Clustered Gemini DC

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RocketboyG80

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I was thinking of getting an Estes Gemini DC and clustering it by putting an 18mm motor in each of the "parachute tubes". I already know I will have to add some noseweight (at least 1oz) to keep it stable because I hear they are hardly stable with one motor. :rolleyes: But I was wondering if I cut those outboard tubes down a few inches will that will help to push the CG farther forward? Also, what do you think about igniting a 24mm motor in the center along with the two 18mm motors, this may require adding more length to it too. Or I could just ignite the two 18mm outboards and vent their charge through those little holes that are already there if I plug the rear of it with a used motor. This rocket has a lot of potential. Any suggestions??
 

teflonrocketry1

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I have done quite a bit of work on the simulation of the Estes Gemini DC. As far as I know this design simulates stable with a static margin around 2.3. Attached is my RockSim Version 7.03 file that includes side pods with nosecones, allthough the program won't draw the nosecones where they belong. The areodynamic forces (CP) of the side pod are simulated as a set of four phantom fins spanning the diameter of the side pod tube. The side pod tubes are simulated as inside tubes that were changed to outside tubes using a word processor to give the correct CG. Transitions whose features were edited with a word processor serve as the side pod nosecones.

Bruce S. Levison, NAR #69055
 

teflonrocketry1

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Here is a screen shot from the simulation.

Bruce S. Levison, NAR #69055
 

teflonrocketry1

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Here is another screen capture with 3D view of Gemini DC.

Bruce S. Levison, NAR #69055
 

teflonrocketry1

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This design should fly nice on a 3 motor cluster. With a 24mm core motor and only 0.5 oz of added nose weight to achieve a static margin of 1.79; expect around 2600 ft AGL with 2 x C6-7's and one E9-8.

See the RockSim version 7.03 file attached.

Bruce S. Levison, NAR #69055
 

RocketboyG80

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Thanks for that info. With that in mind I might go for the three motor cluster with a central 24mm (I could also use the motor mount as an adapter to fly using a central 18mm motor). Although, I don't know how the rocket will hold up to the thrust, after all it's only meant for a C motor at most. Especially if I use a D12 or even E15 central motor. I was thinking maybe I could cut the fin set out of some 1/32" thick G10 fiberglass I have and I can use West System epoxy for the fillets. But my main concern is the strength of the airframe itself. As far as I know no one makes a strong 24mm tube. Although, I could smear some epoxy on the inside of the tube which might help the strength factor. I'm not really concerned about too much weight as I could always fly it with an F21 and two D21's! :D
 

Micromeister

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Rocketboy:
Apparently you haven't read any of the other cluster threads in the forum.
You need not worry about the airframe at all. I've clustered dozens of models BT-5 to Bt-101 .013 and .021 wall tube without a single drop of epoxy or fibreglass. primed and painted as any other model. Basswood will do nicely for the fins also. Check out the thread "clustering the classics for many pic's.

If I were going to do this cluster conversion, I'd add a payload section to allow different nose weight slugs to be added for different set-ups. single 18mm motor thru 24mm/18mm booster combination. Depending on the material used for the weights you may only need a bay 2 inches long. Just a thought.

My Geo-Sat and Deep Space transport both use the 24mm to 18mm adaptor mounts you mentioned..works great with C's and D12's
 

RocketboyG80

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Micromister, that's good that I can keep the rocket light and still fly powerful clusters. I was thinking as a first flight I may use a central D12 with outboard C6 motors because they have the same burn time of 1.6 seconds and it should get some good altitude. I like that idea about the payload bay, I think I'll use that because it will make adjusting noseweight a lot easier than trying to get clay and BB's into the little hole in the NC. BTW, I'll remember to look around the forum next time so I don't ask already answered questions.
 

airforce

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Rocketboy,
Don't you worry one little bit about asking questions. These old guys just get a little grumpy some times.;)

I've tried searching and some searches bring back about 40 million hits, like I want to or have time to read all of that.:confused:

Nope, I say ask away.....Your defender, Airforce:D

I'm one of the old ones but I'm nicer!!!!!!
 

