Anchoring the Kevlar in an MD LPR rocket, and other motor mount issues

[neil_w]

Consider a 24mm minimum diameter cardboard rocket. OK it's not really minimum diameter, it's more like a rocket with a full-length MD stuffer tube, but that's near enough the same thing, as far as this question goes. But this is in no way a high-performance rocket. I'm trying to figure out a straightforward way to build a traditional LPR motor mount for it. So that includes a hook, an engine block up front and a Kevlar attachment.

Inserting the engine block from the rear seems like a bad approach, as does pushing it down from the front (down a long tube). A better approach seems to be using a coupler as the engine block, like this:

I think I saw something like this in some Estes instructions. Insert the hook, push the coupler down against it, and then connect the body. This looks pretty good to me, very simple and effective, and no exotic techniques required.

This leaves me a bit stumped about how to anchor the Kevlar. I've seem some different approaches but none that completely satisfy me. What are some good options?

 

[GlenP]

Mount the Kevlar to a baffle, or a similar coupler joint with a purpose built Kevlar support, just a crossbar if you want to keep weight down instead of a full baffle.

 

[BABAR]

I am about 98% low power, once in a while I do an E, and I did a G once, but most of my experience is D and down.

The engine blocks (or motor blocks, if you prefer) used in 18mm and 24mm birds are major overkill as far as thickness is concerned. I haven't used the provided mounts in kits or scratch builds in years (the ones for 18 mm and 24 mm motor ones provided in the kits I use as centering rings for 13 mm and 18 mm respectively.) For at BT-20, I cut a 1/4 inch long piece of BT-20, snip out just enough so it will curl up nicely in the BT-20, put the glue inside the BT-20 just below where I want the ring (WHITE glue, by the way, yellow is likely to lock), then use either the provided pusher or usually a used motor casing marked to the right depth, I smoothly push the ring in place and QUICKLY pull out the pusher. I have NEVER had a motor blow through this. even though it is much thinner than the standard block.

Back to your question, I HAVE started to use a longer piece of tube, wrap the Kevlar around it, put the Kevlar free end through, and push THIS up as my combined engine block AND shock cord attachment. The extra few inches of tubing protect the Kevlar near the tail a bit from the motor ejection charge. YMMV

 

[Hyak]

For LPR. I have used a short coupler section, - length varies for whatever I am doing - with a slight groove in it, (BE CAREFUL TO NOT CUT THE TUBE IN HALF ) just enough so the Kevlar will lay partially in the groove, and extend out of the coupler. I always tie a little knot in the exiting end. Then I epoxy the coupler into the Airframe. It is a snug fit, but I have never had an issue.

Again, I only do this for LPR stuff and so I am not using big, thick Kevlar.

I like BABAR's idea though as well, as it seems similar.

I do a lot of HPR so I epoxy pretty much everything anymore... Even a Mosquito! (Ok so maybe that's been a few years, and I didn't use epoxy on that!)

 

[neil_w]

For LPR. I have used a short coupler section, - length varies for whatever I am doing - with a slight groove in it, (BE CAREFUL TO NOT CUT THE TUBE IN HALF ) just enough so the Kevlar will lay partially in the groove, and extend out of the coupler. I always tie a little knot in the exiting end. Then I epoxy the coupler into the Airframe. It is a snug fit, but I have never had an issue.

I'm not entirely sure how to groove a cardboard coupler enough to make room for Kevlar without cutting all the way through it. It's possible that it can be done, I'm just having a hard time picturing it.

I wonder if I should just lay the Kevlar *inside* the coupler and epoxy it down? Would two linear inches of epoxy joint be strong enough, or does it need to be mechanically anchored in some way?

[edit] or I could put a piece of cut tubing, @BABAR style, *inside* the coupler, and sandwich the kevlar between the two. That would save me from worrying about how to wedge the kevlar between the coupler and the body tube. Yeah, it's starting to sound like overkill, but that has never stopped me before...

 

[kuririn]

I don't understand why you are averse to using a regular thrust ring?
Just cut a groove on the outside and tie a kevlar thread to it.

Use a dowel to lay down some glue in the motor tube.
Then glue the ring into the motor tube using an expended engine casing.
KISS. No coupler, keeps the airframe one piece.

 

[neil_w]

I don't understand why you are averse to using a regular thrust ring?

