All-Thread in Fin Cans

In quite a few threads and blogs that I have seen that document the construction of large, Level 3-type rockets, the builder has tied the fore and aft centering rings for the motor mount together with 3 or 4 lengths of all-thread. I have always been curious about that. I have never seen an explanation of why it is done and I have often wanted to ask about it.

1. What is the purpose of this construction technique?
2. How common is it?
3. Is it used more in clusters or is it also used in single mounts?
4. At what point in the range of rocket sizes or motor impulses does this technique become advisable, necessary or a reasonable option?
5. Is it an example of something that makes the construction of rockets that will launch on M, N and O motors different from all others?

Building a Level 3 rocket isn't anything that will be on my radar (or in the budget) anytime soon. I'm just curious about it, that's all.

I've seen this question before and the back and forth discussion on it. I haven't seen any convincing arguments for having all threads in the fin can. I used the technique in my level 3 project, but didn't in my last big project and that came out fine.

MarkII said:

I have never seen an explanation of why it is done and I have often wanted to ask about it.


[*]What is the purpose of this construction technique?

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To add needless weight to the rocket and move the Cp aft. This allows you to add more needless weight to the nosecone to compensate.

[*]How common is it?

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Unfortunately very common.

Crush rod! (to quote the infamous Rich Pitzerus)

Allthread in compression is nothing but dead weight and provides no benefit. It's a material designed to be used in tension.

Thus, for tying the bulkheads of an electronics bay together, allthread is a good choice. For tying centering rings in place on a fin can, it's dead weight.

Yes, you can use it to hold the centering rings in place while epoxy cures, but why? It's dead weight in the long run, and unecessary when clamps and a jig will do the same.

-Kevin

It's absolutely unnecessary, and serves no purpose whatsoever. It's all too common, unfortunately, and adds weight exactly where you don't want it.

troj said:

Thus, for tying the bulkheads of an electronics bay together, allthread is a good choice. For tying centering rings in place on a fin can, it's dead weight.

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That's pretty much what I was wondering. My take is that, in the event of a hard (very hard) deployment, there could be a huge jerk on the U-bolt anchored to the forward centering ring in the fin can. I wondered if the ring could be pulled (broken) from the motor tube and airframe - ie, if the forward section of the fin can could break off; if it might fail in tension, or if the ring itself might fail, or if the tubes might fail by delamination at the tube-ring joint. If any failure like that could occur, then having the all-threads in place to spread the jerk along the length of the fin can would be beneficial.

I don't have the structural engineering knowledge to do a paper assessment. I do wonder if this has ever really been encountered in a hobby rocket failure. Or if it's really an over-reaction (corrective action) to some other failure encountered by a rocket. For example, a ¼" centering ring failed, so the all-thread was added when the real problem/solution was the forward ring shoulda been ¾".

After that, it's been monkey-see, monkey-do ever since. "Bob had 2 pounds of all-thread in his fin can on his L3. I better do that, too. Better safe than sorry."

That said, lots of us tend to do more over-building on our cert birds than we do the rest of our fleet. Just in case. And, if we're venturing into new impulse territory, there's some added apprehension. We tend to defer to others, whether they really know any more than we do, but they've done it, and got the cert, so we talk ourselves into copying them, into overbuilding

So, it's not needed for the sake of the rocket. But it's needed so that we won't worry about the thing that can't ever happen anyway

That said, even if I don't put any all-thread in my L3 fin can, I may add some wooden dowels in the same fashion, just in case

Doug

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I never saw any practical reason to use it in my level 3 & left it out. I used 7/8" thick wood struts to transfer weight & beef up the coupler section in the Fin can section.

https://www.users.nac.net/jdcluster/APFreak.htm

Scroll down the page to Dec. 10 entry to see pictures of what I did with the wood struts.

JD

Artical in High Power Rocketry, don't ask me the month, Lance Griffin and Ron Urinsco built a special rocket to test a launch tower. They had to build a 5" diameter rocket about 6' feet tall and weighing 50 or so pounds. Urinsco built a motor that pushed this rocket up a rail intended for some liquid fueled rocket as a test. They did many things to add weight to this rocket and one of the things they did was add allthread for the added weight more than anything else. Said it really didn't need it for this flight, the motor was like an L900-something but after recovery they planed more powerful motors so why not basically.





.....I think it ended badly.

Thanks for the answers. That explains why I have seen it in some places (mostly regarding builds that were done several years ago) and not in others (more recent builds). So it was kind of a fad at one time - something done because everyone else was doing it? I had wondered whether it had something to do with beefing up the motor mount area for large motors because of.... - well, I couldn't think of how to finish that sentence. I had wondered if there was some factor or issue that had totally escaped my notice. And it sounds like there was, but just not anything that I had suspected.

