21-inch by 19-foot "Unfinished Business"

[Feckless Counsel]

TRF,

Thanks for your encouragement and excellent advice. I'll definitely follow Troj on the next Sonotube.

Otherwise the topcoat is sanded coarse to 60 grit. That required about 75 minutes. I'll patch and level a few spots then wet sand to a gloss finish.

Feckless

 

[Feckless Counsel]

TRF,

May I share my split / collapsible mandrel concept for your comment?

Referring to the picture below:

1. Mandrel is cut lengthwise to 1/4-inch slot
2. Angle stock (black) is glued to slot edges inside tube (brown)
3. U-channel (green) captures angle stock (black)
4. HDPE strip (white) fills gap for solid vacuum tight seal.

After lay-up the HDPE and u-channel are pulled to yield a collapsible mandrel. Hopefully the tube will release even though it had been vacuum bagged to the mandrel.

Feckless

 

[CarVac]

TRF,

May I share my split / collapsible mandrel concept for your comment?

Referring to the picture below:

1. Mandrel is cut lengthwise to 1/4-inch slot
2. Angle stock (black) is glued to slot edges inside tube (brown)
3. U-channel (green) captures angle stock (black)
4. HDPE strip (white) fills gap for solid vacuum tight seal.

After lay-up the HDPE and u-channel are pulled to yield a collapsible mandrel. Hopefully the tube will release even though it had been vacuum bagged to the mandrel.

Feckless

My main concern is: Is it actually going to be vacuum proof? I guess you can plug the gaps with vacuum tape...

 

[troj]

My main concern is: Is it actually going to be vacuum proof? I guess you can plug the gaps with vacuum tape...

Get some two inch wide flash tape, and put that over it.

My concern is more about the bond of the channel to the cardboard. Vaccuum bagging is going to put pretty decent pressure on that, and unless it's a good, solid bond, at some point it may come loose. Once it comes loose even a little bit, I think it's going to be like a zipper, and just go all the way down, at which point the mandrel is going to collapse.

Sonotube isn't all that strong; I even if the bond to the metal channel doesn't fail, the tube itself may tear under the load.

-Kevin

 

[Feckless Counsel]

Kevin,

Good observation regarding the strength of Sonotube. Pulling the HDPE strip and u-channel will shear the angle stock / tube joint. Going to be a tug-o-war for sure. I'll try to strain relieve that by clamping angle stock to mandrel end caps or, perhaps, placing a few pop rivets outside the layup area.

Feckless

 

[Eric1]

Just curious, Do you really have to pull a vacuum? I do hand lay up all the time and I have never had to pull a vacuum.

 

[Feckless Counsel]

Eric,

Thanks for your reply and I really appreciate this question. The short answer is not necessary but please note the goal here is efficiency - big rocket flies high on smallish motor.

In the previous project, Anima Mundi, airframes were formed as open lay-ups. Recall those were 21.75-inch outside diameter at 0.062 wall thickness. Without rings it was easy to deflect airframes to oval and there was a lot of discussion towards periodic buckling in flight. At this scale more rings, stringers and rest add runaway mass defeating the goal.

Now I wish to double wall thickness and, therefore, quadruple stiffness at equivalent mass using 2mm Spheretex core material. By open lay-up I will certainly over-saturate that spongy material for a nasty mass penalty. Vacuum bagging onto a bleeder will otherwise result in low resin content, neatly compacted and superior strength airframes.

Airframe goal is 20.75 inch OD, wall thickness 0.120 at 12 pounds per 48 inch section.

I look forward to your comments.

Feckless Indeed

 

[Feckless Counsel]

TRF,

It's been a busy first half at the office. Projects complete there. Hopefully I can make some progress here. For those with interest I would make an update and, as always, invite you to comment on my feckless practice in rocketry.

To address Kevin's observation I've treated the mandrel's "split line" with 12-ounces Smith's penetrating epoxy. After stripping a line from the interior coating the epoxy was applied as pools from the inside (picture 1 & 2). Not sure how much penetration achieved. Will know when the cut is made?

Mandrel is mounted and ready to split today. Note the overhead router sled made from 1010 extrusion. That has some flex in the span but a slow ride across the piece should keep the burr from wandering (picture 3 & 4).

I've added a stanchion clamp "break" to the winding station (picture 5). That is a long needed feature that prevents unwanted lash and rotation of the mandrel.

So any words of wisdom before I split the thing?

