Two-Part Foams

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tg08

tg08

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I'm looking to strengthen my TTW fins with expanding foams, does anyone have recommendations? If it matters, my rocket is 4" in diameter and roughly 6 feet tall, flying for my L1 certification.
 
I used the stuff below to foam a 6” Wildman Ultimate. It worked fine.

But I’m not sure I’d use foam again. It adds weight (albeit not much) exactly where you don’t want it, and I’m not convinced it really adds that much strength. I think it is not as widely used as it used to be.

IMG_4211.jpeg
 
Thanks, I might just use it anyways. I can counterbalance the weight of the foam with some nose weight, I'm not trying to send my rocket hurtling way into the sky, so I can afford the extra weight. In fact, I'd prefer a growling low and somewhat slow flight.
 
tg08 said:
I'm looking to strengthen my TTW fins with expanding foams, does anyone have recommendations? If it matters, my rocket is 4" in diameter and roughly 6 feet tall, flying for my L1 certification.
Click to expand...
are your TTW fins now breaking loose with your current method?

Tony
 
I use Sika Post Fix, decant the each half of the bag into Ziploc Quart Screw Top containers, measure equal parts by volume, and its about 2-3lb per cu ft density...somewhere I did a thread on it....its available at my local Lowes store for like $12....
 
tfish said:
are your TTW fins now breaking loose with your current method?
Click to expand...
No they aren't, however I've gotten lots of suggestions at my current launch site to use foams for my L1 build. I may have misworded it, I'm not trying to repair my rocket, but rather strengthen it for when I fly it.
 
I've used the above foam from Amazon. Worked well, but do a small test cup so you get a feel for its expansion and time frame. Feels solid! Does add weight, but like you said, if you're not looking to go real high, not a problem. I also used it to fill the nose cone, which holds the weights up to the front well. Work fast and neat! Protect the motor tube from getting foam into it. Be ready to dump excess out as it expands. I then used a dremel tool to grind off any excess (cures hard, so a knife won't help you much). This pic is when I finished prepping it, right before gluing the aft ring into place.IMG_20230919_124654_kindlephoto-614034880.jpg
 
tg08 said:
No they aren't, however I've gotten lots of suggestions at my current launch site to use foams for my L1 build. I may have misworded it, I'm not trying to repair my rocket, but rather strengthen it for when I fly it.
Click to expand...

Foam adds little or no real value. Assuming TTW fins, you'd be much better served by getting fillets onto the fins where they meet the motor mount.

Foam is really only good for filling volume, not for bonding.

-Kevin
 
I've used PML foam twice and it worked out well for me, but it did take up time for planning, prep, pour, watching and cleanup.

One application was on a 4" fg Wildman Demon and I chose foaming because it was a rush build to make a club drag race (FWIW I ended up being the only one that finished on time!).

I had epoxied the fins to the motor tube and also had fin fillets prior to the foaming (so foam doesn't come out at the slots). I later applied bigger and stronger fillets and it looks plenty beefy now.

20231113_124758.jpg

The end result was very good and while I still had some reservations about using foam and also not sure if I'd use it routinely, it was surprisingly strong. I plan to fly the Demon more next year and will report back if I have any issues with the foamed can.

The second time was to fill a 4" Exocet 3D printed nosecone since I wanted to strengthen it. It also worked out well for me.

YMMV
 
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OK, tried not to but have to weigh in on this one......

I have to start with a little background, I used to have a company where we molded structural cores for composite parts (and added resin transfer molded skins). We made the cores for the Pegasus Rocket fins for example. And yeah i know your friend has been doing this for years and it works great.....

