Fiberglass/foamcore board.

the rocketry show
madcow rocketry
Wildman Rocketry
shop deals on amazon
The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
RAHagen

RAHagen

Well-Known Member
Joined
Jun 23, 2014
Messages
647
Reaction score
15
So yesterday, while I was talking about the rocket I'm building for my Jr. Lv. 1 certification, my Dad suggested a different type of material one could make fins out of. He suggested taking foamboard (the stuff displays for school are made of), cutting them into strips, gluing them back together with epoxy and then coating it in a layer of fiberglass on both sides. He works on airplanes a lot and says that's how people make sparred wings for composite aircraft.
Well... I tried it and it seems like it might be a viable fin material!
In the second photo is the board after it was cut and glued back together. In the first photo is it after it has been fiberglassed on both sides. I'm not entirely sure about how strong it is, but preliminary measurements show it having a density about .2 grams per cubic centimeter less than birch plywood. I plan to do some destructive testing on both the board and plywood of similar size. I'll make sure to post the results here. I'm not sure anybody else has done this or not but either way I'd love to hear about it if you have!

1406151436692.jpg

1406151547902.jpg
 
Last edited:
I have done it with gator boards and a home freeze packing vacuum unit I picked up at the Salvation Army Surplus store. The board styed intact with alternate layers of glass at different angles. I used balsa edges for aerodynamic streamlining but I 'm not happy enuff to fly those on a "J" plus they are fat and clunky looking. I was going to cut them on a wet tile saw so I don't screw up my band saw. I obtained carbon fibre arrow shafts and kite tubes (rods), but have yet to try a new layout. I'm going to try foam board inserts in BB for a stubby rocket with 1/4' fins later this summer. Keep us posted!
 
Chintzy foam core laminated with glass or carbon is the cats meow! I made some fins that way laminated with 2 layers of 4oz glass and they worked flawlessly. I flew the rocket several times before a crash into hard salt. It tried to desert dart but the only part that survived was the last 1 to 1.5 feet (fin can). I rebuilt and flew again. Fell out of the sky non ballistically this time and nary a dent on the fins! Lesson learned? Foam core fins are pretty light and tough AND Vmax motors often (4 or 5 times now for me) burn too quickly to ignite the delay grain.
Forgot about the Bad Axe rocket. Suffered the same fate at the little hands of the H410 recently (2014) but fin can is OK! Here is a build thread.
https://www.rocketryforum.com/showthread.php?29717-Bad-Battle-Axe-Built-Thread
-Ken
 
Last edited:
I've tried similar for other things, never yet for fins, and it certainly is stiff and light. Why would you cut it up then glue it back together - that can only weaken it?
 
pythonrock said:
I've tried similar for other things, never yet for fins, and it certainly is stiff and light. Why would you cut it up then glue it back together - that can only weaken it?
Click to expand...

Kinda interested myself. Guessing it makes a thin rib to increase the stiffness of the stuff. Criss-cross them to make plyfoam!
 
Cutting the board and gluing them back together with epoxy makes spar-like ribs making it almost like re-bar. That's at least what I've found right now.
 
I've experimented with something similar and was pleased with the results--One day I'll get around to working that into the actual rocket I'd intended...



Later!

--Coop
 
Fiberglassing over a far less dense substrate (such as foam board, balsa, or honeycomb) can make for a very light and strong composite structure.

However, I don't get the cutting into strips part. You may gain rigidity in one direction if the joint between strips is greater than that of the substrate, but you would be giving up some of the smooth surface properties of the structure (unless the board is warped).

Greg
 
GregGleason said:
Fiberglassing over a far less dense substrate (such as foam board, balsa, or honeycomb) can make for a very light and strong composite structure.

However, I don't get the cutting into strips part. You may gain rigidity in one direction if the joint between strips is greater than that of the substrate, but you would be giving up some of the smooth surface properties of the structure (unless the board is warped).

Greg
Click to expand...

Exactly. I don't see any gain from the strips either.


Sent from my iPhone using Rocketry Forum
 
I have been looking I to doing this myself for my L1 rocket. Having recently read a LOT about fiberglassing, the core simply needs sheer strength. As long as the material's sheer strength is enough, the main strength is from tension side of the fiberglass.

The area I want to explore is what happens to the edge if one slams the fin straight down on the edge. I was thinking perhaps using square dowel might be needed there. If nothing else, it might work for allowing rounding the edges.


Kirk
 
Last edited:
Personally, I'd cut the foamcore to shape, use fiberglass rod or wood dowel to edge each one, glass them once, glue 'em in place then tip-to-tip glass them. I'd started to do this with a 4" BlueTube Red Max but then Estes released the kit.
 
GregGleason said:
Fiberglassing over a far less dense substrate (such as foam board, balsa, or honeycomb) can make for a very light and strong composite structure.

However, I don't get the cutting into strips part. You may gain rigidity in one direction if the joint between strips is greater than that of the substrate, but you would be giving up some of the smooth surface properties of the structure (unless the board is warped).

