SAFETY QUESTION - possibly "ALERT"

Hello,
Has anyone out there noticed an increase in the number of unstable flights of the Estes Gemini DC kit?
If so, have you identified the root cause?
Did they change something in the kit (nose cone lighter? pod cones heavier?)
We've had a rash of completely unstable flights with these models. One was probably caused by a canted side pod, but the others had no obvious building errors. And there was no sign of "Poopy Clay" nozzle erosion and vectored thrust. AND they were all launched from a 1/8 inch diameter x 4 foot long steel launch rod.
I'm tempted to make everyone glue on 2 additional small fins on the aft end of the main body on the locations not occupied by the side pods. And use B6-2 or B4-2 motors only.
-Fred Shecter NAR 20117
-- """Remove "zorch" from address (2 places) to reply.
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Come to think about it, I don't recall ever seeing one fly stable in the last 4 years. I always thought it needed more nose weight.
Ted Novak TRA#5512
Fred Shecter wrote:

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On Mon, 24 May 2004 13:52:46 -0500, nedtovak

Interesting, mine's always flown perfectly. Not often, though, pain to prep.
tah
--
Tod A. Hilty
Hilty Information Systems
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I've never seen one fly right. I've seen them fly wrong though.
On Mon, 24 May 2004 13:52:46 -0500, nedtovak

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It must be the atmosphere. They fly quite well in the thick humid air in southeast Alabama.
We had one launch with seven or eight Gemini DC's taking part a contest. The winner for duration with both parachutes fully deployed. I believe all were built stock except for the tube reinforcement where the ejection charge hits the BT-20 tubes.
We had one that was a little squirrelly and concluded it was from vectored thrust. The remaining flights were nominal, or mostly so.
Vectored thrust....reminds me of:
Here's our clearance, Clarence. Roger, Roger. What's our vector, Victor.
Still makes me laugh.
John
wrote:

flights of the Estes

heavier?)
was probably caused

And there was no sign

launched from a 1/8

end of the main

B4-2 motors only.

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Could be they just get "stuck" in the air. ; )
Randy
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Humid air is thinner (less dense). It only feels thick.
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Average 350 feet ASL here.
John

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it could have been "Poopy Clay Vectoring" that many years ago. I hope the Estes lurkers are lurking and reading. If I confirm a design flaw I will officially contact them and inform them.
They really need larger margins of safety (more calibers of stability) on a lot of kits.
-Fred Shecter NAR 20117
-- """Remove "zorch" from address (2 places) to reply.

Estes
up,
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On Mon, 24 May 2004 19:38:13 GMT, "Fred Shecter"
<snip>
What's beginning to scare me is that the above term is starting to become widely used as a "technical" term to describe thrust vectoring.
I think we should just call it the "Shecter Effect"...
<vbg>
tah
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Tod A. Hilty
Hilty Information Systems
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wrote:

Shouldn't that be Shred Vector effect?
:-)
John

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Isn't the Gemini DC the ONLY Estes kit in release designed by Matt Steele?
Jerry

--
Jerry Irvine, Box 1242, Claremont, California 91711 USA
Opinion, the whole thing. <mail to: snipped-for-privacy@gte.net>
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If so, can he or anybody else, post a copy of the instructions (or e-mail them to me). Then I can compare the "should be built like this" condition to the "kid built it like this" condition.
-Fred Shecter NAR 20117
-- """Remove "zorch" from address (2 places) to reply.

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The one I had didn't come with clay. Are they supposed to? I know they could sure use it.