Most of the 2 part foams people are using are not really doing what they think they are because of how they are being mixed / applied, and because of how they are being used. Couple of things people don't really understand about 2 part cores,

  • What you are doing is called free rise where the foam is allowed to expand without any constraints.
    • Free rise foam density will change within a pour, the density and strength of the foam will vary - a lot- with a pour
    • Foam rising along a side of a 'cup' dosen't bond nearly as well as you think
    • the density is VERY dependent on the moisture within the foam, the temperature of the foam and the temperature of the part, plus the relative humidity.
  • Foam Strength
    • 2 part FREE RISE room temperature cured foams are very fryable (yes that is a word for the brittleness of foam) the tensile strength is very low and the toughness is low too,
    • The foam reaction is exothermic (gets hot) and needs to be for the chemical reaction to work, problem is since it is exothermic, it will shrink when it cools putting the foam in tension (see note above about tensile strength of hand mixed free rise foam).
    • The cured resin shrinks as the reaction progresses. After the part is solidified it wants to keep shrinking a little, not from CTE but from the chemicla reaction. It shrinkage causes the foam to be loaded up in tension (see note above about tensile strength)
      • heating the cured foam helps develop stronger properties but it still shrinks. This can happen much later, for instance when the rocket is in the car or out in the sun. OR AFTER YOU HAVE A HOT MOTOR IN THE MOTOR MOUNT.
      • Listen to a fincan that you molded foam in, you can hear the 'rice crispies' noise from the foam breaking under tension after cure
    • Free rise foam is VERY anisotropic (properties are different in each dimention), this is exacerbated by pouring it in long tall cavity (like a fin can). Even if the strength is ok like that, when it heats or given time, it wants to shrink even more in that dimension, causing the foam to be loaded in tension (see note about foam strength in tension)
    • The strength of foams (molded or free rise) is NOT proportional to the density, 2 lb per ft^3 foam is about 1/8 the strength and stiffness of 4lb foam.
  • Foam Stiffness vs epoxy stiffness
    • Foam has a low modulus, much lower than epoxy. Unless you have a really big thickness between the bodytube and the motor mount, the body tube fin joint will fail before the foam can actually do much. Better to pour an internal fillet of eply into your fin can then use foam (or best do a cartridge design fin can, absolutly will be stronger than one with foam!).
  • Foam Bond
    • At the foam molding company we could get skin to foam bonds that never failed, really never, they were always stronger than the foam. With a free rise foam in a fincan the bond may look to be good but usually it fails easily.
    • Pouring into a deep pocket will trap air, the air will get pressurised by the expanding foam and will cause the foam to debond from the sides over time.
  • Foam Mixing
    • VERY few people that use the 2 part foams on here mix them well, they might get hard but just because it is hard dosent mean that it is mixed correctly.
      • Stick in a cup is a non starter
      • You need to use a high shear mixer in a LARGE cup, we would use popcorn buckets
      • High Shear mixer is some thing like a jiffy mixer
      • The drill needs to be a high speed drill (yeah i know the cordless drill you use has 2 speed... but the fast one is not so fast) look for a 2400RPM as a minimum, we used air tools 4,000 rpm under load - minimum)
      • you need to mix the foam for 20-30 seconds at full speed while working the sides and bottom of the container - WEAR GOGGLES (like highs school chemistry class kind not safety glasses, oh and you will ruin your clothes....
      • Foam should be 40-60 F not higher before you mix.
      • DONT SCRAPE OUT THE SIDES OF THE BUCKET, this foam is likely not well mixed. let it sit in the bucket and watch it expand.
Foam can be used as a structural element if it poured or molded correctly, and structurally attached to a part.
Mike - foam paid my mortgage - K
 
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I also have a job where I have designed and used foam core composite materials and kramer714's comments give a good summary of the different considerations. What I find is that using 2-part polyurethane foam only seems to make sense with larger body diameters and larger annular gaps. All the things Mike mentions tend to work against one trying to fill a tall thin space.

The thing that I find the 2-part foam does well is add stiffness rather than strength. Large tubes are more flexible than one may think, and long fin slots make this worse. Fillets at the body tube and at the motor tube I consider a minimum, with foam being not a great alternative [but sometimes a useful addition].

Any brittle material whether it is 2-part foam, epoxy or other solid materials will not do well in tension, so it is prudent to design so that their tensile properties are not important. Foam core construction is typically best suited to carrying shear loads, with compressive strength a secondary useful function.

The chemical reaction generates steam so materials need to tolerate hot and humid local conditions. Cardboard does pretty well but not perfect. Also the 'free rise' is not quite in a free state so some internal pressure is typically realized that also needs to be accounted for. In rockets from 6 inches to 12 inches I have not encountered any significant shrinkage issues in cardboard or carbon fiber, but bulging is definitely a concern in anything large with fin slots.

