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  #1  
Old 07-25-2007, 02:47 AM
A Fish Named Wallyum A Fish Named Wallyum is offline
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Default BT-80 Photon Disruptor

DISRUPTED!!!
This was one of the NARAM projects that I have all but abandoned since last week's vacation went south on me. (I was sick the whole week. ) As it turns out, it would have been a stretch to call it an upscale anyway since I went BT-80 to BT-70, not quite on the same scale as BT-50 to BT-20. I got the two sections joined with the help of two homemade CR-8070 centering rings, and the motor mount installed, but what to do about the fins?
As many of you know, the Photon Disruptor has T-shaped fins that mount onto the lower body tube. My problem came when I started thinking about cutting the fins. The widest balsa I could find locally was 6" x 1/8", which means that the tip plate of the fin is going to have to be cut with the grain running parallel to the main fin. This is going to make the whole structure iffy on landings. I thought about papering the two sides of the tip plate before attaching them to the main fin sections, but then I have to wonder about the possibility of the paper delaminating at the main fin. I didn't plan to fly this on anything more aggressive than an Estes E9, so I wanted to keep the weight down. As it stands now, the rocket is complete except for attaching the fins. Anyone have an opinion on the possibility of this project being a fin damage queen?
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Build floor: Centuri Design Contest F-150 Hurricane Estes - Low Boom SST Semroc - Gee'Hod, Shrike, SST Shuttle

In paint: Canaroc Starfighter Scorpion Estes F-22 Air Superiority Fighter, Solar Sailer II Semroc Cyber III

Ready to fly: Estes - Multi-Roc, Solar Sailer II Semroc - Earmark, Snake Jumper
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  #2  
Old 07-25-2007, 08:06 AM
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sandman sandman is offline
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Bill, bring it with you to NARAM.

With that many people there will be no shortage of "free" advice.
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  #3  
Old 07-25-2007, 11:08 AM
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CPMcGraw CPMcGraw is offline
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Quote:
Originally Posted by A Fish Named Wallyum
DISRUPTED!!!
This was one of the NARAM projects that I have all but abandoned since last week's vacation went south on me. (I was sick the whole week. ) As it turns out, it would have been a stretch to call it an upscale anyway since I went BT-80 to BT-70, not quite on the same scale as BT-50 to BT-20. I got the two sections joined with the help of two homemade CR-8070 centering rings, and the motor mount installed, but what to do about the fins?
As many of you know, the Photon Disruptor has T-shaped fins that mount onto the lower body tube. My problem came when I started thinking about cutting the fins. The widest balsa I could find locally was 6" x 1/8", which means that the tip plate of the fin is going to have to be cut with the grain running parallel to the main fin. This is going to make the whole structure iffy on landings. I thought about papering the two sides of the tip plate before attaching them to the main fin sections, but then I have to wonder about the possibility of the paper delaminating at the main fin. I didn't plan to fly this on anything more aggressive than an Estes E9, so I wanted to keep the weight down. As it stands now, the rocket is complete except for attaching the fins. Anyone have an opinion on the possibility of this project being a fin damage queen?


Study the patterns attached below. This problem is similar to one faced by RC airplane builders when they use slabs of 1/4" balsa to make up the stabs and vertical fins. The large balsa steets tend to "cup" across the grain, which is not a good thing.

It's not bad for the grain to run parallel to the pylon for the bulk of the fin, it's just bad if the fin cups across the grain. I show two pieces of balsa glued together to make up the center of the fin; I'd suggest doing this anyway if the width of the fin is over 4". The end caps should be about 1/2" wide.

The alternate version gives you more cross-grain in the center than the first version, and may have greater strength for the fin-to-pylon connection. The edge caps perform the same function, however. They resist the cupping that would otherwise happen across the wide end grain.

