Mounting recovery harness in 4" 75mm mount fiberglass rocket fin can?

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How would you attach the recovery harness anchor point in a 4" rocket with a 75mm motor mount?

  • Wildman type Kevlar strap to one side of motor mount

    Votes: 1 1.9%
  • Wildman type Kevlar strap to BOTH sides of the motor mount

    Votes: 19 36.5%
  • OBH type mini eye bolts to both sides of the motor mount, Kevlar Y harness

    Votes: 8 15.4%
  • Eye bolt to top of the motor closure

    Votes: 22 42.3%
  • Other (please explain)

    Votes: 2 3.8%

  • Total voters
    52

Banzai88

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I've found the rocket kit that I wanted for my 4"/75mm project. Scored a previous generation fiberglass Madcow DX3XL for a crazy good deal! 4" diameter, 75mm mount, 92" tall!

Since this is the first really heavy rocket that I've done, figured I'd ask here about how you build your rockets of this size. I'm at an impasse on how to mount the fin can recovery harness, and what material to use.

How did/would YOU do it?
 
Depends on what you fly most of the time...if you are only going to fly 75s, then go eyebolt. If you want to also fly CTI 54s, go harness (but don't epoxy anything to the motor tube, you're asking for repair, replacement, and overall strength issues).

I have this exact airframe (love it) and have a 2-point MMT harness I use when I adapter down to 54 for small field flights and then I go eyebolt for my big 75mm M flights.
 
Also recommend the Wildman style epoxying of the Kevlar loop to two sides of the motor mount with slots in the upper CR. I like to run a large swivel in the loop and knot it so the middle of the swivel comes to the edge of the booster tube.

I have no issues with using eye-bolts and u-bolts in the upper ring, but this config (75mm MMT in a 98mm frame) leaves you with barely a 1/2" between the MMT and the airframe. It is difficult to mount an eye bolt in that narrow space very well. Epoxying good strong kevlar to the MMT has not failed me yet.

There is an argument that epoxying Kevlar is bad because creates a sharp shear point for the Kevlar. This is probably true, but going overboard on the Kevlar is easy and not expensive. You can find 5000+ pound rated tubular Kevlar for less than $5 a yard, so no need to skimp and that heavy duty Kevlar won't shear in that size of rocket. It will also last a lifetime.

https://giantleaprocketry.com/collections/fins-1/products/kevlar®-tubular-shock-cord
https://www.amazon.com/gp/product/B077VQFJZJ/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
Also, if you haven't gotten shears to work with Kevlar, save your sanity - get a pair of shears designed to cut Kevlar. These look chintzy, but they cleanly cut Kevlar like butter:

https://www.amazon.com/gp/product/B01MY6WPT2/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1
 
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I’d recommend a U bolt to the forward CR and loop harness your kevlar shock cord (if it already has the stitched loop even better) to the U bolt and your all set. No fuss no muss.
 
The only shock cord I’ve ever broken was a Kevlar one epoxied in like that. Much heavier than needed, but the epoxy edge creates a very high stress point. It wasn’t even an abnormal ejection.
I’ll always include some kind of hardware that allows me to replace the shock cord.
But for 75 mm motors I just bolt the shock cord to the forward closure.
 
I’d recommend a U bolt to the forward CR and loop harness your kevlar shock cord (if it already has the stitched loop even better) to the U bolt and your all set. No fuss no muss.

I am a big U-bolt fan, when there is enough room for one. In this case, Tom is using a 75mm MMT in a 98mm frame - that is darn near minimum diameter and I am not aware of a U-bolt that would fit in that very small space between the the MMT and the airframe (without going over the open end of the MMT).

I have tried eye-bolts in this rocket config, but ended up with numerous problems. First, it is difficult to even mount the eye-bolt in the CR effectively. Assuming the minimum size eye-bolt you would try to use on this rocket would be some kind of forged/welded 1/4"-20 eyebolt, you are going to be left with barely 1/8" of CR on either side of the eye bolt shaft. No room for washers and barely enough room to even get a hex nut on the threads. This doesn't give a good base for the bolt. Probably the best bet would be to epoxy the nut on the back then just pour a bunch of epoxy forward of the CR to create an epoxy dam. Then, the eye-bolt is going to basically be flush against the side of the airframe (it is going to be over the end of the MMT if you try to mount the eye perpendicular to the airframe). I made this mistake once and didn't think about the fact that there was no way to get a quick link through the eye bolt. So, it you do go down this path, make sure you secure that Kevlar to the eye-bolt before you install the MMT into the airframe (also ensuring the knot doesn't get in the way of a long motor).

