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.
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
"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.
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.
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.
but that will kill the beautiful lines of this rare from estes lately gorgeous rocket.
just toss a nickel in the nose (glue a disc inside as a block drop in nickel glue in nose cone.
you will get your balance issue resolved and it will retain its clean gorgeous lines
hmmm lets make sure we are talking about the same ship :-)
blue with 2 side pods 4 sweeping fins central motor 18mm and side pods eject chutes out the rear ? thats what we are talking about right ?
I love that rocket. one of my all time favorites. its just a clean pretty rockets.
some mods I make SOP when building them.
angled panel in the ejection case path. block glued into main tube just above ejection ports.
the block decreased the amount of space that has to be pressurized ad the angles panels direct the ejections gases where they need to go and also prevent you from accidentally shoving wadding and chute past the point where the cases enter (which would burn up the chute and prevent ejection. see this a lot)
this also provides another firewall so that ejections cases do NOT directly inpings the inner wall of the side pod and burn through
I also lather the CA or epoxy on this angled panel for better fire retardation (never had one fail even after 50 some odd flights)
I want to use kevlar for the chute shock cord but I fear it will tear up the tube edges. so I mount the shot cord close to the top where it will be easy to replace and monitor etc.. for damage.
its important to lay it nice flush and flat though so it does not get in the way of the ejecting parachute.
I also sometimes upgrade it to D engine power. :-) this required 4 nickels in the nose (with the other mods included)
I was also thinking of 2 18 mm motors in the pods ad a central chute. have not got a change to try that yet.
this rocket is so pretty and has such nice lines that it is one of the few that I go the extra mile when it comes to painting. it just looks sooo nice. I would never want to alter it with addons and extra fins. would ruin the gracefullness of this rocket.
Same rocket. You propose mods during construction. I am looking for RSO fixes for the rockets that show up at a launch already built. They must be fixed to be allowed to fly, since 5 in a row have gone unstable. 5 Different Gemini DCs from different builders.
Nothing ruins the gracefulness of a rocket like it crashing into a kid's EYE or a luxury automobile in the parking lot or starting a brush fire resulting in the loss of a launch site.
There's always the "foam orbiter solution" - ban them from flight. I prefer to fix them if possible. There was no proven fix for the foam-orbiter-of-fiery-death.