Best place for shock cord mount on L3 bird: centering ring or motor case?

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jcsquire

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I'm looking for advice on where to put the shock cord mount on the lower airframe tube of my L3 certification bird - plain-vanilla design with a total loaded weight of just under 50lbs, 8" diameter G12 fiberglass and slightly over 12' tall. Two options:

1) Do what I did for my Level 2 rocket and place a couple of 3/8" U rings on either side of the forward-most centering ring of the motor mount.

2) I'm planning to use a CTI 98mm motor, and the forward end of the casing is tapped to accept a 3/8"-16 bolt 1" deep...I can get a nice forged bolt of the right dimensions from McMaster Carr.

By my calculations, both options would hold a shade over 1,000 dead pull, likely more than that in a dynamic tug. The single forged ring looks simpler and would avoid a few stress points in the drilled hole locations of the forward centering ring (although with generous epoxy I doubt that makes much diff), but...I don't see people doing it. Most of the builds I see on the forum use double U mounts.

Any thoughts?
 
Both have the tether connected to motor and U bolts keep everything together
 
g zilla said:
Both have the tether connected to motor and U bolts keep everything together
Click to expand...

Gary, I'm unclear about your comment. Both ways do involve the tether connected to the lower airframe but one way it is connected to the centering rings using U bolts and the other way it is connected to the forward end of the motor casing using a forged eye bolt. Are you saying it makes no diff? That's what I'm inclined to believe too.
 
I would say that mounting to the motor is the safest bet, and you say that both options could hold 1000 lb but can the centering ring itself hold 1000 lb?
 
For a 50lb rocket you might consider a zipperless design where the forward retention for the motor goes to the rear bulkhead of the coupler and the shock cord is attached to the front bulkhead of the coupler.
 
I glue 1in Kevlar strap [4500lb] to motor mount tube for back-up recovery point.

Load is now off CR and on a substantial anchor point.

Strap is just long enough to protrude from fin can a few inches. TN is attached there.

Just notch top CR enough to fit over strap and encapsulate Kevlar to tube.

Has proven simple, cheap & failsafe for many, years on dozens of large rockets.
Drop harness down empty tube....attach to eyebolt on motor install motor and go. Both motor and recovery gear attached to harness.

I am a TAP & recommend this technique. Both motor & fin can as back-up support recovery gear. AND sometimes ya forget your eyebolt....this way ya still fly.
There are others that work quite well, but this one employs K.I.S.S!

100_2323_2.jpg

100_2326.jpg

100_2389_2.jpg
 
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+1 on the 1" Kevlar strap ... doing the same for my L3 build.
 
3stoogesrocketry said:
Who are your TAP members and what have they suggested ? Eric
Click to expand...
Good point. My L3CC member (I'm NAR and Tripoli, and decided to certify NAR and use Tripoli comity) is Jerry O'Sullivan. Great guy and very experienced/knowledgeable, but he's the only L3CC member in my state and I'm reluctant to make him my first line of defense for every little question about my L3 build. Instead, my plan is to ask my many little questions like this to the rocketry community at large, digest all the (hopefully different!) ways of looking at the problem, and make an informed decision myself. Then once I have a fully fleshed out concept, write it up and let Jerry comment on it before I begin to build. If there are any points that I'm trouble making up my mind about after getting community input though, I have no problem asking Jerry for specific advice in advance...I just don't want that to be my first approach for all my questions! Besides, I get more different ideas from the forum, and I've been really impressed as a newbie at how good the advice is here. The signal-to-noise ratio is way higher than I'm used to seeing in a community forum.
 
When I get to my L3 build I Plan to use both motor and motor tube retention. If I go with the new single use motors then I will go with one motor tube and one u bolt on the centering ring for retention.
 
