Shock Cord Failures
homer101
I am relatively new to model rocketry, but have had about 50 launches in
the last two months. After a few flights with the same rocket, I am
experiencing several instances of shock cord failure. Since I fly only
low powdered rockets, C's and under, I must not know the tricks for shock
cord longevity.
Any suggestions from the pros?
Thanks,
Mark
Wolfram v.Kiparski
> After a few flights with the same rocket, I am
> experiencing several instances of shock cord failure. Since I fly only
> low powdered rockets, C's and under, I must not know the tricks for shock
> cord longevity.
How did your shock cord break? What went wrong?
Did it snap?
Did it burn?
Wolf
LarryC
homer101 (home...@ole.net) wrote:
> I am relatively new to model rocketry, but have had about 50 launches in
> low powdered rockets, C's and under, I must not know the tricks for shock
> cord longevity.
> Thanks,
> Mark
Hardly a pro, I - more like an internet.bore, but you're certainly fair game
for such an one :-)
I'm one of the few who actually *likes* the old Estes all-rubber shock cords.
They're fairly strong, and they are *much* more resistant to heat than the
light weight elastic they use in the E2X series. Dollars to doughnuts (less
than even odds in some parts of New York and LA :-) your problems concern
mostly models like the Dagger, the Bandit, and whatever clone they have out
now. If that's the case, order some old Estes shocks, glue them into the inside
of the tube ala Stine, and throw away the cords supplied with the kit. Chuck
out the shock cord mount, which adds much drag, and cannot be used with a
replacement cord anyway. While you're at it, chuck the draggy plastic
launch lugs they give you, and use the old kind. You can fill in the body
tube holes with epoxy after gluing a piece of paper on the inside. (With
the lugs and shock cord mount, the Dagger has a Cd of about 0.87! - not
that it doesn't go high enough on a C.)
You'd have to be pretty eccentric to take that advice, so here's another
suggestion. Reinforce your bouncy shck cord with a slack piece of Kevlar
fishing line. Arrange it so that the kevlar kicks in when the bouncy shock is
just taut. Never tried this with an E2X, but would bet that both cords could
fit in the draggy, inefficient mount :-)
Just finished building two balsa Alphas this way. They will be optimal mass
E25 attempts next month. Since optimal mass is quite high for this
configuration, my last attempt had a heavily-weighted nose, which ripped
right off the shock. (Much better than my previous attempt, where I weighted
the model with too much tracking powder, which prevented the nose from
blowing out. The walls did instead...). Anyway, I'm hoping that the
reinforcement will allow intact recovery before I run out of E25-10's.
You might order your cords, Kevlar, and even your models from Quest. That
works too :-)
Best Regards,
-Larry Curcio
Jonathan Sivier
homer101 <home...@ole.net> writes:
>I am relatively new to model rocketry, but have had about 50 launches in
>the last two months. After a few flights with the same rocket, I am
>experiencing several instances of shock cord failure. Since I fly only
>low powdered rockets, C's and under, I must not know the tricks for shock
>cord longevity.
>Any suggestions from the pros?
I'm not a pro, but I play one on TV. ;-) You don't say what is breaking
your shock cords, are they burning through or breaking from too much force?
One rule of thumb is that longer shock cords last longer. The ones supplied
by Estes and other manufacturers in kits are generally much too short. They
are often too wimpy as well. On most smaller rockets I use 6' of 1/4"
elastic with a Kevlar shock cord mount. If your shock cords are burning
through you might need more wadding. If they are breaking due to too much
force you might be using the wrong delays for your motors, so ejection occurs
when the rocket is moving too fast.
Jonathan
----------------------------------------------------------------------
| Jonathan Sivier | Secretary, Central Illinois Aerospace |
| j-si...@uiuc.edu | NAR #56437 |
| Flight Simulation Lab | Tripoli #1906 |
| Beckman Institute | Home Address: |
| 405 N. Mathews | 5 The Summit |
| Urbana, IL 61801 | Champaign, IL 61820 |
| 217/244-1923 | 217/359-8225 |
----------------------------------------------------------------------
John H. Cato, Jr.
homer101 wrote:
>
> I am relatively new to model rocketry, but have had about 50 launches in
> the last two months. After a few flights with the same rocket, I am
> experiencing several instances of shock cord failure. Since I fly only
> low powdered rockets, C's and under, I must not know the tricks for shock
> cord longevity.
