Motor Mount experiment

[jamjammer53150]

Back when i was a kid , not to date me but i did watch the moon landing , anyway a guy down the street built rockets , and i remember watching him . When he built his motor mounts he wound string around the moter tube against the centering rings ( and even then he attached picture frame wire there for his shock cords) then soaked them with waterd down carpenters glue.
So i did an experment
I took 1/4 hobby plywood , a heavy wall tube 38mm Id 3mm thickness made a centering ring
I made 2 sets of theese
one set i epoxyed using 5 min generic ( I roughrd everything up ) and did normal fillets , using 2 batches everything pretty normal
The other i wound some hemp , the size kids use for necklaces ( stolen from my daughter) wound 20ish turns , then saturated with ultra thin ca .
I lef both sit for 3 days , weighed them , the string one was 2.8 grams lighter .

I then very scientifically put a pipe over them and proceeded to bash with a hammer
Both held with a significant beating. the epoxy one started delaminating the ply but by then any consinstancy was gone in my experement

So is this a valid technique? it is lighter ? anyone else ever do this?
Besides loosing fingerprints the CA string method is quicker and less of a pain

and since i use 150 lb stainless downrigger line to attach shock cords (on bigger rockets), i can wind them right in !

[Mark II]

Wait a minute, what's the purpose of winding string around the motor tube again? What is that meant to do?

Mark K.

[mycrofte]

It makes the engine tube a lot stronger but I never launch anything bigger than C6-5...

[jamjammer53150]

It creates somewhat of a mechanical fillet , for the glue to impregnate. some what like soldering , you always want some sort of mechanical connection , then the solder to hold it all together
I gurss the theory is there is way more surface area for the glue to work with

[STRMan]

The technique makes sense. It's sort of like fiberglass. Polyester resin is basically weak and brittle, but when it saturates glass fibers, it forms a strong and resilient material. I like it.

[jamjammer53150]

I guess I somewhat overbuild everything anyway , like I always put a tube coupler ahead of the upper centering ring.
If you have ever seen a motor drive through a rocket on a rod , which looks really cool, is somthing i wish to prevent .
RIght now Im building a bt80 patriot , with 3 24mm motors , the kit came with those black cardstock rings for the mm. So I already hace beefed that up as if some time i try 3 f's in it the could be easly 60lbs of load at that point .

I guess building sucessfull rockets is eliminating every potential point of failure , or at least reducing it to the most practical margin

[GregGleason]

The technique makes sense. It's sort of like fiberglass. Polyester resin is basically weak and brittle, but when it saturates glass fibers, it forms a strong and resilient material. I like it.



That is correct. It is a form of a composite structure. Fiberglass cloth is better suited for this and so is laminating epoxy.

The problem is CA is a "brittle" adhesive and may not perform well in some applications. I saw someone's rocket have a liftoff problem (IIRC, it was maybe an "H" or "I" HPR rocket) where his CA joint failed and I think it was his MMT. He said that it had worked on previous flights.

Greg

[jamjammer53150]

So the ultimate would be epoxy , and string . not Kevlar mind you , the string should be able to absorb the ashesive
In my test neither failed , however I suspect CA with no string would have failed under the bash test .

[nvrocketeer]

I wonder if this technique is some kind of attempted defense against motor casing failure.

[jamjammer53150]

If you wound the whole motor tube with string ( enough of it ) it would support a casing failure , however the pressure would go either up or down in that condition .

As a side note , string wrapping of ariel charges for fireworks is a closley guarded family secret of several Itialian old timers.
I have personally seen an old man do very complex string winding on a hand made 6 inch 9time report shell . The production time on the shell was weeks to account for drying time , and they proformed flawlessley every time . I have hand light such shells and never had a worry . now chineese shells .....

[ghrocketman]

I build flying tanks. Nothing wrong with STRONG LIKE OX ! Then again my rockets almost never have fins busticated off tubes like many others do, even upon a BALLISTIC recovery !!!
I don't build light nor will I ever as I do not compete for altitude or safety; I build to recover in ONE piece, no matter the landing/recovery parameters.

[Mark II]

I guess I somewhat overbuild everything anyway , like I always put a tube coupler ahead of the upper centering ring.
If you have ever seen a motor drive through a rocket on a rod , which looks really cool, is somthing i wish to prevent .I use common sense and build my rockets well without going overboard with excessive reinforcement. I use quality components, top of the line adhesives and I take the time and effort to get good strong bonds. Among other things, I apply mild heat to curing epoxy and wood glue, which makes both of them noticeably denser and harder. In hundreds of launches, I have never had a motor come loose and fly up through the rocket nor have I ever had the mount break loose and get ejected. I have never shredded a parachute nor have I ever zippered a tube. I have never had a nose cone come off and become lost. I have never had a shock cord separate at deployment. I snapped a fin in the very first flight of my very first rocket back in 1967. If I ever break another one, I'll be sure to let everyone know. Last fall, my Maxi Alpha clone core sampled from a height of about 600 feet when the 'chute got tangled up with the Nomex cloth. There was a slight crumple in the top 2" of the unreinforced BT-80, and no other damage. The worst mishap that I have ever experienced was last July when only 2 out of 3 motors in the cluster lit in my FSI Hercules clone. (Similar in size to a Maxi Alpha but heavier.) The rocket went nearly horizontal at about 75 feet up, powered into a sturdy old oak tree downrange, and then fell nose first onto a paved driveway. It had a sickening sound, like a large log being split. As a consequence, I will need to repair the tip of the balsa nose cone and one cracked fin fillet and I will need to patch two or three gouges in the airframe, and then it will be returned to my active fleet. Other than papering some balsa fins, I have never used composite construction on any of my rockets. I don't hesitate to reinforce where it is necessary and makes sense, but I don't build tank rounds. Too often these discussions devolve into a debate over the merits of ultralight construction vs. armor plating. I do neither, but rather construct my rockets to be somewhere in the middle of these extremes. My rockets are reasonably sturdy, but no one would ever describe any of them as being even slightly heavy. Anyway, the cord wrapping technique that you described sounds interesting and ingenious, and I will probably try it sometime when it seems appropriate. Thanks for sharing it.

MK