2Stage fin design

[Karl]

It's 00:11am and I've got round to add another rocket to my project list.
Shes a 2" bodytube with 4mm balsa fins in a 2 stage design (Like the BSD Thor)

Karl

 

[Karl]

The fillets drying.
Karl


PS: Sorry about the messy workspace
Karl

 

[rabidsheeep]

AHHH!!

SAND THOSE FINS!!!

cool looking rocket

 

[Karl]

Thanks for the comment The fins are sanded believe it or not , they just look a little fuzzy. The fillets are looking perfect I just need a balsa nosecone now.....
Karl

 

[powderburner]

Karl,
I hope you merely mis-spoke and those fins are made from plywood. In that case the grain direction in your photos doesn't matter as much.
But if those really are 4mm balsa fins, it looks like you are headed for trouble. The grain direction needs to run inbd-outbd, not fore-n-aft. Balsa is weakest in cross-grain bending.
If you have not flown this bird yet, it would probably be worth it to remove the fins and start over. (They are going to come off soon anyway.) You could line up the wood grain with the fin leading edge, or the trailing edge, or the mid-chord line. Most folks like to line up the grain with the LE in order to get a little smoother finish there, for lower drag.

 

[Stymye]

... those things happen when you stay up so late...


just kidding

doesn't look like a 2 stager tho

 

[Karl]

Thanks for that , She was going to fly on a E18 tomorrow aswell The rear fins have the grain running the way you said , but not the top ones. Are you trying to say she will most proberley shread?
Karl

 

[Karl]

Originally posted by stymye
... those things happen when you stay up so late...


just kidding

doesn't look like a 2 stager tho

The lower fins are going to sperate and the upperfins act as the uperstage.I thought about using a D12-0 to put her up to about 100ft and have a timer make a AP motor kick in....Xavien make some nice timers
Karl

 

[Rick James]

Originally posted by Karl
Thanks for that , She was going to fly on a E18 tomorrow aswell The rear fins have the grain running the way you said , but not the top ones. Are you trying to say she will most proberley shread?
Karl

Both sets of fins are wrong... the grain needs to run parallel to the leading edge of the fin. If they don't shred they will break on landing.

 

[powderburner]

Rick James understates:
"If they don't shred they will break on landing."

Karl, you have a dangerous situation where those fins could come apart anytime. You could end up with a burning E stuck in your ear (or any other part of your anatomy).

I think you need to look very closely at your own photos. In the second one, the grain sure does look like it is going the wrong way. If it is currently covered with paint, you could check this by pushing on your fin a little and seeing if it bends (relative to the BT).

 

[Karl]

Thanks for that , she is already painted One of the bottom fins has the grain going the oposite way from the other 2 bottom fins I might be able to rip them all off and reaplace with Plywood.......hmmm, It will be an even bigger job than building her but if it's going to be un-safe the way it is, then I guess it's worth it.
Karl

 

[Rick James]

Karl,

A couple more comments... First, I want to say that I really like the design... I've seen fin designs like this a lot, but for some reason I never thought to apply it to a 2 stage rocket... I don't get it, I guess my brain just doesn't work that way... but this is something I would like to try. With 24mm motors, however, I would definetly use basswood instead of balsa (but that's just me). And I would cut the tube at the stage separation and put in the stage coupler before putting the fins on.

Second, my first rocket back as a BAR was the Estes Broadsword (upsacle of Big Bertha) and I cut the fins in the wrong direction. Fortunately no one got hurt, but the ricket was destroyed on the first flight... very sad - I spent a lot of time on the finish.

Good luck with this... I would like to see the finished product.

 

[MetMan]

Karl,

The fins look pretty thick. Could you glue dowel stock to the leading and trailing edges? Sand the paint off first, attach with wood glue, then fill with FNF. Should take the flex out.

MetMan