Clustering Question

Being still a relatively new BAR...I am beginning my first venture into clustering, by building a clone of the classic Estes Astron Cobra. Reading articles
and reviews by other rocketeers, I often see how these people mention using extra wadding, to compensate for the extra ejection charge of multiple motors. But, I ask...how can this matter? What are the odds of all two, three, or more, motors firing their ejection charge at the same time? Not only would the rated time delay have to be 100% accurate, and consistent...but all the motors would have had to ignite at the same precise moment as well. I would think that in all likelihood, the motors would *not* fire their ejection charge at the same instant. The first charge would deploy the recovery system, while the following charges...even if a fraction of a second later...would be redundant, and pose little "threat" (if one's fears are of extra power from a multiple ejection charge). Is my thinking correct here?
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Greg Heilers
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Greg Heilers wrote:

Estes used to recommend that you stuff a little wadding on top of the ejection charge of every motor in a cluster. That way if one should not light it's ejection charge will not be lit when the others go off, and you have a reverse burn (not a pretty sight !).
There can be a plus or minus 20% difference in delay times and still be in spec, so even if the motors all lit at the exact same time they could go off at different times. The extra wadding makes sense, as most cluster birds are bigger in diameter, and "blow by" , especially if you use dog barf wadding is more likely.
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writes:

I wonder if/why this is still necessary. The motors all have ceramic ejection caps. Can an ejection charge set off another that is behind the ceramic cap?

This is pretty clear in flight. Even with simultaneous ignition, the ejection charges sound like popcorn with multiple reports. My favorite is a friends "D80", a cluster of 8 A10-3T motors in a BT-70. Sounds pretty cool with all those ejection charges.
--
Bob Kaplow NAR # 18L >>> To reply, there's no internet on Mars (yet)! <<<
Kaplow Klips & Baffle: http://nira-rocketry.org/Document/MayJun00.pdf
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Bob Kaplow wrote:

I think it would depend on the motor used. If you use a D12-0, there is no cap on the fore end of the motor, is there?
I wouldn't use a zero delay motor for all clusters, but as a booster stage in a cluster, its very possible to use them.
-Aaron

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But you can't plug the boosters because they are needed to ignite the upper stage. I've seen more than one TARC booster ignite the back end of a D12-0 at staging...
So I repeat: why is the wadding still needed in a cluster on top of the ejection clay charge cap?
--
Bob Kaplow NAR # 18L >>> To reply, there's no internet on Mars (yet)! <<<
Kaplow Klips & Baffle: http://nira-rocketry.org/Document/MayJun00.pdf
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There can be some black powder dust residue along the inside of the motor tube above the clay cap & between the clay cap and casing. If this burns or smolders it could lead to the ejection charge and get the unignited motor ignited backwards. I've seen it happen.
I also think I (and dozens of other witnesses) saw another cluster ignition oddity. Estes 36 D Squared with 2 D12-5 motors. Only one ignited on the pad. Rocket flew about 30 degrees from vertical. Just after motor buyrnout, the second motor ignited! This thing had Estes igntiers and plugs. We examined the motors and rocket carfully.
Here is my theory: Plugs popped out when igniters fired and only one motor ignited on pad (or uniginted motor's igniter was pulled out when rocket lifted off). At motor burnout, the delay produces a HUGE flame with low velocity. If you ever static tested motors as I did in the MIT blast chamber, you would seriously appreciate the delay flame and afterburn. Anyway, I suspect the airflow around the base of the rocket allowed the heat from this delay afterburn to flow into the nozzle or shine into the nozzle and ignite the propellant from the IR.
Motor showed no odd scorches nor was it installed upside down.
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-Fred Shecter
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snip
Greg,
When we fly our 5 engine Saturn V, we use the same delay on all engines and they go pretty much together. In 20+ flights there has been less than .3 seconds from 1st to last. I can usually count 3 separate ejections which means at least 2 are at the same time. There is a LOT of fire & smoke pushing the upper stage off the lower stage. We use 4 pieces in each stuffer, 2 laid flat on top of the stuffers, a piece or two between each of the 3 chutes and I still have had chutes that were singed.
You do need plenty of wadding. Might not be as bad with just 2 engines. I haven't seen yours (bt 60?) but if your building the Estes version your chute compartment is pretty small for 2 ejections.
Randy www.vernarockets.com
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I like clusters, too and fly them fairly often. Only a small percentage of the time do I notice non-simultaneous ejection, so I've been assuming they're pretty close. So:
Senario 1: Non-simultaneous ejection with extra wadding - you "waste" a little wadding.
Scenario 2: Simultaneous ejection without extra wadding - possible flame damage, failure to deploy, even reverse burn.
Which would you rather risk?
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On Wed, 22 Feb 2006 10:26:42 -0500, Scott Schuckert wrote:

Well...it is not the extra wading that I was questioning, as I have always been one to use about twice what is recommended, as well as wrapping the chute in an additional sheet. I was just questioning the "triple-strength ejection charge" concept...as I was having a hard time believing that motors are manufactured to such a consistency, that multiple motors would actually set off their charges at the same instance.
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Greg Heilers
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I'm a bit of a cluster hound (I have clustered A3-6T's in something I call Cheap Thrills) and have never really noticed any problems. I use dog barf wadding and have never had a problem. I did notice a kind of echo effect when flying my LOC Viper IV on 4 E9-8's, but it caused no problem with chute deployment.
Bill Sullivan
"You know, just once I'd like to meet an alien menace that wasn't immune to bullets."
The Brigadier, in "Robot"
Greg Heilers wrote:

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