Quantcast

L3 Success: Compulsion, 4-in Fiberglass Frenzy XL build

The Rocketry Forum

Help Support The Rocketry Forum:

kevinkal

Insatiable Hobbyist
Joined
Feb 13, 2016
Messages
598
Reaction score
22
This will be a build thread to share, learn through feedback, and document my first "M capable" High Power Rocket build and subsequent flights.

I ordered the 4-inch Fiberglass Frenzy XL from Madcow Rocketry in November, but did not start construction until recently.
It will be roughly 93" long with a 75mm motor mount and should have a take-off weight around 26.5 to 30 lbs.

Kit Contents:
IMG_2887.jpg

The airframe tubes and nosecone are nicely fabricated filament wound G12 fiberglass. I was impressed with how well the tubes fit, and how square the tube cuts were.

The bulkheads and centering rings are 1/8" G10 fiberglass. There is room for improvement with the centering rings, as the 3 units that shipped with my kit were not cut well. The inside diameters were all roughly 0.015 to 0.020" too large vs. the motor tube diameter. Also, the inside diameters were not well centered. The cut edges seemed to be rough and discolored brown, as if a dull tool might be to blame.

The fins are 3/16" G10 fiberglass. The fins are cut out nicely. The forward fin leading edges are beveled, and the aft fin trailing edges are beveled. The beveling is adequate. I would prefer that the bevels are more uniform and symmetric between fins.

All said, I'm happy with the kit and look forward to flying it this summer in the Black Rock Desert, Nevada.
 

kevinkal

Insatiable Hobbyist
Joined
Feb 13, 2016
Messages
598
Reaction score
22
I'll be using Glenmark G5000 RocketPoxy for the majority of the fiberglass bonds. I like their pigments, and will be using the Florescent Red:

IMG_2923.jpg

I debated leaving out the pigment for the Avionics Bay Bulkheads, but went with it as it allowed me to see and get all the air out of the bond:
IMG_2926.jpg

I'll also be using Aeropoxy ES6209.
Here's the Avionics Bay switch ring bonding using ES6209:
IMG_2960.jpg

With the 2-inch thick switch band, it was difficult to get all the air out and adhesive in.. I worked this back and forth for about 10 minutes until all the air was out.
IMG_2965.jpg
IMG_2970.jpg
 

kevinkal

Insatiable Hobbyist
Joined
Feb 13, 2016
Messages
598
Reaction score
22
reserved.. perhaps for Open Rocket Model and Simulation Results
 

crossfire

Lifetime Supporter
TRF Lifetime Supporter
Joined
Jan 16, 2011
Messages
4,540
Reaction score
538
Your lucky I don't think that Frenzy is even offered anymore. You got the 75MMT and thicker fins. I was wrong the 75MMT Frenzy is still there its the RW Broken Arrow
with 75MMT that is not there anymore. I am sure Mike could make one up.
 
Last edited:

ChrisAttebery

Well-Known Member
Joined
Feb 16, 2014
Messages
2,453
Reaction score
60
Looks good Kevin. I'm interested to hear how injecting the fillets works out.
 

Nathan

TRF Supporter
Joined
Apr 20, 2012
Messages
2,129
Reaction score
437
I'm also interested in the epoxy injection. Some photos would be nice. I built that same rocket and did the internal fillets with a dowel which was a real PITA to do with a 3" motor tube in a 4" airframe. And split fins.
 

kevinkal

Insatiable Hobbyist
Joined
Feb 13, 2016
Messages
598
Reaction score
22
Looks good Kevin. I'm interested to hear how injecting the fillets works out.
I'm also interested in the epoxy injection. Some photos would be nice. I built that same rocket and did the internal fillets with a dowel which was a real PITA to do with a 3" motor tube in a 4" airframe. And split fins.
I did a little test yesterday to see how well Rocketpoxy would flow inside a 3/8" diameter polyethylene tube.
I used a 60CC Syringe to suck approximately 8g of Rocketpoxy up into the 3/8" poly tube, then I simulated injecting lengths of epoxy. From a batch of 40g of Roketpoxy I able to make seven 4.0 g lines of epoxy on a sheet of cardboard. This was to simulate placing epoxy inside the fin can between the 75mm motor mount and the 4.0" body tube.