Installing the ring from the back means applying the glue from the back, and trying to nail the exact location without getting any extra on the interior of the motor mount. Likewise, you then need to push it to the exact right spot so the hook will butt up against it. I have done this before, it's a bit fiddly. I was trying to think of a more bulletproof procedure.

I have never seen such wide thrust rings before. Where did you get those?

 

[dhbarr]

I'm not entirely sure how to groove a cardboard coupler enough to make room for Kevlar without cutting all the way through it. It's possible that it can be done, I'm just having a hard time picturing it.

I wonder if I should just lay the Kevlar *inside* the coupler and epoxy it down? Would two linear inches of epoxy joint be strong enough, or does it need to be mechanically anchored in some way?

[edit] or I could put a piece of cut tubing, @BABAR style, *inside* the coupler, and sandwich the kevlar between the two. That would save me from worrying about how to wedge the kevlar between the coupler and the body tube. Yeah, it's starting to sound like overkill, but that has never stopped me before...

Just sand a flat side onto the outside of the coupler, then press in on it until it creases a tiny bit.

 

[kuririn]

I have never seen such wide thrust rings before. Where did you get those?

These were BT-5 thrust rings from a previous build thread, but the same applies to larger thrust rings.

 

[lakeroadster]

I'm not entirely sure how to groove a cardboard coupler enough to make room for Kevlar without cutting all the way through it. It's possible that it can be done, I'm just having a hard time picturing it.

I use a small triangular file, it's made for cutting steel.



Thrust Rings
In regard to thrust rings, I like using them. They help keep the body tube round and add rigidity to the fin can on minimum diameter rockets.

I put one on the outside of my X-Wing, for aesthetics and to reinforce the BT-50.



Kevlar Mount

I used an Estes style trifold mount on the X-wing... but it's buried down inside the BT-50, adjacent to the motor.

 

[Scott_650]

EDIT

Per kuririn’s suggestion use the thrust ring as an anchor for the kevlar cord, just don’t use it as a thrust ring/motor block since inserting it to precise location seems to be what you’re trying to avoid. Another technique that I’ve used several times is doing a good old fashioned trifold mount but instead of doing it just below the shoulder of the nose cone glue it at the aft end of the body tube above the motor mount. That gives you the flame resistant advantages of the kevlar and a much lower profile trifold if you’re working with smaller tubes - plus it’s near impossible for a chute or streamer to hang up on a trifold mount if it’s lower in the tube.

Oops - didn’t see that lakeroadster already suggested this - and included some awesome diagrams to boot!

 

[neil_w]

A trifold or thrust ring above the coupler is certainly plausible.

 

[GlenP]

It can be a pain when you are trying to get the glue in there and accidentally hit the inside of the tube in the wrong spot. If you are installing an 18mm motor block, you could put a drop of glue on the end of a skewer or small dia dowel, and shield it inside of a smaller dia tube like a BT5. You insert both to the proper depth, then the inner tube prevents glue from hitting the part where the motor goes. You pull the glue dowel inside the sheath, then out of the 18mm motor tube. I usually just do a small drop at a time and have a pencil mark on the dowel, drop a small bit in quickly and spread it around. If you are careful and spin the dowel slightly you can avoid dripping glue. The smaller tube insert, or even a paper liner would help catch the drips. It takes a steady hand for this operation!

 

[neil_w]

It can be a pain when you are trying to get the glue in there and accidentally hit the inside of the tube in the wrong spot. If you are installing an 18mm motor block, you could put a drop of glue on the end of a skewer or small dia dowel, and shield it inside of a smaller dia tube like a BT5. You insert both to the proper depth, then the inner tube prevents glue from hitting the part where the motor goes. You pull the glue dowel inside the sheath, then out of the 18mm motor tube. I usually just do a small drop at a time and have a pencil mark on the dowel, drop a small bit in quickly and spread it around. If you are careful and spin the dowel slightly you can avoid dripping glue. The smaller tube insert, or even a paper liner would help catch the drips. It takes a steady hand for this operation!

Good thoughts, thanks. Seems doable.

 

[K'Tesh]

I did a little essay about why you might want to stick with rubber shock cords...

https://www.rocketryforum.com/threads/using-protection-yes-rubbers-for-your-rocket.158586/
That said... I would recommend a combination of the notched engine block, along with my "U Loop" shock cord idea.