> Any suggestions from the pros?
> Mark
Mark,
If you're talking about Model Rocket stuff (C's and under like you say), I
would look into something like some heavy Kevlar (tm) twine or some such.
The rubber band cords typically used in Estes stuff (do they still use
this?) is not at good as the narrow 1/8" elastic - but neither can take that
much abuse (heat damage and such). To be sure, Kevlar twine does NOT have
any 'boing' in it - but with good, secure mounting and longer lengths, it is
not necessary. By longer, I mean at least 3 to 4 times the length of an
elastic material it replaces. Also, a good trick is to make a short (6" or
so) bridle of some of that braided Stainless Steel leader material as the
'base anchor' in the rocket body. Simply provide a good quality snap on
the end of the textile material to connect to a formed loop (w/ crimped
sleeve) in the SSt bridle. This will allow you to replace the textile
part when it gets worn or shows other signs of weakening and not have to
tear apart your rocket. Again, this 'mentality' of 'boing' in your shock
cord is pretty much an 'old wives tale' - it is not really needed with
proper design & construction.
Hope this helps...
-- john.
homer101
LarryC wrote:
>
> homer101 (home...@ole.net) wrote:
> > I am relatively new to model rocketry, but have had about 50 launches in
> > the last two months. After a few flights with the same rocket, I am
> > experiencing several instances of shock cord failure. Since I fly only
> > low powdered rockets, C's and under, I must not know the tricks for shock
> > cord longevity.
> > Any suggestions from the pros?
> > Thanks,
> > Mark
>
Thanks for the advice. The rockets that have recently failed include:
Estes Pegasus ( it also has a nasty habit of melting the fin assy.),
Hijax( sans payload) and Supershot.
BTW, I tried using cellulose insulation in place of wadding in my tiny
Sparrow today. It imbedded itself a good inch and a half into a baked
clay ground, the epitome of a lawn dart. It'll require surgery to
remove the top inch or so of the fuselage. Maybe it'll fly even higher
now. ;-)
Mark
Dave Lyle
cur...@telerama.lm.com (LarryC) wrote:
>Just finished building two balsa Alphas this way. They will be optimal >mass
>E25 attempts next month. Since optimal mass is quite high for this
>configuration, my last attempt had a heavily-weighted nose, which ripped
>right off the shock. (Much better than my previous attempt, where I
Could someone explain to me the concept of 'Optimal mass' and how it is
determined? I suspect it has something to do with the trade offs in
boost/coast altitudes.
Dave
LarryC
Dave Lyle (da...@execpc.com) wrote:
> cur...@telerama.lm.com (LarryC) wrote:
> Could someone explain to me the concept of 'Optimal mass' and how it is
> determined? I suspect it has something to do with the trade offs in
> boost/coast altitudes.
Excisely. The coast altitude/launch mass curve has a peak. At masses below
peak coast distance mass, the rocket is too easily overcome by drag. It's
momentum is from high initial speed, and drag is proportional to the square
of the speed. At masses above peak coast distance mass, the rocket starts
out at too slow a speed to coast very far. Meanwhile, boost altitude
decreases monotonically with launch mass.
Optimal mass occurs near the end of the increasing portion of the coast
altitude/launch mass curve. At optimal mass, the change in boost altitude
from a small change in mass is just offset by the (opposite) change in
coast altitude. Note that around O.M., the launch mass/altitude curve is
rather flat - because the balance of effects still holds approximately.
You can approximate optimal mass by bracketing the drag coefficient of
your rocket. Using the eyeball method, come up with a maximum and minimum
possible Cd. Find the optimal mass at each of these values. Take the average
of these masses, and then compute the altitude at the two bracketing Cd's
with this mass. The higher of the difference between these altitudes and the
true optimal mass altitudes at the ends of the intervals is a maximum
credible altitude penalty for misestimation. Because of the aforementioned
curve flatness, the altitude penalty should be trivial. If that's the case,
the average is your working optimal mass.