Heres the test "pallet":

IMG_3052.jpg

The idea here is that I can use a longer tube, 24-inches, to carefully place epoxy fillets on the internal fin mounting points. The 3/8" OD tubing is just small enough to fit between the motor mount and body tubes.

As the length of epoxy sucked into the tube increased, the force required to draw it in increased substantially. At 8 inches, the force was significant, and the flow rate was very slow. I think I'll be able to fillet the forward fins on both the motor tube side and internal body tube side using 2 or 3 four gram injections. The aft fins will only require 2 four gram injections. This should give nice fat fillets.

When I place the fillets, I'll be sure to take pictures and post the process here.
 

cherokeej

Well-Known Member
Joined
Feb 16, 2009
Messages
835
Reaction score
51
There is an alternative method...

Not a knock to this method, just a suggestion of another way. (I've done it both ways. Doesn't really matter which method you use, it's still gonna be a PITA.)

Split the bottom of the air frame. Use a Dremel to extend the slots all the way to the end of the tube. Then, with the forward c/r glued in place, a second ring floating ahead of the fin position, place and position the mmt tube in the a/f, temporarily place the rear c/r, then tack the fins in place one at a time with some fast cure epoxy. When that's set, pull the entire mmt assembly out, and the internal fillets become a breeze. Slide the mid-centering ring down against the fin tabs, same for the rear ring, and fillet those, too. Then you can add some epoxy inside, slide the assy back into the a/f, use a clamp on the back end to hold things round, and do the external fillets. Then add your motor retainer system of choice, and you can fill the kerfs left by the Dremel with epoxy paste filled with fused silica (aka Cabosil.)

Like I said, just another way to go. 75mm mount in a 4" a/f doesn't give ya a whole lot of room.
 

rocketsam2016

Well-Known Member
Joined
Aug 31, 2016
Messages
270
Reaction score
4
Suscribed, looking good! I do like the red epoxy....

For the electronics in the nose cone, does the wood brace against the side of the cone at the top or is it only touching at the base? If it only touches at the base, that is quite the long lever arm so I'd try some drop tests to make sure it'll survive hitting the ground at various angles under chute. The nose mounted tracker housing ive been working on wasn't strong enough at first and broke onlanding. And it was much much shorter than yours, I was pretty surprised it broke. I've built it now to survive free fall from 7 feet or so. You may be fine though, mine is built to be lightweight.
 

timbucktoo

Well-Known Member
Staff member
TRF Supporter
Global Mod
Joined
Jun 13, 2014
Messages
8,109
Reaction score
1,274
Location
Cocoa Beach
For the electronics in the nose cone, does the wood brace against the side of the cone at the top or is it only touching at the base? If it only touches at the base, that is quite the long lever arm so I'd try some drop tests to make sure it'll survive hitting the ground at various angles under chute. The nose mounted tracker housing ive been working on wasn't strong enough at first and broke onlanding. And it was much much shorter than yours, I was pretty surprised it broke. I've built it now to survive free fall from 7 feet or so. You may be fine though, mine is built to be lightweight.
Also curious why the GPS mounting board is so long! I have also experienced failure so I ended up with a short piece of G10.
IMG_3157[1].jpg
 

timbucktoo

Well-Known Member
Staff member
TRF Supporter
Global Mod
Joined
Jun 13, 2014
Messages
8,109
Reaction score
1,274
Location
Cocoa Beach
When I place the fillets, I'll be sure to take pictures and post the process here.
Have you considered injecting fillets using a thinner epoxy (e.g. West System) mixed with chopped carbon fiber. Personally I think its much easier!
IMG_2203.jpg
 

soopirV

Well-Known Member
Joined
Sep 1, 2010
Messages
1,157
Reaction score
6
All these frenzy builds are making me envious! Might have to join the surge, watching with interest.
 

kevinkal

Insatiable Hobbyist
Joined
Feb 13, 2016
Messages
598
Reaction score
22
There is an alternative method...

Not a knock to this method, just a suggestion of another way. (I've done it both ways. Doesn't really matter which method you use, it's still gonna be a PITA.)

