Replacing the shock cord trifold mount with a better system.

So I dug around and found the "sand the trifold mount out" thread, sounds good, but I don't want to go back with a trifold.

What are my options? Drill a tiny hole through both centering rings and thread some kevlar thread through those? Thinking about using a piece of clothes hanger as a long drill bit. But I would bet a slathered glue on both sides of both centering rings.

I can feel the frustration rising and I haven't even started yet.

TIA, Phil.

What is the rocket, what is the current status, what is happening exactly?

IF this is a built rocket, the method below won't work.

wrapped around motor mount, glued, and through two notches in the ring

@hcmbanjo had a trick on his blog, you used a QTip tube (cut the cotton ends off) that ran from front to back trough the centering rings. You threaded your Kevlar through the tube, had a loop around the rear end of the motor mount. The advantage was that you could pull the shock cord out and check it periodically (Kevlar is temp resistant but it ain’t completely flameproof) and easily replace it.

BABAR said:

@hcmbanjo had a trick on his blog, you used a QTip tube (cut the cotton ends off) that ran from front to back trough the centering rings. You threaded your Kevlar through the tube, had a loop around the rear end of the motor mount. The advantage was that you could pull the shock cord out and check it periodically (Kevlar is temp resistant but it ain’t completely flameproof) and easily replace it.

Click to expand...

You can get that from Apogee, here:

https://www.apogeerockets.com/education/downloads/Newsletter338.pdf

I did basically what you're proposing on my Big Daddy. It's been a few years, so I don't remember the exact steps, but I drilled holes through both centering rings and threaded some kevlar through them. If memory serves, I used some thin wire as fish tape to pull the kevlar though.

A plastic coffee stirrer can be used instead of the Q-tip, assuming you can find the plastic straw type. Of course, the long skinny wooden ones are useful for putting adhesives deep into a body tube.

To remove the old tri-fold mount, I cut the shock cord as close as possible, then gently peel up the mount. I've done this successfully several times when refurbing older rockets. Just be patient and don't rush.

neil_w said:

What is the rocket, what is the current status, what is happening exactly?

Click to expand...

The trifold is boogered up and interferes with chute ejection. I have several I would like to convert, but they are an 18mm motor in a bt-30 tube and I think I'm going to have work the trifold down flatter. It's a Hi Jinx.

As I'm reaquiring skills after a 14y hiatus, I started with some cheapos before moving onto the nicer ones. Still it bothers me to remember after the fact what I should have done differently.

Thanks for verifying what I should have done.

Phil.

First, what's BT-30? I've never heard of that size.

Second, rather than using a wire hanger as a long drill, I suggest you acquire a long drill like these or similar.

Another option is, as you mentioned, to do the trifold over again, patting it down flatter this time. If you're doing that, may I suggest also trying to push the mount further down the body tube so that it's below the stowed position of the parachute so and the 'chute never has to be pushed past it? You'll want a metal rod or wooden dowel of a good 1/2" or so diameter to help position the mount and to roll it down flat.

When you do a trifold, how do you lay the elastic? If I remember right, Estes shows the elastic right down the middle of the paper trapezoid. I like to lay it parallel to one side so that when it's folded over it makes a V inside the paper; that way it doesn't lay on top of itself and doesn't build up as much thickness.

+1 on everything jqavins said above.

In general, for small LPR rockets already built, I wouldn't bother with trying to retrofit a Kevlar attachment to the motor mount. Just put in a good trifold mount and it should work fine. If you have a trifold that's messed up, try to scrape it out and put in a nice new one (I don't know how hard it is to get an old one out.)

For your future builds, you can switch over to Kevlar + elastic, as desired.

jqavins and Neil,

Sorry, I typo'd that, BT 50., Love those drill bits. Those were probably stacked, before I read about doing a V. It's amazing what you can learn here.

I may wind up re-doing the trifold, but I'm thinking about drilling through the thick centering rings, just to see if I can. I'd rather learn on these and make better builds in the future.

THANKS! Phil.

Well, in that case, here's what I'd do, and keep in mind that there are likely better ways.

• Drill through the centering rings.
• Install a very small tube. I've used the cotton swap sticks, but remember to buy the cheap ones, as the better quality ones have solid, rolled paper sticks that don't do you any good here. I've tried the hollow coffee stirrers and found them too small.
• Use a piece of piano wire as fish tape. You can attach the Kevlar cord to the end of the wire with a drop of CA, but be sure to cut off and discard the end of the cord that's been glued. Kevlar and CA do not like each other.

Good luck.

jqavins said:

Well, in that case, here's what I'd do, and keep in mind that there are likely better ways.

• Drill through the centering rings.
• Install a very small tube. I've used the cotton swap sticks, but remember to buy the cheap ones, as the better quality ones have solid, rolled paper sticks that don't do you any good here. I've tried the hollow coffee stirrers and found them too small.
• Use a piece of piano wire as fish tape. You can attach the Kevlar cord to the end of the wire with a drop of CA, but be sure to cut off and discard the end of the cord that's been glued. Kevlar and CA do not like each other.

Good luck.

Click to expand...

Yes, I saw the cotton swab technique. That is what I will try. I'm struggling to find the right piano wire vs the trial-and-error web-ordering... In addition to this I'd like it strong enough to hold up a fin while painting with CWF (Apogee video).