You find O.M. by fiddling with simulation programs, varying launch mass to
get the maximum altitude. DIGITRAK does that automatically.
Regards,
-Larry Curcio
Rick Taylor
The Silent Observer wrote:
> I haven't managed to keep a rocket long enough (yet) for the shock cord
> to part due to damage from ejection, but based on the way they get
> brittle, I think it's only a matter of time. I'm planning to start
> building a loop of steel fishing leader into the rocket in place of the
> standard Estes shock cord mount; I can then attach both ends of the cord
> with snap swivels (from the same fishing outlet) and replace it when it
> get too tired.
I just bought some 30# test 12 inch steel leaders with a swivel on one end and a
snap hook on the other. I plan on using these somehow to replace the standard
shock cord mount.
OTOH I also have some 150lb test stainless (for fishing) that may also work
into a shock cord mount method somehow.
(experimentation is part of the hobby -- someone has to come up with new ideas!)
Awards1
I don't think that any exposed shock cord can be flown more than 3 or 4
times safely.
Get in the habit of replacing them if you see any brittleness or get ready
to replace your rocket.
On larger diameter rockets, some sort of blast sheild works good, such as
a nomax heat shield or just plain leafy cabbage.
A friend of mine has been wrapping alluminum foil around the shock cord in
the body tube and taping it on tightly so that it doesn't fall off (for
the benefit of not having any cows eating it)
I always double up my shock cords and when it breaks, I have the other
cord to catch it. I've never lost a rocket to shock cord failure.
Tracy Dungan
Tripoli OK
Midwest Trophy MFG
awa...@aol.com
Joe Pfeiffer
I am relatively new to model rocketry, but have had about 50 launches in
the last two months. After a few flights with the same rocket, I am
experiencing several instances of shock cord failure. Since I fly only
low powdered rockets, C's and under, I must not know the tricks for shock
cord longevity.
Any suggestions from the pros?
Here are a few from an amateur --
First, are the shock cords in question all-rubber? If so, switch to
sewing elastic. Also, while you're switching shock cords, switch to a
longer one: at least twice the length of the body tube.
Second, rather than using the
folded-paper-glued-to-the-side-of-the-body-tube Estes shock cord
mount, use some kevlar string attached securely to the motor mount,
with the shock cord tied to it. This way, if the shock cord starts to
look old, you can change it easily.
--
Joseph J. Pfeiffer, Jr., Ph.D. Phone: (505) 646-1605
Assistant Professor FAX: (505) 646-1002
Department of Computer Science http://www.cs.nmsu.edu/~pfeiffer
New Mexico State University
Las Cruces, NM 88003
Elaine Nowak
> Here are a few from an amateur --
>
> First, are the shock cords in question all-rubber? If so, switch to
> sewing elastic. Also, while you're switching shock cords, switch to a
> longer one: at least twice the length of the body tube.
>
Beware of the kind of elastic band you use!!!
Last weekend I had a bit of a mishap with my Sidewinder when it turned
into a lawn dart. The shock cord seperated about 3 inches above where it
is mounted to the body tube..and it spiralled down 900ft into the ground
and burring itself about 6 inches and straight as an arrow!...I'm glad
the sidewinder spirals and that it didnt suffer any major damage. The
only real damage it suffered from a riped engine hook, which was easily
reparied (thanks to my sweetie :-) )
After trying to find out what ripped the shock cord (we first thought it
might have been parachute swivel got caught or something), we then
realized the cord actually became brittle. We then started to look into
what these elastic materials were made of, the most common is a
polyester/rubber blend. To be exact 30% rubber and 70% polyester. The
polyester doesnt hold up to the punishment of the ejection gases!!