Split the bottom of the air frame. Use a Dremel to extend the slots all the way to the end of the tube. Then, with the forward c/r glued in place, a second ring floating ahead of the fin position, place and position the mmt tube in the a/f, temporarily place the rear c/r, then tack the fins in place one at a time with some fast cure epoxy. When that's set, pull the entire mmt assembly out, and the internal fillets become a breeze. Slide the mid-centering ring down against the fin tabs, same for the rear ring, and fillet those, too. Then you can add some epoxy inside, slide the assy back into the a/f, use a clamp on the back end to hold things round, and do the external fillets. Then add your motor retainer system of choice, and you can fill the kerfs left by the Dremel with epoxy paste filled with fused silica (aka Cabosil.)

Like I said, just another way to go. 75mm mount in a 4" a/f doesn't give ya a whole lot of room.
Have you considered injecting fillets using a thinner epoxy (e.g. West System) mixed with chopped carbon fiber. Personally I think its much easier!
View attachment 319593

Thanks for the suggestions. It's clear that you two have done it that way before. Those were definitely options that I was considering.
Before arriving at the plan to use a syringe to inject epoxy from the aft end of the rocket to form internal fin fillets, I spent many hours reviewing techniques and considering their unique implications in my case. I considered a number of other options including:

1. Per Madcow Instructions .. mount the motor tube with the two forward centering rings, tack in the fins, then pour epoxy from the back of the rocket along the fins, using a dowel to form fillets.
- While it sounds simple enough, I did not think that I could ensure that I'd get enough epoxy all the way down the fins and in the correct position and thickness. From the aft of the rocket to the tip of the forward fin root is roughly 20 inches! Getting a dowel down there to try to position epoxy would be extremely difficult, as the epoxy would just end up getting distributed over unintended surfaces etc.

2. External Injection .. Build up the motor mount within the body tube with all centering rings including the aft. Tack the fins in, then use a syringe to inject epoxy though small holes cut in the body tube at the root of each fin. Some might call this "Wildman Style" injection given it seems to be the norm for his build instructions.
- While this would work, I think that control of where the epoxy flows is not good. I was not certain that I could use this technique and get good fillets on both fin-motor mount and internal fin-body tube. Also, given that the fins are split, this technique would be prone to leak epoxy between the fins unless I add additional centering rings.

3. Fin Pockets .. like the pockets that rocketsam2016 built into his Frenzy XL... Build "fin pockets" between centering rings on the motor mount, install the motor mount and all centering rings, then fill the fin pockets with epoxy and insert the fins... basically giving a "square" fillet between the motor tube, fin and inside body tube.
- This seems like a good way to go. A downside, however, is that it will add more weight than needed as the epoxy at the center of the "pocket" on each side of the fin is not necessary.

4. Split the Airframe .. Extend the fin slots all the way though the back of the booster tube aka "split the bottom of the airframe" technique.
- While this does offer the best access to the bonds at the motor mount, I don't like the idea of making that cut to the booster tube. I think there are number of downsides to this technique. The 3 "flaps" between fins slots will no longer serve as an axial reference guide. Breaking the full circle fiberglass around the fins seems like it would weaken the fin can. This technique doesn't make it any easier to place internal fillets between the fin and the body tube.

Regarding option 4, splitting the airframe.. My two forward centering rings are NOT a snug fit to the inside of the booster tube and they are only 5 inches apart. Thus, when I place the motor tube into position, there are 17 inches of unsupported motor tube that will slop around and align down with gravity about 1/8" until I add the aft center ring. The aft centering ring uses the rigid body tube to provide a center reference for the motor tube. Should I split the airframe, the 3 remaining fiberglass "flaps" would not provide a reliable center reference.
 

kevinkal

Insatiable Hobbyist
Joined
Feb 13, 2016
Messages
598
Reaction score
22
IMG_3127.jpgIMG_3137.jpgIMG_3138.jpg

Suscribed, looking good! I do like the red epoxy....

