Swivels

I am wanting to add a swivel to my harness above the BT. Can't help but wonder how people size their swivels, is there a rule of thumb? X lb rocket = Y lb rated swivel?

Easiest method would be weight of rocket time maximum expected g's, that gives you a working load for the swivel.

rharshberger said:

Easiest method would be weight of rocket time maximum expected g's, that gives you a working load for the swivel.

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Thanks for the quick reply. And how do I determine max G's?

Thrustcurve / Openrocket will give you an estimate for each motor.

viciouspeanut said:

Thrustcurve / Openrocket will give you an estimate for each motor.

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Not what he's looking for.
He wants to know the maximum load applied to swivel/harness under deployment.

timbucktoo said:

Not what he's looking for.
He wants to know the maximum load applied to swivel/harness under deployment.

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Duh sorry! I totally under thought that.

timbucktoo said:

Not what he's looking for.
He wants to know the maximum load applied to swivel/harness under deployment.

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Thanks Tim. So say I have a 6lb rocket and OR says it has a vertical acc. of 150ft/sec sq at deployment. What swivel rating should I use?

timbucktoo said:

Not what he's looking for.
He wants to know the maximum load applied to swivel/harness under deployment.

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Actually it will give a maximum gee's in the event of a drag separation which is good to know, and is likely higher than than a properly size ejection charges forces. As a default I use 50gs for most sub-mach flights and 100g's for mach+ flights, and it has worked for me. Working loads are lower than breaking load limits for hardware (but with non-hoisting and rigging hardware who knows what the actual ratio is).

Teddy of One Bad Hawk can probably help. For what it's worth, he recommended me a 5/16" swivel for a 29 lb. rocket

dhkaiser said:

Thanks Tim. So say I have a 6lb rocket and OR says it has a vertical acc. of 150ft/sec sq at deployment. What swivel rating should I use?

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Why is the acceleration so high at deployment? Is it still under thrust?

This is a cool problem. Lots of moving parts. Seems like there ought to be a rule of thumb -- like 10x the weight of the rocket + 100 lb, or something.

Assuming that the swivel is under no significant stress during ejection.

If deployment happens at apogee, when the speed of the rocket through the air is nearly zero, then then the falling rocket will accelerate until the drag force is equal the weight -- so that the greatest tension on the swivel would be the weight of the rocket.

If deployment happens earlier, or later, than apogee -- the drag force will be more or less proportional to the square of the speed through the air (at high speed) and proportional to the cross section of the parachute. So, if you really want the swivel to survive a catastrophic failure of the rest of the recovery apparatus, it should be rated according to the force exerted on the fully opened canopy at the maximum speed of the rocket.

What is the cost-benefit here? Going too-heavy on the swivel will cost you a few grams of mass. The benefit is that the rocket might come down intact from a high-speed deployment.

EDIT -- What is the ultimate strength of the next weakest part of the existing recovery apparatus? Does it make sense for the swivel to have a higher tensile strength than the shock-cord (for instance)? That's not rhetorical -- I am really asking.

here is a thread I found that might shed some light!

https://www.rocketryforum.com/showt...el-strength-ratings&highlight=swivel+strength

timbucktoo said:

here is a thread I found that might shed some light!

https://www.rocketryforum.com/showt...el-strength-ratings&highlight=swivel+strength

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I like Bob Krech's response best in that thread, use 100G's for all calculations when determining swivel/hardware size.

100G isn't too bad of figure to use, but you can't design every piece of the recover system to be the strongest, something has to be the weak link. And maybe the swivel is the best piece to be the weak point.

That is all said with several assumptions.

• The swivel is attached to the fin can
• The rocket is DD.
• The apogee charge isn't oversized. (no blow it out or blow it up type charges)

Under these conditions the only thing that could put a large load on the swivel during a nominal flight is if no drogue is used and the opened rocket is still falling ballistic (payload section straight down and pulling the fin can behind it on the shock cord). In any case where the main is pushed out downward and opens with little load until the rest of the rocket falls a significant ways past it (overly long shock cords). When the fin can hits the end of the shock cord, there is a huge load on the recovery system and it might be better to break the swivel then tear anchors out of the fin can or payload/av-bay. Since it's only falling from main deployment altitude, the fincan may be undamaged or very fixable afterwards.

Other types of failures during other parts of the flight or during non-nominal decent will probably cause extensive damage to much of the rocket and a broken swivel will be a minor issue.

You have to make your own cost/risk analysis.

Personally I use the #10 ball bearing ocean fishing swivels rated about 400 lbs on the fin cans of DD rockets to 15 lbs. For my 55lb L3, I use a 30KN man rated climbing swivel on the fin can.
I have not see a need for a swivel on a motor eject rocket, but it's possible I'll use one on the fin can of motor eject rockets that routinely use the JLCR. I haven't had one that needs it yet.

To actually answer the OP question.

dhkaiser said:

I am wanting to add a swivel to my harness above the BT. Can't help but wonder how people size their swivels, is there a rule of thumb? X lb rocket = Y lb rated swivel?

Click to expand...

I search the Wildmand site assuming you want the swivel for your L1 Drago. Is your Drago the standard Drago, 41" tall and 29mm MMT? Motor eject?

If that's the case, I would not bother with a swivel unless you use a JLCR and have a problem with the spinning BT twisting up the cord and chute as it drops. If you do need one, I would use a #10 ball bearing ocean fishing swivel rated about 400 lbs and put it about 3" to 4" above the top of the BT.

I that isn't what you want the swivel for, see above.

No not the drago, I am finishing a 3" Avalanche. I intend to use motor eject and a JLCR. Don't plan on anything bigger than a 38mm motor.

I use something in the "middle" 70G's x weight of rocket. I actually own two sizes. 600 Lb. big ones, and 380 lb for the rest. Most mid powers get 380, some light HP rockets as well. most of my larger (up to 6-7 lbs) get the 600 lb. it's the weakest point of my recovery, but I've never had one fail.

I'm using the 600 lb. swivel for what it's worth on my Avalanche. Looks like it'll come in ~6lbs.

swatkat said:

I'm using the 600 lb. swivel for what it's worth on my Avalanche. Looks like it'll come in ~6lbs.

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Thanks, that is what I decided to use as well.