minimal warps in each front and aft fin so that they lined up when all assembled. So, perhaps that is why the grains all matched. Here's pictures .. note that the grain of the front fin flows right into the aft fins for each of the 3 sides:


Note that I only rounded the leading edge of the forward fins. I just removed the burrs off the other fin edges. I hope this whistles!!

It's sad to paint this nice wood grain.. but I need to see the rocket so they're going to be some bright color... I'm leaning toward Rustoleum 2x Key Lime Green and a metallic Hot Pink from Krylon... I hope those paints are compatible.

 

[kevinkal]

I decided to try the inexpensive Madcow 29mm Motor Retainer.


1/4" precision holes drilled per instructions:


The darn brass inserts cracked and delaminate just about every layer of plywood! Note how little clearance there is from the edge of the centering ring.
This is not good. I think Madcow should improve this design. I can only see this working well in soft wood, not high layer quality birch plywood.


Not only did the inserts crack the heck out of the plywood.. they also wound up not going straight in.
I had to grab the screw and force the inserts to level out before doing what I did next...


After fretting and considering dumping the Madcow Motor Retainer and going for a nice Aeropack retainer... I decided I could fix this mess with some 30 minute epoxy...
I carefully staturated all the split plys with epoxy, with the insert fully seated. Then I coated the doublers with liberal epoxy.
This will pretty much make the inserts unremovable, but that is ok. This rocket can use a little aft weight since it is overstable.


To clean up the cracks and chips on the outside surface, I used some glazing putting and sandpaper. It's nice and flat now.



Finally, the motor retainer is installed.. and will look nice on the outside while being ugly but solid on the inside.

 

[AlfaBrewer]

Next time, skip the threaded inserts and use tee nuts.

I'll try to remember and check the stability numbers on my Athena when I get home.

 

[DrewD]

Thanks, I'd be interested to know where you ended up. Is your build single motor deploy or other?
I just check the Stock Motor Deploy Open Rocket Model with an I200W / H268R load in the back, and the stability is showing 2.88 cal. Those are about the biggest 29mm motors I can see putting in, they used the RMS 29/360 hardware.

Checked the relation between my CP CG and it looks like 3.75 cal with no motor. 2.6 cal with an I200. Weighing in at 47oz minus motor and paint. I set mine up for single deploy using a RRC2 for deployment at apogee and motor for backup. I may experiment with a chute release in the future. The back end of mine has some additional weight. I laminated the fins with 1 layer of fiberglass for strength, thickness, and to counter some warping (which didn't help). I also foamed the fin can. Mostly because I was too lazy to fillet the inside lol. But I was pleased with the results. In addition to all that, I was also sent fins that were cut for a 38mm mount. So rather than seek the correct fins, I opted to upgrade to the 38 mmt and centering rings. Fortunately LOC fixed me up with what I needed. I'm still torn on whether or not to round the leading edge of the forward fin. But from what I gather, It helps enhance the whistle. I also used Rocketpoxy for the first time on the external fillets. They appear to be super strong and durable. Very different from the West Systems epoxy I'm use to!

 

[kevinkal]

Checked the relation between my CP CG and it looks like 3.75 cal with no motor. 2.6 cal with an I200. Weighing in at 47oz minus motor and paint. I set mine up for single deploy using a RRC2 for deployment at apogee and motor for backup. I may experiment with a chute release in the future. The back end of mine has some additional weight. I laminated the fins with 1 layer of fiberglass for strength, thickness, and to counter some warping (which didn't help). I also foamed the fin can. Mostly because I was too lazy to fillet the inside lol. But I was pleased with the results. In addition to all that, I was also sent fins that were cut for a 38mm mount. So rather than seek the correct fins, I opted to upgrade to the 38 mmt and centering rings. Fortunately LOC fixed me up with what I needed. I'm still torn on whether or not to round the leading edge of the forward fin. But from what I gather, It helps enhance the whistle. I also used Rocketpoxy for the first time on the external fillets. They appear to be super strong and durable. Very different from the West Systems epoxy I'm use to!

Thanks for the data.
I'll bet you hit that stability number of 2.0 that you're looking for when you put a 38mm motor in there. Sounds nice with the glassed fins. What weight glass did you go with? If I do build up a 38mm booster for this, I'll take a hard look at glassing the fins too.