For the electronics in the nose cone, does the wood brace against the side of the cone at the top or is it only touching at the base? If it only touches at the base, that is quite the long lever arm so I'd try some drop tests to make sure it'll survive hitting the ground at various angles under chute. The nose mounted tracker housing ive been working on wasn't strong enough at first and broke onlanding. And it was much much shorter than yours, I was pretty surprised it broke. I've built it now to survive free fall from 7 feet or so. You may be fine though, mine is built to be lightweight.
Also curious why the GPS mounting board is so long! I have also experienced failure so I ended up with a short piece of G10.
View attachment 319592
You two saw that issue right away! Yes, that plywood is cantilevered from the base that it's embedded in. It will deflect significantly in bending across the flat side if I impact the nosecone in that direction. The plywood sled will not break even if the end impacts the side of the nosecone, but I don't think the tracker will survive these impacts. I have not bonded the coupler in the nosecone yet, as I want to be confident that I have a good solution to this issue first. I have in the past simply placed foam just below the tracker between the nosecone and the plywood sled to resist displacement. This exact tracker and long mounting sled have flown twice in my other 4" FG rocket which has the same mounting ring in it's nosecone. The foam solves the bending problem.. but it's not the clean solution I hope to achieve.

The reason I made the plywood sled so long was to get the GPS antenna out from underneath 2 bonded layers for filament wound fiberglass. I figure that there may be better reception under just a single layer. Also, the tracker can easily be repositioned on the sled. So, when I cut the sled I cut it long knowing that I could make final adjustments later.
 

kevinkal

Insatiable Hobbyist
Joined
Feb 13, 2016
Messages
598
Reaction score
22
I worked to get the centering ring holes concentric to the outside diameter within a few thousands of an inch for both forward centering rings. Next I used a dremmel tool to notch and radius slots in the top centering ring where a 1" Kevlar anchor strap will exit on opposite sides of the motor tube. After deciding what depth I wanted the Aeropack 75mm motor retainer to reside within the rocket, I marked then bonded the centering rings to the motor tube using Rocketpoxy. My motor retainer will protrude 5/8" out the aft end of the rocket, which should give comfortable room to grip the retainer for removal, and also give a little more ground clearance for the aft fins such that the motor retainer will impact the ground first more often. There's 5.25" space between the upper two centering rings, and the top ring is recessed 3/8" from the top of the motor tube, leaving room for a full fillet. Here's some pictures:

Dry fit top two centering rings and 1" Kevlar Anchor Strap
IMG_2995.jpg

So that I could see my position reference marks, I did not fillet the bottom until the first batch of Rocketpoxy had set.
IMG_3035.jpg

For the bottom fillets, I left space for the forward fins to butt up into the middle centering ring.
IMG_3040.jpg

After the sloppy application of Rocketpoxy fillets, I spent a good amount of time making sure the fillets only spanned about half the width of the centering ring so that there would be room for the body tube fillets when I install the motor tube.
IMG_3057.jpg

To reduce wear on the Kevlar strap where it exits the top centering ring, I put a 1.5" length of clear heat shrink tubing around the Kevlar.
IMG_3061.jpg

I left a "no epoxy buffer" zone on the Kevlar strap for about 1/4" below the top centering ring with the understanding that it should reduce wear on the Kevlar strap connection at this location. This left about 4.75 inches of bonding length for the 1" wide Kevlar strap. Thus each side has 4.75 square inches of bond area using AeroPoxy ES6209.
IMG_3076.jpgIMG_3089.jpg

Using nitrile gloves, I carefully worked the epoxy into the weave of the Kevlar strap using liberal ES6209 epoxy:
IMG_3088.jpg

Finally, wrapped a 5 mil thick piece of mylar around the upper motor tube and used tape to hold it tight until the epoxy had set.
IMG_3091.jpgIMG_3100.jpg

I'm out of picture space for this post. so I'll add the final pictures of the Kevlar anchor strap in the next post.
 

rharshberger

Well-Known Member
Joined
Oct 13, 2014
Messages
9,793
Reaction score
1,764
Location
Pasco, WA
Thanks for the suggestions. It's clear that you two have done it that way before. Those were definitely options that I was considering.
Before arriving at the plan to use a syringe to inject epoxy from the aft end of the rocket to form internal fin fillets, I spent many hours reviewing techniques and considering their unique implications in my case. I considered a number of other options including:

1. Per Madcow Instructions .. mount the motor tube with the two forward centering rings, tack in the fins, then pour epoxy from the back of the rocket along the fins, using a dowel to form fillets.
- While it sounds simple enough, I did not think that I could ensure that I'd get enough epoxy all the way down the fins and in the correct position and thickness. From the aft of the rocket to the tip of the forward fin root is roughly 20 inches! Getting a dowel down there to try to position epoxy would be extremely difficult, as the epoxy would just end up getting distributed over unintended surfaces etc.

