L3 build questions

I'm finally getting around to building my L3 project, a Polecat 10" Thumper DD that that's been in my home for over two years. Everything is going well so far but I do have a couple of questions for those more experienced at building bigger projects.

1. I'm using US Composites epoxy and, with regards to how easy it is to work with, I like it better than Aeropoxy. This is primarily due to it being thinner which makes it a bit easier to pour/pump and measure. However, I do sometimes wish I could thicken it up a bit just before pouring it so I've tried some chopped/milled fiberglass that I got from my local hobby store. Unfortunately, when I go by the directions listed (¼ to ½ teaspoon per ounce of epoxy), it doesn't' seem to be any thicker. If you have used this stuff to thicken epoxy before, what ratio did you find to be optimal? I did try to search for this here but the first 8-10 threads I found bore no fruit.

2. The instructions for building out the av bay/payload section are making me raise and eyebrow or two and I'm not real keen on proceeding as documented without some kind of affirmation from others who've done this. Specifically, the instructions say to epoxy the upper av bay bulkhead as well as the coupler into the payload section. I've never done this with an av bay before but, hey, this is a 10" rocket so I have PLENTY of space to get my hands in there and work so I'm ok with doing it on this rocket...especially since there was no switchband included and I don't feel up to cutting one from a 10" body tube with minimal equipment. And, besides, it'll be quicker to slap epoxy in there than it will be to measure and drill out holes for retention pins.

But here's the real problem: the instructions say to epoxy only the first 2" of the coupler into the payload section. Umm...considering the coupler is more than 13" long, I can't think of a good reason for this. Imagine only gluing only the first 1" of a coupler for a 5" diameter rocket...you just wouldn't do that. I'm leaning towards putting it 4" deep (or maybe more). Has anyone w/ a DD Thumper (or ANY Polecat kit with similar instructions) ventured away from these instructions with any problems? Has anyone followed the instructions and experienced subsequent structural failures?

If I were building the Thumper I would go with 6" into each tube. Plus install a 1" wide switch/vent band in the middle of the coupler. Its just a 1" section of body tube glue on to the coupler tube. I always use screws or plastic rivets to hold main chute tube to the coupler. I would not glue coupler in main chute tube. Its much easier to work on av-bay with out it being glued into the tube. Use 2 threaded rods to hold bulk plates to the coupler tube. The kit should be a fun flyer. My L-3 only went a mile high so I just popped the main at the top. It all depends on how large of a recovery are you have.

Have fun and fly safe

I built the 7.5" Thumper with Polecat av-bay which is very similar. The reason they give you that instruction on the AV-bay is so you have adequate coupler length for a stable fit. I wouldn't go less than 9" exposed in a 10" dia. tube. This leaves you 4" to epoxy into the payload section. The other option would be to have like a 20" bay which is far too long for the stubby Thumper. Regarding access to the AV-bulkheads, you can leave them both free and remove the whole assembly on both ends. Other option is to glue in forward (inside) bulkhead. This is what I did. It is really no problem on an 8" or 10" dia. short rocket to get in and maintain/install wiring as needed.

On the US Composites... It sounds like you have the 635 system. This is good for laminating and, with some milled FG, good for gluing in couplers as above, but it isn't structural epoxy like you'd want for major junctions or strong fillets. If you are trying to use it for this second application, the only way you are going to get it thick enough is with tons of West 405 filler or microballoons the second of which, bring more effective at thickening, but you'll loose strength. You are better served by going out and getting some real structural epoxy like System Three T-88.

dixontj93060 said:

I built the 7.5" Thumper with Polecat av-bay which is very similar. The reason they give you that instruction on the AV-bay is so you have adequate coupler length for a stable fit. I wouldn't go less than 9" exposed in a 10" dia. tube. This leaves you 4" to epoxy into the payload section. The other option would be to have like a 20" bay which is far too long for the stubby Thumper. Regarding access to the AV-bulkheads, you can leave them both free and remove the whole assembly on both ends. Other option is to glue in forward (inside) bulkhead. This is what I did. It is really no problem on an 8" or 10" dia. short rocket to get in and maintain/install wiring as needed.

On the US Composites... It sounds like you have the 635 system. This is good for laminating and, with some milled FG, good for gluing in couplers as above, but it isn't structural epoxy like you'd want for major junctions or strong fillets. If you are trying to use it for this second application, the only way you are going to get it thick enough is with tons of West 405 filler or microballoons the second of which, bring more effective at thickening, but you'll loose strength. You are better served by going out and getting some real structural epoxy like System Three T-88.

Click to expand...

+1 the T-88 epoxy, its easy to work with and if you only need 8ozs or so they sell it in a u-Tah tube that fits in a standard caulk gun, as long as you dont use the mixing tip there is not much waste and the tube can be recapped. It mixes easily by hand and is about the consistency between honey and molasses.

dixontj93060 said:

I built the 7.5" Thumper with Polecat av-bay which is very similar. The reason they give you that instruction on the AV-bay is so you have adequate coupler length for a stable fit. I wouldn't go less than 9" exposed in a 10" dia. tube. This leaves you 4" to epoxy into the payload section. The other option would be to have like a 20" bay which is far too long for the stubby Thumper. Regarding access to the AV-bulkheads, you can leave them both free and remove the whole assembly on both ends. Other option is to glue in forward (inside) bulkhead. This is what I did. It is really no problem on an 8" or 10" dia. short rocket to get in and maintain/install wiring as needed.

Click to expand...

You echo many of my own thoughts and leanings. An exposed length of 1 caliber should work which would give me about 3.5 inches to epoxy into the payload section. And, like you noted, there is PLENTY of room to work in there. If this a 4" rocket, epoxying it wouldn't even be an option for me.

On the US Composites... It sounds like you have the 635 system. This is good for laminating and, with some milled FG, good for gluing in couplers as above, but it isn't structural epoxy like you'd want for major junctions or strong fillets. If you are trying to use it for this second application, the only way you are going to get it thick enough is with tons of West 405 filler or microballoons the second of which, bring more effective at thickening, but you'll loose strength. You are better served by going out and getting some real structural epoxy like System Three T-88.

Click to expand...

Actually, I have the 150 system, not the 635. I opted for the 3:1 medium speed hardener.

PropellantHead said:

Actually, I have the 150 system, not the 635. I opted for the 3:1 medium speed hardener.

Click to expand...

OK, so the 150 is much thicker than 635. So you are close. I'd suggest West 406 (I mistakenly said 405 before) or I believe that US Composites has Cabosil/colloidal silica for far cheaper. Both those will give you more "substance" than the milled FG. If you need even more thickness then go to the microballoons (US Composites sells the red phenolic). The last step will sacrifice strength but provide much more workability. For critical joints though keep using the FG strands (1/4") as they serve a different purpose than the filler and really add to the strength.

When my rockets were mainly cardboard or phenolic, I used to glue the coupler and bulkhead in all the time. It makes it very strong. On 4-inch rockets it was a real pain getting the match wire through the bulkhead hole, so I started doing the Smoking' Rockets anti-tube style. This was my favorite, though I'm not sure how well it would scale to a 10-inch rocket (my 7.5-inch level 3 had two anti-tubes). With fiberglass kits I use the removable coupler/avionics bay/PML rivets. Either way you chose will work, but if you don't like the removable bay you can always epoxy it in place. It is very difficult going the other way.