Different Epoxy for Different purpose??

[OverTheTop]

As SMM said, opinions vary. I am leaning towards just buttering up the fin and poking it in these days, and not getting too hung up on fillet size and quality.

In the past I have shaped wooden dowel to sit at the location for the internal fillets on the motor mount tube. They were just glued in place with West Systems. They effectively became the fillets. One side was shaped to conform to the motor mount radius. You just have to figure out how to hold them in place as they dry.

As for internal fillets on the inside of the airframe (under the outside fillets), I have never bothered.

[RainierWolfcastle]

Personally I would not use west systems because of the viscosity. It's my understanding that anything you add to the epoxy will weaken it to some degree. To me west is more practical for rolling tubes or doing tip to tip.

If starting with a straight epoxy such as west systems the correct additives such as 403 actually make it stronger for filleting. Most of the thicker epoxies already have additives in the hardener so may differ, and the basic 1:1 5 minute jobbies do have thickeners which usually result in reduced strength.

All that said OTT's idea for dowel sounds much easier and given the expected loading on that join would be either trying to pull straight out or move sideways seems to give you strength exactly where you need it.

[drew]

Personally I would not use west systems because of the viscosity. It's my understanding that anything you add to the epoxy will weaken it to some degree. To me west is more practical for rolling tubes or doing tip to tip.

If starting with a straight epoxy such as west systems the correct additives such as 403 actually make it stronger for filleting. Most of the thicker epoxies already have additives in the hardener so may differ, and the basic 1:1 5 minute jobbies do have thickeners which usually result in reduced strength.

All that said OTT's idea for dowel sounds much easier and given the expected loading on that join would be either trying to pull straight out or move sideways seems to give you strength exactly where you need it.

And just to add to this I wouldn't use West Systems for rolling tubes or tip to tip. Granted most of my scratch built rockets are destined for high velocity minimum diameter flights but I find West Systems less optimal for these use cases when compared to Araldite. For low and slow it's perfectly fine but I'd steer well clear of it for more aggressive flight profiles.

[OverTheTop]

For low and slow it's perfectly fine but I'd steer well clear of it for more aggressive flight profiles.

My Orange Revolution went to Mach 1.8 no problems, and 21500'. I guess low and slow are are subjective terms

[RainierWolfcastle]

And just to add to this I wouldn't use West Systems for rolling tubes or tip to tip. Granted most of my scratch built rockets are destined for high velocity minimum diameter flights but I find West Systems less optimal for these use cases when compared to Araldite. For low and slow it's perfectly fine but I'd steer well clear of it for more aggressive flight profiles.

Is that based on West System Tg? Not 100% sure if I read the specs right, but it seems to be around 50-60C for both, however the 3600 can be raised by a post cure baking?

[drew]

For low and slow it's perfectly fine but I'd steer well clear of it for more aggressive flight profiles.

My Orange Revolution went to Mach 1.8 no problems, and 21500'. I guess low and slow are are subjective terms

Commercial FWFG airframe with West tip to tip though, right? I've seen West Systems laminated t2t fray from Mach 2+ flights. I'd also argue that below Mach 2 t2t is more cosmetic than necessary (assuming your fin thickness and design isn't prone to flutter at your top speed).

[OverTheTop]

Commercial FWFG airframe with West tip to tip though, right?

No tip-to-tip. Just minimum diameter.

[RainierWolfcastle]

My Orange Revolution went to Mach 1.8 no problems, and 21500'. I guess low and slow are are subjective terms

Very true, for design and structural consideration I guess there are different problems which come up at different increased speeds and how easy you find solving them probably has a slight effect on what you consider 'fast'. Everything from CP shift, fin flutter, compression loading on frame, shear force acting on fins and lastly (and importantly for the discussion on choice of epoxy) temperature from sustained time at supersonic speeds.

[joeman]

Hi,
Yeah, I guess asking if a glue is stronger or weaker is not always a good question to ask. Depends upon what stresses are upon...and the nature of their application - cyclic, etc.

From what I've read, I guess the choice of glues also comes down to what is practical as well as what the builder is familiar with. I do like the sound of the dowel (as a fillet) trick that OTT mentioned. Thanks.

Anyhow, because of the lack of space I have to deal with, I've ended up bogging up the fin ends (between MM and AF) with 105/206/403. Seemed like it would be a sensible decision because then I know whatever epoxy I ultimately use will be confined to the spot where it is required AND I can use runny epoxy that I know will infiltrate the surfaces being bonded (including the scoring marks I made on the Tang). All I'll need to keep the rocket level. Seems simple.

On another note....
One thing that I've discovered is that I can shine an LED torch through the fibreglass (it is translucent) and I can very easily see how things look. Easier than trying to shine a torch where my head is! Yeah, pretty obvious/simple, but really nice to be able to do.

Cheers

Joe

[OverTheTop]

From what I've read, I guess the choice of glues also comes down to what is practical as well as what the builder is familiar with.

Definitely. We all have differing skillsets, equipment, and knowledge. Work with what you have to achieve your success. There are a lot of ways to do things and a lot of opinions. Don't take them as gospel, think them through.

Don't take things in isolation either. I didn't do tip-to-tip on Orange Revolution, but other work meant I didn't need to have it to take the forces. I view tip-to-tip as a bit of a band-aid solution, but that is just me (and maybe a few others ).

Also, if someone says something can't be done, just keep thinking about it. Be skeptical. I am not a conspiracy theorist, but I will say "Question everything!".

[drew]

Definitely. We all have differing skillsets, equipment, and knowledge. Work with what you have to achieve your success. There are a lot of ways to do things and a lot of opinions. Don't take them as gospel, think them through.

Agreed 100%. Whatever works best for you is the best way to go (within reason of course). And critically thinking through varying opinions is always the best course of action.

Don't take things in isolation either. I didn't do tip-to-tip on Orange Revolution, but other work meant I didn't need to have it to take the forces. I view tip-to-tip as a bit of a band-aid solution, but that is just me (and maybe a few others ).

Properly scoped fins (from a thickness perspective) combined with appropriately sized fin fillets eliminate the need for tip to tip in effectively all use cases below Mach 2. That's my opinion at least.

[drew]

For low and slow it's perfectly fine but I'd steer well clear of it for more aggressive flight profiles.

My Orange Revolution went to Mach 1.8 no problems, and 21500'. I guess low and slow are are subjective terms

I was referring to West Systems for airfame construction and T2T. I got really confused there as I seemed to recall Orange Revolution was a commercial FWFG airframe so I naturally assumed you were referring to T2T on your Orange Revolution! I just did a bit more digging and saw that you used Wests for your fin fillets. I don't reckon that would ever be a problem with a commercial motor single stage flight to be honest primarily because shock waves from Mach don't collapse on the fillets, at least not with the same force when compared to the NC and fin edges. And when I refer to seeing T2T fray when combined with high speeds and Wests that's specifically referring to the leading edge of the fin.

On a different note, I noticed the aluminium jig you made for your router table to fin bevelling in your Orange Revolution build thread. Can you break that down for me a bit more from a jig design and construction perspective? I've got a router table very similar to yours that I've never used but given my new workshop I now have the space to leverage it in build works. A fin jig like that would be ideal for my workshop!