Fat, Stubby Rocket Needs Shorter Delay

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rocketgroupmike

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Hi all, I've got a bunch of parts for a 3" dia. project I have in mind. The problem is that shortest delay for all the 29mm motors I've tried is still a bit too long for my design. (according to the demo version of Rocksim, anyway) Should I just give up? The rocket will be heavy, too...
 

rstaff3

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Must be pretty heavy to be 3" and stubby! I have flown purty heavy rockets on G80-4s with good success. In the 48-52 oz range. I wouldn't give up. Build a good recovery harness and add an anti-zipper device to the cord and go for it. Separate chutes on the cone and body may help too.
 

rocketgroupmike

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Around 50 ounces is just what Rocksim says... It's a bunch of PML parts (NC, MMT tube, tailcone, etc.) that I planned on buying three .062" G10 fins and a short (~20") piece of QT for. Now that I think about it, I better check all the part descriptions because something is not right. I'm not planning any 'glass or anything... It can't weigh that much, can it?
 

rstaff3

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Sounds heavy to me, but a lot depends on if you need a lot of nose weight. My 3" PE Ultra Fatboy has plastic nose and tail cones, small ply fins, and a 15.5" tube. It comes in at about 48oz unloaded, about 52 with a G80. It has alot of noseweight due to small fins that are mounted on the tube in front of the tail cone. The nose weighs over 20 oz and sets the CG for an I211.
 

lalligood

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Here are a few suggestions:

1) It sounds like you are going purely off RockSim. (Do you have the components on hand?!?) Double-check what you've done in RockSim & make sure that there aren't any hidden mass weights (like you might have put an extra one where you already had one!) If you have the parts on hand, weigh them yourself to make sure that what RockSim has is accurate. Sounds silly but you might find your answer that way.

2) I would suggest maybe reassessing the components you wish to use. PML makes some great parts but they might be overkill for what you have in mind. A simpler design mated with paper tubes should yield a lighter design as well...

3) Make the noseweight adjustable. There are multiple solutions that can be done to allow you to adjust the noseweight to properly compliment the motor being used (i.e., larger motor = more weight; smaller motor = less weight).

4) Along with different components, consider different build techniques. There are a couple of good threads going on right now in the Techniques forum if I'm not mistaken discussing adhesives. Epoxy works but you might have other adhesives that will work too but with a lesser weight penalty.

HTH,
 

DynaSoar

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Originally posted by rocketgroupmike
Hi all, I've got a bunch of parts for a 3" dia. project I have in mind. The problem is that shortest delay for all the 29mm motors I've tried is still a bit too long for my design. (according to the demo version of Rocksim, anyway) Should I just give up? The rocket will be heavy, too...
The shortest is 4, AFAIK. How many seconds coast do you get (apogee in seconds after burnout), and how much negative Z (speed downward) at 4 seconds?

You running these sims on G64s and G80s? I find it difficult to believe you'd have more than 2 seconds between apogee and ejection on those. If you are, I think you're just underpowering it.
 

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