# of Layers of Carbon for L3?

Art Upton said:

Thanks for the info Boris,

then I wonder why your tube failed and the others I have flown and seen flown by others have not failed?

Did you have a vent hole in the rocket?

Could the nose cone have been poped off in flight and the tube section from the nose cone down zipped by the recovery cord/harness?

38mm LOC Kraft tube has been flown naked to mach and more for quite a long time, just ask Barry at LOC

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I had checked with Barry at LOC before building it, and he did say he thought it would hold up. Bob Krech did a detailed anaysis and determined that the force of the engine pushed the tube right to its limits, when you add turbulence - shred. It was moderately windy that day, perhaps that was enough to make the difference?

There was a vent hole. The NC was pretty snug. Not sure what the failure trigger was.

Light glass cover with strong slow epoxy should prevent re-occurrence with only slight weight gain.

Ft = M2/(M1+M2)*F

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You know Penny;
if in a vacuum, you put a feather on top of a bowling ball and accelerate the two as one system; If a=g, the force of the feather on the bowling ball is just the weight of the feather and not the force that is accelerating them both!

But can the feather tell if it's in an accelerating inertial frame of reference or a gravitational frame of reference?

Art Upton said:

You know Penny;
if in a vacuum, you put a feather on top of a bowling ball and accelerate the two as one system; If a=g, the force of the feather on the bowling ball is just the weight of the feather and not the force that is accelerating them both!

But can the feather tell if it's in an accelerating inertial frame of reference or a gravitational frame of reference?

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*****

One of the best combinations I have seen used, was 1 layer of kevlar sock, covered with 1 more layer of carbon fiber sock, in a 4in min diam rocket. The fins were held on with epoxy putty fillets.

This thing survived a ballistic fall from 11,000ft, when the shock cord failed at deploy. The alt. was recovered 7.5 mi away, with data intact ,till deploy, where something went major bad.

However this thing punched a 4in hole through 5in of asphalt and survived. The motor was ruined when it punched through the retainer and hit the asphalt plug,collaping the forward end of the motor, which was buried under the rocket.

The fincan was pulled from the road, with only 4 in. of tube needing to be cut off, to make it ready to fly again, after new motor retention.

The kevlar gives you hoop strength around the tube, and the carbon gives compression strength.
If I remember right the rocket only weighed 7lbs pad ready sans motor.

View attachment 20061112oops.JPG

If I may -

I'm quite a fan of using unidirectional fibers when building airframes. I typically like to use carbon fiber in the axial direction and kevlar in the hoop direction. finally, put a lay of thin S-glass over the outside for easy finishing..

Like Blackjack, we had an interesting experience with one of our tubes...though ours were only 54 mm tubes...

We were trying to get the tube off the mandrel by hammering out the mandrel, which required a wooden block between the hammer and the mandrel. We were slamming this thing so hard with a huge hammer and it wouldn't budge more than a few inches... at one point the block slipped and we hammered the block into the end of the tube. The solid oak split right down the middle but didn't leave one trace of damage on the composite tube.

We ended up getting the mandrel off by soaking it all in liquid nitrogen...


Quite incredible stuff. 0.6 mm goes a long way...

Wow! This has turned in a great discussion. It'll take me a while to understand those formulas. Hopefully, one of my engineering classes will discuss this in more detail. Can anyone suggest a class to take on this?

My goal in this rocket design is one that is light enough to fly primarily on 38 mm and 54 mm motors, but still strong enough to take an M1850. Seems I calculated the thickness of three layers of 5.6 oz CF wrong. It is 0.036" without epoxy, which should make it about 1/16" or close I think after adding epoxy.

I had a cheap RTF Quest rocket that flew on C motors. I only had 3 flights on that and it failed completely after the third flight, but I think that was from the landings. I noticed that the failure point was the spiral groove. With that in mind, I looked a bit more closely at the rocket picture posted by Bob. It looks like that's where Boris's failed too. Maybe a previous flight landing buckled the airframe just enough at that spiral groove for it to fail on the next flight. Also, if the pressure relief hole is drilled right on the spiral groove, might that weaken the airframe?

Mike

sailmike said:

Wow! This has turned in a great discussion. It'll take me a while to understand those formulas. Hopefully, one of my engineering classes will discuss this in more detail. Can anyone suggest a class to take on this?
Mike

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Statics
Dynamics
Fluid Mechanics

No one seems to have answered your concern about the centering rings taking the force of the motor thrust. Just as they are named they are centering pieces not thrust bearing elements. It’s like pushing on the center of a pie plate. You need to transfer the thrust from the motor to the walls of the tube and at the same time shoring up the tube walls. Easy to do with the fin tabs being glued to the motor tube. (Ahh you do plan to add through the wall fin tabs... eh.) At the same time cut them to butt up against the centering rings. Also other gussets can be added to centering ring further forward. See these two articles from Tim Van Mulligan at Apogee. They center (forgive the pun) on paper rings and gussets for LPR but the engineering concepts apply to HPR.

https://www.apogeerockets.com/education/downloads/Newsletter63.pdf

https://www.apogeerockets.com/education/downloads/newsletter104.pdf

fin tabs
https://www.apogeerockets.com/education/downloads/Newsletter165.pdf

To shore up more of the tube you could add thin spars of spruce running parallel to the inside walls of the tube. This would be lighter then making the fiberglass thicker.

