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How strong does it need to be?

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KilroySmith

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Starting on a couple of L2 rockets (one self-designed), and despite months of reading, I don't know how to answer the question. I've read a lot of build threads. There's the "Build it the way it's designed, it's strong enough" crowd, the "I use epoxy on everything, including when I help the cub scouts build their first rockets" crowd, and the "I just keep building it up until it looks about right" crowd. But how do I calculate how strong a: 1. motor tube centering ring, or a 2. stage coupler for a dual-stage rocket, or a 3. Fin and attachment, or a 4. shock cord, or a 5. shock cord anchor need to be? Sure, I can just always install a 1/2 welded-steel eyebolt to anchor my shock cord, and use epoxy on everything intended for a motor larger than a 'B', but there must be a better way. I've picked up a couple of high-power rocketry books - but haven't been impressed so far. A lot of "How I built this rocket", and "do it this way", and not a whole lot of "this is why we do it this way". Any help? /frank
 

blackbrandt

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Well.... I kinda eyeball it. It's honestly something you just get the feel for. I look at other build threads, and knowing who builds heavy, and who builds light, I can make a determination as to how most rockets are built.
1. Motor tube centering ring: For 38-54mm motors, I use an 18mm casing for the fillet.
2. Interstage coupler. This is one place where you DO NOT want to skimp. This should be strong enough to support the rocket completely on its side.
3. Fin and attachment: I always go overkill here, but it's a bad habit. I tend to use a 18-24mm Estes casing for fillets, and also inject fillets a la Crazy Jim style.
4. A rule of thumb I always hear 5-7 times the length of the rocket. As for strength, I use 750lb 1/8" Kevlar for most stuff up to about 5 pounds, and then I use nylon/kevlar strap for anything bigger.
5. Shock cord mount: This can just be glued to the side of the motor tube (read any of Wildman's instructions, these explain it really well).

Hope this helps. Another thing to remember, if you prep the surfaces correctly, you don't need much epoxy. There's a bonding thread in the high power forum, read that. You'll see that you don't need all that much epoxy if you sand your surfaces, and clean them.
 

eggplant

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You can start by calculating the forces that will be applied to things, such as aerodynamic forces and shock loads from recovery. Once you have these numbers, just look at your options for parts and find some that are rated or tested to meet your needs. Most people in the hobby prefer to use someone else's empirical data about what materials and techniques work, because it is usually the case that copying someone else's construction will get you similar results. I usually only see someone do this sort of calculation if they are really pushing the limits with a high performance flight, because otherwise cardboard and wood glue (or anything stronger like fiberglass) works well for just about any hobby flight and is very light. On the other hand, you see people using much stronger materials to build hobby rockets (or epoxy on cub scout rockets) for a variety of reasons. Some recovery surfaces are unforgiving enough that cardboard tubes don't fair well without reinforcement, which is one reason why you might see people use materials that are overkill for the vast majority of hobby rockets.
 

GregGleason

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Your question is one of design engineering and materials science. That is well beyond the scope of many in the hobby. As a result, many rockets are overbuilt.

The issue is that the materials used have a wide spread of variation with regard to strength. Unlike metals, which have a lot of data regarding strength, the materials that we use to build hobby rockets usually do not have that data or the standard deviation is so wide it is not helpful. So we are left to see what others have done that works.

Is that the end of the discussion? No. But with rocketry, the answer is often "it depends". A rocket that may fly fine on an H123 may fly apart on a K550. I have seen a rocket turn into confetti in the blink of an eye. So we do the best we can with educated guesses. For example, if you have a motor that will kick of the pad with a 100 pounds of thrust, you better have a thrust structure in place that can handle the sudden g-shock. If your rocket is going to spend more than a moment in the transonic region (M 0.83-1.2), you better have done your homework with regard to fin flutter and how the fins are attached.

I know that this might not be the most satisfying answer, but that's the world as I see it. In the words of the famous philosopher "Dirty" Harry Callahan ...