As far as using all thread to strengthen a fin can for a hard deployment, a lot of people have switched from using u-bolts in the top CR to threading a forged Eye-Bolt into the top of the motor, and securing the harness to that.
You don't ever have to worry about losing your motor case, and there's no way for the booster to slide past the motor!
One Caveat to this method, you need to either loctite or use a jam nuts on the eye-bolt against the bulkhead so the eye-bolt doesn't unscrew itself while under chute... Seen that before!

Ron

plano-doug said:

That's pretty much what I was wondering. My take is that, in the event of a hard (very hard) deployment, there could be a huge jerk on the U-bolt anchored to the forward centering ring in the fin can. I wondered if the ring could be pulled (broken) from the motor tube and airframe - ie, if the forward section of the fin can could break off; if it might fail in tension, or if the ring itself might fail, or if the tubes might fail by delamination at the tube-ring joint. If any failure like that could occur, then having the all-threads in place to spread the jerk along the length of the fin can would be beneficial.

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Keep in mind, though, that to properly load them, you have to make sure every piece has its nuts in the right spot and properly (and evenly!) torqued. Or you lose a lot of the benefit.

If a ring is epoxied to the motor mount and to the airframe, the ring is likely to fail before any of the joints will.

...and the primary effect of all-thread is to add weight in one of the worst places in a rocket to add weight -- the aft end!

-Kevin

I used 3/8" all thread in my L-3 Rocket. It's purpose was to provide a positive hard point for the recovery syatem. It worked and it worked well. The added weight was not an issue.

Now days I use forward retention, that is the threaded forward closure on the motor casing. At this point I no longer need all thread, except to span the distance between the forward closure and the forward bulkhead (I always use the zipperless design).

It did serve it's purpose well, overkill? I think so, but its all history now.

No two projects are the exact same, sometimes one setup will work and sometimes another. Allthread is jut another tool in the builders box.

Quick burst has it correct. Allthread does NOTHING to make the rocket stronger during thrust.. (ok a little but negligible) what it is good for. RECOVERY.

The jerk force of a 10 foot parachute opening can be TREMENDOUS. and it could be even worse if something goes wrong and you have a less than optimal deployment.

So my opinion. if you can afford to use allthread. DO IT. and hook it to your recovery system. I have seen WAY to many big rockets come in without a chute because the recovery point pulled through a 1/4 ply bulkead. if you allthread now you are anchored in four or more spots, and it spreads that force out to the entire rocket not just the one little spot where your eyebolt goes through.

Another thing I tend to use the allthread sticking out the back of the rocket as a way to anchor my motors in. again hard deploy could rip the rocket apart. but at least your chute will still be attached to the motor case.. and that is the one thing I am sure you want to get back if your rocket was to be destroyed.

Mark, right on the money, but that's why I use the motor to anchor my recovery harness. Most rockets I've built, including the 16" Diablo had the fins tacked in, two 1/2" Baltic Birch Ply CR's, then I 2-part foamed the whole shebang. No fillets at all, no all-thread, no 'glass, kevlar or anything. The foam basically makes the entire booster one solid piece. Is it heavier than the all-thread... who knows. I don't think it is, but I also know the foam is not featherweight either!
It IS however, VERY quick and easy.
Are a lot of rockets overbuilt?? Maybe, but so what. We build it how WE want to build it, which is what this hobby is all about. It allows our creative sides to come to life. I always read these threads, and almost always learn something clever that I wouldn't have thought of otherwise. It's all about building OUR birds the way WE want to build them. TRF and RP give us all a place to display our ideas and ingenuity. You'll always find someone who thinks you should have done it another way, usually their way!

LOVE this hobby!!

Ron

Interesting discussion. I'm glad that both sides of the issue are being presented. Whenever I thought about possible reasons for it, one that I never came up with was to fortify the recovery harness anchor. I'm not inclined to ever try the technique, and it is hardly an issue for me right now (putting it mildly), but I had wondered what the actual rationale was for it. It always struck me as an odd and rather extreme thing to include in a rocket's construction. (Rockets of the kind that we fly, anyway.) I have been able to readily grasp nearly every other feature or technique that I have seen used in the construction of big honkin' rockets, but that one always threw me for a loop.