2. External Injection .. Build up the motor mount within the body tube with all centering rings including the aft. Tack the fins in, then use a syringe to inject epoxy though small holes cut in the body tube at the root of each fin. Some might call this "Wildman Style" injection given it seems to be the norm for his build instructions.
- While this would work, I think that control of where the epoxy flows is not good. I was not certain that I could use this technique and get good fillets on both fin-motor mount and internal fin-body tube. Also, given that the fins are split, this technique would be prone to leak epoxy between the fins unless I add additional centering rings.

3. Fin Pockets .. like the pockets that rocketsam2016 built into his Frenzy XL... Build "fin pockets" between centering rings on the motor mount, install the motor mount and all centering rings, then fill the fin pockets with epoxy and insert the fins... basically giving a "square" fillet between the motor tube, fin and inside body tube.
- This seems like a good way to go. A downside, however, is that it will add more weight than needed as the epoxy at the center of the "pocket" on each side of the fin is not necessary.

4. Split the Airframe .. Extend the fin slots all the way though the back of the booster tube aka "split the bottom of the airframe" technique.
- While this does offer the best access to the bonds at the motor mount, I don't like the idea of making that cut to the booster tube. I think there are number of downsides to this technique. The 3 "flaps" between fins slots will no longer serve as an axial reference guide. Breaking the full circle fiberglass around the fins seems like it would weaken the fin can. This technique doesn't make it any easier to place internal fillets between the fin and the body tube.

Regarding option 4, splitting the airframe.. My two forward centering rings are NOT a snug fit to the inside of the booster tube and they are only 5 inches apart. Thus, when I place the motor tube into position, there are 17 inches of unsupported motor tube that will slop around and align down with gravity about 1/8" until I add the aft center ring. The aft centering ring uses the rigid body tube to provide a center reference for the motor tube. Should I split the airframe, the 3 remaining fiberglass "flaps" would not provide a reliable center reference.


To be honest all these are good methods, personally I prefer using either fin pockets or injecting fillets and have done split fin rockets both ways. Most people seem to build fin pockets as a wedge which does use more epoxy in the upper section of the pocket, mine the dams are parallel to the fin between 1/8" and 1/4" or less between the dams and the fin sides, and I have yet to have a failure and it seems to use about the same amount of epoxy as traditional fillets and since the dams are balsa they have negligble weight from the damns. By using the fin pockets I can use a fairly thick structural epoxy like Rocketpoxy or on my L3 bird System3 T-88 in the U-TAH tube (two part epoxy that fits in a standard caulk type gun).

Injection can add just as much weight as the other methods too, it is among my favorite methods for certain.

My least favorite method is goop on a stick or some variation of it.

Picture below is a Mini-Frenzy fin can with fin pockets, fillets will be about 1/16" each side of fin, this rocket will be flown on a I600R, but is still light enough to fly under FAR101 rules (even set up dual deploy and with nosecone tracker). I use the same methods on bigger rockets.
MiniFrenzyXLdd4.jpg
 
Last edited:

kevinkal

Insatiable Hobbyist
Joined
Feb 13, 2016
Messages
598
Reaction score
22
Some pictures of the final result of bonding the Kevlar Strap using Aeropoxy ES6209. I'm happy with the result.
The bond should be stronger than the strap itself. Aeropoxy ES6209 was found to have excellent lap shear strength numbers per John Cocker's test results.
For G10 to G10 Fiberglass lap shear joints, it failed at 1,600 psi. Simplisticly, each 4.75" by 1" side of bonded Kevlar has 4.75 square inches of bond area and thus the bond should be stronger than the Kevlar strap. I bought the strap from Wildman, who did not list its strength rating. However, from my online research, I believe the strap is rated for 2,400 lbs.