-Dirty-Harry-Callahan-dirty-harry-34346969-200-200[1].jpg

Greg
 

DavidMcCann

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It's harder to find, but look for people reporting on their failures. That is typically the best source of info
 

Rex R

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figure your recovery gear for a 50grav load(5lb rocket * 50G = 250 lb load) and round up
Rex
 

rharshberger

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figure your recovery gear for a 50grav load(5lb rocket * 50G = 250 lb load) and round up
Rex
Or your can use OpenRocket and it can simulate the maximum acceleration/deceleration in Gee's. Then based on the max G's figure the requirements as Rex R stated.

Learning to use OR, Rocksim, and RASAero II, will simplify life greatly and will help with building lighter better optimized rockets.
 

byoungblood

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I wasn't too crazy about simply epoxying the shock cord of my Darkstar Jr to the MMT, so I notched the top two centering rings, then tied the Kevlar to the MMT using a clove hitch (it tightens under load). Then I put some epoxy over that. For my 4" rocket I used the eye bolts included with the kit (started as a Madcow Patriot, made some custom fins to turn it into a Standard Missile). All comes down to how much room you have to work with in the body tube.

Fins get internal fillets, plus external ones, reinforced with milled fiberglass. Same for any centering rings that can be reached for fillets. I use a small amount of microbaloons on the external fillets to reduce weight a little and to make it more sandable for finishing purposes. I use off the shelf Bob Smith epoxy on all my MP and HP rockets. Maybe if I ever decide to go.for my L3, I may need something better but for what I do it works.

Those construction techniques held up well for both my L1 and L2 flights, plus half a dozen others on my Standard.
 

Kruegon

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As the OP stated. We all seem to have a category we fall in to. I overbuild. It's just where I feel comfortable.

The long and short of it is: there's no one right answer. Each rocket will have its own requirements. Each motor will have the same. For the biggest part, go with your gut so long as your gut feels like you might be overdoing it a little.

I built my L1 & L2 the exact same way. BSI 30min epoxy, internal and external fillets. I used 1/4" forged, stainless steel eye bolts for the L2. My only material variation. And my L1 was a single altimeter. I went redundant on my L2.

The biggest issues you face are:
1) What size parachute do I need?
2) Will this one break Mach?

As previously said, OpenRocket, RockSim, or RASAero II are your friends. I have run my Estes PSII kits on an H165 and I200. Found out my building techniques were definitely strong enough.

If you have specific questions ask them directly. One of us will be happy to assist. Probably more than one.
 

jeff_j_black

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Fly some rockets and get a feel. It depends on the overall weight and what you will subject it to. SIM it right and a rocket that flies G's can fly H's.
 

rharshberger

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Sometimes I believe we underestimate the capabilities of the materials we are using. A rocket constructed of Loc airframe tubing and aircraft birch plywood fins (similar to Binder Design, Loc, and Madcow kits) are capable with K motors. Wood glue is stronger than either material, and the only places epoxy is really desirable is in the external fin fillets for cosmetic not strength reasons, and when bonding or doubling couplers, I also like it when inserting motor mounts as epoxy wont seize like wood glue. One item I believe adds greatly to the rocket is the use of a thrust plate the same diameter as the airframe as part of the motor mount, then the centering rings are not carrying any of the load between the MMT and the Airframe (or not very much). With a thrust plate the motors thrust is applied directly to the exterior of the airframe in along the strongest axis of a cylinder. I make my thrust plates usually of 1/4" baltic birch plywood, but if one so chooses they can be made of aluminum or fiberglass too. The pictures below are of two thrust plates, the all aluminum one is for a 98mm to 75mm and matches the exterior diameter of the Loc 5.38 airframe (the 12 holes match an Aeropack 98 flanged motor retainer) and the aluminum is .125" thickish (its actually 10 gauge iirc) and was more than strong enough for a M1297 (the motor retainers are overkill at 6 of them, 3 would have done the job). The 3" Frenzy XL clone is actually FG wrapped Loc airframe with 1/8" baltic birch plyfins laminated with a layer of 4 or 6oz FG and a layer of 2oz FG on each side. There are two thrust plates on this rocket one is under the red motor adapter andis 1/4" baltic birch, the other is the the red one thats part of the 54mm to 38mm motor adapter. The reason I like thrust plates is that the motors thrust ring bears on the plate, which in turn bears on the airframe very little force gets transferred through the adhesives in the MMT. The Frenzy XL's largest motor to date is a J354 White Thunder, and the rocket weighs 6lbs ready to fly. The recovery harnesses are 25' of 1/4" Kevlar from Wildman, and the Quick links are 1/8" quick links. While flying on a I218R the rocket suffered a drag seperation of the fincan section (its zipperless design) and suffered no damage,to any components. Knowing what I know now I could have saved at possibly as much as a pound mostly in epoxy and reinforcements, and still have flown any motor up to a K and built the rocket almost entirely with wood glue.

