Shock Cord Length Revisited or Shock Cord Length in Dual Deploy

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gary7

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Yes, there are general rules of thumb or guide lines for shock cord lengths. But I can not seem to find them for rockets using dual deployment. If I want to do dual deploy with the total lenghth of a 15 pound rocket almost divided in half at the AV-Bay, is there a general rule of thumb for the length of each cord?
 
I do 4X to 5X the rocket length on each section. If I'm somehow constrained in overall shockcord material, I'll be more generous with the drogue section.
 
about 4 or 5 times, like Dixon said

but the second part depends on drogue or no dougue.
I fly drogueless most of the time and I have a 4" that has 50 feet of Kevlar, I would normally made 40 feet , but Kevlar don,t stretch well so added 10 feet. Drogue section got 20 feet and the remained to the main section (30). Now the only problem I see is everyone has a different formula for BP to use(how much), I use very little on the drogue section, key is those ground tests to get just enough to split them part.

We have one person in the club that uses like 15 to 1 on cord and throws in a few dozen chutes to boot!
 
also depends on how everything will hang on the way down. You want to make sure that your nose and tail don't hang at the same level, to keep them from banging into eachother.
 
I disagree on the 4x - 5x length. I think it's far too long, and a properly packed/deployed parachute doesn't need it, and it can actually create other problems.

-Kevin
 
I disagree on the 4x - 5x length. I think it's far too long, and a properly packed/deployed parachute doesn't need it, and it can actually create other problems.

I don't know... I rubber band the SC for slow release along with using a deployment bag on most flights. I always seem to have great spacing and contolled deployment. Shorter may work, but longer may help if something goes wrong (late drogue firing for instance). I think I'll stick with what works for me.
 
I disagree on the 4x - 5x length. I think it's far too long, and a properly packed/deployed parachute doesn't need it, and it can actually create other problems.

-Kevin
Do you disagree on the 4x-5x length all rockets or just dual deploy as was in the original post? As I have found the 4x-5x widely quoted. What do you feel would be a good example of shock cord length for (x length rocket at x dia. at x Lb.s) for dual or single deploy?

-Scott
 
I agree with the 4-5x the length of the rocket,all my vulcanites have 20 foot shock cords,55 inch rockets x 4.5 = about 20 feet,works for me!! The shock cord in my Arcas is 36 feet long!
 
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Yes, there are general rules of thumb or guide lines for shock cord lengths. But I can not seem to find them for rockets using dual deployment.

Next time your at a launch,watch the flights.
Take note of rockets with short cords vs long.
Judge for yourself which are most successful & trouble free.
Take note of those that use shearpins & those that don't.
Ask how much bp they're using.

A basic 3in rocket.....is a 3 in rocket, no matter how fancy or plain it looks, the internal volume is basically the same for most payloads or close enough to do some ciphering.
Pay particular attention to rockets like yours, or very close in size.

Then decide for yourself.

It's all very relative. Those who use more powerful ejection charges, tend to use more cord to scrub off separation speed.

Blah, blah, blah. I could go on & on.

It boils down to this.Those that are most successful flying rockets ask & then filter advice. Apply it to their own situation and ground test their equipment.

It is a rocket & it IS rocket science! Do the homework.
 
Yes, there are general rules of thumb or guide lines for shock cord lengths. But I can not seem to find them for rockets using dual deployment. If I want to do dual deploy with the total lenghth of a 15 pound rocket almost divided in half at the AV-Bay, is there a general rule of thumb for the length of each cord?

It is a balancing act! also consider that there are different kinds of dual deploy, such as zipperles.

For me, I like to eject the fin can at apogee(rather than the aft or drouge section of airframe). This is different than you see in something such as a wildman kit where there is substantial airframe attached to the fins. the more airframe - the more stable it will fall.
what i see here, is the more stable the rocket wants to decend, the longer cord you can afford. however, when its just a spininning unstable fin can as on my rockets. shorter is better. less acceloration between the main section and drouge sections. "they like to go different directions" a long cord lets them get more energy going different directions and can cause zippers and shake mains out. a drouge can be added at the airframe to combat this.. but i have found shorter is better here.
for my sons darkstar dd. we used 20 of 1/4 kevlar, with a 12" drouge, it fell FAST....and fairly stable without much activity.

I only use about 10- 15 to 20 feet of shock cord on the main. its not really needed. all you need is enough lenght to shake off the energy of the ejection charge. So if you can eject and get all the laundry out with 2grams.. put in 2.5 grams for good measure, and enough shock cord so on your ground test, the nose cone goes as far as to near the end of the shock cord, but not act like a dog running on a chain....

