Quantcast

Level 3 FG rockets - allthreads through the CRs - pros and cons.

The Rocketry Forum

Help Support The Rocketry Forum:

Bat-mite

Rocketeer in MD
Joined
Dec 5, 2013
Messages
10,890
Reaction score
1,637
Location
Maryland
I know that a lot of people reinforce their motor mount in very large fiberglass rockets with some allthread through the CRs. This obviously adds weight at the wrong end, but adds structural stability.

For those who have done this, can you list the pros and cons (other than what I just mentioned)?

I guess my bottom of the line question is: should/must/ought I do this for my RW Formula 200 build?
 

ksaves2

Lifetime Supporter
TRF Lifetime Supporter
Joined
Nov 25, 2009
Messages
5,981
Reaction score
294
I believe it's overkill. I emailed Doc Damerau who used to sponsor the www.rocketmaterials.org site and his opinion was it added unneeded weight to the backend. One can still find the site on the Wayback archive.
For a standard L3 project I believe liberal use of a decent grade of epoxy plus the usual TTW fin construction is all that's needed. Now if one "wants" to add weight to the backend and concomitant nosecone weight to limit
altitude (to say stay within a waiver) sure all-thread away.

For a very large rocket with a built up construction o.k. I could see it. I know of a full scale Phoenix that flew fine structurally on an M and N without all-thread. Kurt
 

CZ Brat

Well-Known Member
Joined
Jan 3, 2013
Messages
611
Reaction score
3
I am not L3 and am by no means an expert. But I thought the reason for the all-thread was to distribute the shock cord forces across 2 CRs instead of just one. This is to minimize an eye bolt pulling out or breaking a CR. Though it is added weight, I imagine that it doesn't have to be a lot. For an all-thread rod to have the same tensile strength of the weakest link in your recover system, it probably doesn't have to be very wide. 1/4" will probably be overkill.

http://www.almabolt.com/pages/catalog/bolts/proofloadtensile.htm
 

dhbarr

Amateur Professional
Joined
Jan 30, 2016
Messages
6,707
Reaction score
1,215
I'm guessing same answer opposite vector for high initial thrust, plus ability to ratchet CRs on top and bottom of fin roots.

Doesn't seem necessary to me, but *shrug* I might do it while assembling a fincan to keep the glue lines thin.
 

Bat-mite

Rocketeer in MD
Joined
Dec 5, 2013
Messages
10,890
Reaction score
1,637
Location
Maryland
I am not L3 and am by no means an expert. But I thought the reason for the all-thread was to distribute the shock cord forces across 2 CRs instead of just one. This is to minimize an eye bolt pulling out or breaking a CR. Though it is added weight, I imagine that it doesn't have to be a lot. For an all-thread rod to have the same tensile strength of the weakest link in your recover system, it probably doesn't have to be very wide. 1/4" will probably be overkill.

http://www.almabolt.com/pages/catalog/bolts/proofloadtensile.htm
Same thing could probably be achieved by making an additional top CR out of plywood, and bolting the U-bolt through both pieces. Would have to see which is lighter.
 

ksaves2

Lifetime Supporter
TRF Lifetime Supporter
Joined
Nov 25, 2009
Messages
5,981
Reaction score
294
Use a thicker forward CR for more adhesive surface area and I would expect that one would only have to be concerned with a "minimal" stress deployment at apogee using the electronic means. U-bolts are good as long as one doesn't decide to use
a longer motor casing later on! :facepalm: For main deployment, I'm aware of a large standard DD rocket where the main deployment ripped the ebay to bits. Builder mentioned to me he should have used more than two parallel pieces of all-thread
and thicker plywood. Kurt
 
Last edited:

jd2cylman

Still not Carl... ;-)
TRF Supporter
Joined
Jun 17, 2011
Messages
4,700
Reaction score
590
My PR Madd Max was built with 3 CR's. The top CR has two generous fillets on it and 2 u-bolts (one on either side of the motor tube), to distribute forces equally. Since it's an 8" diameter rocket, there's a little extra room in there :wink: Just use a bridle harness with enough distance between the "reins" to clear your longest motor. Mine was long enough, then I went and bought a 98/15360 case... Gotta make a longer bridle now. And had to make a doughnut AV bay. That motor is long! And the CTI 6XL is longer!!!!