As for the High Power books, I agree they are good reading and give some great ideas but they are seriously lacking in the department of material strengths, but that in itself is a technical subject.

L3Build111.jpg LastDayofBuild2.jpg
 
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Rob702Martinez

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Learn the limits of your materials and what they do at the limit.
Learn the limits of your adhesives and what each one will or won't do, compositions, additives.
Learn from others at a launch rather than books or forums.
Learn why something failed or has held up.
Read or search more about related topics to the materials/adhesives/properties/methods you use or plan to use.
Become your own expert.
Dont be afraid of OpenRocket
Ask questions to learn.
Question everything, to understand.

First few things that ran to mind.....hope it helps. ,
 

thomas

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While I agree, that most rockets could be build with muss less strength, I do not like the term overbuild.

A rocket does not fail because you overbuild it.
My top priority is to build a safe rocket, and overbuilding is only unsafe if your rocket crashes into something.
On the other hand if your rocket is not strong enough this can lead to very unsafe situations.
If you loose a fin, or your body tube fails, or you rip your shockcord it is dangerous.
If your rocket flies a few meters less because you build it stronger than it need to be that is not a security problem.
That is also a reason why I do not like competition rocketry, it forces you to build rockets at the strength limit.
Maybe that is less a problem if you launch somewhere in the desert, where nobody is anywhere close, but on the small fields we have here safe recovery is mandatory.

You have to be sure your rocket will survive the flight and the recovery, if you can build a light rocktet which will do that go for it.
I have seen a RSO reject a rocket for a L2 flight on a small J motor because its body tube was phenolic without reinforcement.
That is of course nonsense because phenolic is strong as hell, it is only brittle.
Good plywood and woodglue are also extremely strong.

If I am unsure how strong a material is, I will ask somebody, most likely an engineer.
 

tomsteve

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has anyone ever said
" I built this rocket way too strong?"
 

DavidMcCann

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While I agree, that most rockets could be build with muss less strength, I do not like the term overbuild.

A rocket does not fail because you overbuild it.
Sure it can. overbuilding can lead to a host of problems.


has anyone ever said
" I built this rocket way too strong?"
yes.

I don't exactly build things on the razors edge, nor do I think everyone should. But I think it's good to look at any build plan and think "Am I building one part of this stronger than another/ will i get any actual benefit from this?"
 
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Binder Design

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The biggest problem is that most kits do not come with adequate instructions if they include them at all. My suggestion is to find a rocket manufacturer with proven kits that include a detailed instruction manual with each kit, and follow the instructions.
 

DavidMcCann

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The biggest problem is that most kits do not come with adequate instructions if they include them at all. My suggestion is to find a rocket manufacturer with proven kits that include a detailed instruction manual with each kit, and follow the instructions.
I think the difficulty in providing instructions is the wide range of methods that could be used depending on goals. You could sit down and write 5 different build manuals for a 3" kit with 54mm mount and they'd all be "proper" depending on the rocketeers individual goals. It'd be cool to see that style manual done, but that's quite a bit of work.