Blah blah blah.. like Crazy Jim said... your mileage will vary, watch, listen, trial and error....Just remember, when you watch them fall, you want to see the aft end lead the way, and the main section to follow. you dont want them fighting eachother for the lead....(shock cord length can change this dynamic.) People have told me drouge chutes can actually make a rocket fall faster than without drouge chutes... i thought they were full of it... but... I have since learned differently...

launch a few to 1500 to 2000 feet so you can see it all up close.. change a little bit each time and see how it goes.
 
Do you disagree on the 4x-5x length all rockets or just dual deploy as was in the original post? As I have found the 4x-5x widely quoted. What do you feel would be a good example of shock cord length for (x length rocket at x dia. at x Lb.s) for dual or single deploy?

Yep, I disagree with it. One time, and only one time, I followed that. Every other time, I've used 6 to 12 feet, and even that's typically excessive.

The challenge is the "blow it up or blow it out" mentality, and the fact that far too many people basically shove a wadded parachute into the tube, and hope for the best.

If your ejection charges aren't horribly oversized, and you're not trying to shoot a piece of tubing into the next county, you find that 13 miles of cord isn't needed.

I've seen many rockets where the flier uses long cords, and on ejection, the parachute comes out, inflates, and stops, while the rocket is still in free-fall underneath. Then the rocket, which is continuing to accelerate, hits the end of the cord with a hard jerk. 'tis fun to then watch the rocket, now separated from the parachute, free fall to the ground.

Moral of the story? Use appropriately sized ejection charges, and learn to pack a parachute.

On our big projects (over 100lbs recovery weight), we use enough cord to get the lines a couple feet beyond the end of the tube, and no more.

-Kevin (whose only recovery failures have been when the recovery came out on the way up, or when the recovery didn't come out at all)
 
BTW, take a look at NASA and military recovery systems... Based on this logic, the SRBs should have what, 450 feet of recovery harness before the parachutes?

The military dumps heavy cargo out of aircraft and doesn't use 13 miles of recovery harness on those, either.

The trick is in the terminology -- from the model rocket world, we have "shock cord" which is elastic, and absorbs ejection load. In high power rocketry, it's imperative to think of it as what it is -- a harness. It's not meant to absorb a load, it's meant to hold things together under the recovery device.

-Kevin
 
The challenge is the "blow it up or blow it out" mentality, and the fact that far too many people basically shove a wadded parachute into the tube, and hope for the best.

If your ejection charges aren't horribly oversized, and you're not trying to shoot a piece of tubing into the next county, you find that 13 miles of cord isn't needed.-Kevin (whose only recovery failures have been when the recovery came out on the way up, or when the recovery didn't come out at all)

and an unqued moment of silence.........

Blow it up--- doesnt put out a parachute... but.. i have never broken recovery shock cords (oops i mean harness) either! Kevlar is cheap in relative terms... soo... i like to call them Exuberant charges... not appropriate.. BUT>>> i dont wadd up my parachute either. there are good ways of packing to "unfurl" the parachute... I never use less than 3 grams.. even in a 54mm darkstar...

big and heavy is deferent than a dragster on the strip...
 
I never use less than 3 grams..

Jeepers creepers...3 grams? Of BP?! On a 54mm airframe? Holy kah-blammo Batman. The 29/180 loads come with 1.5 grams and that is more than enough to push out a chute on my single deploy 30"x4" airframe. Heck, I have dual deploy bird with a main section about the same size (30"x4") that I use 2 grams on and it's enough to bust two shear pins AND push out the laundry.

-Dave
 
Jeepers creepers...3 grams? Of BP?! On a 54mm airframe? Holy kah-blammo Batman. The 29/180 loads come with 1.5 grams and that is more than enough to push out a chute on my single deploy 30"x4" airframe. Heck, I have dual deploy bird with a main section about the same size (30"x4") that I use 2 grams on and it's enough to bust two shear pins AND push out the laundry.

-Dave

well, its not like your charging a bullet.! where too much BP will explode the case and all mayhem breaks loose exploding recievers and bolts screaming at you...

all you are doing is presurizing a volume of air.... to create "X" expansion.... causing a force equal to the force required for separation.

so.. lets take my sons darkstar (i use that cause people on here are familiar with that rocket all my others are scratch built.)
for the apogee charge... the charge is placed on the avbay at the top of the drouge complartment, where (2) 2-56 nylon screws act as shear pins..
it only takes a finite amount of force to shear these two pins, and over come the friction on the coupler that is present in flight(not much if the pins did thier job). This force again is finite and can be "proven" with testing.