Thanks, Phil

Don't know how far along you are with this, but I would offer up a couple of changes to the motor mount attachment. First, make your holes out toward the body tube, not along the motor mount. Keep the kevlar on the outside toward the body tube if you can. Next, I use coffee-stirrer straws instead of qtips. A shop near me actually has extra-long ones. The extra length works for E and F mounts, they are small diameter perfect for lightweight Kevlar, and presumably a bit more temperature resistant. After all they were made for hot coffee. And they're FREE.
Even if you're retro-fitting, adding the straw is probably just as easy as not using it. Just use a piece of fine wire to pull the kevlar through.

Edit: oh, and my pull wire is made from .023 welding wire. just go find a friend to cut you off a piece.

Another alternative for tubes to run Kevlar through is the igniter carrier tubes that come with Aerotech reloads.
Just the right size for the 3/16" stuff. To make a longer tube just use thin CA on a join.
And welding wire is the way to go. I've got a nice piece of s/s filler wire.
Cheers

I've used piano wire for the purpose. This, plenty stiff to push through*, and cheap enough. I always have some on hand to make LP engine retainers anyway.

Yes, it seems floppy in a full length, but just try making a sizable bend in a few inch span. You can, but you'll feel the force it takes.

The way I 'fish' the Kevlar through the small tubes is to use a needle and thread. I put some fine thread through a needle with a dab of super glue to hold it. Push that through the small tube and with a small few taps the needle falls through and pulls the fine thread with it.

Then I can attach the Kevlar string to the end of the thread carefully and then you can pull the Kevlar string through.

The OP referred to a common problem, that of a ragged trifold snagging a chute. I've used the balloon clamp method with excellent results - a perfectly smooth and flat trifold every time. Insert a small party balloon into the tube so the balloon end is below the trifold (may need an long balloon used in making balloon animals). Blow it up - it will press the trifold firmly into the tube, the pressure will ensure an even distribution of glue, and the balloon will peel right off when you pop it the next morning.

Wow, Kevin what a great idea! And here I was pushing down the mount with my middle finger for five minutes.

For retrofitting low power smaller diameter rockets a great option is the Stine shock cord mount. Designed for round elastic shock cords, it will also work with thin elastic cords. Knot one end and thread the cord through the holes. Then glue the mount inside the tube. Only one thickness of paper and elastic, rather than three.

A brilliant variation of this comes from "Blackshire" at YORF. Read about it on Chris' blog here:
https://modelrocketbuilding.blogspot.com/2011/04/more-shock-cord-mount-ideas-part-1.html

Both the tri-fold and Stine mounts would benefit from a layer of glue on top applied after the installation. This smooths the mount lessening any snags as well as protects and strengthens it from the ejection gasses.

And of course, for new builds a Kevlar leader attached to the motor mount is best.

I should mention the Loc method on some kits. A length of nylon cord knotted on both ends. The ends are embedded in a layer of epoxy forming a loop anchored in the body tube. Probably overkill for smaller LP birds.

And the Centuri method on some older kits: Cut a short rectangular segment of body tube, knot the end of the shock cord, and glue to the main tube.
Is that enough to think about?



Ho, ho, ho!

Those style mounts work well and are low-profile.

While I rarely use the trifold any more I really liked the "Balloon Clamp" Idea!

Remember the balloon clamp method, as it has other occasional applications. I received an Estes Solar Flare kit many years ago with its rings (1 inch long pieces of BT-80, I think) badly out of round. I soaked them with Aerogloss dope and used balloons to push them uniformly outward, forcing them round. The next morning when I popped the balloons the rings were perfect.

I'm sure there will be other applications every now and then. Just keep it in your tool box next to that basin wrench; you'll hardly ever use it, but when you do it's just perfect.

OMG!

kevindcornwell said:

may need an long balloon used in making balloon animals

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As Grandma would say, "what will they think of next"!

LOL I learned a little trick with the balloon thing!

Went to the party store and got some of the balloons for making animals and stuff.

Got them home and tried to blow one up. I couldn't, well not with developing an aneurism

So I thought and looked around and spotted a can of canned air, like what you use to clean keyboards and such. Wrapped the balloon around the thin tube and POOF it fills right up!

Also you can leave the balloon like half empty and move the bubble around. So you can drop the thinner end of the balloon into the tube then just push the air down into the tube!

I’m using the yellow tubes used for setting thrust rings in 18mm and 24mm diameter rockets. A piece of coupler could be used in larger diameter rockets. I looped kevlar through a 1/2” lug. Glued the lug inside a 1” piece of yellow tube. Glued tube above thrust ring.

sl98 said:

Glued tube above thrust ring.

Click to expand...

That sounds like it would work. After carefully removing the trifold... and CAREFULLY putting glue in the right place without smearing it all over the place...

Or even using a piece of an engine...

Thanks.

Kuririn- Has it right, the card stock or index card method where you cut holes or slots and thread the elastic through like a buckle. Forget where I pick it up, must be Harry Stine's book of Model Rocketry and I used it for all my low power builds...no bulges. Shock cord damage or broke...glue another. Drilling through cardboard rings, all i see is damage to the rings or dislocating the motor mount.