Harness gets so, so tangled up

[ChicagoDave]

Recently I've noticed an issue with my Kevlar harness getting completely tangles up. I mean it's so bad I have to to take it off the quick links and de-tangle it. These are typically 2.3, 2.6 or 3" rockets I'm flying. In some cases the tangles get so bad that it keeps the chute from opening fully and the rocket lands in a flat spin. Typical apogee is between 4 and 6k so the rocket as some time to fall before the main opens.




To combat these I've taken to putting swivels on each quick link. MY working theory was that the booster spins during the decent and causes the harness to get knotted up, but the swivels don't seem to isolate the booster from the harness enough. All that's done is get the swivel tangled up in the harness. If you look in the photo I posted you might be able to make out a swivel tangled up in there.... :|

What can I do to reduce these knots?

~Dave~

 

[David Schwantz]

what type of swivels are you using? Make sure that thee spin freely. You can always use a powered dry graphite for a lube in them.

 

[kbRocket]

Has anything changed recently? Just add the Jolly Logic?

I frequently drop rockets from 5 to 20 k feet without any twisting and no use of swivels at either end of the Kevlar. I suspect your problem is twisting induced from the parachute. It probably has imperfections that cause it to spin a lot more than the air frame, which should be pretty well aerodynamically balanced.

I mention the Jolly Logic chute release because I think it has as lot of potential to cause tangles. Before it releases it may prevent the swivel on your parachute from spinning if it is attached to the tether. This might be OK. After it releases it becomes a pendulum at the end of it's tether. It can swing around and easily tangle with a parachute shroud line and keep it from spinning. It is tangled up right there in the mess.

I have not put a lot of flights on my Jolly Logic but I have observed it tangling. I think next time I use it I will attach an extra length of cord between the knot where the parachute lines come together and the swivel. The Jolly Logic will attach to this cord and be far enough from the swivel that it cannot get to it after release.

Like this:



Before release the parachute bundle can spin and the swivel will work. After release the parachute can spin , the swivel will work, and the Jolly Logic will not snag. You could even put a swivel at each end of the extension cord. Then the Jolly Logic wouldn't have to rotate with the parachute and there would be two swivels that could relieve the parachute twisting.

Have you watched your rockets falling to see when the twisting happens?

 

[Handeman]

I have a rocket where the fin can spins over 100 RPM on the way down. It never twisted up the shock cord in 50 flights. I use a ball bearing swivel on the fin can and enough of a drogue to keep a little tension on the shock cord. I've never used swivels on chutes. They are pretty much useless there.

I tried a barrel swivel once and it deformed and didn't spin. It got trashed after the first flight. I only use ball bearing swivels.

There is always going to be some amount of friction in any swivel. Without any tension stretching the shock cord, the free floating shock cord and swivel will both spin together with the fin can and twist up. You need enough drag from a drogue chute or JLCR bundled chute to keep the tension on the shock cord higher than the spinning friction in the swivel. That way the spinning of the fin can overcomes the friction in the swivel and the swivel spins without twisting the shock cord.


It really doesn't matter if you use the JLCR, DD, or any other upper recovery system. You need the swivel on the fincan/booster. Its those fins that spin and twist things up.

 

[kbRocket]

I have a rocket where the fin can spins over 100 RPM on the way down.

I wonder why your fin can spins so quickly? Nearly twice a second seems difficult to achieve on descent even if you were trying.

A few years ago I stopped using swivels at the ends of my tethers and never have any twisting problems. Multiple rockets, high flights, some DD and some main at apogee. I have a good assortment of ball bearing swivels of various sizes and use them only at the parachute attachment. Before I started making my own Kevlar tethers I would cut the swivels off the commercial ones I purchased because I didn't see much value added. They were barrel swivels which don't twist well, add snagging potential, and are another point of potential failure.

At a launch last month I had four flights in the 14 to 16k' range. Two different rockets. All of them main at apogee. Average descent time was just under six minutes while the aft end of the rocket just dangled. No noticeable twists.

For me this is the norm on all of my rockets. It's interesting your experience is so different.

I wonder what would cause a fin can to even want to spin? On average air should blow evenly around both sides of the rocket causing a balanced force/torque. I could see an extremely asymmetrical or crooked set of fins causing spin, but nobody would build that into a normal rocket.

One key word in ~Dave's~ post is 'recently'. It implies that he didn't have a twisting problem and something changed causing one to develop. That is why I asked when the Jolly Logic was added. Multiple rockets recently developing the same twisting symptom should have a root cause.