Now.... that said the amount of BP is irrelevant to the force to cause separation.. (only that it is enough) - extra, is only vented out of the end of the airframe... just like a bullet no longer accelorates out of the barrel of a gun, neither would the main section of the rocket AFTER separation.
THAT is not saying, the extra expansion created during the shearpins shearing and the coupler being free of the airframe doesnt allow acceloration.. because it does. (back to the arguement on a longer drouge harness to shake off energy of exuberant charges, not saying the kevlar would break... but 120g's is rough on anything)... but, the extra BP doesnt cause problems with the "exploding the rocket" as some claim...if the rocket has a failure from the BP, its from somehting else failing before the force of the shear pins being sheared. (ever pulled a parachute out of a nose cone!!) or someone putting a charge that more resembles a bomb than a "flame front generator" which a charge should be to cause air expansion, not BP detonation.... You could use 10 grams in a 54mm darkstar... you might get some "flash" out of the end of the airframe... but wouldnt "BLOW UP"...if you did it correctly..
wheras - we allow the bp to generate a flame front causing expansion... not detonation...
(how to make a charge like that is another thread)
Likewise- if you have no air the only way the flame front can travel is through detonation!!! so if you are preping a rocket, and push out all the airspace. 2grams will exlode your rocket...
goto a cold air ejection....

my 4" birds all get 4 grams....
 
Well...whatever floats your boat. I used to be in the "blow it out or blow it up" camp until I put a little extra powder in a charge and when it came time for the main to come out it destroyed a nice nylon chute and the cord snapped off because all the excessive charge burned through the cord. Probably could've used better protection from the charge like a chute protector but even so that event looked like it was hard on the airframe. So now, I do ground tests and only put in enough bp to do the job comfortably.
 
Blow it up--- doesnt put out a parachute... but.. i have never broken recovery shock cords (oops i mean harness) either! Kevlar is cheap in relative terms... soo... i like to call them Exuberant charges... not appropriate.. BUT>>> i dont wadd up my parachute either. there are good ways of packing to "unfurl" the parachute... I never use less than 3 grams.. even in a 54mm darkstar...

big and heavy is deferent than a dragster on the strip...

Kevlar will handle the load, but it's hard on the U-bolts, quick links, airframe, and every other piece of the recovery system and rocket that's involved, as well.

I've seen altimeter downloads where the accelerometer data shows the rocket hitting the end of a recovery harness with horrendous forces.

-Kevin
 
Gotta use enough to overcome air pressure from the velocity of the rocket falling - something not taken into account if all you do is ground test a rocket. Crazy Jim is right on when he says talk to the successful flyers and make decisions based on observing how they do it & how much BP they use... but a little too much is better then not enough, right? :cry:
 
Gotta use enough to overcome air pressure from the velocity of the rocket falling - something not taken into account if all you do is ground test a rocket. Crazy Jim is right on when he says talk to the successful flyers and make decisions based on observing how they do it & how much BP they use... but a little too much is better then not enough, right? :cry:

Operative word here is "little." See this rocket here:

[YOUTUBE]gh18PSYhNNU[/YOUTUBE]

We had an "oops" on the way down. 150 pounds of 20' long, 12" diameter tubing came in ballistic from 8k. The apogee charge failed to blow. If it would have worked perfectly the charge for the chute cannons would have come out at 1300. So, at 400+ mph the rocket separated with only the main charge going off. Guess how many grams of BP it took to separate that rocket.

:wink:

-Dave
 
To be fair...this was a charge in a 4' long 4" tube that butted right up against the bulkhead of the forward section. The tube was essentially filled with chute.
 
Kevlar will handle the load, but it's hard on the U-bolts, quick links, airframe, and every other piece of the recovery system and rocket that's involved, as well.

I've seen altimeter downloads where the accelerometer data shows the rocket hitting the end of a recovery harness with horrendous forces.

-Kevin

Yeah, but you definitely don't want to use too little. I've found 4 grams in a typical 5 inch rocket to be about right, with 2-3 grams in 4 inch rockets. I don't have any 6 inchers that use BP, so I don't know how much I would use in those.

(Oh, and I absolutely agree that more people need to figure out how to have a controlled deployment. I've recently been using deployment bags, and it's amazing how little shock you can have when deploying a main at 100ft/s)
 
Kevlar will handle the load, but it's hard on the U-bolts, quick links, airframe, and every other piece of the recovery system and rocket that's involved, as well.

I've seen altimeter downloads where the accelerometer data shows the rocket hitting the end of a recovery harness with horrendous forces.

-Kevin

i have those same horrendous forces on my data, IF my shock cord is not long enough! BUt, not near the horrendous forces of a balistic recovery....

