Radial balance: should it be a concern?

[Curtis Enlow]

I am designing my avionics bay sled which will include prime & backup battery/altimeter systems.

I understand the importance of longitudinal balance. But I am questioning the wisdom of putting batteries AND electronics, all on one side of the central longitudinal line.

Should this be a concern? Should I make an effort to distribute the weight, or is the arm/moment so small as to negligible?

 

[Steve Shannon]

I put the batteries on the other side of the sled, but not for balance. If one comes loose (should never happen but stuff happens) I don’t want it beating up the electronics. I also use lightweight clips and long battery leads so they don’t pull loose.

 

[JohnCoker]

I try to center things, but don't get obsessive about it. Generally I put the plate off-center and put everything on the side nearest the centerline. For very small bays, I do as Steve suggested and put the batteries on the back. (Generally the batteries are the heaviest part.)

To avoid batteries coming loose, I use zip ties.

 

[Curtis Enlow]

Thanks, guys. Makes sense. The more i thought about it the less concerned I was, but it doesn't hurt to keep as balanced as possible.

 

[SammyD]

I make my AV Bays balanced as much as possible, but I'm unable to find photos right now, even created one sled that looking at the all-thread and platform look like a "Z" so it was straight down the MIDDLE of the AV Bay. I build by avionics onto the sled in such a way that both sides of the sled get a mirror image on both sides using redundant altimeters, batteries, switches, and wiring. Does it help with longitudinal balance? I'm not sure, but anything I can use to make the rocket go STRAIGHT UP, I'll incorporate into my builds...

 

[Alan15578]

I am designing my avionics bay sled which will include prime & backup battery/altimeter systems.

I understand the importance of longitudinal balance. But I am questioning the wisdom of putting batteries AND electronics, all on one side of the central longitudinal line.

Should this be a concern? Should I make an effort to distribute the weight, or is the arm/moment so small as to negligible?

Most sounding rockets are spin balanced, and some are even designed with pockets for balance weights. I don't think even large HPRs ever bother with that. Sport rocketeers will often attach one camera to the exterior side of their rocket. So much for symmetry and balance...

 

[Curtis Enlow]

Sport rocketeers will often attach one camera to the exterior side of their rocket. So much for symmetry and balance...

I always find that surprising, and I am constantly surprised the rockets go as straight as they do. If I were to mount a camera and had to use an external aerodynamic blister I would definitely have an opposing blister to balance it out (I assume many of the straight flyers, in fact, do).

I make my AV Bays balanced as much as possible, but I'm unable to find photos right now, even created one sled that looking at the all-thread and platform look like a "Z" so it was straight down the MIDDLE of the AV Bay. I build by avionics onto the sled in such a way that both sides of the sled get a mirror image on both sides using redundant altimeters, batteries, switches, and wiring. Does it help with longitudinal balance? I'm not sure, but anything I can use to make the rocket go STRAIGHT UP, I'll incorporate into my builds...

I agree, Sammy. In as much as it is possible one should, at least, shoot for the theoretical optimum: For one, it's fun and a good learning experience, and it makes a difference in performance. I understand that issues with Reynolds Numbers at small scale, and all of that, but why not geek out and play it like the big boys do?

My batteries and avionics just about offset each other, and while the sled itself is offset by the central post (I'm only using one all-thread post for weight - it's a relatively small, low-performance MP/HP rocket) I will put them on opposing sides and it will help equalize the weight distribution.

 

[SammyD]

I hear you Curtis. Try using aluminum all-thread and aluminum nuts - you'll be amazed at the weight savings in 3" and 4" builds. In fact, it's all I'll use. 9" of steel all-thread isn't light; 2 sections of 9" all-thread is even worse...

 

[Curtis Enlow]

I wanted to use aluminum all-thread, but couldn't find any at any of the local supply places (the same with aluminum eye-bolts and quick-links). I probably could have ordered some all-thread, but wanted to proceed to try and get ready in time for the June fly-ins. I actually bought some aluminum rod and a small tap & die set from HF. Some of their stuff is good, some of it...well, let's just say the die chewed the 1/4" rod down to a 3/16" rod with no threads ;(

If you have any good sources for aluminum fittings and all-thread, do share! I would definitely like to try that on my next build.

 

[Keisling]

McMaster-Carr (https://www.mcmaster.com) and Fastenal (https://www.fastenal.com) both carry a full line of aluminum allthread, nuts, washers, etc. If you have a Fastenal branch nearby, you can order online and save shipping by picking it up at the branch.

 

[Curtis Enlow]

McMaster-Carr (https://www.mcmaster.com) and Fastenal (https://www.fastenal.com) both carry a full line of aluminum allthread, nuts, washers, etc. If you have a Fastenal branch nearby, you can order online and save shipping by picking it up at the branch.

Thanks, Dave! That's great to know.

 

[Curtis Enlow]

McMaster-Carr (https://www.mcmaster.com) and Fastenal (https://www.fastenal.com) both carry a full line of aluminum allthread, nuts, washers, etc. If you have a Fastenal branch nearby, you can order online and save shipping by picking it up at the branch.

Fastenal has aluminum 1/4-20 for $11.67 for a 6' foot length. Not too shabby! Unfortunately, aluminum eye nuts do not seem to occur in nature, but it's still quite a reduction in weight, especially for larger rockets that might require two rods.

 

[Nytrunner]

I've heard titanium bike spokes are in fashion with some flyers

 

[boatgeek]

I wanted to use aluminum all-thread, but couldn't find any at any of the local supply places (the same with aluminum eye-bolts and quick-links). I probably could have ordered some all-thread, but wanted to proceed to try and get ready in time for the June fly-ins. I actually bought some aluminum rod and a small tap & die set from HF. Some of their stuff is good, some of it...well, let's just say the die chewed the 1/4" rod down to a 3/16" rod with no threads ;(

If you have any good sources for aluminum fittings and all-thread, do share! I would definitely like to try that on my next build.

If you want to go to a physical store, you might check with Tacoma Screw. (https://www.tacomascrew.com/locations) They have dang near everything, though I stumped them once asking for a couple of 0-80 screws to replace ones that were lost from altimeter terminals. Also, the people at the counter know what they're talking about. When I went to get shear pins, the conversation went like this:

Boatgeek: I need some #2 nylon screws, about 1/4" long.
Salesperson (not missing a beat): Do you want pan head or flat head? Also, white or black nylon? Is slotted OK? We don't have Phillips at this store.

It's great to shop in stores where half or more of the customers are in coveralls--the store knows that they need to be fast, right, and knowledgeable to stay in business.

I would call first just in case they don't have it.

 

[Curtis Enlow]

If you want to go to a physical store, you might check with Tacoma Screw. (https://www.tacomascrew.com/locations) They have dang near everything, though I stumped them once asking for a couple of 0-80 screws to replace ones that were lost from altimeter terminals.

I have a 6' section of 1/4" - 20 coming for $11.67, and that should keep me happy (for a couple years!). I can keep using the steel section I have for mock-up purposes until then. But, yup, I am learning the advantages of checking the industrial sources for things. I love my local country hardware store (they have a huge bulk & misc fastener/doodads section), and will sacrifice a little $ to keep their doors open, but some stuff - like the allthread - it's not worth it. My 18" section of steel from them probably cost as much as 6' of aluminum from Fastenal. It might take a week to get here, but that's a couple rockets worth even with double-rod installations.