Feckless Counsel

 

[Eric1]

Only words I can offer is," Go Slow"!

 

[Feckless Counsel]

TRF,

Mandrel is split with a nice straight line. I've found some delamination as predicted by Kevin. Penetrating epoxy was not effective as applied to the tubes interior surface. Perhaps it will be more effective on the "end grain"?

Feckless

 

[Feckless Counsel]

TRF,

Here's a test of the pop rivet idea. That would secure angle stock at mandrel ends. Might that prevent "zippering" as described by Kevin? Using manual force I couldn't rip it from a sample of bare tube. So internal layers of the mandrel could still delaminate locally but not rip away from the assembly.

Hmmmm. Only need it to last 4 layups but not feeling strong here.

Feckless

 

[dixontj93060]

Feckless, you're back!!! Cheers!

 

[cwbullet]

Very impressive. Do you have a parts list for that fiberglassing jig?

 

[Feckless Counsel]

Tim,

Thanks for welcome return. Hope I can make progress before the next office assignments. Really need to scratch this itch.

Chuck,

The winding station is cobbled from parts found on 8020's ebay site. I'll post a parts list but please note 1010 is a bit flimsy at this scale (72 x 36 x 36). I wish I had used 2020. That, however, would have been a budget buster.

TRF,

Has anyone experience with countersunk blind rivets? Perhaps I can reinforce my split mandrel from delamination by placing a few along the length of angle stock and inside the winding area? Grind to flat if necessary. Touch up with thickened epoxy.

Recall the mandrel is a single layer of 16oz cloth over sonotube.

Feckless

 

[cwbullet]

If you post the parts, I will do a build thread with the 2020 if I can figure it out.

 

[Feckless Counsel]

Chuck,

Attached is the basic parts list for that winding station. It's not complicated but you have to deal with the intersection of three members. 8020 sells some expensive solutions for corner joints but I did that with joining plates across all three "surfaces" of the joint. Good luck and let me know where I can help?

Feckless

 

[SMR]

Has anyone experience with countersunk blind rivets? Perhaps I can reinforce my split mandrel from delamination by placing a few along the length of angle stock and inside the winding area? Grind to flat if necessary. Touch up with thickened epoxy.

Recall the mandrel is a single layer of 16oz cloth over sonotube.

I tried pop rivets on glassed Sonotube just once and vowed never again. While they work great on sheet metal, I found they tend to pull through the glass. While part of the problem may have been in the quality of my glassing, IMHO I believe the break tension of the pin is higher than the ability of the glass to hold the rivet head above the surface. And I was using traditional, flat-bottom, dome-toped rivets. Countersunk rivets would be even harder to keep afloat, as the beveled bottom surface would tend to wedge itself through the hole. YMMV.

Sather

 

[Feckless Counsel]

Sather,

I tested several countersunk rivets that all pulled through under tension. Not only did the glass / cardboard give way but the rivet's head also deformed. Rivets are not appropriate to glassed Sonotube applications.

How about 6-32 countersunk screws? Test looks promising. Using a trim piece I assembled angle to tube using a 6-32 screw and nut. Screw is driven just sub-flush (pictured below). That assembly passed the "crank test" without pulling through and with little distortion to the tube's surface. Plan now is to drive six of those along the angle and within the winding area. Cover with thickened epoxy and sand flush.

Any comments before I commit to that process?

Feckless

 

[Eric1]

I think the screws will be the way to go.

 

[Feckless Counsel]

TRF,

Glued angle bracket and placed end screws. Pictured below is the rough fit of tube, angles and u-channel.

Feckless

 

[Feckless Counsel]

TRF,

Slow going bit I managed to make some progress today. Pictured below are cuts made to mandrel's plywood end caps. First cut was a 1/2-inch wide dado to receive aluminum u-channel. That cut leaves the u-channel sub-flush to accommodate rabbets on either side of the dado. Rabbets allow angle stock to sit flush and, hopefully, in the tangent line of the mandrels surface. Note end caps are cut together to assure symmetry.

I wanted to share an assembly photograph but that would require the assistance of a few sober friends. As few of my friends are sober there may be a wait for those still interested.

Feckless

 

[mpitfield]

Way above my level but I'm subscribed...looking forward to updates.

 

[chuck5395]

I wanted to share an assembly photograph but that would require the assistance of a few sober friends. As few of my friends are sober there may be a wait for those still interested.

And so we wait. Good thing we don't have to be sober too.