Shock Cord Length Re-Visited

[DRAGON64]

In my search, 3X the length of your rocket is an accepted 'rule-of-thumb' length for shock cord design. But it is a little vague on dual deployment; so is the 3X(Length of rocket) to be yet divided again for each deploy event, or does each deploy event get a 3X length of cord?

In other words: (3X 11)/2=16.5

or 2X(3X 11)= 66

This is in reference to a DX3 Massive I am building, which has become a bit...massive!

I am looking at 1" Tubular Nylon (something colorful)

 

[tim cubbedge]

I usually keep my cords 4-5 times the rocket's length (read that here somewhere, some time ago) and typically keep the drogue cord longer than the main so for a 5 foot rocket i make the drogue about 25' and the main about 20'. Opinions will vary!

 

[DRAGON64]

My 'Feng Shui' tells me 33' for main, and 55' for drogue.

 

[Orien]

I went extra long on my 5' Pike - about 35' for the drogue. I did this after analyzing onboard video of apogee separations. A lot of force occurs when that drogue snaps open and decelerates the upper section while the booster is still hurtling forward. With the extra long shock cord, the booster slows way down before being yanked about. In one flight, the booster and upper sections never even passed each other. I'm going to add length to my other rockets now.

 

[waltr]

I also have been going 4-5 times rocket length on both Drogue and Main cords.

Another thing I have been doing is do three z-fold bundles along the cord. I do 3-4 folds with a small piece of masking tape around the folds. This does two three things:
1- easier to pack into BT
2- prevents tangling during ejection
3- Absorbs energy during deployment. This is the rocket sections being blasted apart and the force of the chute opening.

 

[MidOH]

A rocket with a 25' total of cord, can get 25' closer to a power line, than a rocket with 50' of cord, and not become flammable confetti.

So it's flying field dependent for me. A ding on the booster is not as bad as a complete loss in a tree or powerline.

On a 4" DD rocket, I'll run about 20' of cord on the booster, and 6' on the main. Some Z folds, taped onto the booster cord.

 

[rharshberger]

Talk with Teddy from @Onebadhawk see what he recommends.

 

[Titan II]

113" X 5.5" 42 lbs at launch.
Use 40 on drogue and 25 on main, both with z-folds.

 

[Viper3]

For dual deployment I would use approximately 3X for the drogue. For the main I would use a shock cord some what longer than the total length of the main chute and attach the chute to the nose cone eye bolt. I would also z- fold both shock cords and hold them together with rubber bands.
There will be almost as many different answers as there are Rocketeers.

 

[manixFan]

I went extra long on my 5' Pike - about 35' for the drogue. I did this after analyzing onboard video of apogee separations. A lot of force occurs when that drogue snaps open and decelerates the upper section while the booster is still hurtling forward. With the extra long shock cord, the booster slows way down before being yanked about. In one flight, the booster and upper sections never even passed each other. I'm going to add length to my other rockets now.

I second this. After analyzing both video of deployments and accelerometer data, longer is better, as long as it's not tooooo long! I used to go by the 3x rule, but I found from the accelerometer data that there was a big jolt when the payload bay hit the end of the cord, a longer cord has reduced that by quite a bit. But from watching videos, too long and there is a good chance that things get tangled up on deployment and the chutes then never fully deploy. So I have settled on about 4-5X the length of the main sections, depending on size and weight.

The amount of weathercocking also is a factor - with a lot of weathercocking a shorter cord seems better - less chance to get tangled up. I've got a couple of videos of fins getting ripped off by shock cords that were overly long and the rocket getting tangled up during deployment, and then when the chute opens, it yanks the fin right off. It happened to me with a PML Little Lunar Express, which was a big bummer - I never found the fin. So, I don't think there is a single rule of thumb that can apply to all situations. Which, of course, makes sense.


Tony

 

[DRAGON64]

Talk with Teddy from @Onebadhawk see what he recommends.

I have a number of emails into Teddy, but he has not responded since the first email, I hope he is doing well.

 

[Rocketjunkie]

I use 50' at apogee, 10-15' for the main. This is for rockets 4-12" diameter, 5-100 lb. recovery weight.

 

[DRAGON64]

I use 50' at apogee, 10-15' for the main. This is for rockets 4-12" diameter, 5-100 lb. recovery weight.

Do you use tubular nylon or Kevlar? To save room, I may have to opt for tubular Kevlar to get 50+ feet plus drogue fitted in.

 

[DRAGON64]

Talk with Teddy from @Onebadhawk see what he recommends.

Contact has been re-established, apparently Ted was on vacation for the past two weeks. I am now waiting to see what Ted has to offer.

 

[Rocketjunkie]

Tubular Nylon. Even though Kevlar (aramid) is stronger, it has less stretch.than Nylon. 3% for Kevlar, 10% for Nylon. Since there is less stretch with Kevlar, the loads on the attachment points is about 3X higher. The total energy to break is about the same (jerk load). Kevlar will zipper even G12 or G10 airframes.Kevlar does have better heat resistance.

 

[DRAGON64]

Excellent points, thank you. One Bad Hawk and I are back in discussions, and evaluating options. I belive tubular Nylon is still on the table.

 

[DRAGON64]

Working with Ted at One Bad Hawk, I came up with this configuration:

35' 3-loop for the drogue and main, for total of 70' of recovery harness. I wanted the 25' configuration for the main, but these only came in a 2-loop. It was explained that the 3rd loop was not needed due to the main chute attaching to the hardpoint at the nose cone, but I have not attached a chute to the nosecone since my days of building Estes rockets. I prefer a bit of service line for the cone to 'dangle' for lack of a better term seperate from main chute attachment.