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"Poopy Clay" refers to the motor nozzle clay material, NOT to "clay" (actually plastiscene) inserted into nose cones for nose weight - which it could obviously use! Long cut&pasted standard motor failure message follows:
Here is why temperature cycling can cause Catastrophic failures (CATOs). The nozzle, propellant and the casing all expand and contract at different rates. Since the motors are so small, this is only a problem if the temperature that the motor "sees" swings between wide extremes. When this happens, we see several effects:
1) The propellant and the clay nozzle develop a crack at their interface. This actually results in *Lower* peak pressure and peak thrust because the motor can begin the end-burning earlier than it should (never forming the "big dome" of burning surface area that we should get at normal peak thrust).
2) The casing and the propellant can de-bond. They aren't really bonded in a "glue" sense, but the mechanical bond is weakened from the stretching and contraction. (For wet rammed motors, there may be a tiny glue-like "bonding", but the cycling will break that bond). The flame can propagate along the entire inside of the casing and propellant interface and result in a huge overpressure. This leads to a casing split (if the delay is still "grabbing" the casing tightly) or a "blow through" which is like a Roman Candle.
The two of these can combine to form different CATO scenarios:
a) Blow through at ignition or just after ignition (on the pad/rod). Clearly a sign of a nozzle/propellant interface crack allowing the flame front to reach the debonded casing to propellant interface at or just after ignition.
b) CATO above the pad (like 50 feet up). Clearly there was no crack along the propellant/nozzle interface and the flame front had to wait until it naturally reached the casing wall and then propagate up the de-bonded propellant/casing interface.
A final scenario is the cracked propellant grain. These can go BLAM (or KA-PLOW) quite spectacularly since they really overpressurize the casing big-time and can happen with a perfect casing to propellant bond. A defective tool used to form the centerbore of the propellant can cause these. The C5-3 had such a problem when a tool was mis-manufactured. I believe the root cause was a lack of radius on the tip, which formed a sharp edge, which led to cracking. Motors also could be cracked if any contaminant got on the tool or in the propellant during ramming, but dropping or rattling will not cause a crack!
As for the temperature cycling - avoid firing a motor at a temperature 75 degrees F lower than the highest temperature it has ever seen. If fired while too cold, the propellant will be contracted away from the casing and it will probably fail. Folks launching in cold weather can do so if they store their motors in their warn car or in their toasty parka inside pockets. (Is that an F100 in your pocket or are you just happy to see me?)
Why would a normally stable rocket fly unstable when using a motor that it flew stable with before?
Did you look at the nozzle? We have had several VERY scary "flights" where the rocket had little thrust and/or veered into cruise missile mode. After crashing and putting out the brush fire, we examine the nozzle and find that it is either too wide (wider than normal at the throat) or it is eroded asymmetrically. The asymmetric erosion is bad and you can clearly see the exhaust residue all over the missing area of the nozzle indicating that it disappeared at ignition or shortly thereafter.
All unstable flights with Estes motors from years "A" and "B" and maybe "C" need to be inspected and if the motor/nozzle is the cause, a M.E.S.S. form filled out and the manufacturer notified. The least that will happen is a package of replacement motors and a kit. The most that will happen is an improvement in materials used in manufacturing and a product that performs like we remember for decades and decades.
I hope this info helps folks.
-Fred Shecter NAR 20117
-- """Remove "zorch" from address (2 places) to reply.
wrote:

kits.
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Perhaps it's due to over building where the ejection gasses are ported into the side pods. Mine uses standard nose blow recovery, and flies perfectly stable all the way up to C motors. There have been lots of reports of the rear ejection not working, burning through, ect., and I would say any one who has read reviews of the kit will tend to overbuild it, thus throwing the CG/CP relationship off. Just my thoughts, David

of the Estes

heavier?)
probably caused

there was no sign

launched from a 1/8

end of the main

motors only.

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the problem I imagine is CG
the model is borderline anyway. with enough residual build up after many flights OR with heavy glue use in the rear it could shift the wrong way. I just glue a nickel in the nose and have never had a problem since.
Chris Taylor http://www.zodiacreview.com /

of the Estes

heavier?)
probably caused

there was no sign

launched from a 1/8

end of the main

motors only.

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I'm tempted to either add a few small fins at the back end between the existing fins (2 or 4) OR I might just box in the existing fins with a nice tight rectangular masking tape loop. 1/5 inch wide. It will add a lot of effective fin area for very little added weight.
Or maybe some spin tabs - but that might just get me a spinning, unstable rocket....
-Fred Shecter NAR 20117
--
"""Remove "zorch" from address (2 places) to reply.
"Chris Taylor Jr" < snipped-for-privacy@nerys.com> wrote in message
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I fly mine with 13mm motors in the pods, added weight to the nose and it flys great !
------------------------------------- Jim Chambers ------------- NAR 80299 L1 ------------------------------------- F.A.R.T. Friendly American Rocket Technologies Friendly American Rocket Team --------------------------------- God............ Grant me the Senility To Forget the People I never Liked, The Good Fortune to run into the Ones I Do, and the Eyesight To Tell The Difference ! ---------------------------------
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Would that be stable? I'd think the tape would start flapping in the breeze and really cause you some problems.
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William Smith
ComputerSmiths Consulting, Inc. www.compusmiths.com
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