Composite Warehouse 5" Bullpup or Concept 125 (54mm x 3) - Which one needs built first?

[rfjustin]

Makes sense. Bummer cause that design ends up really strong. I'd have new fins cut. Especially if you're going to fly 3xK2050 or L1090's

No worries, I'll end up doing some stout carbon/glass "tab" work, then fillets, then glass fin to fin over all that. Just cant bring myself to toss the fins in the spare parts bin. Speaking of, anyone need a 75mm motor tube and (2) 5"-3" centering rings?

 

[Theory]

Confident it will be built strong enough to survive any combination he loads in it.

that said, it is unfortunate it was supplied with fins that really do not fit

 

[TheTank]

How much of a gap when seated in valley? Something i have done in the past on a similar issue was measure and mark the gap on the motor mount tubes. Tape off the ends and fill the gap/valley to the desired height with epoxy/glass fibers. Then drop the fins in and fillet. That said… that could be considerable weight on this size rocket and perhaps more than a fin to fin layup… but i figured id mention it. Several ways to slay this dragon.

 

[rfjustin]

Confident it will be built strong enough to survive any combination he loads in it.

that said, it is unfortunate it was supplied with fins that really do not fit

The supplied fins are perfect for the 75mm motor tube that came with the kit, but I could not leave it alone.

 

[rfjustin]

How much of a gap when seated in valley? Something i have done in the past on a similar issue was measure and mark the gap on the motor mount tubes. Tape off the ends and fill the gap/valley to the desired height with epoxy/glass fibers. Then drop the fins in and fillet. That said… that could be considerable weight on this size rocket and perhaps more than a fin to fin layup… but i figured id mention it. Several ways to slay this dragon.

Right at 3/4" of gap to make up in order for it to "connect."

 

[3stoogesrocketry]

You could reuse the rings and motor tube for a booster . L2500 staging to 3 CTI 54 L mellow loads.........

 

[OzHybrid]

You could 3d print an insert to bring the height back up. Happy to assist and do that for you. Just flick me an email with dimensions.

Or you could cut some 1mm Fiberglass and bridge across the tubes at the right height then drop the tab onto that. There's significant strength advantage from a full tab. There's extra work and weight to get that strength back with a f/glass layup that you'll need to do with no tab.

 

[rfjustin]

You could 3d print an insert to bring the height back up. Happy to assist and do that for you. Just flick me an email with dimensions.

Or you could cut some 1mm Fiberglass and bridge across the tubes at the right height then drop the tab onto that. There's significant strength advantage from a full tab. There's extra work and weight to get that strength back with a f/glass layup that you'll need to do with no tab.

Thank you for the kind offer. I'll chew on it for a bit and see where I end up.

 

[DaHabes]

You could 3d print an insert to bring the height back up. Happy to assist and do that for you. Just flick me an email with dimensions.

I would also build slots into the inserts for the fin tab to lock into. If the 3D parts were printed with 30% infill they would be plenty strong, not very heavy and the fins would be rigidly held.

 

[OzHybrid]

Something like that? Bottom needs a tidy. In metric of course.

 

[OzHybrid]

If you give me your proper tube and fin dimensions, I can just dump the numbers in and you can see a real one and get a weight estimate on the printed item.

 

[rfjustin]

Than

If you give me your proper tube and fin dimensions, I can just dump the numbers in and you can see a real one and get a weight estimate on the printed item.

Thank you for the offer. I've decided to lop the fin tabs and carbon/glass reinforce as needed.

Best,

 

[OzHybrid]

I love that orange tube. Might have to get some. 125 is a nice size, you can get your arm in there. Good luck with the build. Looking forward to seeing the finished beauty. Is it flying naked? That tube deserves it......
Norm

 

[rfjustin]

Nose cone and fin can will be black. Will paint the fin can to cover the tip to tip and internal work, the rest will be left as is. I agree, the orange tube is purdy!

 

[ChuckH]

Sorry, I'm late to the game. Now what is this that you are building that you are going to fly once and sell?

 

[rfjustin]

Sorry, I'm late to the game. Now what is this that you are building that you are going to fly once and sell?

Likely the Bullpup.... LOL

 

[rfjustin]

Fin tabs are off, spacing looks good. More to come as time allows... Also, a thank you Payload bay for the tools available to make easy work of a motor retention/drogue recovery hardness plate.

 

[rfjustin]

Needed to cut a 10" coupler into 3 pieces for the motor retention bulkhead to have a solid place to land. More pics in the near future will help tie it all together.

 

[rfjustin]

 

[rfjustin]

 

[rfjustin]

 

[rfjustin]

Rail guide 10-24 PEM nuts are in as well as the motor mount. Fins go on next...

 

[crossfire]

Nice work JF.

 

[rfjustin]

 

[rfjustin]

 

[ThePlmbr]

It's looking good.

 

[Bowman]

Well thought out and masterful build as usual

 

[rfjustin]

Rather surprised how stiff the fins are with internal fillets only... adding large JB Weld fillets and moving forward. I would be very surprised if this were to shred even under full load.

 

[DaHabes]

Rather surprised how stiff the fins are with internal fillets only... adding large JB Weld fillets and moving forward. I would be very surprised if this were to shred even under full load.

Fin flutter isn't a factor of the strength of the bonding to the airframe, it's a function of the shear modulus of the material used for the fins, the speed of the airflow over the fin and by the fin size and shape. Especially important is the length of the tip chord and thickness of the material.

No need to guess. You can use this spreadsheet (found elsewhere on TRF) which provides a very conservative estimate of fin flutter speeds (based on a Peak of Flight newsletter - I added the shear modulus tab from internet searches). If you have AeroFinSim, it provides another data point, which I have found to generate a higher fin flutter speed.

Last point, if you use internal fillets, then the external fillets are really for improving aerodynamics and not need for bonding to the airframe. For that, JB Weld is overkill in terms of weight. Rocketpoxy is my go-to for external fillets.

 

[rfjustin]

Fin flutter isn't a factor of the strength of the bonding to the airframe, it's a function of the shear modulus of the material used for the fins, the speed of the airflow over the fin and by the fin size and shape. Especially important is the length of the tip chord and thickness of the material.

No need to guess. You can use this spreadsheet (found elsewhere on TRF) which provides a very conservative estimate of fin flutter speeds (based on a Peak of Flight newsletter - I added the shear modulus tab from internet searches). If you have AeroFinSim, it provides another data point, which I have found to generate a higher fin flutter speed.

Last point, if you use internal fillets, then the external fillets are really for improving aerodynamics and not need for bonding to the airframe. For that, JB Weld is overkill in terms of weight. Rocketpoxy is my go-to for external fillets.

Thanks for the input.