We were thinking of trying a cotton/rubber elastic band material..or
follow up on a tip that was given to me earlier from Bill:
He wrote this tip:
At Walmart. Look in the camping gear section, they have
a package they call a "shock cord repair kit" for replacing the rubber
cord inside of some tent poles. It's a roll of about 20 feet or so of
nice 3/16" diameter white shock cord and some hardware and wire.
Has anyone tried any of these ideas?
Elaine
PaulDiming
In article <31FD0D...@lexis-nexis.com>, Rick Taylor
<Rick....@lexis-nexis.com> writes:
>
>(experimentation is part of the hobby -- someone has to come up with new
>ideas!)
>
>
Sometimes the hard part is the "new" ideas. I believe a number of folks
have used steel wire leaders. In one case, they connected the leader to
the engine mount (like Quest). I'm not sure about the other variations on
how to use metal leaders. But I do know a bunch of people have tried
and/or are already doing this. But do enjoy the fun of trying something
new for you. Let us know how your experiments turn out. Eperimentation
IS fun!
Paul.
Fly Baby Fly!
John H. Cato, Jr.
Elaine Nowak wrote:
>
> After trying to find out what ripped the shock cord (we first thought it
> might have been parachute swivel got caught or something), we then
> realized the cord actually became brittle. We then started to look into
> what these elastic materials were made of, the most common is a
> polyester/rubber blend. To be exact 30% rubber and 70% polyester. The
> polyester doesnt hold up to the punishment of the ejection gases!!
>
> We were thinking of trying a cotton/rubber elastic band material..or
>
> follow up on a tip that was given to me earlier from Bill:
>
> He wrote this tip:
> At Walmart. Look in the camping gear section, they have
> a package they call a "shock cord repair kit" for replacing the rubber
> cord inside of some tent poles. It's a roll of about 20 feet or so of
> nice 3/16" diameter white shock cord and some hardware and wire.
>
> Has anyone tried any of these ideas?
Elaine,
One thing that many do not know is that the rubber compounds go through
chemical changes each time they are stretched - outgassing Hydrogen gas,
as I recall - and, effectively, making them weaker and weaker each time
they are used. This does not even take into account any chemical
reactions that (most likely) *do* occur with interactions of ejection
gases.
On the small scale of model rockets, it is not as big a deal - but the
'chemical realities' apply, regardless of the scale. The best approach is
to mount in the rocket a short, stainless steel 'leader' material (from
the fishing supply places) permanently in the rocket, such that whatever
'shock cord' is used, it can be simply replaced when worn or aged. And,
the most important thing is to not try to 'push' that shock cord too far -
when it begins to look bad, scorched, brittle, etc. - trash it.
As far as the 'tent pole' repait kit - yeah, the round 'bungee stuff' is
better than flat elastic - but not by much. Take a short piece of this
and pull the rubber bands out and look at what you have left and you will
see what I mean. Conservatively, with what I said earlier about the
chemical changes that go on in rubber bands, the rubber 'part' should be
considered to add ZERO to the ultimate strength of the whole 'cord'.
The repair kit is one way to get ahold of this - but I would imagine it is
a lot more expensive approach. The round 'bungee' cord can be had in bulk
- either from 'climbing shops' (I think?), some hardware stores, or from
Industrial Supply places such as McMaster-Carr - all the way down to 1/8"
size. From my experience, and in my opinion, 'bungee's' UPPER limit (of
rocket mass) is about 5 lbs - maybe 10 - but not a pound more.
For small rockets, some of this heavy Kevlar twine (0.020-0.040" dia) will
do nicely (several 100 lbs breaking strength), weighs less, has higher
temp resistance, and can be packed much more compactly. Into the bigger
rockets (above, say 2-3 lbs), the tubular nylon stuff begins to find
better application (up to 4000 lbs test). Neither of these materials has
the stretch that the elastic materials do - but that is easily solved by
using 2-3 times the length of an elastic material, and just doing a decent
job of anchoring the ends.
Just FWIW - from my perspective.
-- john.
Darrell D. Mobley
John, an update here on the bungee materials.
Ky Michaelson has some nylon covered bungee that is small and yet strong.
He also mentioned that he is getting some that is Kevlar covered...