Greg
Click to expand...
I get your point. What I did in order to keep the surface smooth was I glued the pieces together and then sandwiched them between two metal plates and placed twenty pounds of weight on top. the glue either oozed out the sides or flattened out on the top of the foam board. the result was nearly as smooth as the original board. As for strength, this is just the first attempt. I'm thinking that the next time I make one, I diagonally cut the board, and then fiberglass. The whole basis behind it is making a grain-like structure, just like in wood or the ribs in an aircraft wing. I guess in order to really know if it makes a difference I would have to fiberglass plain foam board and compare the results of some strength tests. One thing i did notice though is that after fiberglassing the board the thickness is actually more similar to 1/4 inch plywood than 18th inch plywood. So I will probably have to compare the results with 1/4 inch plywood instead of 1/8th inch like i originally planned.
 
The theory behind sandwich panels would indicate that you will not get an efficient gain from the ribs. Let me see if I can explain why.

Let's consider an I-beam for a moment. This cross section has a lot of material on the top and bottom of the stiffener but very little material in the middle. The material on the top and bottom take the compressive and tensile bending stresses, while the material in the middle transmits the sheer between the top and bottom portions. Think of it like a deck of cards - it will bend easily if the cards are allowed to slide (shear) against each other, but if you glue them together that 52-card deck is going to be very difficult to bend. Another very important point is that the ability of a structure to handle load goes up with the square of its thickness. So a 1" wide by 4" tall piece of solid steel is sixteen times as strong (loaded vertically) as a 4" wide by 1" tall piece of steel, despite the fact that they use the same amount of material.

The trick with an I-beam is the recognition that it doesn't take very much material to transmit the shear loads between the top and bottom surface. So they put most of the material mass at the top and bottom (the flanges) and much less mass in the middle (the web).

A sandwich-cored panel does the same thing. Two very strong layers of fiber-reinforced plastics (such as fiberglass) are laminated to a very light-weight core (typically foam, wood, or a honeycomb structure). Provided the two different materials are well-adhered, the core is capable of transmitting all the shear stresses to the outer layers, which take up the tensile and compressive bending stresses (in the plane of the outer surface). When you cut the core, depending on how it is constructed, you can actually reduce the ability of the foam to transmit sheer in one direction (perpendicular to the 'ribs'), weakening the sandwich plate. If you have vertical ribs of fiberglass between the pieces of foam you will get an increase in strength along the axis parallel to the 'ribs', but it will come very inefficiently, as it will not be as strong as it would have if that weight in fiberglass were used to beef up the outer layers.

Let me know if that doesn't make any sense.
 
I should point out a couple more things about the ribs in an aircraft wing:

(1) They are basically making an I-beam there, with the upper and lower surfaces being the flanges and the ribs being the web. Keep in mind that they typically use the interior of the wings for storing wiring, cables, fuel, etc.

(2) The ribs DO make a huge difference in the torsional rigidity of the wings (i.e. they keep them from twisting).
 
wfcook said:
The theory behind sandwich panels would indicate that you will not get an efficient gain from the ribs. Let me see if I can explain why.

Let's consider an I-beam for a moment. This cross section has a lot of material on the top and bottom of the stiffener but very little material in the middle. The material on the top and bottom take the compressive and tensile bending stresses, while the material in the middle transmits the sheer between the top and bottom portions. Think of it like a deck of cards - it will bend easily if the cards are allowed to slide (shear) against each other, but if you glue them together that 52-card deck is going to be very difficult to bend. Another very important point is that the ability of a structure to handle load goes up with the square of its thickness. So a 1" wide by 4" tall piece of solid steel is sixteen times as strong (loaded vertically) as a 4" wide by 1" tall piece of steel, despite the fact that they use the same amount of material.

The trick with an I-beam is the recognition that it doesn't take very much material to transmit the shear loads between the top and bottom surface. So they put most of the material mass at the top and bottom (the flanges) and much less mass in the middle (the web).

A sandwich-cored panel does the same thing. Two very strong layers of fiber-reinforced plastics (such as fiberglass) are laminated to a very light-weight core (typically foam, wood, or a honeycomb structure). Provided the two different materials are well-adhered, the core is capable of transmitting all the shear stresses to the outer layers, which take up the tensile and compressive bending stresses (in the plane of the outer surface). When you cut the core, depending on how it is constructed, you can actually reduce the ability of the foam to transmit sheer in one direction (perpendicular to the 'ribs'), weakening the sandwich plate. If you have vertical ribs of fiberglass between the pieces of foam you will get an increase in strength along the axis parallel to the 'ribs', but it will come very inefficiently, as it will not be as strong as it would have if that weight in fiberglass were used to beef up the outer layers.

Let me know if that doesn't make any sense.
Click to expand...

I'm pretty sure I understand your analogy. So... Even if you were to glue the board together with something stronger than the original board (Like 2-part epoxy) it wouldn't be any stronger than the original board? I would have thought that by gluing the strips back together with epoxy would increase the ability of the core to transfer the weight from the top layer of glass to the bottom.(or does it?)
 
Well, you're talking about two different types of load here.

