shock cord length

What length of shock cord would I need for this rocket? Is there a formula
for this?
approx. weight 2.5 - 3 lb
length 48"
chute 36" 1/4 round (umbrella)
shock cord used is 1/8" kevlar bungee attached at the motor mount.
Thanks
Mike
Reply to
Mike
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I've been told "if there's extra room in your recovery bay, your shock cord is too short".
Gary
Reply to
GGoldy
Hi Mike...
On my 5.5x Streak (33 inches long) I am using two ten foot 3/8 tubular kevlar shock cords. I have a streamer attached one foot below the altimeter bay (and nine feet above the fin can). The parachute attaches one foot below the nose cone (and the same nine feet above the altimeter bay).
My back-of-a-beer-soaked-cocktail-napkin analysis is; at apogee, the Pro38 ejection charge separates the fin can from the altimeter bay. With the streamer closer to the altimeter bay, it *should* descend with the fin can hanging down and not banging into the front half. At altimeter deployment (hopefully) the whole thing will come down with the fin can lowest, the altimeter bay in the middle and the nose cone dangling loosely from the parachute.
I'm using the RDeH8 method to attach the kevlar tube to the u-bolts, with a couple of stitches of kevlar thread added as 'worry-proofing'.
John
Reply to
John Bonnett
Yep. And that goes for HPR too! ; )
Randy
Reply to
<randyolb
Kinda puts my little "Lil Man" rocket to shame.....(You should have seen it next to a PML Ultimate Endeavour)
The rascal has 6ft of kevlar line, a 2in Square piece of kevlar blanket, a tiny fishing swivel and a 12inch plastic chute........ All that gets crammed into a recovery bay space of 65mm x 18mm.............about 52mm x 18mm if ya want to get technical and allow for the eye screw... With such a small amount of space even an A8-3 looks like a cannon going off!!!
Does adhere to the other posts rules though..........there's certainly no space left....!!!
>
>> What length of shock cord would I need for this rocket? Is there a > formula >> for this? >> >> approx. weight 2.5 - 3 lb >> length 48" >> chute 36" 1/4 round (umbrella) >> shock cord used is 1/8" kevlar bungee attached at the motor mount. >>l >> Thanks >> Mike >> -- > Hi Mike... > > On my 5.5x Streak (33 inches long) I am using two ten foot 3/8 tubular > kevlar > shock cords. I have a streamer attached one foot below the altimeter bay > (and > nine feet above the fin can). The parachute attaches one foot below the > nose > cone (and the same nine feet above the altimeter bay). > > My back-of-a-beer-soaked-cocktail-napkin analysis is; at apogee, the Pro38 > ejection charge separates the fin can from the altimeter bay. With the > streamer > closer to the altimeter bay, it *should* descend with the fin can hanging > down > and not banging into the front half. At altimeter deployment (hopefully) > the whole > thing will come down with the fin can lowest, the altimeter bay in the > middle and > the nose cone dangling loosely from the parachute. > > I'm using the RDeH8 method to attach the kevlar tube to the u-bolts, with > a > couple > of stitches of kevlar thread added as 'worry-proofing'. > > John
Reply to
CJC
I have found there are practical limits. In a dual-deploy EZI-65, ~75 feet was "too much", as the shock cord had more drag than the drogue and made a big "M" in the sky with the drogue as the lowest point. This pulled the bottom and top sections of the rocket together, and I feared a mess. ~45 feet is more than adequate for that particular application.
Hey, I had to know if there was such a thing as too much. Got my answer, too. ;-)
Reply to
Tweak
That's tweaky.
Reply to
Jerry Irvine
Over here the trend has tended to be drogueless from apogee and then main at 1000 or 800'
I've found that shockcord length as a minimum should be 2.5 x rocket length
Damian
Reply to
Damian Burrin
I ran sims both with and without the streamer, and didn't like the descent rate unless the streamer was installed. I don't want the chute popped out at much over 100 FPS. Without the streamer it was coming down at ~190 FPS.
John
Reply to
John Bonnett
I thought Tweaky was Buck Roger's robot.
Randy
Reply to
<randyolb
Well, I guess if the altitude is not great enough to allow the shock cord to fully deploy, it could be construed as too much.... I guess.... I suppose... I guess....
Ever seen a Yogi Bear deployment where the rocket lands, bounces, and then ejects the chute? ; )
Randy
Reply to
<randyolb
Been there done that and saw The Wiley Coyote Special core samples and then the rear section of the rocket is blown out of the hole by the ejection charge about a second after inpact. Was funny as all get out but you wanted to cry for the guy too, it "was" a beautiful rocket.
Reply to
nitram578
According to Sivier's Rule (TM) the correct amount of shock cord and parachute (or other recovery material) is just a bit more than will actually fit in the available space. ;-)
Jonathan ----- Jonathan Sivier Secretary, Central Illinois Aerospace jsivier AT uiuc.edu NAR #56437 Tripoli #1906 CIA Web Site:
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"Remember to always keep the pointy end up."
Reply to
Jonathan Sivier
At my first launch 2 years ago, after returning to the hobby, the shock cord on my Commanche-3 broke (I gave the farmer who owned the property the core sample, figuring he could put it to better use than I). The advice I got that day from the club I've followed ever since. "Take the kit's shock cord and pitch it! Then get another one triple that length to replace it." Yesterday I gave similar advice to the 3 TARC teams I'm mentoring (btw, they all have in their qualifying flights and at least one is going to the nationals IMHO)............
"There's no such thing as too much shock cord if there's room in the rocket for it, high power rockets routinely use 100' or more. Long shock cords can be a real advantage for TARC because it let's the booster section touch down first, reducing the weight, and thus allowing the payload section with the eggs to land slower/softer."
Chuck
Reply to
Zathras of the Great Machine
care to brag?
Reply to
tater schuld
And make it twice as heavy...
Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!
Reply to
Bob Kaplow
And three times more likely to snag on power lines, trees, and other hazzards.
Reply to
Alan Jones
Wasn't it Twiggy?
Reply to
Tweak
Oh no, it was fully deployed WAY up. Worked fine for a bit, then you could watch the two sections start making big arcs in the sky, drawing the rocket pieces together.
I am noticing some doubt from people around here, like I would go to the trouble to make up a fib. So I say try it. Take a < 5 lb dual deploy rocket and run 75ft (or more) from the fincan to the upper section with a drogue stuck somewhere in the middle (not exactly in the middle, which I don't need to explain for most, but mentioning it will give the pedantic one less thing to pipe up about).
Yep.
Reply to
Tweak
I could stuff enough shock cord in a 7.5 inch rocket to let it hit the ground before the main deployed.
That begs the question...just how much 1/2" TN could you stuff in a 3 foot section of 7.5" tubing?
I don't know anyone who uses that much shock cord. I know I don't. At the most maybe ~60 feet from fincan to upper section, then 15-20 for the main in a L3 class rocket
Reply to
Tweak

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