Newest design after 20+ years of absence....

rocket sok
Onebadhawk
eggtimer rocketry
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Looks good to me! I like upswept fins. What will you make the fins out of?
 
Looks good! I think it will be approved by the boss. :)

My daughter was looking over my shoulder one day while I was using Rocksim, and asked what it was. I told her I use it for designing rockets, and she said "COOL! Can I try it?" She went right to work and designed a rocket, and with a little help from me built it, and we flew it this spring. Flies like a champ. She beat me....I have yet to build or fly my own designs. :rolleyes:
 
Rocket86 said:
Looks good! I think it will be approved by the boss. :)

My daughter was looking over my shoulder one day while I was using Rocksim, and asked what it was. I told her I use it for designing rockets, and she said "COOL! Can I try it?" She went right to work and designed a rocket, and with a little help from me built it, and we flew it this spring. Flies like a champ. She beat me....I have yet to build or fly my own designs. :rolleyes:
Click to expand...

Well? Where are the photos???
 
qquake2k said:
With balsa fins shaped like that, I would definitely paper them. Because of the grain in balsa, I'd be afraid they'd snap off.
Click to expand...

[POW]Eagle159;226742 said:
Or do a 2-part fins...if your worried.
Click to expand...

Definitely worth considering. I've been considering Lexan as well.
If I go Balsa, I will probably coat them with wood glue, bondo scrape them smooth while wet, let them dry, then sand them.
I may do the Paper/glue coating. Worth considering.:cool:
 
GDJ said:
Definitely worth considering. I've been considering Lexan as well.
If I go Balsa, I will probably coat them with wood glue, bondo scrape them smooth while wet, let them dry, then sand them.
I may do the Paper/glue coating. Worth considering.:cool:
Click to expand...

Or use 1/8" plywood. It's not all that heavy.
 
GDJ said:
I'm sure I can find it.

Just for S**ts and giggles, I ran a simulation with birch fins:

It's a bit faster with Balsa, but the stability co-efficient goes up to 3.99. It's a good trade off.
Click to expand...

I've never weighed the same size balsa and plywood fins. I'd be curious to know what the weight difference is.
 
Yep, but a trade-off is a thinner wing with Birch and one can gain back some altitude by lowering the drag.
Lexan is a bit over 1 gram per cc, but it's pretty strong stuff. Fins with a thickness of 1mm are possible, theoretically.
 
GDJ said:
Yep, but a trade-off is a thinner wing with Birch and one can gain back some altitude by lowering the drag.
Lexan is a bit over 1 gram per cc, but it's pretty strong stuff. Fins with a thickness of 1mm are possible, theoretically.
Click to expand...

Very doable with carbon fiber plate. IIRC, that is close to about 2 to 3 layers of 6oz carbon fiber. It has a higher density at about 1.54 gram per cc, though.

Greg
 
You guys are right that these fancy materials would probably work, but GDJ might also want to keep this project on a cost and schedule that runs a little under the next NASA heavy-lift design

Plain old balsa would work fine with a few minor fixes. Starting with the original fin planform:

fin 1.jpg
 
It definitely looks like this fin design might run into some flutter problems.

Just to be clear, for structural reasons the balsa grain direction would need to run parallel to the fin leading edge, like:

fin 2.jpg
 
The problem with this is that the main part of the fin would only have a limited portion of the root that would be available (the purple bar).
It doesn't really matter what you do with the forward part of the fin (ahead of the red dotted line) as the main fin would have a grain direction where the wood would be put into bending across the grain (the weakest direction) and with any significant airload the main part of the fin would snap off right behind that joint (the red dotted line)

fin 3.jpg
 
If you added a doubler layer (actually, two, one on either face of the main fin) that bridged across the root, you would add considerable structural support for the cantilevered aft portion of the fin. The doublers would have a grain direction running along their own 'leading' edge (or outer edge) and would end up creating cross-plies at the base of the main fin.
If you made the doublers match the planform of the forward part of your fin design it would look something like:

fin 4.jpg
 
Balsa will be fine, and, as qquake2k noted, papering will help to strengthen them. That would be especially helpful if you have a "freak" landing on the fins while the body tube is parallel to the ground.

Greg
 
This would give an effective transfer of airloads from the main fin into the doublers and spread the attachment loads all along the new, longer root edge (still the purple line).
It would also make the fin root thicker and give you more bonding area for the fin root joint.

And if you tweaked the leading edge angle of that front end just a bit, and made it wider, you could make the reinforcement area a lot bigger on the inboard end of the main fin. The doublers would even help beef up the fin trailing edges to reduce the amount of damage there from landing and hitting those edges of the fin.

fin 5.jpg
 
Excellent information gentlemen!

So, what I will do is mod the fins so the root is larger, and probably reduce the sweep forward slightly.

I'll run sims with other materials just in case.........
 
Doubler material could be a couple layers of heavy cardstock.

Much better would be to cut the doublers from some good dense balsa sheet (1/16 th)

Better yet, but might be overkill, would be to use some basswood sheet (1/16 th)

Definitely overkill would be to make the doublers from model airplane plywood (1/32 or 1/16 th)

I just can't feature that you would need to make the entire fin from plywood. If you did, and used 1/16 th, it would be pretty vulnerable to warping. If you used 1/8 th, it would still be vulnerable to warping (probably not as much or not as badly) but would start getting heavy for a model rocket.

Balsa will work, it just needs a little engineering....
 
powderburner said:
This would give an effective transfer of airloads from the main fin into the doublers and spread the attachment loads all along the new, longer root edge (still the purple line).
It would also make the fin root thicker and give you more bonding area for the fin root joint.

And if you tweaked the leading edge angle of that front end just a bit, and made it wider, you could make the reinforcement area a lot bigger on the inboard end of the main fin. The doublers would even help beef up the fin trailing edges to reduce the amount of damage there from landing and hitting those edges of the fin.
Click to expand...

+1 This is the most beneficial method to keep the weight down. It's a small rocket. However, you must plan for the 5X version when your daughter reaches high school! :eek:
 
Okay, I redid the fins and the root is about 4X longer, with only about 5% increase in Drag (I'll do a super polish on everything and it makes up for it) and roughly the same stability.

I'll see if I can hit the hobby shops tomorrow and get the parts I need and as close as possible to the design.



EDIT: Design finalised (with B4-4 Engine). Genevieve Approved. Off to the store with her to get parts! Wish me luck.
 
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