With fins, we are primarily interested in the bending loads.

If you are talking about compression loads (the load you would get by laying the fin flat on a table and setting a weight on top of it), it is true that some core materials may not handle that particularly well. However, (1) this is not a load case that you are typically concerned about with rocket fins, and if you have compression loads high enough to cause the core to fail you have much bigger problems, (2) some cores do in fact handle these loads very efficiently (honeycomb cores for example), and (3) a thin layer of epoxy between the strips will provide almost no help in that case anyway and certainly not enough to justify the weight.

The point here is that the epoxy in the center is a brittle substance that weighs a lot and provides, at best, very little strength at all for any load case in which you are interested. At worst, it can weaken the panel. If you were to make the ribs out of fiberglass, they would likely increasee torsional rigidity, but again that is not a load case you would concern yourself with on a rocket as much as you would on an airplane.

Having said all that, I think that the panel you made will be fine and i don't mean to be critical at all - I think it's great that you are doing this. Next time, you can simplify the process a bit.

I do have enough experience with large sandwich-cored structures to have at least a clue, although I am far from being an expert on the subject.
 
Okay that makes a lot of sense. It was very constructive criticism. I probably will still put the material through some destructive testing just because its small size... and i'm curious :p... I learned a lot in this thread, and I might want to use this material on the next scratch built rocket I make. I'll make sure to take a look at your experience with it as well. It seems like this can be a very capable material in the rocketry hobby and I look forward to studying it further. Thanks for the help!
 
RAHagen said:
Did you help build that!?!
Click to expand...

I was part of the technical team for three campaigns, including that one. Not a day goes by that I fail to thank the good Lord that I'm not doing that any more. It was fun while I was young and single. Sort of.

Anyway, my point there is that we didn't have any ribs in the core :)

In this case, those boats are a carbon fiber sandwich with an aluminum honeycomb core. I always considered that to be as crazy as can be, because carbon (graphite) and aluminum are on opposite ends of the galvanic scale, and if any saltwater gets in there you get a very corrosive battery...but the boats are built to race, and performance is the primary consideration, so if they fall apart it's generally thought of as OK as long as they cross the finish line first. I'm not kidding.

I was mostly on the performance side of things and not part of the structural team, but I am a Naval Architect/Marine Engineer. I participated in all the meetings and discussions and I am trained in the subject. I do understand what the structural issues are. (Not quite as bad as "I'm not a doctor, but I play one on tv", but maybe a little too close for comfort there).

I'm glad you recognize the discussion as constructive, because my purpose is not to criticize AT ALL. Good luck and I'll be following your progress on the forum, so keep posting!
 
Last edited:
I’d ditch the Foam core and use Gator Board instead. It is much easier to cut and carve and it doesn’t warp.
 
Huh. Never used gator board. I'l have to look into that. Thanks. :)
 
I would go with one of the offerings from fiberglast.com or cstsales.com
 
I have been watching as I had the same thought as well. Vacation came and went, need to get some cups to mix epoxy in to start my testing.

As for the Garorboard suggestion, I will skip it myself. The cost is way more expensive then cheap foam board. My understanding is that foam board is available from Dollar Tree for well $1.00.

Further, my research indicates that the core only needs some sheet strength to work well as a core, and I have seen a write up where someone built a canoe with end grain balsa and 6 oz cloth inside and out.

My idea is cheap, easy obtain ability, easy to work with. Balsa meets this, but on a practical side is limited to a four inch width without making a joint. Foam board also meets the criteria with no need to glue pieces together.


Kirk
 
1tree said:
I have been watching as I had the same thought as well. Vacation came and went, need to get some cups to mix epoxy in to start my testing.

As for the Garorboard suggestion, I will skip it myself. The cost is way more expensive then cheap foam board. My understanding is that foam board is available from Dollar Tree for well $1.00.

Further, my research indicates that the core only needs some sheet strength to work well as a core, and I have seen a write up where someone built a canoe with end grain balsa and 6 oz cloth inside and out.

My idea is cheap, easy obtain ability, easy to work with. Balsa meets this, but on a practical side is limited to a four inch width without making a joint. Foam board also meets the criteria with no need to glue pieces together.


Kirk
Click to expand...

Also note that to be an effective core, the balsa needs to be end grain balsa, which is not as readily available and not as cheap. A good cheap foam core board will serve you well for rocketry, provided it can handle contact with the epoxy and solvents it may encounter during the build without actually dissolving.
 

Similar threads

86mustang408w
Foam Board Fin Template.
Replies
20
Views
896
neil_w
neil_w
neil_w
Need a covering for my particle board workbench
2 3
Replies
63
Views
1K
NTP2
NTP2
ep29030
Not a Normal Tool Organizer
Replies
20
Views
735
lakeroadster
lakeroadster
M
Fiberglass cloth lifting off plywood when removing mylar sheet
2
Replies
31
Views
2K
jahall4
jahall4
JoeFilgas
SNARKY by Dyna Star (Apogee Components) Build Log
2
Replies
45
Views
3K
Cape Byron
Cape Byron

Latest posts

Back
Top