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Zebedee said:
For a 50lb rocket you might consider a zipperless design where the forward retention for the motor goes to the rear bulkhead of the coupler and the shock cord is attached to the front bulkhead of the coupler.
Click to expand...
If I understand this correctly, then I'd place the drogue inside the coupler, instead of inside the whole lower airframe? That would work if it would fit, and would certainly make it much more zipper-resistant. My plan was to use the inside of the coupler as my avionics bay, but I could just design with a longer coupler to make room for the electronics pretty easily....let me think about that. I like the way that approach avoids vectoring an off-axis line of shock cord tension through the lower airframe.
 
If you have enough room you can still use the lower section for the droge. I plan to do it that way with a kevlar blanket.
 
blackjack2564 said:
I glue 1in Kevlar strap [4500lb] to motor mount tube for back-up recovery point. Load is now off CR and on a substantial anchor point. Strap is just long enough to protrude from fin can a few inches. TN is attached there. Just notch top CR enough to fit over strap and encapsulate Kevlar to tube.

Drop harness down empty tube....attach to eyebolt on motor install motor and go. Both motor and recovery gear attached to harness.
Click to expand...

Very cool idea Jim; thanks! And thanks for uploading the photos. I see how this is much stronger than a U-bolt through a CR approach (which wouldn't work anyway for lack of space in a near-minimum diameter build like in your photos). I'm still a bit confused about two things:

1) When you say "TN is attached there." What's a TN? I'm still very new to this hobby!

2) You say "Drop harness down empty tube, attach to eyebolt on motor and go. Both motor and recovery gear attached to harness". I want to make sure I understand those sentences. The harness is what I've called (from my days flying model rockets as a kid) a "shock cord", one end of which is attached to the drogue and ultimately the airframe coupler, and the other end is attached to the eyebolt in the motor mount AND the 1" Kevlar strap you've installed on the motor tube? That is, it is knotted around the eyebolt in the forward end of the motor casing and then ultimately knotted to your Kevlar harness epoxied along the motor mount?
 
Jcsquire , you have the right idea as to what Jim is saying . If I use this method , I will make the shock cord ( TN , tubular nylon ) pull tight againt my primary anchor point before it pulls on my motor case . I use this approch so if my motor retention fails , the motor case can only fall out the back of the rocket a few feet .

Eric
 
jcsquire said:
Very cool idea Jim; thanks! And thanks for uploading the photos. I see how this is much stronger than a U-bolt through a CR approach (which wouldn't work anyway for lack of space in a near-minimum diameter build like in your photos). I'm still a bit confused about two things:

1) When you say "TN is attached there." What's a TN? I'm still very new to this hobby!

2) You say "Drop harness down empty tube, attach to eyebolt on motor and go. Both motor and recovery gear attached to harness". I want to make sure I understand those sentences. The harness is what I've called (from my days flying model rockets as a kid) a "shock cord", one end of which is attached to the drogue and ultimately the airframe coupler, and the other end is attached to the eyebolt in the motor mount AND the 1" Kevlar strap you've installed on the motor tube? That is, it is knotted around the eyebolt in the forward end of the motor casing and then ultimately knotted to your Kevlar harness epoxied along the motor mount?
Click to expand...



First the picture was a 75mm mount in a 4in rocket. I don't have pics handy on my larger builds.

It's done same way for 6in...& up.

1. Sorry should have been more clear TN= tubular nylon. I use 1inch on rockets over 4in diameter. Many these days go with Kevlar all the way. I use the Kevlar strap for my Y-harness on the mm tube, Tie an overhand knot on the end , then attach my tubular nylon shock cord to that. I like nylon, it will stretch a bit and help "forgive" the shock load of deployment. Kevlar will not.

2. your complicating it a bit. I just have a quick link on the eyebolt attached to my motor closure. I drop the Y-harness back through the motor mount---out the rear of rocket----then simply attach the quick link to harness and install harness & motor into rocket.
Just another back up so I not only have a solid recovery point, but can't lose the motor if motor retention fails.[as unlikely as it may be]


3. I also have some large 98 motors which have their own 3 ft shock cord attached to the closure eyebolt. The other end has a quick link tied to it. I just install motor and reach into fin can and attach recovery gear [y-harness to that] Larger rockets where you can reach inside , I find are easier to do this way.

