I always thought you wanted to angle your thrust lines through the CG. Especially for a short stubby where the thrust line is so far off the center line. This will mitigate any thrust imbalance. As you're doing it in CAD this should be easy.
Reenforcing surface mount fins
- Thread starter Sooner Boomer
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lakeroadster
When in doubt... build hell-for-stout!
I spent the time making this for Sooner Boomer, based on how he built his 7-motor cluster.
To do what you are proposing could not be done with (7) 18mm motors, within the confines of a BT-80.
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Looks to me like the rear of the motors can move 5or6 mm relative to the inner ones. That makes the asymmetry 12or 13 mm off line rather than the 19? it is now. That makes the issue currently 50% (7/12or 13 depending on how you want to do the math) worse than it could be.
lakeroadster
When in doubt... build hell-for-stout!
5 or 6mm? Try 3 or 4 times that amount.
Here's the visual of the geometry.
You can move it. You just have to make it better. I did not say you would be able to get the thrust line through the CG. But by doing nothing, you're doing nothing...........
lakeroadster
When in doubt... build hell-for-stout!
Ah yes, but you're doing something... changing the goal line. Classic switcheroo. What you said is in black and white, in post 31. Which was "through the cg"
Time for a refresher course...
The rocket works great as it is. Did you watch the video @Sooner Boomer posted previously?
7 motor Fat Boy at Pawhuska 9-25-21 - YouTube
Sooner Boomer
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Thanks, something to think about.
I have actually not built any of them yet to see how they work... This thread motivated me to go back and make the 18mm version. I originally was focused on making these as quarter turn locking cluster mount but I think that the bolt-in mount that Lakeroadster is doing is much easier when using fin tabs.
FWIW - I think I came up with a way to make a twist in cluster mount that would work with fin tabs. The idea is to make internal fin tab mounts that would glue inside the body tube around the fin tabs (I would 3D print these but you could make them from something else). The cluster mounts would have a notch so they could slide around these internal tab mounts (the red bars in the render below) and still twist lock into place. It was easier to fit this all in with 24mm engines but might work with 18mm engines also. Could also do a similar twist lock cluster mount with the centering rings and internal fillets on fin tabs (would preclude the need for bolts to mount).
If you read what I wrote in that post and my next one, you'll find I said you wanted your thrust lines to go through your CG. In the next post I gave you the best possible advice based on the available configuration and provided the basic math for the magnitude of that improvement.
At no point did I say that the rocket design would be prevented from launching, ie MUST have the thrust lines through the CG.
As you have the power of CAD available to do this, it seems like a wasted opportunity not to implement that improvement.
If the OP does not want that for historical accuracy, fine. But I'm sure CAD modeled engine mounts were not available then.
But whatever the OP wants. The same stability or better stability. All it takes is a couple of motors to not light.
Thanks for reading this far . please go back and reread all of what I posted before replying if you intend to do so.
,Norm
lakeroadster
When in doubt... build hell-for-stout!
Sure will, you too. Here's some surface mounted fins on a Fully 3D printed Minie Magg.Have a great day Norm...
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lakeroadster
When in doubt... build hell-for-stout!
Witchcraft...
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Cardstock maybe? If you did a squares-U-shape between each finpair you'd be substantially reinforcing the body as well.
lakeroadster
When in doubt... build hell-for-stout!
You might want to consider basswood with papered fins. I did that with my Advanced TIE Fighter for the solar array wings... and then did a strength test using some extra material. I was amazed that the test showed an 8X increase in strength over raw basswood.
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WOW, you're very talented, I'm highly envious
