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tOD

tOD

Sinking in the quicksand of HPR
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Has anyone tried encapsulating a motor mount with PML foam after the rocket's already built? I've got a DX3 with the 54 mm mount that I'd like to try it with. All the internal joints were filleted with Rocketpoxy when it was built but I'd like to add the foam since I got it for another build. Is it good to just drill some holes and pour it in? What about quantity? 20 ml or so?
 
I looked into the stuff. I decided against it since it was suuuper heavy. I didn't want to add any more mass to my rocket's butt. The strength from some good urethane foam was more than enough for me.

I'll bet it comes in various densities (maybe?), but the structural epoxy foam was too heavy for me.
 
I've used it on a 4x mosquito MMT/fin tabs and to fill vacu-form fin halves and fairings. I like the stuff because it doesn't continue to grow after it cured like spray can foam.
 
iirc isn't the PML stuff "variable" density based on the number of drops of water added. I am with others though, adding unnecessary weight to the "wrong" end of the rocket it not usually recommended.
 
rharshberger said:
iirc isn't the PML stuff "variable" density based on the number of drops of water added. I am with others though, adding unnecessary weight to the "wrong" end of the rocket it not usually recommended.
Click to expand...
Yes it's adjustable density with water. You gotta waste some outside of your project to figure out the best weight strength ratio you're looking for. For filling fin halves and fairings I go with the maximum water for the amount of foam mixed, carve and sand to fit and use foam safe CA for attaching. When I've done MMT encasing I've gone with about half the max water and still had good strength results with not a whole lot of nose weight needed.
 
What I'm getting at is that I've got limited flying experience, having just come into rocketry about a year ago. I got my L1 and have flown the DX3 twice on 38 mm motors using the 38/54 adaptor. I've got an I364 that I plan to use next as kind of a shakedown for an L2 attempt. I'm planning to use the 54mm J250 DMS motor for the cert flight. The rocket will probably fly on some more J's and maybe some smaller K's if I get my cert. I may never get to take it anywhere that I could stick an L in it. I was wondering if the mount is strong enough as I've built it and I'm just being over cautious to think I need more reinforcement. Having done the internal fillets I've already gone beyond what the kit instructions call for. Attached is a photo of the mount before the aft cr was installed. I hadn't heard about the foam as I was building it or I probably would have added it then.
 

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tOD said:
What I'm getting at is that I've got limited flying experience, having just come into rocketry about a year ago. I got my L1 and have flown the DX3 twice on 38 mm motors using the 38/54 adaptor. I've got an I364 that I plan to use next as kind of a shakedown for an L2 attempt. I'm planning to use the 54mm J250 DMS motor for the cert flight. The rocket will probably fly on some more J's and maybe some smaller K's if I get my cert. I may never get to take it anywhere that I could stick an L in it. I was wondering if the mount is strong enough as I've built it and I'm just being over cautious to think I need more reinforcement. Having done the internal fillets I've already gone beyond what the kit instructions call for. Attached is a photo of the mount before the aft cr was installed. I hadn't heard about the foam as I was building it or I probably would have added it then.
Click to expand...

I wouldn’t add additional foam at this point. With good internal fillets it’s simply unnecessary and adds weight to the wrong place without benefit.
 
FWIW I read through the description on PML's site and the foam is polyurethane (same as all the other expanding foams), not epoxy.

I suspect, though this is speculation, that most of the high-density foams could be made adjustable with additional water. The water reacts with the isocyanate to produce additional carbon dioxide.

Best -- Terry
 
Fillets or foam shall be my mantra.
I picked up on another thread after I posted that covered basically the same topic with the same conclusion as Steve and Bat-mite. My clubs next launch is scheduled for 2/19, now if the weather will cooperate...
I also have a completed Wildman Sport to try, foam only, no fillets, maybe H97J, or G77R.
 
My MAC Performance Scorpion is foamed with simple two-part foam. It is a 75mm airframe with a 54mm MMT. That only left about 9mm between the airframe and the MMT. I was not comfortable trying to add fillets in that tiny space. Meanwhile, the rocket got an extended payload upgrade, and it was showing a stability margin of about 3 cal. So foam was an easy substitute for fillets, and it has a ton of flights on it. So I try to let the rocket determine the build technique.
 