 

[Hyak]

Its basically just a scratch in the ring (NOT DEEP!) I have cut to deep more than once! But I only suggested it as I had also understood that you didn't want to use a thrust ring. (Which I also like) (I don't like, Tri-Folds)

Anyway, put a little knot at the end and yes, the Epoxy will hold permanently. Again, LPR only

 

[Grant_Edwards]

(I don't like, Tri-Folds)

If you've ever had to replace a tri-fold in an old rocket you'll never use one again. With kevlar, it's almost excusable but with kevlar there are other methods that are both better and easier. Tri-fold with rubber shock-cord is an especially bad combination, because the rubber cords will dry out and fail after a few years. If I have one complaint about Estes it's that they're still using tri-fold with rubber shock cord in their kits.

 

[lakeroadster]

If you slot the thrust ring o.d. and glue in a short section of a straw, you can then re-thread a new piece of Kevlar if need be at a later date...

I did something similar to that on my Columbine.. except it's a rear discharge spool.





A used oral syringe can be handy for getting glue where you want it...

 

[smstachwick]

I can confirm that the coupler-as-motor-block technique is viable. I just used it in my Super Star Trooper conversion, although those motors were just friction fitted.

No comment on the Kevlar.

 

[bjphoenix]

If you've ever had to replace a tri-fold in an old rocket you'll never use one again. With kevlar, it's almost excusable but with kevlar there are other methods that are both better and easier. Tri-fold with rubber shock-cord is an especially bad combination, because the rubber cords will dry out and fail after a few years. If I have one complaint about Estes it's that they're still using tri-fold with rubber shock cord in their kits.

I did some experiments recently, documented in another thread. My idea was that a rubber shock cord has limited force on the mount since it doesn't transmit impact, but a kevlar cord doesn't elongate so the impact force goes into the mount. So I did some experiments to see how strong a tri-fold mount was, and with kevlar it isn't very strong. I came up with a mount using a piece of cloth instead of folded paper. I was doing this for retrofit in some older kits, for newer builds I think looping the shock cord around the motor mount is better. But since then I did retrofit an older rocket doing this- I drilled holes in the centering rings and fed the shock cord through with a piece of stiff wire.
Another point for consideration- I've read that kevlar is somewhat flame resistant but if connected at the motor mount it would eventually burn through so some people are using stainless steel wire to connect to the motor mount then they connect the kevlar closer to the front of the body tube.

 

[neil_w]

I have a decent idea of some options. Will decide how to do it when/if the time comes.

Sometimes when considering a question like this I like to frame it as: if I purchased a kit, what build procedure would I consider to be reasonable? Normally, a well-designed kit would have a build procedure that is relatively bullet-proof. I believe I saw the coupler idea in some Estes instructions, but they don't do Kevlar so I couldn't find any examples that show that part.

(no I'm not working on a kit)

 

[Grant_Edwards]

My idea was that a rubber shock cord has limited force on the mount since it doesn't transmit impact, but a kevlar cord doesn't elongate so the impact force goes into the mount.

One generally accepted practice is that you make the kevlar cord long enough that there is no impact to transmit. Another alternative is to attach a length of elastic or rubber to the kevlar so that there is something else to absorb any impact. Either way, the idea is that the mount for the kevlar cord doesn't experience significant impact.

I tend to do both: a kevlar cord 2-3 times as long as a typical Estes shock cord and then a shorter length of elastic/rubber that's easily replaced.

Some people also put a piece of some sort of replaceable tubing around the first few inches of kevlar to shield it from the heat. On many of my rockets the kevlar is also replaceable, but that's pretty difficult to do on a minimum diameter rocket.

 

[Grant_Edwards]

But since then I did retrofit an older rocket doing this- I drilled holes in the centering rings and fed the shock cord through with a piece of stiff wire.

I've done that too, and it works nicely. The kevlar can even be replaced if it does get burned. I had to fabricate an extra long drill bit that would reach the forward centering ring from the back. Once you've figured out a way to drill the holes, it's a pretty easy retrofit. It certainly took less time to install the kevlar+elastic than it did to remove the old trifold mount with a round file.

 

[BEC]

The problem with Kevlar, however it is attached, is that it is going to burn through after somewhere between 2 and 30 flights, if it's anchored near the top of the motor as most ideas here have suggested.

Mr. Edwards' comments notwithstanding, Kevlar in a tri-fold made of reasonably heavy paper, installed down a couple of inches from the top of the BT will last a very long time...as long as you also protect it from abrasion by the lip of the body tube. This is especially important if you use thin CA to harden the lip of the tube, and it applies regardless of where the bottom of the Kevlar is anchored.