Steve's suggestion above to use the forward closure as the attach point may be the most elegant solution. The only downside to that method is it pretty much rules out DMS motors as an option, but that may not be an issue for you.

I would still prefer to at least install the "epoxy the kevlar to the sides of the MMT" in this rocket config. You can always use the forward closure anyway (or as a back up), but you can't go back later and epoxy the kevlar to the MMT.
 
Attached is a PDF drawing that shows how the centering ring math works out when using u-bolts with a 4" fiberglass body
tube and a 75mm motor tube. I put this sketch together several month ago for a future build. I wanted to see how tight
things would get. Another challenge are nuts and washers.

Another drawing shows the AeroTech motor, 6400 case, inside the LOC 34" motor tube. The next longer motor case would
take you right up to the point where the bolts start arching over and would hit the case.

I have not built the rocket yet. I plan to see how some "test" rings turn out once I cut them and drill the holes for the u-bolts.
Drawings only show things in a perfect world.
 

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Another good shock cord mount is to make a hardpoint by glueing a short coupler and a bulkplate into the tube, with a u-bolt and quicklink connected to a tether. The advantage is that it's virtually indestructible, the disadvantage is that the motor size is limited by where you glue in the bulkplate. I did this with my L3 Drago XL, and a 3" RW G3... I had an L1000 forward closure fail explosively in the G3, shearing off the Aeropack retainer cap, and it did absolutely no damage to the rocket due to the hard point.
 
Attached is a PDF drawing that shows how the centering ring math works out when using u-bolts with a 4" fiberglass body
tube and a 75mm motor tube. I put this sketch together several month ago for a future build. I wanted to see how tight
things would get. Another challenge are nuts and washers.

Another drawing shows the AeroTech motor, 6400 case, inside the LOC 34" motor tube. The next longer motor case would
take you right up to the point where the bolts start arching over and would hit the case.

I have not built the rocket yet. I plan to see how some "test" rings turn out once I cut them and drill the holes for the u-bolts.
Drawings only show things in a perfect world.



If you are willing to limit the size of case you can use, then this discussion becomes much easier. You can devise a bunch of different ways to get the u-bolt securely fixed in that scenario.

If you want to leave your options open for cases like a CTI 6GXL, you need to figure something else out.

If you don't mind not being able to use DMS motors, the forward closure is an option.

As with most engineering problems, you have to compromise somewhere. None of these options are wrong or right, they are just compromises. I currently have 6 flyable 98mm rockets with 75mm MMT mounts (I have made more over the years, but some of the other ones aren't serviceable anymore). I have tried all the shock cord mounting techniques listed in this thread and they have all worked. The only ones I kind of regretted later were the ones where I compromised on motor length. Somewhere down the road, I always ended up wishing I could fly a bigger motor than I had engineered a solution for.
 
I used an eyebolt in the forward closure of the motor for my L3 cert on Friday. I used threadlocker and a jam nut to ensure it wouldn't come loose.

I originally planned on using a Wildman style Kevlar harness epoxied to the motor mount, but decided not to, mostly because I was lazy. The forward closure attachment is strong and simple, which appeals to me.
 
If you are willing to limit the size of case you can use, then this discussion becomes much easier. You can devise a bunch of different ways to get the u-bolt securely fixed in that scenario.

If you want to leave your options open for cases like a CTI 6GXL, you need to figure something else out.

If you don't mind not being able to use DMS motors, the forward closure is an option.

As with most engineering problems, you have to compromise somewhere. None of these options are wrong or right, they are just compromises. I currently have 6 flyable 98mm rockets with 75mm MMT mounts (I have made more over the years, but some of the other ones aren't serviceable anymore). I have tried all the shock cord mounting techniques listed in this thread and they have all worked. The only ones I kind of regretted later were the ones where I compromised on motor length. Somewhere down the road, I always ended up wishing I could fly a bigger motor than I had engineered a solution for.

As with most engineering "challenges", the compromise usually starts with the "budgetary constraints".
 
I agree with Neutron95 above.