On my Wildman Sport rockets, I foam and don’t even bother with external fillets. I recently built a Darkstar Mini, and the space between the motor mount and the body tube is so small that I just filled it with Aeropoxy instead of foam. Not sure that was the best approach, but it is sturdy, even without external fillets. I might add some external fillets out of Aeropoxy light for aerodynamics, but it doesn’t appear to need anything for strength.
 
tOD said:
Fillets or foam shall be my mantra.
I picked up on another thread after I posted that covered basically the same topic with the same conclusion as Steve and Bat-mite. My clubs next launch is scheduled for 2/19, now if the weather will cooperate...
I also have a completed Wildman Sport to try, foam only, no fillets, maybe H97J, or G77R.
Click to expand...

If you use foam only, be sure to seal the cracks first. Expanding foam goes everywhere you don’t want it.
 
I put foam in my DX3 like you are building. I have less confidence in the foam. I don't "feel" it's as strong as internal fillets. But I have no proof of this. It would be fun to watch someone try to inject the foam in a hole. I don't think you can mix it, fill a syringe, and inject it without it being a comedy routine. But I was wrong that one other time,.
 
roytyson said:
I put foam in my DX3 like you are building. I have less confidence in the foam. I don't "feel" it's as strong as internal fillets. But I have no proof of this. It would be fun to watch someone try to inject the foam in a hole. I don't think you can mix it, fill a syringe, and inject it without it being a comedy routine. But I was wrong that one other time,.
Click to expand...


I use the PML foam on most fin cans that I build. The biggest advantage of the foam is it adheres to all surfaces and produces a solid bond to the entire surface, not just along the mmt and the slot edges. I just foamed my LOC Ultimate by injecting the foam through 1/8" holes drilled in the aft centering ring. Filled all the gaps and added slightly over 12 grams. BTW, yes you can fill a syringe and inject it without it being a comedy show. The syringe actually slows down the reaction. I have even been able to reuse syringes after wiping off the end. Works slick, greater adhesion area, lighter than bulk epoxy fillets. Most importantly - its fun to play with. I also make a lot of multi-tube cluster rockets and it works best for sealing them off in the airframe. But some people don't like it and that is fine. It would be boring if we all agreed.
 
roytyson said:
I put foam in my DX3 like you are building. I have less confidence in the foam. I don't "feel" it's as strong as internal fillets. But I have no proof of this. It would be fun to watch someone try to inject the foam in a hole. I don't think you can mix it, fill a syringe, and inject it without it being a comedy routine. But I was wrong that one other time,.
Click to expand...

Someone...don't recall who...posted about using foam as the sole adhesive in TTW construction. It IS in fact essentially a form of polyurethane glue (similar to Gorilla glue) and the foam grips everything, not just the root edge. As a suspender-and-belt man, I'd fillet anyway. Oh, and drill some holes in the part of the fin that's inside the airframe, more surface to grip.

I've thought about injecting foam with a syringe but can't come up with a way that wouldn't leave me looking like Dan Ackroyd after the Stay-Puft man exploded.

Best -- Terry
 
The Geoff said:
I use the PML foam on most fin cans that I build. The biggest advantage of the foam is it adheres to all surfaces and produces a solid bond to the entire surface, not just along the mmt and the slot edges. I just foamed my LOC Ultimate by injecting the foam through 1/8" holes drilled in the aft centering ring. Filled all the gaps and added slightly over 12 grams. BTW, yes you can fill a syringe and inject it without it being a comedy show. The syringe actually slows down the reaction. I have even been able to reuse syringes after wiping off the end. Works slick, greater adhesion area, lighter than bulk epoxy fillets. Most importantly - its fun to play with. I also make a lot of multi-tube cluster rockets and it works best for sealing them off in the airframe. But some people don't like it and that is fine. It would be boring if we all agreed.
Click to expand...

