Shock cord stuck on fin?

[billdz]

On a flight Saturday, my rocket's chute did not open immediately and it looked like a hard landing was imminent. The chute finally opened maybe 75' from the ground but there were some tense moments. Now trying to figure out what went wrong. See the attached pic, looks like the shock cord or shrouds got stuck on a fin. Is there a way to stop this from happening in the future?
Thanks,
Bill

 

[markkoelsch]

A few questions. First, the rockets configuration- single deploy or dual? Single break point or two? How long is your harness/shock cord?

 

[Q-Aero]

Your chute seem to be attached too far from the upper section, probably with a too long shock cord betwen the upper section and the chute, and this long cord twist with the part that go to the AFT .

 

[Q-Aero]

Your chute seem to be attached too far from the upper section, probably with a too long shock cord betwen the upper section and the chute, and this long cord twist with the part that go to the AFT . Here an image I get on google, , just look how close to the nose is the chute

 

[billdz]

Thanks for the replies. This was single deployment with one break point. The shock cord is about 11' long. The chute is attached near the midway point (as Q-Aero notes, this may be the problem, I did not build this rocket, but the instructions say to attach the chute 3' from the payload section). Attached is a pic from the same flight, with the chute open, just before landing.

 

[Banzai88]

Question 1: What orientation was the rocket in and what was the speed at deployment? The picture of the tangle is a classic photo of a rocket that deploys while still having forward motion, either vertical or horizontal or a combination of both. The deployment charge separates the forward part of the airframe, which begins to slow down, then the aft end of the airframe basically flys into the deploying/unraveling parachute/shock cord.

Question 2: How energetic was the deployment event? A weak deployment charge that does not give sufficient separation to the two halves of the rocket, coupled with your choice of parachute placement in the middle of the shock cord could cause the same issues as above.

If a tangle like that happens, altitude is your only friend and worst enemy, as on one hand it gives time for the slip stream to unravel the tangle, but also will lead to greater damage potential the higher it went. Luckily terminal velocity of an aerodynamically spoiled airframe isn't very high and it's likely to survive a terrestrial impact!

50/50 split of the length of the cord is never advisable, as it allows the upper and lower sections of the rocket to bang together when under the inflated canopy.

I would move the parachute closer to the nose, not much more than about 10-15% of the overall length of the cord down from it. That way, if there is any forward velocity to the aft section, the canopy is likely to be clear of the traveling aft section and/or be effectively deflected off of it while unravelling/deploying. Also, even on a marginal separation charge you stand a better chance of the parachute being pulled free and getting into the slip stream and deploying.

Then again, sometimes weird s*** just happens.

 

[billdz]

I did not have eyes on the rocket at deployment, although it makes sense that an early deployment could cause a tangle. And yes, looking at the pic of the rocket with the chute open, it is pretty clear that the chute should be attached closer to the payload section, otherwise the 2 sections can hit each other during descent, I don't know why the builder attached it in the middle. The instructions say 3' from the payload, I was thinking 2' would be better, and your 10-15% suggestion would mean 1'.

 

[noffie79]

I would personally put a longer harness in it as well.

 

[Handeman]

I wouldn't bother with a longer harness, just attach the chute about 2' from the payload and make sure you pack all the harness in the booster tube before the chute goes in. Then go easy on the ejection charges.

 

[Coop]

I have video of this occurring on my Hellraiser flight. I puzzled at the cause, until I got the flight data. Primary altimeter failed to detect apogee. Therefore, rocket had an appreciable amount of forward speed at separation when the back up charge fired two seconds later. This slightly late deployment caused the shock cord to hang around one of the fins as the rocket fell under streamer. It remained so until Main deployment, which shook it loose. So, I would ask: what was your speed at deployment?

Later!

--Coop

 

[retortec]

I had similar problems with my Ventris using a Chute Release. The airframe was so light with huge fins sometimes it would flat spin. I added a guide chute and moved the connection much closer to the nose cone. Seems to have solved the problem. Here is a great pic from the on board camera.