I do recognize the value in following recommended directions and motors for a kit.... but we stray from that a bit ;)
 

RocketFeller

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Building strong is good, but it often means building heavier.

Heavier rockets require more impulse to achieve the same performance, so they need to be stronger to handle the thrust. Heavier rockets also are more likely to suffer damage on landing, so they need to be built stronger to survive landing. In the end the lighter built rocket will often perform better and survive landings just as well as the stronger, heavier rocket.

I have definitely done my share of overbuilding, especially high-power rockets. The last kit I built (a shortened Estes Argent) I used only wood glue, except for a very small amount of epoxy on the plastic transitions and (upgraded) shock cord attachment. The rocket flies great on a wide variety of motors and I am able to use a smaller (18") parachute.
 

Binder Design

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I think the difficulty in providing instructions is the wide range of methods that could be used depending on goals. You could sit down and write 5 different build manuals for a 3" kit with 54mm mount and they'd all be "proper" depending on the rocketeers individual goals. It'd be cool to see that style manual done, but that's quite a bit of work.

I do recognize the value in following recommended directions and motors for a kit.... but we stray from that a bit ;)
My goal is to provide instructions that would be able to fly on the complete range of recommended motors and to give the easiest way to a safe successful flight. It is frustrating when a customer disregards those instructions and builds the equivalent of a flying boat and then tries to fly it on an H motor. I have customers that get excited about foaming fin cans, then screw up their whole aft end and have to order new parts. They get worried about non welded eye-bolts and add a half pound of U-bolts. If you impart enough force in flight to open up one of these eye-bolts, you've already destroyed your rocket.

If you have not seen a Binder Design instruction manual, they are 11 pages long and we are the only manufacturer that shows the modular fin can construction technique where you slide the whole fin/motor mount section out in order to access fillets. To explain how detailed they are, we even show how to tie a knot.

Obviously after a person has a bunch of experience under their belt, they can build a kit however they want. The OP was not about that, it was about learning. You can find all kinds of wacky ideas about how to build a rocket online. It's important to start with a proven base for success, IMO.
 

Steve Shannon

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has anyone ever said
" I built this rocket way too strong?"
More frequently I hear someone else say that someone built a rocket way too heavy.
Heavy and strong are not synonymous.


[emoji1010] Steve Shannon [emoji1010]
 

Steve Shannon

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Building strong is good, but it often means building heavier.

Heavier rockets require more impulse to achieve the same performance, so they need to be stronger to handle the thrust. Heavier rockets also are more likely to suffer damage on landing, so they need to be built stronger to survive landing. In the end the lighter built rocket will often perform better and survive landings just as well as the stronger, heavier rocket.

I have definitely done my share of overbuilding, especially high-power rockets. The last kit I built (a shortened Estes Argent) I used only wood glue, except for a very small amount of epoxy on the plastic transitions and (upgraded) shock cord attachment. The rocket flies great on a wide variety of motors and I am able to use a smaller (18") parachute.
+1



[emoji1010] Steve Shannon [emoji1010]
 

Buckeye

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My goal is to provide instructions that would be able to fly on the complete range of recommended motors and to give the easiest way to a safe successful flight. It is frustrating when a customer disregards those instructions and builds the equivalent of a flying boat and then tries to fly it on an H motor. I have customers that get excited about foaming fin cans, then screw up their whole aft end and have to order new parts. They get worried about non welded eye-bolts and add a half pound of U-bolts. If you impart enough force in flight to open up one of these eye-bolts, you've already destroyed your rocket.
plus 1
 

boatgeek

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My materials professor from 20 years ago used to say that engineering is the science of good enough and that engineers will be happy to tell you when something is safe but get really squirrely when you ask them when something will break. As discussed up above, everyone has different priorities for what is good enough. Competition people need it to be light and barely strong enough. On the other hand, I'm willing to lose a couple hundred feet of altitude to get a rocket that is pretty bulletproof. The altitude isn't that important to me, but being able to expect to fly it again today even if the chute fouls is.