Smaller or those with long fin cans making reaching inside difficult I drop the Y-harness back as described in 2.

Hope this clarifies things a bit.:smile:


Edit: looks the Eric answered while I was typing....LOL
 
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jcsquire said:
If I understand this correctly, then I'd place the drogue inside the coupler, instead of inside the whole lower airframe? That would work if it would fit, and would certainly make it much more zipper-resistant. My plan was to use the inside of the coupler as my avionics bay, but I could just design with a longer coupler to make room for the electronics pretty easily....let me think about that. I like the way that approach avoids vectoring an off-axis line of shock cord tension through the lower airframe.
Click to expand...

There are (as with most things) many ways to do it. You can make the upper air-frame longer and have the AV bay in the middle of it instead of in the coupler. Or you can have the electronics in the nose cone and just have a simple bulkhead in the middle of the upper airframe with the drogue charge on one side, main charge on the other and break-away wires to the drogue charge. Another way is to go drogue-less and use a cable cutter on the main for dual deploy but that's likely a whole other discussion. :)
 
I doubt you will be able to stay under 50 lbs.
My 6" x 9 ft fiberglass rocket hit the scales at 52 lbs loaded.

It will come in closer to 70 to 80 lbs with everything.

I used 1/4" U-bolts for attachment points in the booster but, also ran a harness to the top of the motor through the 3/8" eyebolt for added insurance.
The payload section had 5/16" U-bolt attachements.

JD

jcsquire said:
I'm looking for advice on where to put the shock cord mount on the lower airframe tube of my L3 certification bird - plain-vanilla design with a total loaded weight of just under 50lbs, 8" diameter G12 fiberglass and slightly over 12' tall. Two options:

1) Do what I did for my Level 2 rocket and place a couple of 3/8" U rings on either side of the forward-most centering ring of the motor mount.

2) I'm planning to use a CTI 98mm motor, and the forward end of the casing is tapped to accept a 3/8"-16 bolt 1" deep...I can get a nice forged bolt of the right dimensions from McMaster Carr.

By my calculations, both options would hold a shade over 1,000 dead pull, likely more than that in a dynamic tug. The single forged ring looks simpler and would avoid a few stress points in the drilled hole locations of the forward centering ring (although with generous epoxy I doubt that makes much diff), but...I don't see people doing it. Most of the builds I see on the forum use double U mounts.

Any thoughts?
Click to expand...
 
IHMO, this is a question for a L1 or maybe a L2 cert, but not a L3. If you are planning a L3, IMHO, you'd have the experience and knowledge to know the answer to this on your own, and be able to explain that choice to your L3CC or Tap member. If you don't, it's time to get more experience at L1/L2.

I'm a hard azz about stuff like this. When I got my L3, understand that no one on the L3cc had gotten an L3 yet (this was when the L3cc was first formed). You had to be able to make your own design decisions, and be able to back them up when questioned. L3 is the "PhD" of certs, after all. It's not a badge, but a journey.
 
Just as a FYI - the eyebolt to the motor case has a potential failure mode:

Long descents under drouge can cause the fincan to "twirl" quite a bit. If the eyebolt is not firmly held in place, for example with stopper nuts in opposition to the motor closure, the eyebolt can unscrew and return your fincan to freefall.

I know some folks have drilled the motor closure (and the eyebolt) to allow placing a pin to prevent this.


All the best, james
 
RecoRocket said:
IHMO, this is a question for a L1 or maybe a L2 cert, but not a L3. If you are planning a L3, IMHO, you'd have the experience and knowledge to know the answer to this on your own, and be able to explain that choice to your L3CC or Tap member. If you don't, it's time to get more experience at L1/L2.