With just a 29mm in there, I'll try to keep her light. I'm only getting 2500' or so with the I200W. Aside from adding about 6oz for the dual deploy setup, configuring the aft set of fins as square edges in open rocket increased Cd a fair amount, lowering altitude from the low 3s.

 

[kevinkal]

I applied the tape just outside the contact points of a large 3/4" tongue depressor. This wound up being 3/8" inch on either side of the fin2tube intersection.
I mixed up 16+16 = 32g Rocketpoxy for a few minutes, and immediately got it soaking into the fillet locations.
I assume having more time to sit there while it is at it's runniest state will help it bond to the porous cardboard and plywood.


Immediately after I roughed in the fillets, I got the heat gun and made a dozen passes over each fillet. I saw mostly little bubbles break out until no more remained.

Then, I stayed with it for about 90 minutes! until the fillets firmed up enough that they would hold most of the concave shape of the tongue depressor. I waited about 30 minutes before making the first pull with the tongue depressor soaked in IPA. I used liberal 99.9% IPA to make pulling/pushing the epoxy easy. If I were to guess, I'd say I used about half of the mixed epoxy. I put about 25 grams on initially, but when I pulled the fillets, I wound up removing a good amount. So, they look nice and wide and concave. Generally smooth. They cover enough cardboard surface area that they should add some strength.
I can't say I'm looking forward to the next two rounds of fillets.. but hopefully they'll go a little easier. I don't think I can speed it up since it was the time that it took the epoxy to firm up that force the 90 minute interval.


Rocketpoxy has a specific gravity around 1.5 g/cc. Which is ~50% greater than the specific gravity of the BSI epoxies. It's noticeably heavy. Fortunately, I need some more weight in the aft section. It would be nice to get the stability number below 3 cal.

I calculated the combined fillet weight for the first 4 fins at 15g. (I weighted 17g of excess epoxy in the cup and on paper towels.) Perhaps I'll do a before and after weigh of the booster when I put on the next 4 fillets.

 

[Nick@JET]

I've been working with RocketPoxy as well, so far I'm having good results with mixing, then letting it sit for 45 min before applying, but still have to stay on top of it for 15-30 min or so. I found when I heated it that is was even runnier. Still think the strength gain is worth it.

Your fillets look very good

 

[Banzai88]

Your fillets are good. RocketPoxy is some pretty strong stuff. Try some ProLine 4500 for fillets and you'll NEVER go back!

 

[kevinkal]

I've finished the fillets on all fins. I had a little issue when I was a bit too aggressive with the heat gun in getting a large bubble out of a setting fillet. The Rocketpoxy in that location cured significantly faster than the other section of the fillet, and I was not able to remove and smooth the fillet as well as I would have liked too. Thus, I needed to sand that particular fillet after it cured. I went ahead and sanded all the fillets using a round dowel to fit the fillet contour. I also used the thin yellow frog tape to mask off the areas that I didn't want to accidentally remove material from... the root of the fins and the cardboard body tube. The tape was thin enough that I was able to get the fillets sanded right down close to the tube and fins without any damage. I plan to hit this with high fill primer to take care of the remaining minor surface irregularities in the fillets and plywood fins.

Wooden dowel with sandpaper wrap too sand fillets:




I also drilled the G10 Fiberglass bulkheads for charge cups, terminals and also a pezio speaker on the drogue side. I cut the bolts such that there will be only 1 thread protruding when the nylock nut is tightened down. Notice that I also cut down the 1.5g charge cups at an angle. I anticipate needing no more than 0.7g of 4F black powder, and thus I could afford to cut down the cups and still have room for the 4F powder, ematch and wadding. I also cut them at a 30 deg angle with hopes that the flame front and pressure wave will be directed to the center of the body tube, and not so much into the side of the body tube next to the charge cup. I will have the slanted edge facing the center. Picture below:

Drilled G10 bulkheads and intended hardware to install.

 

[DrewD]

Looking good! I like the idea of cutting the charge well at an angle. I may have to try that. After some ground testing I ended up with 1g bp as an optimal charge. A little on the energetic side, but is sure to get the job done. How soon do you plan on flying your bird?

 

[kevinkal]

Thanks. I'm working hard to have it ready for this Saturday. I have some RMS 29/180 H motors to start with, and I might be able to pick up a few loads for a 29/240. I'm thinking that the punchy RMS 29/180 H238T load will be a good one for whistle.. It should hit hard and burn out in about .7 seconds. and hopefully the Athena will whistle on up to 1,500 ft.
Tonight I worked on finishing up the Altimeter Bay Electronics wiring and bonding the charge wells, screw terminals and the amplified speaker to the bulkheads etc. I need to at least have primer on this whole bird and successful charge tests completed by Friday night. It's going to be close.

