- Jan 30, 2016
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Hey, long time no post!In-flight testing is king, is it not?
My Falcon 9 flew fine with the stock fins, but those occasional rougher-than-normal landings is what cracked them *every time*
I would personally go with one step thicker than that.
I've used a replaceable bamboo skewer mounted between the motor mount and the BT that sticks out 4 or 5" to protect fragile fins at landing.Because they're not all the way in back, they're not nearly as effective, so I was hoping at least hoping to save some weight.
Looking at your fin shape.Hey, long time no post!
That is valuable input.
It does look like the F9 has those swept-back pointy-tipped fins which would seem to have a lot more vulnerability to landing impact. My tail will be more like this:
View attachment 397075
In theory the fins should be subjected to much less landing impact shock. I'm still fiddling around with the final fin shape. Because they're not all the way in back, they're not nearly as effective, so I was hoping at least hoping to save some weight.
Realistically, I could make them 3/32" (which would definitely be plenty strong IMHO) and all it'll cost me is a bit more nose weight, so I probably shouldn't worry about it.
References online say 450-550 ksi, which matches up with the 3 GPa along the grain in the chart above. That goes down to 0.1-0.2 GPa for stiffness across the grain. As already mentioned, natural materials are pretty variable, and this seems especially true of balsa from other users I've seen here. I don't have a lot of experience, but I see people talking about buying stiff balsa or soft balsa.So what is Young's modulus for balsa wood?
I'm getting 2.3 GPa from the linked website for unfilled polycarbonate, so nominally less than balsa along the grain but much more than balsa across the grain. One interesting aspect of this is that with the grain aligned along the leading edge, spanwise flexing of the fin is a combo of along the grain and across. That makes an engineering analysis get really unpleasant really quickly, as it is with most non-isotropic composites. Best to stick with the push-on-it-with-your-thumb test!Hmm, those diagrams show balsa having a *higher* modulus with the grain than Lexan, but values I've looked up elsewhere show Lexan having 3-4x the modulus.
This website says 3 GPa for balsa, vs. 13-23 GPa for Lexan.
It certainly feels to me like the 3/32" Lexan I have from Home Depot is much stiffer than 3/32" balsa.
All true.I am assuming that the fins are supposed to be invisible. and also assuming that you are not shooting for altitude, so DRAG efficiency isn't a big deal, but CP efficiency IS a big deal.
Constraints are as follows:Note sure how you are picking the root chord length, but figure you have a fixed length in mind.
In OR, squaring off the front, given the same fin root, brings the CP *forward*.That being the case, would a square or rectangular fin shape (take out the delta in front) allow you to use a shorter semi-span for the same CP benefit?
Wow, Okay, I hadn’t figured on the fins being THAT close to the CG.In OR, squaring off the front, given the same fin root, brings the CP *forward*.
The real one will look better.Seriously though, looks really cool so far Neil! That eraser tip looks perfect. What is your plan for attaching the fins?
Longer is the best answer I think. I added a few inches to the front, and gained some margin and was still able to reduce the semispan a bit. Small amounts of nose weight should be sufficient to make it all work if it becomes too tail-heavy during construction.Does making the rocket a little bit LONGER (move the nose further forward) throw the “scale-pencil-look” off to much?
Great idea, only concern is to round the rear ends or put rubber caps on them.I've used a replaceable bamboo skewer mounted between the motor mount and the BT that sticks out 4 or 5" to protect fragile fins at landing.
Just a thought...
A simple design that could be nice with a good paint job. I started with much more complex rear fins but found it looks way better with the simpler ones. I did this one in white because the unfinished appearance was distracting. Dunno why I found that to be the case here but not elsewhere, but whatever.
View attachment 327580
How big are the fins for the plasma dart? Although you said I should be able to figure that out, I can't. Or do you have an ork file you could put up in a post?Here's a good side view (click for giant version):
View attachment 327707
From front to back: PNC55BB, 2" BT55, 1" 5560 transition, 1" 6050 transition, 10" BT50, 1" 5060 transition, 4" BT60, then 24mm motor mount extends another .5". 26" total.
I think you can derive all remaining dimensions from that, including the fins (in combination with the drawing.) Interior structure is not designed, but you don't need my help for that anyway. I'd imagine a BT50 stuffer tube all the way through to the frontmost transition. I'd also imagine you're going to upscale it.
Look forward to seeing it!
Yeah, sorry there chap. You got caught in the crossfire of long-running thread banter. Jqav was right, that was directed at Neil.