I use stick in a cup, and while I have no illusions that I am achieving commercial quality uniformity, I get a result that seems fully reacted and satisfactory. Not scraping the cup is prudent advice.

Something I would look into for reducing tail weight is to 'turbo' the foam with added water that I have read about. One should expect to need to do some process development, but 1lb/cubic foot would be a nice to have, as the foam does not have a hard job if it is only there for stiffness.

br/

Tony
 
TonyL said:
I also have a job where I have designed and used foam core composite materials and kramer714's comments give a good summary of the different considerations. What I find is that using 2-part polyurethane foam only seems to make sense with larger body diameters and larger annular gaps. All the things Mike mentions tend to work against one trying to fill a tall thin space.

The thing that I find the 2-part foam does well is add stiffness rather than strength. Large tubes are more flexible than one may think, and long fin slots make this worse. Fillets at the body tube and at the motor tube I consider a minimum, with foam being not a great alternative [but sometimes a useful addition].

Any brittle material whether it is 2-part foam, epoxy or other solid materials will not do well in tension, so it is prudent to design so that their tensile properties are not important. Foam core construction is typically best suited to carrying shear loads, with compressive strength a secondary useful function.

The chemical reaction generates steam so materials need to tolerate hot and humid local conditions. Cardboard does pretty well but not perfect. Also the 'free rise' is not quite in a free state so some internal pressure is typically realized that also needs to be accounted for. In rockets from 6 inches to 12 inches I have not encountered any significant shrinkage issues in cardboard or carbon fiber, but bulging is definitely a concern in anything large with fin slots.

I use stick in a cup, and while I have no illusions that I am achieving commercial quality uniformity, I get a result that seems fully reacted and satisfactory. Not scraping the cup is prudent advice.

Something I would look into for reducing tail weight is to 'turbo' the foam with added water that I have read about. One should expect to need to do some process development, but 1lb/cubic foot would be a nice to have, as the foam does not have a hard job if it is only there for stiffness.

br/

Tony
Click to expand...
we would adjust the foam density with water, couple of comments on that;
a little goes a long way, we would add one cc per gallon to get significant density changes.

It is really important to mix the water in VERY well (and in the right component).

One other thing about water, you need to be VERY careful about letting moisture in when you pour. We used to nitrogen purge the air space over the liquid in buckets, for folks on here it is more like don't open the can on a really humid day and re seal it.
Plus it is VERY important to clean the top of the can or the threads of a pour spout. Water in the air makes the resin act like gorilla glue and will bond the lids shut.

YMMV
 
Mike - a very nice detailed article about the foam you work with, on the projects you work with. But few of us are launching these things into space at mach 3. The couple I have done (a Bucky Jones and a scratch built) have gone well. I do add epoxy fillets inside the fin can and to the motor mount before the foam. Not sure how an air pocket forms when I am pouring it to the bottom of the cavity before it expands. But I do have a more rigid structure that seems to adhere very well (which is why I had to grind the rock hard foam that was stuck to the sides and mmt with a dremel). Also, in the quantity we are working with, a high speed mixing whatever in a bucket is just unnecessary and absurd. You only have 20sec before the stuff starts to expand, so best not to mix it like a birthday cake. Proof in the pudding (so to speak) is that the above Bucky Jones had a tangled chute and landed pretty hard. 2 of its large fins cracked significantly, one of them almost completely off. But the damage stopped at the body tube. Everything inside was still hard as a rock and didn't budge a bit. That made it an easier fix (flown twice since then).

There are many different epoxy materials out there, used in many different applications. Not saying that you are wrong (I'm sure you know more than me about this stuff), but it's worked well for me and many others in this application.
 
TonyL said:
What I find is that using 2-part polyurethane foam only seems to make sense with larger body diameters and larger annular gaps.
Click to expand...
Would a 4" diameter BT with 4 5/8" slots justify the use of foam?
TonyL said:
Fillets at the body tube and at the motor tube I consider a minimum, with foam being not a great alternative [but sometimes a useful addition].
Click to expand...
Yes, I plan to apply fillets as well as foam.
 