 

[DrewD]

Let us know how how everything goes if you get it done on time! Mine is still in the primer phase. I'm hoping to make it out to our next club launch next Saturday.

 

[kevinkal]

I decided to add a few layers of fiberglass on the aft flange of the body tube and the aft motor tube protrusion. I hope this will make these tubes more resistant to hard surface landings. Here's a few pictures of the effort and results.. nice and strong, for very little weight:
Materials cut. I started using 15 min epoxy, but it cured too fast. I switched to 30 minute zpoxy for much improved results:


I warmed the epoxy to help it penetrate the glass weave:


Hobby knife to cut off the excess before the epoxy got too hard:


Body tube flange done:


I used the same process to glass the lower motor tube protrusion:

 

[kevinkal]

Duplicolor High Fill Primer for fins and fillets:


Lots of sanding to minimize weight and leave the filler in the right places:


I used Rustoleum 2X flat whte primer on the filled model and nosecone:


My daughter helped me pick out colors that were fitting for Athena, Silver and Pink.

I then painted the base color on the rocket, Krylon Shimmer Metallic Silver. The Silver was difficult to get an even smooth texture on the large booster tube. The paint seemed to setup too quickly, such that when the next side was sprayed, the overspray would add paint dust texture on the previously painted side. The results were good.. but not perfect.
I

I painted the nosecone Krylon Shimmer Metallic Pink. The pink looks very good with a perfect glossy finish.


I ran out of time and was not able to add the pink streaks down the body tubes before Saturday's Launch. I will have to finish the paint over the next week.

 

[kevinkal]

With and AeroTech RMS 29/240 H250G-M loaded, the Athena3 Dual Deploy weighed in at 55.2 oz.

I went with 1.0g of 4F in both the drogue and main charge cups. I put 1.1g of 4F in the motor ejection charge, as an increased potency backup charge.

Here's a shot on my prep table:

On the pad:


Thanks to Benno, here's some nice launch and recovery photos:

H250G ignition.. nice green flame:


H250G 20G Punch Off the Pad with Mach Diamonds if you zoom in:


Zoomed in:


The motor ejection fired significantly early at just 6.5 seconds after burnout. It was supposed to be a 10 second delay. Fortunately, the small 9" drogue didn't put the brakes on too fast and nothing was damaged. You can see that it was still traveling at 120 ft/sec when the charge fired.

The StratoLoggerCF fired the main canopy charge at 700 ft AGL and all was good:


It was a hazy day in central California.


Here's the flight profile plot. Note how the motor ejection fired early, while still at 120 ft/sec upward velocity. I was impressed with the high quality of this data, little noise and very smooth. I think the small, but correctly sized, altimeter vent holes should get credit for this.



While the boost of the H250G was excellent, I had 2 problems with the motor:
1. early ejection at 6.5seconds, not 10 as it was supposed to be
2. Slight Blister to my RMS 29/240 case.
After the successful flight, I found that my new never-been-used Dr. Rocket RMS 29/240 case had a slight blister near the forward seal.
I posted details on the motor problem here: https://www.rocketryforum.com/showthread.php?133802-aerotech-case-blistering&p=1613701#post1613701

After the 0.9 second motor burnout, the Athena3 whistled nicely up until the motor ejection occurred. I have some ground based video that I hope to edit and post soon.

 

[DrewD]

Nice! The finished product looks good. Glad to hear the flight went well with exception of the early motor ejection. To know that yours whistled is encouraging. I'd love to see the video!.
Recently I've started placing the aft centering ring 1/8" forward from the aft end of the air frame. Then filling in that last bit with epoxy. It moves the motor back along with the CG a little bit, but the results have been good! 55oz loaded w/ dual deploy is a good weight. Mine is a bit of a pig at 50oz, no motor, and only single deploy!

 

[kevinkal]

Thanks to Jay, I have a short but sweet video with audio of the whistle. Unfortunately, the way the pads were oriented to the launch station, the Athena flew through the sun.. making it impossible to see after the boost.
This was on an H250G-M, AUW was 55.2 oz.

[video=youtube;wVvqSK_0ZqA]https://www.youtube.com/watch?v=wVvqSK_0ZqA[/video]