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