Dear Mr. The, we posted almost simultaneously. It appears to be true, great minds run on the same track. ;)
 
You pull the piston out of syringe. place finger over tip & pour slurry into sryinge. Replace piston & inject.
Must work fast . I always place parts A & B in refrigerator, to cool down both parts for about 20-30 minutes.
Slows down expansion time to give you more work time to inject.
Trying to inject at normal room temps is suicide.....starts to foam 45-60 seconds after mixing.
Cooling it down gives 4-8 minutes work time before it fires off foaming.
Find my thread on building Wildman mini's with foam, its all in there.
 
blackjack2564 said:
You pull the piston out of syringe. place finger over tip & pour slurry into sryinge. Replace piston & inject.
Must work fast . I always place parts A & B in refrigerator, to cool down both parts for about 20-30 minutes.
Slows down expansion time to give you more work time to inject.
Trying to inject at normal room temps is suicide.....starts to foam 45-60 seconds after mixing.
Cooling it down gives 4-8 minutes work time before it fires off foaming.
Find my thread on building Wildman mini's with foam, its all in there.
Click to expand...
I'm looking at building a Darkstar Junior and was wondering about foaming the fin can. This sounds like it's the information I was looking for. I'll look for the thread.
 
tOD said:
I'm looking at building a Darkstar Junior and was wondering about foaming the fin can. This sounds like it's the information I was looking for. I'll look for the thread.
Click to expand...

The space between the MMT & BT is very small on the DS Jr. Stick with the injected epoxy method as suggested..
 
blackjack2564 said:
You pull the piston out of syringe. place finger over tip & pour slurry into sryinge. Replace piston & inject.
Must work fast . I always place parts A & B in refrigerator, to cool down both parts for about 20-30 minutes.
Slows down expansion time to give you more work time to inject.
Trying to inject at normal room temps is suicide.....starts to foam 45-60 seconds after mixing.
Cooling it down gives 4-8 minutes work time before it fires off foaming.
Find my thread on building Wildman mini's with foam, its all in there.
Click to expand...

You aren't wrong about temperature, my shop is cold so I never thought to mention it. I actually just suck the mix into the syringe and wipe the end, much less messy. I always do the main mmt fillet but not the inner airframe. Then do a minimal one on the outside.
 
I use both internal fillets and 2 part foam in the fin can-the weight is not that bad if you use 5lb density. I bought it on amazon for like 30$ and it is about 32oz of part a and part b. I have built a rocket and added foam later by drilling holes through the aft ring and measuring (10x expansion) and pouring through a cheap kitchen funnel or small graduated cylinder (they have nice pour spouts)-once finished drying just cut the excess off. The caliber isp I just added it to moved my cg aft about 1/2”-no big deal and I can feel more confident in using a much bigger motor. The fin can feels very solid under lateral pressure now with foam.
 
It's been a while since I've played with 2 part foam, and therefore haven't tried that yet, but nowadays I'd use a static mixer and a dual cartridge to dispense that stuff. This should take a away most of the "hurry up" factor. The components and applicators can be found cheap at places like Ebay and Aliexpress (example, while link lasts) and some folks have also made their own equipment out of 3d printed parts and regular syringes.


Reinhard
 
Steve Shannon said:
If you use foam only, be sure to seal the cracks first. Expanding foam goes everywhere you don’t want it.
Click to expand...
When I foamed the Sport the fins were such a tight fit in the slots that only a tiny bit seeped out. Cleaned up in about 10 second with a Dremel.
 
roytyson said:
I put foam in my DX3 like you are building. I have less confidence in the foam. I don't "feel" it's as strong as internal fillets. But I have no proof of this. It would be fun to watch someone try to inject the foam in a hole. I don't think you can mix it, fill a syringe, and inject it without it being a comedy routine. But I was wrong that one other time,.
Click to expand...
Is yours the cardboard and plywood DX3 with the 54 mm mmt? What kind of motors has it flown on so far? I'm limited to 38 mm using the adaptor for now. I may try to certify on a J250 when the weather improves.
 

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