Another problem in rocketry is that the calculations are actually quite challenging. We can easily say that 1/8" birch plywood fins are fine up to J/K impulse, but calculating what the aerodynamic loads on the fin are takes the better part of an engineering degree. Even if you base it off of the average drag numbers from OpenRocket, you aren't accounting for dynamic effects, fluttering, and the like. If you designed to the razor's edge based on average drag, the rocket would probably come apart. Calculating actual shock cord loads would likewise be really hard, since it's all dynamic and shock loading, which is some of the hardest engineering to do. On the other hand, 50:1 on the weight of either the rocket or the heaviest component seems to work pretty well in the field.
 

Oberon

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1) It is definitely possible to overbuild, and for overbuilding to be unsafe. As was mentioned, every time you add weight, you need to add more strength to carry the additional load - which usually means even more weight. It's a vicious cycle. Overbuilding costs you money (materials, plus you need to buy bigger motors and parachutes). All else equal, a heavy indestructible rocket is going to cause more damage when it hits the ground in the most common failure scenarios (lawn darts, tangled chutes, etc.)

2) You're only as strong as your weakest part. No sense getting a 1000lb rated forged eyebolt if it's going to be installed in a cardboard rocket on a lite ply bulkhead - any force large enough to open a standard eyebolt is pretty likely going to severely damage the other components. A lot of overbuilding seems to come from an attitude of "if a little is good, lots is better!" when in reality you could just be adding non functional weight. E.g. huge internal epoxy fillets when a modest amount of wood glue already creates a stronger bond than the fin itself. Even in composite bonding, there's basically a max useful bonding area - extra epoxy is just going to get squeezed out and/or be essentially non load bearing ballast.

3) Unless you're going high mach or using super high thrust motors, flight itself probably won't be the nastiest force on your rocket - either landing or deployment shock, both of which may be greater loads at off-axis angles, will probably be the worst and most damaging events. Again, overbuilding might be bad here, since more weight will put more loads on your recovery system at deployment.

4) As a result of 3, it might make sense to "overbuild" for flight loads in a few key areas, more for durability than flight safety. For example, if you fly a wet field, fiberglass might be desirable just to avoid a soggy rocket even if you don't need the strength. If you fly over a hard surface, more advanced fin reinforcement techniques might make sense to prevent landing damage. Or maybe you just tend to get a lot of "hangar rash" so you use stronger fins and more durable finishes.

5) Epoxy actually has a lot of good qualities you might like even if it's not the "best" adhesive for your application. It makes nice aesthetic fillets because it doesn't shrink. Quick-cure versions give you more working time than super glue but don't take nearly as long to set as wood glue. It doesn't bind up when sliding couplers or centering rings into a tube.

6) As others have said, build for what you want to do. Do you want a tank that will survive a hundred flights, even if you have a few mishaps? Build strong. Do you want a good performer? Build efficient. But whichever you do, build smart - if you want to deviate from the instructions, ask yourself "what am I adding by doing this?" Increasing durability for off-nominal cases, reinforcing for a bigger motor, or even ease of construction are all valid answers. "I want the fin can to survive the detonation of a small nuclear device, even if the fins themselves and everything north of the motor tube has been vaporized" is rather less valid.

Short version - I'm with Binder on this one. I've not had a chance to build a Binder to see his instructions, but I will say I've honestly never had a rocket fail during flight when built according to kit instructions due to lack of strength (and assuming you don't wildly exceed the motor recommendations). Even the much maligned Estes "tea bag" shock cord mount has served me well on LPR and PSII rockets (I've burned through shock cords with much use, but never ripped out a tea bag). All my rocket damage has occurred in off-nominal events (chute fails to open, CATO, delay was poorly timed, etc.)
 