Build up a set of three without trying to get the shape exactly right, then trim and sand after they're assembled. You can stack-sand the three fins and get a closer shape than trying to cut each one separately.
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  #4  
Old 07-25-2007, 10:44 PM
A Fish Named Wallyum A Fish Named Wallyum is offline
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Quote:
Originally Posted by CPMcGraw
Study the patterns attached below. This problem is similar to one faced by RC airplane builders when they use slabs of 1/4" balsa to make up the stabs and vertical fins. The large balsa steets tend to "cup" across the grain, which is not a good thing.

It's not bad for the grain to run parallel to the pylon for the bulk of the fin, it's just bad if the fin cups across the grain. I show two pieces of balsa glued together to make up the center of the fin; I'd suggest doing this anyway if the width of the fin is over 4". The end caps should be about 1/2" wide.

The alternate version gives you more cross-grain in the center than the first version, and may have greater strength for the fin-to-pylon connection. The edge caps perform the same function, however. They resist the cupping that would otherwise happen across the wide end grain.

Build up a set of three without trying to get the shape exactly right, then trim and sand after they're assembled. You can stack-sand the three fins and get a closer shape than trying to cut each one separately.


This is something like what I had in mind. The edge caps are a nice touch. I was looking over the airframe today and started thinking about a whole new rocket with a fin pattern that might stand up to the hard landings. That BT-80 to BT-70 thing is going to look odd as a Photon Disruptor. We'll see.
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Bill Eichelberger
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http://wallyum.blogspot.com/

I miss being SAM 0058

Build floor: Centuri Design Contest F-150 Hurricane Estes - Low Boom SST Semroc - Gee'Hod, Shrike, SST Shuttle

In paint: Canaroc Starfighter Scorpion Estes F-22 Air Superiority Fighter, Solar Sailer II Semroc Cyber III

Ready to fly: Estes - Multi-Roc, Solar Sailer II Semroc - Earmark, Snake Jumper
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  #5  
Old 07-26-2007, 10:19 AM
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CPMcGraw CPMcGraw is offline
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Quote:
Originally Posted by A Fish Named Wallyum
This is something like what I had in mind. The edge caps are a nice touch. I was looking over the airframe today and started thinking about a whole new rocket with a fin pattern that might stand up to the hard landings. That BT-80 to BT-70 thing is going to look odd as a Photon Disruptor. We'll see.


What about a BT-80 to BT-60 drop down? I don't have any 80 on hand, but the difference in diameters should be more pronounced than just going to the 70. Another possibility is to use some of Carl's Series 175 tubing, which is OD 1.84". This is just a little larger than BT-60, but still smaller than BT-70. And, being a little thicker-walled, would form a stronger "backbone" that you could use TTW fins with.
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  #6  
Old 07-26-2007, 11:32 AM
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JRThro JRThro is offline
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Quote:
Originally Posted by CPMcGraw
What about a BT-80 to BT-60 drop down? I don't have any 80 on hand, but the difference in diameters should be more pronounced than just going to the 70. Another possibility is to use some of Carl's Series 175 tubing, which is OD 1.84". This is just a little larger than BT-60, but still smaller than BT-70. And, being a little thicker-walled, would form a stronger "backbone" that you could use TTW fins with.

This would end up being a totally different, and smaller, rocket than the one Bill is building, but the ratios of the OD's of BT-70 to BT-60 (1.34 to 1) is pretty darned close to the ratios of the OD's of BT-50 to BT-20 (1.28 to 1), as compared to BT-80 to BT-70 (1.18 to 1) or BT-80 to BT-60 (1.59 to 1), or even BT-80 to ST-175 (1.41 to 1).
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  #7  
Old 07-30-2007, 11:15 AM
wyldbill wyldbill is offline
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Another take on the fin problem is to do a laminate. I've used very light (.5 oz?) CF scrim (unwoven mat) and laminated it to balsa using a Food Saver with very good success. You get very light, flat, stiff panel with a no grain to fill. It may be a bit hi-tek for this application, but it works nicely.

FWIW,
-bill
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