IMG_3140.jpgIMG_3141.jpgIMG_3144.jpgIMG_3145.jpg
 

kevinkal

Insatiable Hobbyist
Joined
Feb 13, 2016
Messages
598
Reaction score
22
To be honest all these are good methods, personally I prefer using either fin pockets or injecting fillets and have done split fin rockets both ways. Most people seem to build fin pockets as a wedge which does use more epoxy in the upper section of the pocket, mine the dams are parallel to the fin between 1/8" and 1/4" or less between the dams and the fin sides, and I have yet to have a failure and it seems to use about the same amount of epoxy as traditional fillets and since the dams are balsa they have negligble weight from the damns. By using the fin pockets I can use a fairly thick structural epoxy like Rocketpoxy or on my L3 bird System3 T-88 in the U-TAH tube (two part epoxy that fits in a standard caulk type gun).

Injection can add just as much weight as the other methods too, it is among my favorite methods for certain.

My least favorite method is goop on a stick or some variation of it.

Picture below is a Mini-Frenzy fin can with fin pockets, fillets will be about 1/16" each side of fin, this rocket will be flown on a I600R, but is still light enough to fly under FAR101 rules (even set up dual deploy and with nosecone tracker). I use the same methods on bigger rockets.

I think the parallel dams for the fin pockets is the right way to go when using pockets.
Your Mini Frenzy is going to disappear on that I600R.
 

cherokeej

Well-Known Member
Joined
Feb 16, 2009
Messages
835
Reaction score
51
Like I said, just an alternative. But I find that temporarily using a short piece of coupler just behind the rear c/r holds the a/f quite round while applying external fillets.

And just for the record, internal fillets to the a/f are a waste of effort, and un-amended epoxy for any fillets is too. Epoxy is an adhesive, not a structural material. If you're not putting chopped carbon, glass, or Kevlar in your fillets, then you're making decorative fillets.

Next time you mix some epoxy, pour some into a soda straw, and let it cure. Then pick it up, and push with your thumb. Note how easily it snaps. Epoxy is an adhesive, not a structural material.

Let the flames begin.
 
Last edited:

rocketsam2016

Well-Known Member
Joined
Aug 31, 2016
Messages
270
Reaction score
4
You two saw that issue right away! Yes, that plywood is cantilevered from the base that it's embedded in. It will deflect significantly in bending across the flat side if I impact the nosecone in that direction. The plywood sled will not break even if the end impacts the side of the nosecone, but I don't think the tracker will survive these impacts. I have not bonded the coupler in the nosecone yet, as I want to be confident that I have a good solution to this issue first. I have in the past simply placed foam just below the tracker between the nosecone and the plywood sled to resist displacement. This exact tracker and long mounting sled have flown twice in my other 4" FG rocket which has the same mounting ring in it's nosecone. The foam solves the bending problem.. but it's not the clean solution I hope to achieve.

The reason I made the plywood sled so long was to get the GPS antenna out from underneath 2 bonded layers for filament wound fiberglass. I figure that there may be better reception under just a single layer. Also, the tracker can easily be repositioned on the sled. So, when I cut the sled I cut it long knowing that I could make final adjustments later.
Sounds like you have this in hand, but I bet the foam will be necessary. And definitely definitely test it on the ground dropping point first and sidewise! (Equation for speed hitting the ground if dropped from height h is v = sqrt(2*g*h), or use this drop table) My tracker bay saga is as follows:

1) Built "clever" design that would be very lightweight and easy to install out of hobby plywood, wood glue and rocketpoxy. Assumed would be strong enough. It broke on landing during the first flight
2) Made clever design stronger. Used washers instead of tracker to sim weight but was conservative and used much more weight than the tracker, and put it in the nose cone for a rocket I was ground testing. The nose cone probably went up 12 feet, which would mean it hit the ground at 28fps. The tracker housing broke
3) Made clever design even stronger! Tested by dropping nose cone from 5 feet onto its side onto grass. Tracker housing broke.
4) Bought 6 ply aircraft grade plywood, made design stronger and covered surface of critical areas of epoxy. This has held up to 5+ foot drops landing sideways and 7 foot drops hitting point first on grass (18fps and 21fps respectively), with 25% more weight simulated with washers than the actual tracker weight. I should probably test it landing sidewise from higher but I'm scared.... :) and my nose cone will be descending point first onto grass at around 17fps.

My biggest problem was crappy plywood layers shearing, but it was amazing to me at first how many tries it took to make it strong enough. And all bets are off if you hit a rock sidewise...
 

kevinkal

Insatiable Hobbyist
Joined
Feb 13, 2016
Messages
598
Reaction score
22
Like I said, just an alternative. But I find that temporarily using a short piece of coupler just behind the rear c/r holds the a/f quite round while applying external fillets.

