What length of shock cord would I need for this rocket? Is there a formula
approx. weight 2.5 - 3 lb
chute 36" 1/4 round (umbrella)
shock cord used is 1/8" kevlar bungee attached at the motor mount.
On my 5.5x Streak (33 inches long) I am using two ten foot 3/8 tubular
shock cords. I have a streamer attached one foot below the altimeter bay
nine feet above the fin can). The parachute attaches one foot below the
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
closer to the altimeter bay, it *should* descend with the fin can hanging
and not banging into the front half. At altimeter deployment (hopefully)
thing will come down with the fin can lowest, the altimeter bay in the
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
of stitches of kevlar thread added as 'worry-proofing'.
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
Does adhere to the other posts rules though..........there's certainly no
>> 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
> 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
> of stitches of kevlar thread added as 'worry-proofing'.
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
I ran sims both with and without the streamer, and didn't like the descent
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.
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...
Ever seen a Yogi Bear deployment where the rocket lands, bounces, and then
ejects the chute? ; )
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.
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. ;-)
Secretary, Central Illinois Aerospace
jsivier AT uiuc.edu
CIA Web Site:
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."
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).
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