KilroySmith

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They get worried about non welded eye-bolts and add a half pound of U-bolts. If you impart enough force in flight to open up one of these eye-bolts, you've already destroyed your rocket. ... You can find all kinds of wacky ideas about how to build a rocket online.
This is exactly where I was hoping the conversation would go. I know how to build a rocket that can double as a jack stand for changing a car tire, and that's a good skill to have - but I'm trying to understand how to go the other direction without causing undue risk. I'm not that interested in seeing how light I can build a rocket, but I am interested in determining how to right-size the components and techniques used for building a rocket. As a simple example, the 4" cardboard-and-plywood rocket kit that I last flew on a J270 has an Avbay that weighs over a pound, with 1/4" plywood endcaps on it. That seems vastly overweight and overdesigned for what it needs to be - and my curiosity wants to know why it's built the way it is, and if there's a more right-sized way to build it. How strong do the endcaps really need to be? Does it really need two 1/4" all-thread rods holding it together? Does it really need a 1/4" eyebolt on each end? Sure, I could dig out my college textbooks (although I'm a CS guy and so would probably do it wrong) and calculate the bending force in the middle of a 4" round endcap from a 15 psi ejection charge, look up the nominal strength of 5-ply 1/4" birch plywood, apply appropriate margins, and estimate the thickness that I really need - then do that for all common building materials - but as someone above pointed out, we're not all mechanical engineers and that's a lot of work. I was really hoping that someone could point me to a resource that, perhaps, had simplified equations or tables, or even real equations collected in one place to determine these things. I was hoping to avoid the "someone else's empirical data" approach, precisely because of the "wacky ideas" out there. Someone, somewhere, back in the mists of time managed to straighten out a Home Depot eyebolt, so now the accepted wisdom is "put in a forged steel eyebolt", forgetting the circumstances surrounding the event that may make the applicability narrower (for example, the event may have occurred on a heavy, high-speed rocket with premature chute ejection using a reinforced mil-surplus chute and a small eyebolt). Many of these bits and pieces of information are available in 20 years of various online forums and 60 years of various printed magazines, but finding uncovering those bits and pieces is difficult. I guess I'm not thrilled with the challenge of rediscovering limits that have been rediscovered over...and over...and over again. There are other aspects of the sport that I find more interesting... /frank p.s. And someday I'll figure out how to put in paragraph breaks.
 
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soopirV

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What is the groups feeling on motor adapters (not the aeropack ones, the cardboard ones that can be purchased from LOC or scratch built)? My first high power flight was a nasty surprise- I listened to a buddy of mine who made L2, but who ultimately didn't have a ton of knowledge, and used a 29mm CTI H399 in a Norad Pro Max with a 38-29mm cardboard adapter that he built (found out later he used CA) for my L1 cert. RSO didn't have any concerns, but the motor shot through the rocket and disappeared into the desert. I've since learned quite a bit, but am still nervous about cardboard adapters. I built one of my own using epoxy, but haven't flown it. Used a 54-38 Aeropack adapter and felt quite happy with it on a larger rocket, of course, but for the smaller holes, is cardboard still appropriate, or should I buy aluminum?
 

thomas

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The PML adapters are phenolic but something build similar with papertubes will be very strong.
 

