Are External fillets needed?

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Are external fillets needed at subsonic speeds? How fast does a rocket need to go that make external fillets necessary?What other factors should be accounted for?

I’m wondering if I need to apply fillets to the outside of my rocket? I’ve applied both MMT fillets as well as inner BT fillets. How much does this technique give additional strength to the fins?

Sorry if this is been discussed, I’m sure it has... :dontknow:


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David Schwantz

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External fillets will give extra strength. They also add to the looks of the rocket, finish. IMHO. And yes the reduce drag. A 90 degree corner will give the most drag. But is you are looking for a low and slow draggie rocket, don't put them on and leave your fins with square edges.
 
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it's discussed quarterly :)
External fillets will give extra strength. They also add to the looks of the rocket, finish. IMHO. And yes the reduce drag. A 90 degree corner will give the most drag. But is you are looking for a low and slow draggie rocket, don't put them on and leave your fins with square edges.
Hobie1 I'm silly for thinking otherwise! Haha...

90 degree corner produces the most drag! That's interesting!. I’m not going for records but like the idea of making it as drag free as “I can.”

Is there an optimal fillet size per the diameter of the rocket?

I'm sure they do add more strength, as, it’s likely the fillets add extra strength up higher or closer to the tip lets say.

Are there certain "milestones" that make fillets "necessary?" Seems as if "things" change around mach 1.6!? That's just a guess from reading through a number of posts.... not experience. Any other opinions?
 
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cls

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But is you are looking for a low and slow draggie rocket, don't put them on and leave your fins with square edges.

yeah those big fins look draggy. this rocket will be a great low'n'slow


oh btw I agree with this:
What's not needed is the internal BT fillets.

When a fin breaks, that extra glue makes it more difficult to fix.
 
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What's not needed is the internal BT fillets.

yeah those big fins look draggy. this rocket will be a great low'n'slow


oh btw I agree with this:


When a fin breaks, that extra glue makes it more difficult to fix.

So attaching to the MMT and external fillets are the ideal building methods. That gives the most strength and “easiest” method for repair as well.. love it thanks for letting me know, why I ask this type of stuff.

It’s extra work I know…but it will be sanded heavily and I think it’ll look pretty good when complete.

Also was my first attempt at fiberglassing the cardboard tube!
 

Banzai88

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The standard formula for optimal fin fillet radius = 4 to 8% of fin root length. I think it's from Stine's book. Regardless, as a guide it works well.
 
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The standard formula for optimal fin fillet radius = 4 to 8% of fin root length. I think it's from Stine's book. Regardless, as a guide it works well.

So hypothetically: rocket fin root length 10in

10 * (.04-.08)= roughly 13/32nds-25/32nds is the desired fillet thickness at its widest part. See The small red line in photo.

Thanks Banzai for that!


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Banzai88

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So hypothetically: rocket fin root length 10in

10 * (.04-.08)= roughly 13/32nds-25/32nds is the desired fillet thickness at its widest part. See The small red line in photo.

Thanks Banzai for that!


View attachment 547728
Yeah, don't overthink it, as most of us do it more by 'what feels/look right' rather than strict math. With thick fins/sub-sonic it's less structural and more aerodynamic.

For your rocket I would get a piece of 1" copper pipe coupler as a form tool and go to town with it!
 
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Yeah, don't overthink it, as most of us do it more by 'what feels/look right' rather than strict math. With thick fins/sub-sonic it's less structural and more aerodynamic.

For your rocket I would get a piece of 1" copper pipe coupler as a form tool and go to town with it!


Awesome will do! It’s mostly just to keep improving in construction techniques and abilities as well as general rocketry knowledge.

I really appreciate any and all help!
 

samb

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If the model lends itself to internal fillets then that handles the structural requirements. If the rocket is a missile or space vehicle then I’ve gone the no external fillet route. Looks more scale-like to me.
 
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If the model lends itself to internal fillets then that handles the structural requirements. If the rocket is a missile or space vehicle then I’ve gone the no external fillet route. Looks more scale-like to me.


The kit is 4" LOC AMRAAM. I feel the real one lacks the fillets in the pictures I've looked at. Can't remember now I'll have to look again.

My fin bevels aren't great at all. However, they are more consistent than my previous build. I feel a great tool for the best fin bevel results would be a router table.

I'm going to add fillets to this build, as I feel the mid mounted fins should have fillets and to keep with the consistency, I'll model the rear fins the same as the front.
 
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