 

[Handeman]

I wonder why your fin can spins so quickly? Nearly twice a second seems difficult to achieve on descent even if you were trying.

A few years ago I stopped using swivels at the ends of my tethers and never have any twisting problems. Multiple rockets, high flights, some DD and some main at apogee. I have a good assortment of ball bearing swivels of various sizes and use them only at the parachute attachment. Before I started making my own Kevlar tethers I would cut the swivels off the commercial ones I purchased because I didn't see much value added. They were barrel swivels which don't twist well, add snagging potential, and are another point of potential failure.

At a launch last month I had four flights in the 14 to 16k' range. Two different rockets. All of them main at apogee. Average descent time was just under six minutes while the aft end of the rocket just dangled. No noticeable twists.

For me this is the norm on all of my rockets. It's interesting your experience is so different.

I wonder what would cause a fin can to even want to spin? On average air should blow evenly around both sides of the rocket causing a balanced force/torque. I could see an extremely asymmetrical or crooked set of fins causing spin, but nobody would build that into a normal rocket.

One key word in ~Dave's~ post is 'recently'. It implies that he didn't have a twisting problem and something changed causing one to develop. That is why I asked when the Jolly Logic was added. Multiple rockets recently developing the same twisting symptom should have a root cause.

Once the main is out, the fincan and the rest of the rocket just hangs under the main and there really isn't any twisting or spinning at all. You really don't need any swivels for that.

Its the fall from apogee before the main is deployed where all the fin can twisting and dynamics occurs. When you have that inverted V shape under the drogue, or especially a floating fincan falling without a drogue, that's when it starts to spin or fly. If the fincan is pointed down about 30-45 degrees below vertical and falling fins first without spinning, it can act as a glider and pull the whole rocket with it as it heads off in one direction or another. If you are lucky, it heads upwind and shortens your walk. I've seen one that went almost twice as far crosswind then the distance the it went downwind.

On several of my rockets the fincan spins very fast. My L3 is a prime example. I have a climbing swivel on the fincan and it usually lands with the lower shock cord having very few twists. On one flight the swivel got hung up in the bridle and couldn't spin and the shock cord twisted up so bad it was less then 1/2 it's normal length and the drogue chute was completely twisted up inside the shock cord. It looked like the rubber band on a rubber band powered model air plane when it's wound up and ready to fly.

 

[Bat-mite]

Didn't read all replies all the way thru ... just adding my thumbs up for a ball-bearing swivel.

 

[BF Rockets]

One solution is to build rockets with 4 fins. I've only had the spinning problem with 3 finned rockets. I don't usually use a drogue chute, and most of my rockets have 4 fins partly for this reason among others.

 

[Buckeye]

One solution is to build rockets with 4 fins. I've only had the spinning problem with 3 finned rockets. I don't usually use a drogue chute, and most of my rockets have 4 fins partly for this reason among others.

Just the opposite for me. My only 4-fin rocket is the MAC Firestick XL, and it also has a boat tail. The fin can spins like a whirling dervish, where as my 3-fin rockets do not.

I upgraded to better kevlar, a quality swivel, and a small drogue chute. This helps reduce the twist in the drogue cord.

 

[Handeman]

Mine don't seem to matter if they are 3 or 4 fin. I have some of both that spin like crazy.

 

[Steve Shannon]

Do you fold your shock cord next to your parachute or wind it around the chute when prepping?

 

[ChicagoDave]

Interesting discussion!! In my case I'm flying 4 fin rockets (e.g. LOC Sandhawk, LOC IRIS). I'm using the Jolly Logic Chute Release to control the main chute opening altitude (typically 700' AGL but it always seems lower ).

Now that it's been mentioned I don't recall having this same issue with my 3 fin rockets but I've not flown many recently. But that might be something to investigate.

I usually wrap the chute in Nomex and put it in first, followed by the shock cord folded accordion style. But sometimes I try putting the shock cord in first with the chute on top. Neither method seems to make a difference. In either case I always fold the shock cord accordion style -- I feel it makes it less likely to bind up as it deploys.

Also I have tried using a drogue but it didn't seem to make any difference. In fact if anything it made the situation worse.

What I'm going to try next flight is using a small drogue in addition to the swivel in between the quicks link that connects the shock cord to the booster (eye bolt -> quick link -> swivel -> quick link -> shock cord). My working hypothesis is that I need some tension on the swivel and my current configuration isn't providing enough to let the swivel do it's job.