I'm a hard azz about stuff like this. When I got my L3, understand that no one on the L3cc had gotten an L3 yet (this was when the L3cc was first formed). You had to be able to make your own design decisions, and be able to back them up when questioned. L3 is the "PhD" of certs, after all. It's not a badge, but a journey.
Click to expand...

RecoRocket, I have to agree with almost everyting that you've said! No need to apologize for being a "hard azz", and you're probably speaking for several viewers. You see someone trying to quickly go through the stages and unaware of what TN stands for, and you have a right to say "slow down." And there are those out there to prove something to themselves by trying to snatch up certifications. And it's not obvious I'm not among them from the questions I've asked. I'm not one of the badge-collectors, but I am new. My story is that in one of my businesses I build demos for large documentary movies. I've had to turn down two jobs in the past five years because I did not have an L3, and did not want to go the amateur route. Last job was taken by a guy with less experience than I and ended up in the hospital (and 20k of damage to his rental vehicle, another story). So my goal is indeed to go through all the licensing steps relatively quickly so I can take the next job when it inevitably rolls around.

But I'm not taking the path of least resistance, and I'll be more than able to defend every part of my design. If you're an NAR member, you'll read an article I just wrote in the next issue describing a free software package I wrote that lets Rocksim talk to Google so you can directly do Monte-Carlo simulations of hundreds of flights from the regular Rocksim with direct Google topographic plotting of the results. (Click here if interested; it does a lot of other iterative analyses also, like quickly finding the optimal launch rod angle given the current environmental and wind conditions to minimize range). I'm not building a kit L3; it's my own design, with a custom printed circuit board I also designed that houses all the primary and redundant electronics, already field tested on my L1 and L2 birds, and I have an amateur extra FCC license for my high-power GPS telemetry. I have an ATF explosives license so there's no question about the use of BP ejection charges, and my professional engineering license for insurance purposes. I'll be running the model through an 18k industrial-strength aeronautics simulator before flying. Although I may have some surprising holes in my knowledge, like TN, the part that matters, the rocket, will be designed very thoroughly, which I admit is not at all evident by the question I asked.

The only thing you've said that I disagree with is that I should have the knowledge to answer the question on my own. Many people have voiced different opinions on the subject, indicating there is no consensus on the topic...which is what makes building things fun! You have a lot of experience in the field, being L3 certified for a long time; what are your thoughts about my question - do you use a ring on the casing, an epoxied strap to the MM, a U on the CR, or some combination of these as a primary or backup? :)
 
Fdog said:
Just as a FYI - the eyebolt to the motor case has a potential failure mode:

Long descents under drouge can cause the fincan to "twirl" quite a bit. If the eyebolt is not firmly held in place, for example with stopper nuts in opposition to the motor closure, the eyebolt can unscrew and return your fincan to freefall.

I know some folks have drilled the motor closure (and the eyebolt) to allow placing a pin to prevent this.


All the best, james
Click to expand...

Brilliant. I was actually wondering about this...makes a lot of sense.
 
jcsquire said:
Brilliant. I was actually wondering about this...makes a lot of sense.
Click to expand...

Using a lock washer on the eyebolt would do the same thing, and avoid modifying the motor case(which can disqualify you during a cert flight).
 
jcsquire said:
RecoRocket, I have to agree with almost everyting that you've said! No need to apologize for being a "hard azz", and you're probably speaking for several viewers. You see someone trying to quickly go through the stages and unaware of what TN stands for, and you have a right to say "slow down." And there are those out there to prove something to themselves by trying to snatch up certifications. And it's not obvious I'm not among them from the questions I've asked. I'm not one of the badge-collectors, but I am new. My story is that in one of my businesses I build demos for large documentary movies. I've had to turn down two jobs in the past five years because I did not have an L3, and did not want to go the amateur route. Last job was taken by a guy with less experience than I and ended up in the hospital (and 20k of damage to his rental vehicle, another story). So my goal is indeed to go through all the licensing steps relatively quickly so I can take the next job when it inevitably rolls around.