Capt. Eric said:
Also, in the quantity we are working with, a high speed mixing whatever in a bucket is just unnecessary and absurd. You only have 20sec before the stuff starts to expand, so best not to mix it like a birthday cake.
Click to expand...
Phew, I was getting a little overwhelmed by all the information and strict processes.
 
Capt. Eric said:
Mike - a very nice detailed article about the foam you work with, on the projects you work with. But few of us are launching these things into space at mach 3. The couple I have done (a Bucky Jones and a scratch built) have gone well. I do add epoxy fillets inside the fin can and to the motor mount before the foam. Not sure how an air pocket forms when I am pouring it to the bottom of the cavity before it expands. But I do have a more rigid structure that seems to adhere very well (which is why I had to grind the rock hard foam that was stuck to the sides and mmt with a dremel). Also, in the quantity we are working with, a high speed mixing whatever in a bucket is just unnecessary and absurd. You only have 20sec before the stuff starts to expand, so best not to mix it like a birthday cake. Proof in the pudding (so to speak) is that the above Bucky Jones had a tangled chute and landed pretty hard. 2 of its large fins cracked significantly, one of them almost completely off. But the damage stopped at the body tube. Everything inside was still hard as a rock and didn't budge a bit. That made it an easier fix (flown twice since then).

There are many different epoxy materials out there, used in many different applications. Not saying that you are wrong (I'm sure you know more than me about this stuff), but it's worked well for me and many others in this applic
Click to expand...
If whatever you are doing is working for you and you are happy with it keep at it.... part of my point is, an internal fillet is a better way of holding a fin than foam, and most folks on here really don't know what the foam is really doing for them (if anything). If you want to use foam. best to make sure it is cured well and mixed right so it lasts........
 
tg08 said:
Phew, I was getting a little overwhelmed by all the information and strict processes.
Click to expand...
Not that hard, cool foam, keep track of time, make sure it is mixed. BTW we would mix lab samples with less than 100 gram with a 1" jiffy mixer. reproducible results. Free rise density exactly what was called out on the can.
 
I quit using foam in the fin can and only use it in the nose cones now for nosecone av bays and such or for 3d printed nosecones. Fincans with foam puts extra weight at the wrong end of the rocket.
 
tg08 said:
Correct me if I'm wrong, but you are basically saying that foam will work if all of the proper procedures are followed to a tee?
Click to expand...
no, what i am saying is low density urethane foam will give reproducible results if you mix it right. Plus will have properties that don't change over time. From my experience, hand mixed with a stick doesn't give you great properties and may degrade over time.
 
Foaming the fin can will move the CG back...but by how much?

Rough estimate: a 98mm MMT in a 7.5" airframe, with space of 18" to be filled with 2 lb/ft^3 foam between the centering rings, would add about 300 grams to the fin can. For a smaller rocket, say a 4" airframe + 38mm MMT, 10" length to be filled, the foam would weigh about 50 grams.

For an altitude attempt, drag race, or other exceptional project, those weights would absolutely need to be considered. But for an 'ordinary' rocket that's being built for fun and sport flight it's unlikely that foaming would make any significant difference in either altitude or stability. (A check with OR or Rocksim is always a good idea though.)
 
prfesser said:
Foaming the fin can will move the CG back...but by how much?

Rough estimate: a 98mm MMT in a 7.5" airframe, with space of 18" to be filled with 2 lb/ft^3 foam between the centering rings, would add about 300 grams to the fin can. For a smaller rocket, say a 4" airframe + 38mm MMT, 10" length to be filled, the foam would weigh about 50 grams.

For an altitude attempt, drag race, or other exceptional project, those weights would absolutely need to be considered. But for an 'ordinary' rocket that's being built for fun and sport flight it's unlikely that foaming would make any significant difference in either altitude or stability. (A check with OR or Rocksim is always a good idea though.)
Click to expand...
I would add that this is even better for rockets with tapered tailcones like a V2 - where you usually can't build the fin to motor tube assembly separately outside the body tube and internal fillets are also more difficult to apply (Edit: at least in the case of the LOC 7.5 V2 due to the construction requiring a tailcone centering ring be locked and glued in place before the motor tube and fins are glued in).

Given the taper, there is less total volume to fill (vs. a straight body tube) and even less added weight at the far end of the rocket.
 
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