rharshberger

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What is the groups feeling on motor adapters (not the aeropack ones, the cardboard ones that can be purchased from LOC or scratch built)? My first high power flight was a nasty surprise- I listened to a buddy of mine who made L2, but who ultimately didn't have a ton of knowledge, and used a 29mm CTI H399 in a Norad Pro Max with a 38-29mm cardboard adapter that he built (found out later he used CA) for my L1 cert. RSO didn't have any concerns, but the motor shot through the rocket and disappeared into the desert. I've since learned quite a bit, but am still nervous about cardboard adapters. I built one of my own using epoxy, but haven't flown it. Used a 54-38 Aeropack adapter and felt quite happy with it on a larger rocket, of course, but for the smaller holes, is cardboard still appropriate, or should I buy aluminum?
I have so far used cardboard and birch ply motor adapters for nearly every build so far, the trick is making a thrust shoulder so that none of the stresses are imparted on the motor tube itself. Somewhere I have
photos of a 38mm to 29mm adapter that is similar to the 54mm to 38mm in the last two pictures. I don't use many screw on retainers like Aeropacks so I have to make each adapter for each rocket. The first two pictures are of a 98mm to 75mm adapter and a 98mm to 54mm adapter both built identically, to carry the thrust forces I used a .125" aluminum thrust plate the diameter of the airframe, the tubes with CRs are simply held in place with masking tape. The adapters have flown so far with a M1297W and a L1000W and performed perfectly. The 54mm to 38mm is the one I use for my L2 rocket and is made of a .25" baltic birch ply thrust plate and some 38mm tubing with a 38mm to 54mm CR on the end. The thrust shoulder of the motors are fully supported on the thrust plates.






L3Build109.jpgL3build85.jpgLastDayofBuild2.jpgL2projectDay12-2.jpg
 
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soopirV

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I have so far used cardboard and birch ply motor adapters for nearly every build so far, the trick is making a thrust shoulder so that none of the stresses are imparted on the motor tube itself. Somewhere I have
photos of a 38mm to 29mm adapter that is similar to the 54mm to 38mm in the last two pictures. I don't use many screw on retainers like Aeropacks so I have to make each adapter for each rocket. The first two pictures are of a 98mm to 75mm adapter and a 98mm to 54mm adapter both built identically, to carry the thrust forces I used a .125" aluminum thrust plate the diameter of the airframe, the tubes with CRs are simply held in place with masking tape. The adapters have flown so far with a M1297W and a L1000W and performed perfectly. The 54mm to 38mm is the one I use for my L2 rocket and is made of a .25" baltic birch ply thrust plate and some 38mm tubing with a 38mm to 54mm CR on the end. The thrust shoulder of the motors are fully supported on the thrust plates.






View attachment 296424View attachment 296425View attachment 296427View attachment 296428
Those beefy thrust plates are gorgeous- the paper ring on mine was definitely the weak spot.
 

KilroySmith

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Well, in keeping with the thread, let's look. A CTI H399 has about 555 N of peak thrust; that corresponds to about 125 pounds (or about 56 Kg in earth's gravity) of force on the motor mount. That's a significant amount. Let's assume that you wrapped paper to create the thrust ring on your adapter. Let's also assume that the thrust ring failed due to the shear force causing two layers of paper (or a layer of paper and the inner motor tube) to separate, ripping the inner motor tube out of the thrust ring. The circumference of the adapter will be about 2*3.14*(29 / 2) = 91 mm. Let's assume the thrust ring is about 5 mm wide, giving us about 455 mm^2 of area gluing one wrap of the thrust ring to the next. As long as the paper-to-paper bond can sustain a (125/455) = 0.27 pound/mm^2 (or 175 psi) shear force, it'll work. According to Titebond (http://www.titebond.com/Libraries/LiteraturePDFs/FF683_GlueGuideTB.sflb.ashx), Titebond II has a shear strength around 3750 psi. Cyanoacrylate appears to be about 2100 psi when bonding oak to oak. Both are a goodly amount above the 175 we need. It's possible the paper itself could fail in shear; unfortunately I can't find a source for paper shear strength, but let's assume that it's strong enough. Why would your thrust ring fail? My guess is workmanship. In order for the thrust ring to be "strong enough", every layer of paper must be fully bonded to the layer above, and the layer below. If one layer doesn't have sufficient bonding to the next layer, the strength of this one layer sets the maximum thrust that the thrust ring can handle. My guess is that your friend had one or more "dry" layers where little or no adhesive was holding two layers of paper together, the layers slipped, the unglued paper ripped, and the thrust ring failed. For a commercially made adapter, where automated machinery applies the adhesive and winds the paper, I'd have no concern about strength at this power level. /frank
 
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