And just for the record, internal fillets to the a/f are a waste of effort, and un-amended epoxy for any fillets is too. Epoxy is an adhesive, not a structural material. If you're not putting chopped carbon, glass, or Kevlar in your fillets, then you're making decorative fillets.

Next time you mix some epoxy, pour some into a soda straw, and let it cure. Then pick it up, and push with your thumb. Note how easily it snaps. Epoxy is an adhesive, not a structural material.

Let the flames begin.
Here is my understanding on this... not a flame... a discussion...
The epoxy fillets are used to transfer loads between structural members such as the motor tube and the fin. Using thicker fillets does reduce the stress level in the epoxy, but because the epoxy typically has a lower modulus than the composite materials that it's bonding, there will be a greater stress gradient between the root of the bond and the edge of the fillet. The thinner part of the fillet still transfers load, just not as much as the thicker root portion of the fillet.

The manufacturers of common high strength epoxies do provide material properties for their products. They have excellent tensile and compression strengths for the purposes of bonding. Those are structural properties. I've pasted the properties for the adhesive's I'm using below:

Glenmark G5000 High Strength Epoxy, aka. "RocketPoxy"
⦁ Tensile strength 7,600 psi
⦁ ASTM D638 Compression strength 14,800 psi
⦁ ASTM D695 Elongation at break % 6.3%
⦁ ASTM D638 Typical operating temperature -50°F to 175°F
⦁ Maximum use temperature 225°F (107°C)
⦁ Deflection temperature 150°F (66°C)

"DATA SHEET: G5000 HIGH STRENGTH EPOXY
Description: G5000 is a two component filled epoxy with high strength bonds for joining fiberglass and carbon fiber composites with extremely high shear strengths. It also has excellent adhesion to metals, plastics, woods, and ceramics as well. Cures to a very high strength bond that is also nonbrittle to eliminate flexing cracks. Easy to mix 1 to 1 ratio by weight and volume. Mixes to a smooth creamy paste that when applied eliminates drips, sagging, or runoff. Does not require any thickening or strength additives as epoxy is ready to use as supplied. The adhesive cures relatively quickly and can be handled within a few hours. Cures to an easy to paint off white color but pigment can be easily added to provide almost any color desired.

It has excellent mechanical properties, high shear and peel strength, great adhesion, good chemical and environmental resistance, good thermal shock resistance and very low shrinkage. It has low exotherm during cure for filling large mass voids. Uses: Joining and bonded fiberglass, carbon fiber, composites, any where a high strength non-brittle bond is needed. Great for attaching composite rocket fins, bulk plates, nose cone hardware and especially for professional grade fin fillets."


Aeropoxy ES6209
⦁ Tensile Strength 8,000 psi
⦁ Tensile Elongation at Break 7%
⦁ Compressive Strength 11,500 psi
⦁ Tensile Lap Shear, Al to Al
@ 77°F 2,900 psi
@ 140°F 700 psi

"DESCRIPT ION
ES6209 is a translucent, straw colored, two component ep oxy adhesive with a thick, liquid consistency. ES6209 is an excellent structural adhesive for use when good flow and surface wetting are required. Though pourable, ES6209 has a controlled flow viscosity to minimize run-out from the adhesive joint. The 1 to 1 combining ratio by weight or volume makes it easy to measure and mix. The working time of over an hour provides plenty of time to coat and assemble the materials to be bonded.

TYPICAL APPLICATIONS ES6209 has excellent adhesion to the composite and metal parts typically used in kitplane construction. It also bonds rigid foam, wood, ceramics, fiber glass and most metals with equal ease. When cured, ES6209 provides tough, durable structural joints."


The thing about adding chopped fibers into epoxy and making fillets, is that it is next to impossible to control the direction of the fibers. Thus, the strength benefit varies greatly depending on how those fibers align with the direction of the stress though the joint they are reinforcing. Imagine if someone used a fiber amended epoxy for fillets, but after placing the fillets, all the fibers lined up inline with the fillet.. then the fibers add no strength, and all the strength will come from the adhesive properties.
 

cherokeej

Well-Known Member
Joined
Feb 16, 2009
Messages
835
Reaction score
51
Absolutely a discussion. I'm with you, brother.