The reason Estes still uses the tri-fold mount 56 years on (the Alpha, released in late 1965, was the first to use it) is that they are simple and they work, though they don't work all that well in BT-5 or BT-20 models and if the shock cord material dies then they are a pain to remove. In the few cases where I've had that happen I just trim the end of the failed shock cord off, scrub the tube with a combination of a stiff brush and sticky-backed sanding paper on a dowel and put another one in. The two models I've most recently had to do this in it was because the Kevlar wore through.... Both models have over 80 flights each on them.

 

[roytyson]

The problem with Kevlar, however it is attached, is that it is going to burn through after somewhere between 2 and 30 flights, if it's anchored near the top of the motor as most ideas here have suggested.

I couldn't agree more. I spent an entire off-season building low power and attaching the kevlar cords to the motor tubes. Almost every one of them burnt through from the ejection charge. I went back to gluing them into tri-folds near the nose cone.....

 

[teepot]

I'm doing a couple of MD rockets and had the same decision. How to attach the shock cord. I took a centering ring and notched the outside. Then tied the Kevlar around that. I used a dowel to put the epoxy in the tube. Used a long piece of coupler to push it into place. And there you go easy peasy. This is for a 29mm MD which will fly on an I205. I'll use the same method on the 38mm. I plan on flying that on an I500.

 

[kuririn]

Neal, you can have your cake and eat it too.
Go the extra mile and install a replaceable kevlar shock cord system for MD rockets.
Chris M. wrote an article for the Apogee newsletter.
Newsletter343.pdf (apogeerockets.com)
Similar to what Lake alluded to above but with a slip loop and no straw.
Hardest parts that I see about that is precisely laying down the glue so that it does not occlude the slot and threading the kevlar line through the slot after the thrust ring is glued in place.
But I know that's peanuts for you.

 

[neil_w]

Go the extra mile and install a replaceable kevlar shock cord system for MD rockets.
Chris M. wrote an article for the Apogee newsletter.
Newsletter343.pdf (apogeerockets.com)

I never saw that article before, very clever method. I have never done a replaceable Kevlar mount before, and am lucky that so far I have not had any Kevlar breakage. The fact that I fly infrequently and rarely am able to get a large number of flights on any rocket works in my favor here (I think the most I've had is 6 flights on my Solar Warrior, Kevlar still OK on that one).

The question of Kevlar durability and how to best manage it is one I've pondered quite a bit, and will post more here when I have a bit of time.

 

[manixFan]

FWIW, I use very small diameter flexible heat shrink tubing to cover the first several inches of kevlar by the motor mount. I think it also helps by keeping the cord stiffer so you can run it up the side of the body tube and pack the wadding more easily. It's easy to remove and replace - I use a long, flat bladed hemostat to grab it if I need to slide it back up the Kevlar. You really don't need to shrink it - it will stay in place pretty well on it's own if you kink the tubing before sliding it on. On larger diameter or heavier rockets I run it all the way past the end of the body tube to protect the kevlar from rubbing against the edge of the tube during descent. I will put a shorter, second piece right in that area for extra protection as well.

And like Neil in post #28, I've spent a lot of time researching Kevlar and other aramid fibers. In reality, they aren't very good for our uses. but they are better than many of the alternatives. But I bought thousands of feet surplus years ago so I'm stuck with using it for a while.


Tony

 

[kuririn]

FWIW, I use very small diameter flexible heat shrink tubing to cover the first several inches of kevlar by the motor mount. I think it also helps by keeping the cord stiffer so you can run it up the side of the body tube and pack the wadding more easily. It's easy to remove and replace - I use a long, flat bladed hemostat to grab it if I need to slide it back up the Kevlar. You really don't need to shrink it - it will stay in place pretty well on it's own if you kink the tubing before sliding it on. On larger diameter or heavier rockets I run it all the way past the end of the body tube to protect the kevlar from rubbing against the edge of the tube during descent. I will put a shorter, second piece right in that area for extra protection as well.

And like Neil in post #28, I've spent a lot of time researching Kevlar and other aramid fibers. In reality, they aren't very good for our uses. but they are better than many of the alternatives. But I bought thousands of feet surplus years ago so I'm stuck with using it for a while.


Tony

+1
New Way kits include a piece of heat shrink tubing to protect the kevlar near the motor mount.
I should also mention that the thickness of the kevlar affects it's durability (obviously).