 

[Onebadhawk]

I have a rocket where the fin can spins over 100 RPM on the way down. It never twisted up the shock cord in 50 flights. I use a ball bearing swivel on the fin can and enough of a drogue to keep a little tension on the shock cord. I've never used swivels on chutes. They are pretty much useless there.

I tried a barrel swivel once and it deformed and didn't spin. It got trashed after the first flight. I only use ball bearing swivels.

There is always going to be some amount of friction in any swivel. Without any tension stretching the shock cord, the free floating shock cord and swivel will both spin together with the fin can and twist up. You need enough drag from a drogue chute or JLCR bundled chute to keep the tension on the shock cord higher than the spinning friction in the swivel. That way the spinning of the fin can overcomes the friction in the swivel and the swivel spins without twisting the shock cord.
View attachment 427527

It really doesn't matter if you use the JLCR, DD, or any other upper recovery system. You need the swivel on the fincan/booster. Its those fins that spin and twist things up.

This is a good way to explain "The path of least resistance"..
With a belly in the harness it's simply easier for the harness to spin up then for the swivel to spin.
It's a question of where is the twist being applied.
With a belly in the line more twist force is being applied at the belly then at the swivel..
This is a great pat of the reason I make the attachment points the way I do..



As opposed to a single leg to the top centering ring..
Of course spreading the load is the other side of the attachment point equation..

Teddy

Ohh,, and I would never use any swivel but a load bearing swivel to carry load ( any part of the rocket that has substantial mass )..
I historically only use ball bearing swivels on chutes that are out of aerodynamic balance and spin..
But I now have the biggest, heaviest ball bearing swivel I have ever seen.
I just found them, they're insanely cool, I haven't had the chance to put them up on my website yet..

 

[Onebadhawk]

Check here..





I still have to put them on my site.
But if you ask for a Badhawk swivel in an email, this is it..
Prices haven't changed, best around,
The traditional ball bearing swivel is $5,
and the Badhawk swivel is $7..
I don't put much stock in manufacturers weight ratings of these type of swivels..
They say failure rating is 500 and 1500 lbs..
I'm better with WLL ( Working Load Limit ) on load rated swivels,,
An amount of tension that can be withstood without any deformation..
These are #'s you can work a swivel at all day with no concern..
That's a more reliable and realistic number to work with..

Teddy

 

[JimJarvis50]

Here's a funny story. A few years ago, I flew a two-stager at Wayside. Went to 50K or so. When we went to recover the sustainer, we first headed towards the last gps hit, but then we diverted to the RF tracker when we got closer. When we got to the rocket, there was no fin can. wtf? I knew for sure it had been attached. So, assuming it fell off near apogee, we traced the flight path but never found the fin can.

Six months later, I got a call from guy who said he had found the fin can. Woohoo! I asked him where he found it, and to make a long story short, it was under the last gps hit. It had fallen off when the main came out, and if we had just gone to the last gps location, we would have found it. I always wondered what had happened.

Fast-forward maybe 8 years? I did a flight at Argonia - maybe 15K or so. When my wife and I got to the rocket, no fin can. wtf? Humm, where have I seen this before? We looked around the area, and sure enough, the fin can had detached when the main came out and landed about 100 feet away. So, I got a video from that flight and here it is:



I don't exactly know the rpm's on this flight, but it looks like a bunch. I suspect this happens more often than it doesn't happen, and I have several videos showing similar descents. Now I use a swivel. But what I didn't know about this is that given enough time, this spinning action can undo a quick link, and it happened twice to me on two different rockets (both three fins I believe and both with U-bolts on the fin can that are offset from the axis of the rocket). So, now I also put tape around the quick links.

Jim

 

[Handeman]

Here's a funny story. A few years ago, I flew a two-stager at Wayside. Went to 50K or so. When we went to recover the sustainer, we first headed towards the last gps hit, but then we diverted to the RF tracker when we got closer. When we got to the rocket, there was no fin can. wtf? I knew for sure it had been attached. So, assuming it fell off near apogee, we traced the flight path but never found the fin can.

Six months later, I got a call from guy who said he had found the fin can. Woohoo! I asked him where he found it, and to make a long story short, it was under the last gps hit. It had fallen off when the main came out, and if we had just gone to the last gps location, we would have found it. I always wondered what had happened.

