Iceman1979
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- Joined
- Feb 24, 2009
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Hi,
I'm building a couple of rockets, one for myself and one for my Wife. I'm in the design phase of it right now with some of the parts collected so for. I have some question about this that you guys would be able to answer for me.
Background: The rockets are 3" diameter by 49" tall (including nose cone, 36" body tube). I've purchased the tubes, nose cones, motor mount tube, and centering ring. The centering rings are aircraft plywood and are 1.880" thick. The phenolic MMT is 18" long. I got the MMT and centering rings, and plastic nose cone from publicmissiles.com. The BT was purchased from Staples and everything seems to be fitting together just fine.
Questions:
1. Do I need the full 18" of MMT or can I cut that back to say about 12"? It came as a MMT pre cut to that length so I'm not sure if that is supposed to be that length or if it is just that long and you can cut it down to what size you need.
2. I'm using rocsim 8 to design the rocket and it is being modeled after the Aerotech Initiator. The fins are the same size and shape but the rest of the rocket has been adjusted to the part sizes I have. Currently rocsim says my rocket is over stable because I increased the length of the rocket and the size of the fins. I can reduce the size of the fins and get it to not say overstable. The body tube and fins are going to be fiberglassed and I don't see a way to account for that other than overriding the mass of the fins and BT. I wonder if changing the mass of the Fins and BT will change my overstableness of the rocket and if so then how to I calculate the new size of the fins to be stable with out knowing what the mass is going to be?
Thanks
John
I'm building a couple of rockets, one for myself and one for my Wife. I'm in the design phase of it right now with some of the parts collected so for. I have some question about this that you guys would be able to answer for me.
Background: The rockets are 3" diameter by 49" tall (including nose cone, 36" body tube). I've purchased the tubes, nose cones, motor mount tube, and centering ring. The centering rings are aircraft plywood and are 1.880" thick. The phenolic MMT is 18" long. I got the MMT and centering rings, and plastic nose cone from publicmissiles.com. The BT was purchased from Staples and everything seems to be fitting together just fine.
Questions:
1. Do I need the full 18" of MMT or can I cut that back to say about 12"? It came as a MMT pre cut to that length so I'm not sure if that is supposed to be that length or if it is just that long and you can cut it down to what size you need.
2. I'm using rocsim 8 to design the rocket and it is being modeled after the Aerotech Initiator. The fins are the same size and shape but the rest of the rocket has been adjusted to the part sizes I have. Currently rocsim says my rocket is over stable because I increased the length of the rocket and the size of the fins. I can reduce the size of the fins and get it to not say overstable. The body tube and fins are going to be fiberglassed and I don't see a way to account for that other than overriding the mass of the fins and BT. I wonder if changing the mass of the Fins and BT will change my overstableness of the rocket and if so then how to I calculate the new size of the fins to be stable with out knowing what the mass is going to be?
Thanks
John