JeffS

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I picked up a Gemini DC at Hobby Lobby during the last half off sale and was thinking about using two 13mm's and a central 18mm. Wal-Mart has plenty of the "plugged" A10-PT motors for the rocket cars. I thought this might be the way to go so I wouldn't have to fool with all those ejection charges. What do you think?
 

jetra2

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I agree with Sandman. A 18mm central motor and two A10-PT motors would be sweet! I know what I am getting with my next Michael's 50% off coupon! :D :cool:

Jason
 

crashinj

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I've got a DC Gemini in the closet, now I know what I'm going to do with it....aw no, not another project.......I need extra time! Oh well, I know what I'll get at Wallyworld next time I'm there, A10PT's. hehe

CrashinJ
 

RocketboyG80

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That would be easier, but as always, I really want to push this rocket. Yesterday I placed an order with Hobbylinc for a pack of C11-7 motors and two packs of C6-7 motors. I wasn't really sure about whether to get a delay or the -0 motors for the outboards, but I went with the delay. Although, I may plug them some epoxy. Or can I empty out the ejection charge like an AeroTech motor if you drill through that clay?
 

rbeckey

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Speaking with experience here... plugging the delays is a recipe for disaster. I tried this for saucers. The result was one CATO and in the other instances, the sudden thrust of the ejection charge tore up the motor mounts, in one case pushing the motor tube completely out of the saucer! Picture the side tubes of the DC popping off when the ejection charge goes out the nozzle.
 

cydermaster

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The ejection charge can be scraped out of an Estes/Quest motor, and a small slice of wooden dowel CYA-ed in its place to stop any flames etc going into the rocket. This is, however, dangerous and probably illegal (I know it is, in England, without a permit) - so proceed with caution along that path!
 

RocketboyG80

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Hmm. Does anyone know what would happen if I used -0 booster motors in the outboards? I know they must put out some flame to light sustainer motors, but would it still cause problems. Those already plugged 13mm motors would be ideal (as already mentioned) but I really want to use two 18mm's.

Here is a new idea. What if I had the side pod nosecones attatched with Kevlar not glued in? They would pop out at ejection but without deploying any revovery devices; the streamer would be in the main tube. Or just to add to the complexity I could have the main tube without a streamer attatched, and instead have two smaller streamers in each of the side pods.
 

rbeckey

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Heck, just let them eject themselves! Build the booster tubes extra tough, and put wadding in there for each flight, and let them pop out the back. If you want to recover them, attach a streamer to the motor so you can see it fall.
 

Micromeister

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Hey Airforce
I wasn't being grumpy. While I agree with just asking the question..there is so much good info sitting here, its seems a shame to have it all go more of less to waste.
sorry is the brief choice of word sounded "grumpy" rocketboyg80. Sometimes when we're short on time, we sound grumpy giving quick answers. that wasn't the intent:(

Just to take a minute to expand on the real reason I jumped in on this thread in the first place. several folks have talked about the seeming compulsion to "over build" model rockets. Your comments about "fear of the airframe" is an example. I have a 3X upscale Goblin that has a standard BT-101 unreinforced, doesn't even have a stuffer tube model that flys on a 5 D12-5 cluster. the 1/4" Balsa fins are "epoxy rivit"attach, and the motor mount cluster centering rings are 14ply shocard stock. with a turned foam nosecone, 1/4" elastic/ kevlar shock cord and mount, and a 36" nylon hemi parachute. the model weighs in at just over 2 lbs lift-off weight. after more then a dozen flight I have only had to repair one fin cracked on a hard landing under a fowled chute. This is to say Build light, not heavy for LPR and even most LMR's.
 

RocketboyG80

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Rbeckey- That would be the easy way but, the farmer who owns the field that our club flies on seems to be waiting fo a reason to kick us off, and ejected motor casings might just do that. (We already hit his barn roof with a lawndarted midpower rocket.) Even with a streamer attatched they would be a little hard to find in amongst the soybeans.

Micromister-I wondering about if I were to fly it on an E30-7 and two D21 motors, would the DC still hold up? I understand that it could take the force of BP motors but not sure about AP. Also, I didn't take your previous response in a mean way, just terse. :)
 

Micromeister

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E30/D21 combination Humm...As I don't fly AP for sport cause I like to SEE my models fly:) on a strictly structural note, this small model should hold up fine. the only "Extra" I'd worry with would be to epoxy the booster tube/main body joint fillets just to smooth out the transition, assuming your already going with basswood for the fins with epoxy fillets anyway.
 

JeffS

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How would you guys handle the nosecones on the "boosters"? Glue 'em on (I'm using plugged motors anyway) or leave them loose with some kind of short shock cord. I'm leaning toward just gluing them on.
 
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