In their description of the Rocketpoxy... "...two part filled epoxy..." Sounds like they've amended it for you. I must admit I've never used it. Looks like some really good stuff.

I have used Aeropoxy. Also an excellent product. Looks like they are describing it as an adhesive. I note their use of the phrase "...durable structural joints." Seems they're using it as an adhesive.

That's the stuff I amend with the chopped carbon. Random, very short lengths of cut carbon tows I cut with a scissor while I watch a movie or a ball game. A whole lot of little snips into a bowl. And when I say I add carbon, I don't mean a little pinch with the fingertips. I put some carbon in there. To the point it can only be described as black sn*t. Then I add copious quantities of Cabosil, and make my (black) peanut butter for the fillets. Pretty sure I get a good random placement and position of the fibers.

Dave and I folded a piece of PR G10 airframe one day in a Comp4 with a nasty Rx motor, and when we pulled what was left of the bottom half out of the playa, those fins were still attached. Everyone thought we shredded it. It didn't shred, it folded. Nasty motors. :facepalm: When Dave went to salvage the tailcone and ISC receiver, he had to remove the fins with a 4 pound sledge. If Dave makes it to lunch on Friday, ask him about the Insanity 2.5 flight. :wink:
 

ActingLikeAKid

Well-Known Member
Joined
Nov 17, 2015
Messages
1,133
Reaction score
13
The reason I made the plywood sled so long was to get the GPS antenna out from underneath 2 bonded layers for filament wound fiberglass. I figure that there may be better reception under just a single layer. Also, the tracker can easily be repositioned on the sled. So, when I cut the sled I cut it long knowing that I could make final adjustments later.
When I made my tracker sled, I did it so that the top of it gently presses against the inside of the NC. In theory at least ;) the idea is that I don't have to worry madly about how well I've glued the bottom end of the sled down, because the whole thing is held in mechanically (if that makes sense).
 

rocketsam2016

Well-Known Member
Joined
Aug 31, 2016
Messages
270
Reaction score
4
When I made my tracker sled, I did it so that the top of it gently presses against the inside of the NC. In theory at least ;) the idea is that I don't have to worry madly about how well I've glued the bottom end of the sled down, because the whole thing is held in mechanically (if that makes sense).
That seems like a great approach. Isn't easy in my (not OP) case because I designed mine such that for installing the tracker the nose cone bulkhead isn't removed and instead the tracker assembly is slid into a hole in the bulkhead and locked in place with a 90 degree rotation.
 

kevinkal

Insatiable Hobbyist
Joined
Feb 13, 2016
Messages
598
Reaction score
22
I was marking lines for rail buttons last night, and in doing so, noticed that the fin slots do not appear to be well aligned with the booster tube. It appears that both the forward and aft fin slot were cut at the same time and thus both have the same misalignment. It's very hard to measure given the tools I have, an extruded aluminum level that seats on the outside diameter of my tube. However, I've been consistently measuring an alignment offset on all 3 (6) fin slots of roughly 0.025 to 0.030" from the bottom of the aft fin to the top of the forward fin (14.25 inches total fin length). If this is true, then I calculate that the rocket will rotate 1 revolution per every 501 feet of altitude. At it's expected top speed on an M1315W, ~ 1450 ft/sec, that would result in a spin rate of 2.9 revolutions per second. I'm not happy with that.

Rough calculation:
altitude per revolution ~= (tube circumference [radial in/rev]/fin offset [radial [radial in])*(fin length [in]) = (PI*4.03/0.03)*14.25 = 6014 in/rev * 1 ft / 12 in = 501 ft/rev

I plan to verify my measurement of this issue tonight, using whatever means I can find around the house. It's hard finding flat straight surfaces.. and I sure as heck don't trust my door frames.

I'm thinking that to correct this misalignment, I'll have to file the edge of my fin slots.. and thus have a very sloppy fit. Then I thought I could use a thin layer of rocketpoxy to build up the edge with the gap such that it would then produce a snug fit in the proper alignment. Sounds like a lot of work. Not happy with having to do this.. but I don't want my camera spinning at 180 rpm during the flight.
 
Top