Cardboard Tube Reinforcing Conundrum

So you have a 6.5” dia cardboard tube and you want to reinforce it but you can’t wrap the outside in fiberglass, or anything else. Don’t ask why, I have my reasons.

Anybody ever try reinforcing a tube by applying fiberglass to the INSIDE of the tube?

If so; how did you go about doing it?

Any other ideas as to how I might go about strengthening a lightweight cardboard tube, from the inside?

One idea I have is to build an internal frame wrapped around a solid 4" Blue Tube core with the 6.5” cardboard tube just going along for the ride.

This will be using a 38mm motor tube and up to “J” motors.

No need to reinforce it for that power level in my opinion.


Mark Koelsch
Sent from my iPhone using Rocketry Forum

This isn’t a rocket quality cardboard tube such as LOC Precision uses. This is more in the way of the type of cardboard used in food packaging.

It would probably hold together if it was only a skin-over-frame but even a J-350 would probably blow it apart if it was used as the actual load bearing airframe.

What's the wall thickness of the tube?

gerbs4me said:

What's the wall thickness of the tube?

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At this time I can’t tell you as I haven’t received it yet. I just know it isn’t “Rocket quality” tubing, it will require some form of reinforcing.

Just as a reference; I was told that the tube if laid on its side will not hold a round shape under its own weight.

Wow that make the tube wall thickness at 1/32 or less fiber glass would not do anything it would not hold its shape very well

boomtube-mk2 said:

Any other ideas as to how I might go about strengthening a lightweight cardboard tube, from the inside?

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6" Sonotube. Most Sonotube has 2-3 different diameters close to the "nominal" dimension so they can nest them when shipping. Find a concrete supply store in your area and see if they have something close to your lightweight tube's ID.

You can also peel off layers of Sonotube from the inside or the outside to adjust the diameter and/or thickness somewhat.

One problem, though, might be weight. Sonotube is relatively heavy.

Just a thought.

--Lance.

One idea I have is to cut 3” wide strips of fiberglass cloth and apply them individually, and overlapping, inside the tube. I have a couple of thoughts as to how to maintain the shape of the tube while doing this.

The other idea is to use a 4” Blue Tube core with a series of notched centering rings spaced along its length. Into those notches would run wood stringers too which I could attach the outer tube, thus the cardboard tube would not need to be that stout; though I suppose it would be easy to accidently punch a hole in it.

Obviously the fins would run through and attach to the core tube and perhaps even be notched into the centering rings.

another idea ...

If the tube is that flimsy, it must have some artwork or something you want on a rocket, no?

Why not just use an actual 6" airframe and cut the flimsy tube lengthwise and use it as a wrap? Don't make this too hard.

--Lance.

llickteig1 said:

another idea ...

If the tube is that flimsy, it must have some artwork or something you want on a rocket, no?

Why not just use an actual 6" airframe and cut the flimsy tube lengthwise and use it as a wrap? Don't make this too hard.

--Lance.

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That’s a dang good idea but the tube is 6.5 inches and I don’t know of any 6.5” rocket tubes, nor can I wrap it around a smaller/larger tube or the "Nose cone" won't fit.

6" tube, laminate a layer of nomex honeycomb over that, and the a layer of glass. It will be rigid. See John Coker's website for details.


Mark Koelsch
Sent from my iPhone using Rocketry Forum

boomtube-mk2 said:

One idea I have is to cut 3” wide strips of fiberglass cloth and apply them individually, and overlapping, inside the tube. I have a couple of thoughts as to how to maintain the shape of the tube while doing this.

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Lay the fiberglass down and blow up a balloon inside the tube to hold the shape.

llickteig1 said:

another idea ...

If the tube is that flimsy, it must have some artwork or something you want on a rocket, no?

Why not just use an actual 6" airframe and cut the flimsy tube lengthwise and use it as a wrap? Don't make this too hard.

--Lance.

Click to expand...

Im with you on this one with the wrap idea. It will overlap a bit so put the rail buttons were the wrap meets to help hide it. Nosecone not fitting gets me, is it a tube and a nosecone thing your working with?

llickteig1 said:

6" Sonotube. Most Sonotube has 2-3 different diameters close to the "nominal" dimension so they can nest them when shipping. Find a concrete supply store in your area and see if they have something close to your lightweight tube's ID.

You can also peel off layers of Sonotube from the inside or the outside to adjust the diameter and/or thickness somewhat.

One problem, though, might be weight. Sonotube is relatively heavy.

Just a thought.

--Lance.

Click to expand...

I used to work for a construction supply and rental equipment place, one of our big sellers was Sonotube, most sizes of sonotube are available in what we would consider a Airframe Tube and a coupler size tube ie. a 6" sonotube O.D (Airframe)with .25" wall (5.50 I.D.) would be available with 5.50 O.D full length coupler tube (or whatever size made it a sliding fit). I believe they start at about 6" or so and go up to 48" that way, they were shipped nested and they fit pretty snugly. When I think of nesting for shipping they would take one set of 48s and put 36s, 24s, 18s, 12s, 8s, 6s in them.

The inner/outer tube idea is the best. Now, to keep it light make up the cheapest chumpest rings you can, all they should do is hold the inner and outer rings apart. Use something stiff enough to drill holes around the ring, making it as light as possible. Here's the trick. You have to be ready to assemble it all at the same time because you're going to be doing a monolithic pour of 2 part foam. Mix and pour in the foam then slide a ring onto where your calculations show the foam should stop reacting. The holes will allow the foam to lock in the ring when it sets. You should have all you need to do the job in one go. With the foam as the main structural component a rocket can be much stronger and also lighter for the same strength as a balloon-built rocket.

PML has their foam kit but we used Titan brand from Home Depot. You get a lot more for your money and it's the same basic stuff without the mark-up.

Put stringers that run the length of the tube with a ring to hold them in position spaced out the length of the stringer.