Adrian (Oh, bottom line. You'll be fine with out the threaded rod, but if it makes you feel better and don't mind [or need] the nose weight to compensate, why not?)
 

ksaves2

Lifetime Supporter
TRF Lifetime Supporter
Joined
Nov 25, 2009
Messages
5,981
Reaction score
294
My PR Madd Max was built with 3 CR's. The top CR has two generous fillets on it and 2 u-bolts (one on either side of the motor tube), to distribute forces equally. Since it's an 8" diameter rocket, there's a little extra room in there :wink: Just use a bridle harness with enough distance between the "reins" to clear your longest motor. Mine was long enough, then I went and bought a 98/15360 case... Gotta make a longer bridle now. And had to make a doughnut AV bay. That motor is long! And the CTI 6XL is longer!!!!

Adrian (Oh, bottom line. You'll be fine with out the threaded rod, but if it makes you feel better and don't mind [or need] the nose weight to compensate, why not?)
That's the nice thing about large diameter. Can use a method for harness anchoring that facilitates replacement of a worn "burned out" harness. Kurt
 

jd2cylman

Still not Carl... ;-)
TRF Supporter
Joined
Jun 17, 2011
Messages
4,700
Reaction score
590
That's the nice thing about large diameter. Can use a method for harness anchoring that facilitates replacement of a worn "burned out" harness. Kurt
Yep, heck, I could drop a small child in there to fasten something if I needed to... :eyeroll:

Adrian
 

ksaves2

Lifetime Supporter
TRF Lifetime Supporter
Joined
Nov 25, 2009
Messages
5,981
Reaction score
294
Yep, heck, I could drop a small child in there to fasten something if I needed to... :eyeroll:

Adrian
Don't laugh or roll eyes. :surprised: My grandfather made wine during prohibition and my dad said grandpa held him upside down by his feet so he could scrape out the grape pulp in the bottom of
the wine barrel with a small sand pail and scoop!. Kurt
 

astrojase

Loves rockets...
TRF Lifetime Supporter
Joined
Apr 21, 2011
Messages
59
Reaction score
0
Same thing could probably be achieved by making an additional top CR out of plywood, and bolting the U-bolt through both pieces. Would have to see which is lighter.
Or you could screw an eye-bolt into the forward closure of the motor case and attached your shock-cord to it.

And just on the all-thread - I've flown a 6 inch FG kit on a bunch of Ms and an O, no all-thread used in the aft at all, TTW fins, injected epoxy and decent fillets should be ok. Just two lengths of all-thread through the av bay to hold it all together - these days I prefer to use stainless steal all-thread with SS washers and nuts, just for a bit of extra strength and durability.
 
Last edited:

ksaves2

Lifetime Supporter
TRF Lifetime Supporter
Joined
Nov 25, 2009
Messages
5,981
Reaction score
294
Or you could screw an eye-bolt into the forward closure of the motor case and attached your shock-cord to it.

And just on the all-thread - I've flown a 6 inch FG kit on a bunch of Ms and an O, no all-thread used in the aft at all, TTW fins, injected epoxy and decent fillets should be ok. Just two lengths of all-thread through the av bay to hold it all together - these days I prefer to use stainless steal all-thread with SS washers and nuts, just for a bit of extra strength and durability.
Not unheard of having a screw on forward closure unscrew with the attached harness. I've been a witness to that. Kurt
 

blackjack2564

Crazy Jim's Gone Banana's
TRF Supporter
Joined
Jan 18, 2009
Messages
9,064
Reaction score
1,173
Location
Savannah Ga
I have built several 7.5 & 8inch rockets. [8 of them] Simply glued 1in. Kevlar strap [Y-harness ] to sides of motor tube and slid notched CR over. Just be sure Kevlar is not in line with where fins go....

You'll be dead before you ever wear out that 4500lb strap. Ya can always stick a couple of eye-bolts in CR for future use, if it makes you sleep better having back up plan. My L-3 is 13 yrs old with 24 flights on it & no signs of wear.