Also to reduce drag on this rocket switch to a parabolic shaped nose cone. It has a lot less drag then a conical shape.


I tend to think outside the "tube"

Hi Mike,

I just looked at your rocksim file just now, and I have a 4" fiberglass rocket that looks very similar in size with a conical cone.

As to thrust transfer, on min diameter rockets the motor thrust ring transfers the energy to the airframe body. Are not all rockets min diameter with a motor adapter put in them? Just some have glued in the adapter.

When you create a motor adapter to fly it on smaller motors, one of the tricks is to use the bottom ring that is the same size as the tube and use an aeropac retainer that screws to the bottom, or their boat tail model.

Then you can put in the aeropac adapters to get even smaller.

The thrust ring will put the thrust into the bottom ring that that goes into the airframe body tube. When I use the aeropac boattails, they do the same thing.

Many carbon and fiberglass composite rockets will not have thru the wall fins and the fins will terminate to the airframe or in a composite fin can outside the body tube.

The video at the bottom is a rocket 4" fiberglass airframe, fins are carbon that are composited to the body as a curtis fin can.

Its test flight used a motor adapter that consisted of a 3" motor tube containing 2 centering rings that were screwed to the airframe body at the top of the adapter and mid point with three #8 screws.

The Aeropac boat tail transfers the thrust to the body tube, and motor adapter is removeable for 4" motors.

It test flew on a Pro75m M1400 to 996mph

https://www.boostervision.com/wmv/misschv.wmv

Since your tube size of 4.5" doesn't have an aeropac boat tail in that size, you can make the bottom thrust plate the size of the body tube and use the screw on 75mm retainer to the end of it.

That should transfer the thrust of the motor to the thrust ring, to the retainter, to the rear thrust plate, to the outside airframe body, to the ebay vent ring (if you have one), to the payload section, and on to the nose.

JD, help me here if I'm wrong on the transfer of the thrust in all those places.

For stability on the bigger heavy motors you might want to make the bottom about 42-48 inches long and the top chute cannon at least 24 inches to hold the laundry you need to bring it to the ground slow.

Adrian A said:

There are lots of possibilities, from tube dings or defects, to different flight conditions. I think that wind shear could be one of the leading suspects. In the video frame at 0.7 seconds, there is a noticable kink in the plume suggesting either that wind shear just caused the rocket to abruptly change direction, or that the failure already started and the rocket is in the process of folding itself in half.

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At 0.7 seconds the rocket shredded. The reddish puff in the smoke trail is tracking chalk being released. The tail section, with all three fins and engine still under power, continued upwards under thrust for another 0.5 seconds on a less straight trajectory.

boris katan said:

At 0.7 seconds the rocket shredded. The reddish puff in the smoke trail is tracking chalk being released. The tail section, with all three fins and engine still under power, continued upwards under thrust for another 0.5 seconds on a less straight trajectory.

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That is interesting Boris,

that is a similar failure mode the darts I have seen did when they failed at the coupler, or even when the fins come off like in my last 54mm dart that shreded at mach 2.2

Are you sure the crimp in the body tube is from the failure and not from the landing with the motor inside?

In the coupler failures I've had and shared with friends, the motor/fin can keeps going, the chute cannon nose cone falls to the ground with zippers from the cone thru the chute cannon.

The only thing that does not match up to that description is you didn't have a coupler or chute cannon.

That keep makes me wondering if the nose came off, zippered the top part of the tube till the zipper sliced the tube off.

Then the left over booster/fin can reaches a great height and falls back causing a crimp?

Just a wild arse guess with no basis in fact

jderimig said:

Statics
Dynamics
Fluid Mechanics

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For the structural/mechanical stuff we're talking about I'd actually suggest:

Continuum Mechanics - stress/strain, beam deflection, basic failure modes.
Materials Science - looking at microscopic material properties and processes
Structural Mechanics - more in depth load/failure analysis
Structural Testing and Analysis - fatigue and environment equivalence testing methods

Maybe also:
Dynamics of Rigid Bodies


Physics statics and dynamics are assumed.

sailmike said:

Oh yeah, forgot to add that the centering rings, bulkheads, and fins would all be made from a carbon and balsa sandwich, probably three 1/8" sheets of balsa for the bulkheads and two sheets for the centering rings and fins.

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Take care. Sandwich construction techniques deliver great results in the right applications, but there are drawbacks. Shear stress is something that a sandwich won't handle as good as its stiffness against bending may make you believe. I remember laminating some test samples. They consisted of 3mm (~1/8") of balsa or aluminum honeycomb, with a single layer of CF per side. They were really light and stiff but you could still cut them using ordinary scissors.

On my L1 bird, i beefed up the center rings and bulkheads with thicker carbon sheets, so that out of 7mm total thickness, only 3mm were "soft". Otherwise a high speed deployment, or maybe even a high thrust motor, could have damaged the rocket.

Reinhard