Fast-forward maybe 8 years? I did a flight at Argonia - maybe 15K or so. When my wife and I got to the rocket, no fin can. wtf? Humm, where have I seen this before? We looked around the area, and sure enough, the fin can had detached when the main came out and landed about 100 feet away. So, I got a video from that flight and here it is:

I don't exactly know the rpm's on this flight, but it looks like a bunch. I suspect this happens more often than it doesn't happen, and I have several videos showing similar descents. Now I use a swivel. But what I didn't know about this is that given enough time, this spinning action can undo a quick link, and it happened twice to me on two different rockets (both three fins I believe and both with U-bolts on the fin can that are offset from the axis of the rocket). So, now I also put tape around the quick links.

Jim

That sounds a lot like my 4". The lower section was built "zipperless" so the BT on the apogee side was attached to the av-bay and the fin can had a u-bolt offset next to the MMT. That fin can spun like crazy. I say 100 RPM, but I wouldn't bet against +200 RPM. I always had a quick link connected from the u-bolt to a ball bearing swivel with welded rings and the shock cord attached to the other end of the swivel. Never had an issue with a twisted shock cord.
Even hanging from a power line, that fin can spun in a light wind like crazy.

 

[Handeman]

View attachment 427754

The traditional ball bearing swivel is $5,
and the Badhawk swivel is $7..

Teddy

Teddy, is the traditional ball bearing swivel in the upper left freer spinning than the two in the upper right? Are those two on the right actual ball bearing swivels?

The upper left with the welded ring, are those the 12/0 size, about 2 1/2" long?

 

[Onebadhawk]

The top ones are ball bearing.
Load bearing swivels are not..

Teddy

 

[OverTheTop]

Windmilling can be really annoying. I too have had some seriously wound-up string.

The maths says there should be no difference between three and four-fin rockets.

I think the main problem is when the rocket comes in under drogue and is largely horizontal. If there is a little more chute and the airframe is more towards vertical it should be much less.

I have thought of making a rocket with one fin longer (spanwise) than the rest to stop this behaviour.

 

[Handeman]

Windmilling can be really annoying. I too have had some seriously wound-up string.

The maths says there should be no difference between three and four-fin rockets.

I think the main problem is when the rocket comes in under drogue and is largely horizontal. If there is a little more chute and the airframe is more towards vertical it should be much less.

I have thought of making a rocket with one fin longer (spanwise) than the rest to stop this behaviour.

That's an interesting idea. I'm not sure it would work, but please report on it if you do that.

I agree the main problem is when the rocket comes in under drogue and is largely horizontal, but I believe that is because there is no tension on the shock cord and the cord can spin and twist up just as easily as a swivel can spin.
If there is a little more chute and the airframe is more towards vertical , or at the 45° of the inverted V, I don't believe you will get less spinning unless you are very close to vertical, but if you do have a swivel on the fincan, the tension on the shock cord will prevent it from twisting as the swivel will spin instead.

At least, that's been my experience.

 

[OverTheTop]

That's an interesting idea. I'm not sure it would work, but please report on it if you do that.

I will bet your left testicle that it will work .

 

[mlrtime99]

Very helpful thread and reminded me of my last launch. The nylon cord was wound up so tight I was a bit surprised everything made it down in one piece and have been a bit gun shy to launch it again without a swivel, abrasive sleeves, etc.. After reading the comments here and re-watching my launch video with a fresh set of eyes I'm almost positive my drogue was undersized. I never get a good look at the drogue but I see the forward section of the rocket taught to the drogue multiple times but the fincan is providing its own drag and as pointed out in other comments is regularly at 30-45 degrees and the line to the forward section is always slack. This is when the uprushing air really imparts spin to the can. Hopefully if I can get the fincan more vertical it will passively fall as the air moves inline with the fins.

 

[Onebadhawk]

Very helpful thread and reminded me of my last launch. The nylon cord was wound up so tight I was a bit surprised everything made it down in one piece and have been a bit gun shy to launch it again without a swivel, abrasive sleeves, etc.. After reading the comments here and re-watching my launch video with a fresh set of eyes I'm almost positive my drogue was undersized. I never get a good look at the drogue but I see the forward section of the rocket taught to the drogue multiple times but the fincan is providing its own drag and as pointed out in other comments is regularly at 30-45 degrees and the line to the forward section is always slack. This is when the uprushing air really imparts spin to the can. Hopefully if I can get the fincan more vertical it will passively fall as the air moves inline with the fins.

You're correct in that a larger drogue will accomplish what you say.
As the drogure gets larger and larger the legs of that inverted "V" will get closer and closer together
as the booster and payload section both hang straighter and straighter down from the larger drogue..

When you do this where will the nose cone be pointed when it fires ??

A correct attachment point with two legs and a load rated stainless swivel on top is the beginning of fixing the harness spin up,
but not the end.--


Teddy