I understand your fear, but after you build several large projects and witness how well this works....ya quite "overthinking" this stuff.

If you insist on going with E-bolt/U-bolts in top CR...very simple bullet proof way to strengthen is: cut off a 1.5-2 in section of coupler and slide it down on top of CR. Couple of healthy fillets to hold in position. You ain't ever pulling that out.
 
Last edited:

astrojase

Loves rockets...
TRF Lifetime Supporter
Joined
Apr 21, 2011
Messages
59
Reaction score
0
Not unheard of having a screw on forward closure unscrew with the attached harness. I've been a witness to that. Kurt
That would be very bad luck! Add a swivel I'd image would reduce the risk? Some loctite...

I mentioned it because doing it that way, you don't have to worry about reaching down the air frame to attach harnesses etc to the u-bolts. Don't have to worry about permanent fixtures such as u-bolts or having to epoxy a harness to the MMT etc - all which are fine and work of course, just another way to do it... reduce overall weight etc...

But you are correct, there is potential for the eye bolt to unscrew...

Or yeah, just do what CJ says above - that is always the best thing to do :wink:
 
Last edited:

Steve Shannon

Well-Known Member
TRF Supporter
Joined
Jul 23, 2011
Messages
6,467
Reaction score
3,133
Location
Butte, Montana
Totally unnecessary to use all thread in your centering rings. I've never seen a centering ring jerked from a rocket by deployment forces. A good fillet, or if you're nervous, a half inch long coupler ahead of the centering ring your shock cord anchors to, will hold any sane amount of force. If that doesn't hold either your rocket is mid-shred anyway or you did a lousy job of epoxying the rocket together.


[emoji1010] Steve Shannon [emoji1010]
 

pondman

Serenity now......
TRF Lifetime Supporter
Joined
Mar 23, 2011
Messages
712
Reaction score
111
Totally unnecessary to use all thread in your centering rings. I've never seen a centering ring jerked from a rocket by deployment forces. A good fillet, or if you're nervous, a half inch long coupler ahead of the centering ring your shock cord anchors to, will hold any sane amount of force. If that doesn't hold either your rocket is mid-shred anyway or you did a lousy job of epoxying the rocket together.


[emoji1010] Steve Shannon [emoji1010]
Perfect response Steve.
 

cwbullet

Obsessed with Rocketry
Staff member
Administrator
TRF Lifetime Supporter
Global Mod
Joined
Jan 24, 2009
Messages
25,895
Reaction score
3,638
Location
Glennville, GA
I have built one with threaded rods. They can help hold the fin can together while the glue sets, but I see no other reason for them. My level 3 was heavy and I did not use them. It has held up well without them.

I guess you could use them as an attachment point.
 

FredA

Well-Known Member
Joined
Jan 19, 2009
Messages
2,132
Reaction score
318
No real reason to have All-thread anywhere aft of the Ebay.
Just adds weight at the wrong end.
 

FredA

Well-Known Member
Joined
Jan 19, 2009
Messages
2,132
Reaction score
318
All-thread in Ebays does serve to connect the two bulkheads.
In this application it is used in tension, which is the only direction it has any usable strength.
Since, in a traditional DD configuration, the Ebay is "part" of the recovery chain, it has it's merits when used there.
You need something to kept the two bulkheads in place with sufficient strength.
I usually use just one all-thread and use an eye-nut on each end -- I'll drill a hole in the ends so that I can put a cotter pin though to keep the eye-nut from coming loose.
It also provides something to hang your altimeter from although it can be in the way in small diameter bays.

I also use it if I have a big NC that gets a bulkhead on the aft end of the shoulder.
A length of all-thread from the tip though the bulkhead with an eye-nut used to secure the bulkhead and provide a harness attach point is a nice, clean solution.
You can then open the NC easily to insert a tracker. The all-thread doesn't seem to bother the transmitter.
The added usage here is you can take lead balls (fishing weights) and drill a hole though them and slide them onto the all-thread and secure with a nut to form a variable nose-weight system....add more when needed...
 
Last edited:

Latest posts

Top