AZ_Ron said:

Mark, right on the money, but that's why I use the motor to anchor my recovery harness. Most rockets I've built, including the 16" Diablo had the fins tacked in, two 1/2" Baltic Birch Ply CR's, then I 2-part foamed the whole shebang. No fillets at all, no all-thread, no 'glass, kevlar or anything. The foam basically makes the entire booster one solid piece. Is it heavier than the all-thread... who knows. I don't think it is, but I also know the foam is not featherweight either!
It IS however, VERY quick and easy.
Are a lot of rockets overbuilt?? Maybe, but so what. We build it how WE want to build it, which is what this hobby is all about. It allows our creative sides to come to life. I always read these threads, and almost always learn something clever that I wouldn't have thought of otherwise. It's all about building OUR birds the way WE want to build them. TRF and RP give us all a place to display our ideas and ingenuity. You'll always find someone who thinks you should have done it another way, usually their way!

LOVE this hobby!!

Ron

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I have NEVER use expando Foam. but I am willing to try it. seems like it would work.

And sometimes your fincan has no motor tube and is bolted together for disassembly and transport.
https://www.geocities.com/rdh82000/LDRS/Construction.htm#Fincan

Then again you can use kevlar to spread the load:
https://www.geocities.com/rdh82000/L3/#InstallKevlar

I used 2 runs of All-Thread in my L3 project. At the top end, a pair of forged eyenuts are the anchor point for the recovery harness. At the bottom, they tie the aft bulkhead to the rest of the rocket. The motor retainer is bolted to the aft bulkhead. This is entirely done to spread the shock of recovery and tie everything together. I also used all-thread to tie the forward and aft bulkheads of the electronics bay together and have forged eyenuts on either end for the recovery harness attach points.

Not smart enough to just sit this one out....

1) Bonded centering ring, with the RIGHT adhesive and 1/2 inch thick CR on a 5 inch rocket, it will take WELL OVER 5,000 to pull it out. At the day job with a piece of fiberglass bonded in I would count on over 10,000 lbs!! Put a single piece of boat tape in the fillet and the numbers go WAY up from there. This ignores ANY load transfer from the motor tube, this just ups the margins even more.

Still feeling scared? up the thickness of the EDGE to 1" (think ring on top of the CR) and your numbers roughly double.

What is important is to use the right adhesive (don't use laminating resin for structural adhesive)

2) Foam in the fin can. I never really understood the reasoning behind this, I have heard LOTS of anecdotal stories about how the foam is supposed to work, but I haven't hear of one that sounds right from an engineering perspective. Side note, the 'day job' involves structural foam cores for composites. Foam in the can would help prevent buckling but if your fins are TTW and bonded to the body tube, they will prevent buckling better than a foamed fin can. Foam the shape first, and laminate plys on may let you put thinner skins on the body tube.

3) Another use for foam. Where foam could be used REALLY effectively is in an energy attenuator for over-shock from parachute loads. If you can 'decelerate' the energy over even an inch or two the loads will go WAY down. Attenuators are routinely used in commercial applications as a safety. Heck even just sew in a 'zipper' into the shock cord, I have done this on some work applications. https://www.petzl.com/en/pro/vertic...absorbers/energy-absorbing-lanyards/absorbica .

I will go back to work now....

troj said:

Keep in mind, though, that to properly load them, you have to make sure every piece has its nuts in the right spot and properly (and evenly!) torqued. Or you lose a lot of the benefit.

If a ring is epoxied to the motor mount and to the airframe, the ring is likely to fail before any of the joints will.
...and the primary effect of all-thread is to add weight in one of the worst places in a rocket to add weight -- the aft end!

-Kevin

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I recently had a glue joint fail not the centering ring on my Arcas. It was built sometime in 2003 and I do not remember what type of epoxy I used but I know it was not West System. I have been using West Systems epoxy exclusively the last few years though. It was the first fiberglass rocket that I built and I do not know if I roughed up the inside of the tube enough. Most likely, the 50/50 two-part epoxy caused the failure though. The Arcas had a ton of flights on it and the forward centering ring joints failed. The booster came in hot from around 5000 feet. Surprisingly the booster just needed glued back to gather and refinished. She is now ready to fly again.

kramer714 said:

What is important is to use the right adhesive (don't use laminating resin for structural adhesive)

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Do you have a reference part number for a proper structural adhesive? I have been using 5 minute epoxy and/or laminating resin for most things and if there is a more appropriate adhesive, I would be interested in knowing about it. This is mainly for fiberglass and phenolic, as I typically use wood glue for cardboard and wood.

Sandy.