On that note, and a bit off topic, where do you mount your nose cone and main chute on your HPR builds? Main chute at the hard-point NC attachment? Or main chute 1/3 (or so) down the main harness with 'dangling' NC?

 

[Crazyrocket]

I always put my chute at the end and the nose cone down a little ways. I find that it does not swing around as much when compared to the nose cone at the end and the chute lower down.

 

[cwbullet]

Longer is better unless your rocket hits the ground before shock cord is tight or it does not fit in the body tube.

I, too, like my parachute to attach a little down the shock cord from the nose cone.

 

[Conway Stevens]

Working with Ted at One Bad Hawk, I came up with this configuration:

35' 3-loop for the drogue and main, for total of 70' of recovery harness. I wanted the 25' configuration for the main, but these only came in a 2-loop. It was explained that the 3rd loop was not needed due to the main chute attaching to the hardpoint at the nose cone, but I have not attached a chute to the nosecone since my days of building Estes rockets. I prefer a bit of service line for the cone to 'dangle' for lack of a better term seperate from main chute attachment.

On that note, and a bit off topic, where do you mount your nose cone and main chute on your HPR builds? Main chute at the hard-point NC attachment? Or main chute 1/3 (or so) down the main harness with 'dangling' NC?

I attach my main about 3 to 5 feet down from nose cone. Never directly to the nose cone.

Also curious as i understand Teddy /Onebadhawk doesnt do custom length harness anymore. Last time i tried to order one he would only do his standard lengths.

 

[MidOH]

Do you run the OBH cords through the eyelet and loop them, on the forward centering ring, or just use a quick link there?

I prefer the nose cone tied to the shock cord, and the main at the end, with my MPR's. But it's messier to wind up the cord, than nose cone at the end, chute a foot down or on the nose. Especially since I tie my nomex sheet off, very near the tube.

 

[DRAGON64]

I attach my main about 3 to 5 feet down from nose cone. Never directly to the nose cone.

Also curious as i understand Teddy /Onebadhawk doesnt do custom length harness anymore. Last time i tried to order one he would only do his standard lengths.

No custom length cords, as I ended up with the stock options. However, instead of a 25' 2-loop, I opted for a 35' 3-loop. I could have just tied an extra loop for my attchment preference, but I really wanted the quality look and security that is built into the One Bad Hawk harnesses.

Like you, I attach my chute further down the main harness.

Do you run the OBH cords through the eyelet and loop them, on the forward centering ring, or just use a quick link there?

I prefer the nose cone tied to the shock cord, and the main at the end, with my MPR's. But it's messier to wind up the cord, than nose cone at the end, chute a foot down or on the nose. Especially since I tie my nomex sheet off, very near the tube.

I run the OBH harnesses with stainless quicklinks for everything HPR. For LPR to MPR, I generally do like you and tie off my connection points.

 

[DRAGON64]

Longer is better unless your rocket hits the ground before shock cord is tight or it does not fit in the body tube.

I, too, like my parachute to attach a little down the shock cord from the nose cone.

I am hoping that going with the 7/16" Kevlar, that I can stuff 35' of harness and a small 24" drogue in the space just above the long motor.

 

[jahall4]

I am hoping that going with the 7/16" Kevlar, that I can stuff 35' of harness and a small 24" drogue in the space just above the long motor.

How about 50' of 3/8" in just a few inches?

 

[Not Quite Nominal]

I attach my main about 3 to 5 feet down from nose cone. Never directly to the nose cone.

I go the other way. Parachute at the end, nose cone two bundle-lengths down. It hangs as one long string coming down instead of having a nose cone flop.

Controversial opinion: drogue cord length should be based on weight of rocket as well as length. I do 5lbs/ft, a little more for high-density rockets, a little less for low density ones.

 

[bjphoenix]

In my search, 3X the length of your rocket is an accepted 'rule-of-thumb' length for shock cord design

I always wondered why the length of the rocket mattered. You could easily have 2 very similar rockets, similar in diameter, weight and engine size, but one twice as long as the other. Why would the longer one need more shock cord. So for mine I picked what I thought was an average long rocket for a certain class and went 4-5 times that long. So for my BT60 size LPR I use 10' kevlar. I think I could use less, and I do use less with streamers.

 

[Not Quite Nominal]

I always wondered why the length of the rocket mattered. You could easily have 2 very similar rockets, similar in diameter, weight and engine size, but one twice as long as the other. Why would the longer one need more shock cord. So for mine I picked what I thought was an average long rocket for a certain class and went 4-5 times that long. So for my BT60 size LPR I use 10' kevlar. I think I could use less, and I do use less with streamers.

I tend to go a little shorter with LPR.

Shock cord length matters most with dual deploy HPR - you want enough room for the two heavy halves of the rocket to tumble and slow down before you get to the end of the cord.

In LPR the light plastic nose cone isn’t going to load the cord up much. The only load on the other end is the parachute, which can be mitigated with a reefing ring or using a streamer like you do.

 

[bjphoenix]

I tend to go a little shorter with LPR.

I've thought about trying them a little bit shorter. I made a pair at 10' with snap swivels each end so I could move them from rocket to rocket. Then for the streamers I made up a new one at 6', I've thought about trying that one with a parachute. I have a few parachutes with snap swivels too. At the last local launch I launched 3 rockets and used the same parachute and shock cord in all 3.