But I'm not taking the path of least resistance, and I'll be more than able to defend every part of my design. If you're an NAR member, you'll read an article I just wrote in the next issue describing a free software package I wrote that lets Rocksim talk to Google so you can directly do Monte-Carlo simulations of hundreds of flights from the regular Rocksim with direct Google topographic plotting of the results. (Click here if interested; it does a lot of other iterative analyses also, like quickly finding the optimal launch rod angle given the current environmental and wind conditions to minimize range). I'm not building a kit L3; it's my own design, with a custom printed circuit board I also designed that houses all the primary and redundant electronics, already field tested on my L1 and L2 birds, and I have an amateur extra FCC license for my high-power GPS telemetry. I have an ATF explosives license so there's no question about the use of BP ejection charges, and my professional engineering license for insurance purposes. I'll be running the model through an 18k industrial-strength aeronautics simulator before flying. Although I may have some surprising holes in my knowledge, like TN, the part that matters, the rocket, will be designed very thoroughly, which I admit is not at all evident by the question I asked.

The only thing you've said that I disagree with is that I should have the knowledge to answer the question on my own. Many people have voiced different opinions on the subject, indicating there is no consensus on the topic...which is what makes building things fun! You have a lot of experience in the field, being L3 certified for a long time; what are your thoughts about my question - do you use a ring on the casing, an epoxied strap to the MM, a U on the CR, or some combination of these as a primary or backup? :)
Click to expand...

If you don't have the knowledge to answer the question on your own, you need to do more L1-L2 flights UNTIL you know the answer. Like I said, this is the PhD of certs. No cheat sheets allowed - You know or you don't, and for L3, you shouldn't need to ask. I've been a LONG proponent of not only a minimum time between certs, but a minimum number of flights at your current level.

BTW, I used Ubolts in the top CR in my L3 stuff so far, but using a bolt in the forward closure would also work. The choice can also depend on what works best on the design of the rocket itself. For example, if you use ubolts, can you attach the teather when prepping? A 8' long 8" BT, it might be hard to do, while you could drop the tether down the MMT and attach it to the motor and secure the motor that way.

The best way to learn could be to by some l1/l2 kits, and maybe scan the instructions, but then toss them. Then, build them the way you think is best, fly them, and see what works.... You don't like nylon SCs? use Kevlar, and find the way it's best in your view to attach it. If you stick with nylon, can it be replaced if it get's too scorched? You'll get this kind of knowledge with more experience.
 
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@RecoRocket: Makes a lot of sense regarding the difficulty in attaching a tether down the lower airframe. Agreed about the lock washer too; I already have a fitted bowl spring lockwasher specifically for that rear closure. Good idea with the HPR kit instructions; I did that with several Mad Cow rockets after reading Canepa's book and the combination of the two was really helpful. I'm glad you weren't on my dissertation committee; I asked a ton of questions while I earned my Ph.D., and for cheat sheets had stacks of lab notebooks. Like you say, it is journey. I keep learning lots from many different disciplines, including from this generous community (even/especially from it's hard-azzes, who are kind enough to share their experience with me even if they take issue with a newly-certified L2 starting their journey into L3 territory :D)
 
Last edited:
Thanks JD. Must be convergent evolution; I separately chose 5/16" U bolts in my avionics bay area; the 1/4"'s just looked too slender (and had a yield strength that pushes the dynamic loading of a hard ejection if the entire thing weighs 80lbs). Thanks for the heads up on the weight. All my airframe parts (meaning BT, nose cone, MM, CR) come to 35lb, so I SWAG'ged the 50lb overall thinking nearly all else would be motor; the hardware and paint would be minimal, as it was in my L2. Bad assumption I see! And there will probably be a ton of epoxy too. I'll see if I can break out these different weights by proportion (paint, epoxy, hardware, etc.) and post when done to help others accurately estimate.
 

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