All I can really say on the 2-part foam is that it works for me.
I don't have a day job dealing with load factors and all that, I repair
cash handling equipment.
I've built some 4" rockets using G10 fins. Fins were tacked to the motor mount with CA, then I filled the booster/fin can with 2-part foam. These rockets have all been either LOC airframes or Red Arrow Phlexible Phenolic. NO fiberglass at all (I'm lazy!), NO internal or external fillets. They've been through Mach on more than one occasion. What completely sold me on the foam was after one of my Phlexible Phenolic birds zippered down to the top CR. Rather than repair it, I decided I'd just reuse the fins. Well, I wish I'd video'd me trying to get those fins out. I literally beat the living hell out of that fin can trying to get the fins out. Took me over an hour.
In my mind, it all comes down to surface area. Ever square millimeter of motor tube, airframe and fin inside the airframe is locked together in one solid mass. I'm not saying it's the Ultimate, but with being able to do this in a few pours of foam in less than an hour, beats the heck out of spending hours and hours fabricating, laying up internal fillets, 'glassing internals, drilling and lining up holes, expense of all-thread, etc, etc.
Wife is in college and we have 4 kids on my mediocre income... it's about expense and labor time. Foam wins in those categories for me.
I have NO scientific data to corroborate what I'm saying here. Just the knowledge I've gained over the past 13 years. For anyone who's curious about using the 2-part foam... give it a try!
As far as adhesives for construction... my new love is Gorilla Glue. We've done some test pieces of ply and aluminum honeycomb for composite fins... WOW. I'm going to be testing it with Nomex honeycomb and a veneer in the next few weeks in prep for building my 6" BB2.

I guess bottom line is that there is more than one way to skin a cat. (Showing some age here, no one under 35 uses that cliche anymore!)
As was pointed out, I don't think I've ever seen two people build a rocket exactly the same way. There is no ONE right way, although there ARE several wrong ways!

R

Regarding holding G10 fins in with foam....I've had G10 fins pull right out of the foam, leaving nary a scrap of foam on the fin.

Foam isn't an adhesive, and I would recommend against using it as one.

-Kevin

Hi Kev!! I guess I should have included a little more info. I felt as though I was already writing a book, so didn't elaborate.
I used a dremel sanding drum and roughed the hell out of the g-10 where it would be inside the airframe.
I was literally using a dead blow hammer trying to get those fins out of that booster. Keep in mind they were only tacked in with CA on that rocket, it was all foam that was holding them.
You are correct in that if the g-10 isn't very roughed up, the foam will not stick to it. As far as it not being an adhesive... just put a drop on your shirt. Make sure it's not a shirt you like!! It's not an adhesive in the true sense, but it grabs onto airframe material and wood with a vengeance. Just needs a little help sticking to G10, but it WILL work!!

R

I've used foam to lock in a G-10 fin by drilling a bunch of small holes. Seemed to help.

Al, that's also a VERY good idea.
Like I said... there is no one right way. Everyone does things
a little differently. Roughing up the G10, AND drilling some small holes would be the most solid thing possible as far as foaming goes. I've seen other folks drill holes, but it's not something I've ever felt the need to do.

R

Ron,

Good point in the prep, to prep a fiberglass surface best bet is to;

1) Wipe with Solvent
2) break the surface, sanding with clean sand paper works well
3) wipe it (again) with solvent
4) Water break test, put a drop of distilled water on the surface, if the surface is prepped well the water will spread out, if it forms a bead (think waxed car) you don't have a good prep. Working the adhesive on both sides of the bond helps too.

Lots of ways to make things work, not saying that foam wont work. As far as skinning cats, I'm in favor. Drilling holes is a really good idea if you are going to foam fin cans. The foam can build up significant pressure, The holes helps keep the pressure from building up on one side of a fin and 'pushing it over'

As far as adhesive suggestions, for bonding centering rings in I would use 3M DP-190, or DP-420 both are really good for shock and impact, bonds to most substrates. McMaster sells them.

I attached a picture of some fins we made at the 'day job' molded foam core with internal ribs..

Mount your shock cord wildman style: Glue both ends onto the motor tube and encapsulate them in epoxy. This leaves you a nice loop to hook your quick link to. Assuming you glue a solid 8 inches onto the motor tube you'll have to have some really catastrophic failure that all those heavy all-thread rods wouldn't help anyhow

Cbarick, I've tried that method as well, and when building a rocket that will fly with motor ejection, that's my preferred method. It works very well.
The reason I like using a forged eyebolt in the forward closure of the motor is two fold. It's about the most solid anchor you can get, and it has the added bonus of never having to wonder if your case kicks, where it'll land.
I did have one motor slide out at deployment... the motor just hung down below the airframe.

R

skycopp said:

I've seen this question before and the back and forth discussion on it. I haven't seen any convincing arguments for having all threads in the fin can. I used the technique in my level 3 project, but didn't in my last big project and that came out fine.

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Your last "big" project came out fine.....

?



Motor worked too?