I also have a DX3 XL. Just flew it last weekend at MWP on an Aerotech L2200.

Get yourself a threaded forward closure and put an eyebolt in it. Be sure to use a regular nut and a jam nut and some locktite to secure the eye bolt in place. It will add some cost but you'll be happy. No worries about burning/breaking a shock cord you will not be able to easily replace. No holes in the very narrow centering ring. See the threaded insert on the front of the closure in the picture below. I love mine!
 

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I agree with Neutron95[QUOTE="gary7, post: 2341900, member: 1975"]
I agree with [USER=2688]Neutron95
above.

I also have a DX3 XL. Just flew it last weekend at MWP on an Aerotech L2200.

Get yourself a threaded forward closure and put an eyebolt in it. Be sure to use a regular nut and a jam nut and some locktite to secure the eye bolt in place. It will add some cost but you'll be happy. No worries about burning/breaking a shock cord you will not be able to easily replace. No holes in the very narrow centering ring. See the threaded insert on the front of the closure in the picture below. I love mine!


I also have a DX3 XL. Just flew it last weekend at MWP on an Aerotech L2200.

Get yourself a threaded forward closure and put an eyebolt in it. Be sure to use a regular nut and a jam nut and some locktite to secure the eye bolt in place. It will add some cost but you'll be happy. No worries about burning/breaking a shock cord you will not be able to easily replace. No holes in the very narrow centering ring. See the threaded insert on the front of the closure in the picture below. I love mine!
[/QUOTE]


Oh boy do I want a gold forward closure for my 98/10240!!!
 

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My 4" Astrobee D has a 75mm mount in it, and I used a Y Harness with two eyebolts that mount to the upper centering ring. It was custom made for me by @Onebadhawk. Cant recommend this method enough.

IMG_7732.jpgIMG_7733.jpg

I had to do a little bit of work with a file to get it to fit properly, but once it was installed with epoxy, it isnt going anywhere.

IMG_7728.jpg
 
My 4" Astrobee D has a 75mm mount in it, and I used a Y Harness with two eyebolts that mount to the upper centering ring. It was custom made for me by @Onebadhawk. Cant recommend this method enough.



I had to do a little bit of work with a file to get it to fit properly, but once it was installed with epoxy, it isnt going anywhere.

Good pictures! Clearly show the tight quarters one is working with in a 4" body tube and 75mm motor setup.

Here are pictures of the proposed "Y" harness and u-bolt setup for a new build. I opted to go with a choker
arrangement for harness attachment to the u-bolts. I have used this with other rockets. Allows me to attach
harnesses in tight spaces - and not use a Quick link.

Y-Harness-JG-Rocket-01.JPG Y-Harness-JG-Rocket-02.JPG
 
Aren't those u-bolts going to block a long motor?

The longest 75mm case I have is AeroTech's 6400. But the 7680 case will also fit. That case comes within
a 1/4" of where the u-bolts start to "arch over" the motor tube. The legs of the u-bolt are outside the
case's footprint (cross section) until then.

U-bolt-7680Case.jpg
 
Another good shock cord mount is to make a hardpoint by glueing a short coupler and a bulkplate into the tube, with a u-bolt and quicklink connected to a tether. The advantage is that it's virtually indestructible, the disadvantage is that the motor size is limited by where you glue in the bulkplate. I did this with my L3 Drago XL, and a 3" RW G3... I had an L1000 forward closure fail explosively in the G3, shearing off the Aeropack retainer cap, and it did absolutely no damage to the rocket due to the hard point.
I dont bond in the coupler, i use floating nutplates and scew it in. That way it is removable. smaller u-bolt off to one side so the nuts are on the side of the motor forward closure buys some space.
 
Those are some great pics Kris.
And thank you.
Here are some pics of the neatest, cleanest, most professional meant to be looking attachment point and mounting it ever devised..
When I came up with this years ago I did it because I couldn't find anything like it made..

IMG_0041.jpeg Onebadhawk Build 003.JPG

As far as a Wildman style glue in.
They simply work.
They're easy to do, inexpensive, and have an impossible to argue with track record..
If you're concerned about the Kevlar fatigue cracking by flexing back and forth where the epoxy ends,
then end the epoxy under the centering ring the kevlar passes through..
I don't myself sell raw Kevlar, I only sell finished product. I do this as a show of respect for all of the other vendors that do..
I don't have a dog in the "Here's where you can buy raw Kevlar from Amazon" race..