I fly more BP than is required- on the happy side of things... and i test and have proven my methods of deployment to work. (100% success as far as my fleet is concened) (zippers and shaking out mains dont count when they were not a part of deployment. just a mishap during the fall back..)
Yes, i have damaged rockets from these horrendous forces as you so put.. but, they are still flyable without any "fixing".(they were built to take the punishment) A cracked bulkhead- or mmt is very benign damage...even a zipper is an easy fix...

It deffinately took some trial and error to find how to make my charges, how to pack everything, and make it all "work" 99% of it are pages i took from other peoples books...some i got by watching - some by 20 question method of other flyers and most I got from helpful people on the field that offered it up freely.
TEST TEST TEST...
I dont use any math, or any standard rules of thumb...
I test test test. till I like what happens.

:)clay
 
I use 10 feet of tubular nylon in my Vulcanite. My I-ROC EXT has two 25 foot shock cords, because I want to keep the forward section as far away from the booster section as possible. My L3 project will have 5 feet of 2 inch tubular nylon and 30 feet of 1 inch tubular nylon on both ends of the electronics bay.
 
I'll challenge anyone out there in the "right size ejection charge and short recovery harness" camp to a survivability challenge while I use all the BP and harness length I want.

Dual deploy rocket, main out between 500' and 1000', 4" through 8" airframe allowed.

Any takers?

The concequences of too little BP are far worse than those of not enough.

--Lance.
 
I'll challenge anyone out there in the "right size ejection charge and short recovery harness" camp to a survivability challenge while I use all the BP and harness length I want.

Dual deploy rocket, main out between 500' and 1000', 4" through 8" airframe allowed.

Any takers?

The concequences of too little BP are far worse than those of not enough.

--Lance.

Lance, put yer little Red Rider BB gun down...no one is going to have a shoot out at the OK corral over this.

Please note...I NEVER said too little. I said enough to do it "comfortably." Just enough that there is no doubt it will come out. I don't care what anyone says but 3 grams of 4f in a 54mm bird is waaaaaaay too much. I put a gram and a half in my 54mm bird and it stretched the quick-link. :y: 3 grams and I woulda had a hole in the side of the airframe with a creative core sample. I gotta draw the line SOMEHWERE.

Since no one seems to care I am going to tell everyone anyways. The correct answer to my question above...i.e. guess how many grams it takes for a chute cannon to separate a 150# 12" diameter rocket at 400+ mph is...DUHN DUHN DUUUUUHN...

two grams.

:horse:

You do what you think is right in yours and I'll do what works for me in mine. :fly:

-Dave
 
hehehe 2 grams, cool!
when i did my L3 the sims said that I should use 3 grams of BP, but CJ and Tim both said they use 6, so 6 it was. I got no business messing with that kind of experience! Sounds a little loud, but I've never had a problem getting my chute out!
 
hehehe 2 grams, cool!
when i did my L3 the sims said that I should use 3 grams of BP, but CJ and Tim both said they use 6, so 6 it was. I got no business messing with that kind of experience! Sounds a little loud, but I've never had a problem getting my chute out!

your taps didn't make you ground prove your project?
 
Lance, put yer little Red Rider BB gun down...no one is going to have a shoot out at the OK corral over this.

Please note...I NEVER said too little. I said enough to do it "comfortably." Just enough that there is no doubt it will come out. I don't care what anyone says but 3 grams of 4f in a 54mm bird is waaaaaaay too much. I put a gram and a half in my 54mm bird and it stretched the quick-link. :y: 3 grams and I woulda had a hole in the side of the airframe with a creative core sample. I gotta draw the line SOMEHWERE.

Since no one seems to care I am going to tell everyone anyways. The correct answer to my question above...i.e. guess how many grams it takes for a chute cannon to separate a 150# 12" diameter rocket at 400+ mph is...DUHN DUHN DUUUUUHN...

two grams.

:horse:

You do what you think is right in yours and I'll do what works for me in mine. :fly:

-Dave

why did it not separate at apogee?
2grams in a chute cannon, it takes even less if you use explosive bolts.
what did this deploy do to your quick link!

go lite 2 or 3 grams of bp and tell me what the difference is?
 
why did it not separate at apogee?
2grams in a chute cannon, it takes even less if you use explosive bolts.
what did this deploy do to your quick link!

go lite 2 or 3 grams of bp and tell me what the difference is?

As I said, we had an oops with the electronics installation. The ARTS2 was installed upside down and did not detect apogee.

your taps didn't make you ground prove your project?

It was a group project...not an L3 project. 2 grams was used because that was more than enough to get the chute out of the cannon. To our surprise it was also enough to separate everything.
 
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