Most certainly, anyone proposing we purchase anything from Amazon that we can easily buy on the field from multiple rocketry vendors that drive great distances to launches to support us on the fields where we fly and have everything we need
to build and fly rockets right at the launch fields

is well,

those that know me personally completely understand why I'd prefer you complete that sentence yourselves..

I don't think anyone will argue that brick and mortar in all its forms is going by the wayside..
I just know that I enjoy being able to get the thing I need to fly on the field.
I know if that's too continue, I must buy whatever I need at the field,
not just what comes up during the day or is an unforeseen necessity for the day..

Teddy
 
Semi- related comment. Why do you guys make your motor tubes so long? I usually just make the motor tube the length of the fins plus an inch or two.
 
Semi- related comment. Why do you guys make your motor tubes so long? I usually just make the motor tube the length of the fins plus an inch or two.
One of the reasons to do this is to have a smaller volume (and thus a smaller BP charge) to pressurize for apogee separation. I designed this in intentionally into my L2 project so that the drogue and main charges were the same volume, and used the same backup charge volume as well. That way, it semi-mistake proofed dual redundant dual deploy computer charges.

Also, putting the ring up higher acts as a shelf keeping the recovery bundle from shifting aft/down under thrust and getting rid of that 'wiggle' off the rail that sometimes happen.
 
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After some discussion with Ted at OBH, this is what he makes as 'standard recommendations'. I'll be getting my act figured out here soon and placing an order!

Ted writes:
"
Small == 3” airframe..
1/4” Tubular Kevlar — 3/16” Polished Stainless Swivel — 3/16” Welded Polished Stainless Eyes…

Medium == 4” airframe..
3/8” Tubular Kevlar — 1/4” Polished Stainless Swivel — 1/4” Welded Polished Stainless Eyes…

Large == 6” airframe..
7/16” Tubular Kevlar — 5/16” Polished Stainless Swivel — 5/16” Welded Polished Stainless Eyes…

Extra Large == Minimum 8” airframe..
3/4” Tubular Kevlar — 3/8” Polished Stainless Swivel — 3/8” Welded Polished Stainless Eyes…

I can make larger then this Extra Large.."
 
Tom, no kiddin man,
thank you so much for always lookin out..
There are so many that stand by what's right..

I typed this to Tom first..


I haven’t posted anything like this before because the more I say about my technics and methods the more gets copied..
I am sorry for not elaborating more in the past, but this kind of thing wears thin after a while..

These are my loss leader attachment points that I make with harness sets..
They really help with harness sales..

Small, medium, large, Extra Large..

Small == 3” airframe..
1/4” Tubular Kevlar — 3/16” Polished Stainless Swivel — 3/16” Welded Polished Stainless Eyes…

Medium == 4” airframe..
3/8” Tubular Kevlar — 1/4” Polished Stainless Swivel — 1/4” Welded Polished Stainless Eyes…

Large == 6” airframe..
7/16” Tubular Kevlar — 5/16” Polished Stainless Swivel — 5/16” Welded Polished Stainless Eyes…

Extra Large == Minimum 8” airframe..
3/4” Tubular Kevlar — 3/8” Polished Stainless Swivel — 3/8” Welded Polished Stainless Eyes…

I can make larger then this Extra Large..

Tom,
Please post in that thread..
Explain,
If you didn’t understand,
Then I wasn’t clear and others didn’t either..

Copy and paste this whole thing top to bottom including my apologies, not clear..

Thank you so much Tom..


Ted Chernok

Onebadhawk Recovery Harnesses
 
My 4" Astrobee D has a 75mm mount in it, and I used a Y Harness with two eyebolts that mount to the upper centering ring. It was custom made for me by @Onebadhawk. Cant recommend this method enough.

View attachment 545223View attachment 545224

I had to do a little bit of work with a file to get it to fit properly, but once it was installed with epoxy, it isnt going anywhere.

View attachment 545222
Quick question on this. If I use this recovery harness style in a 2.6 with a 54mm. I’m never getting back in the AF to change out the kevlar, in the 2.6. Is that a big deal? Could I move the attachment point up so I can reach it to modify the shock cord length, if needed? Would a forward mm even fit with this?
 
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