Allthread free avbay?

[Moan2]

Hey all, I recently picked up a EggTimer TRS, and in the FAQ section it mentions that you can't use it in an avbay with two allthread rods running through without the range being severely affected. As I am planning on pushing the limits on the range (RW FireFlyer to ~15k, later rockets even higher), I want to keep the range as high as possible. How have people built their avbays without the allthread? I'm not confident in the electronics sled taking the full force of rocket deployment, so I'd like some other ideas. Someone somewhere mentioned running a single length of kevlar from mmt to nosecone, and having the avbay simply attached to that so that it wasn't structural, but I'm still a little stuck on mounting the sled. Brackets on one of the bulkheads maybe?
Thanks!

 

[Yoehahn]

I planned on doing an allthread free avbay for my modular rocket project (posted down below in this subforum) where I have two clamps on the av bay sled holding it in place and then Im using set screws into the bulkplates to attach the coupler putting most of the load on that instead. Mine will be machined aluminum, but I am sure you could do the same thing with thicker bulkplates or blocks of wiid attached to the bulkplates and threaded inserts. I attached the picture and you should be able to see the threaded holes for set screws on the bulkplates themselves, I am using 8-32 threading for everything and it should be strong enough as long as you dont drill right next to the coupler edge.

 

[Tonimus]

I had no issues with my TRS and all-thread. I ran a rubber duck antenna outside of the AV bay and oriented the GPS antenna away from the all-thread.

 

[cerving]

That's a very interesting design. Do you slide it into the AV bay then screw an outside bulplate onto the smaller side with an inner bulkplate? I'd recommend having small hex-head bolts come from the inside (like a stud) and screw the bulkplate to the studs... shear forces like that are better taken with studs.

 

[OverTheTop]

I tied either end of a NC together with a G10 sled. It was screwed onto either end. Nothing going to pull it apart. I would be happy to use a similar method on a regular avionics bay.

NC build thread here: https://forum.ausrocketry.com/viewtopic.php?f=6&t=5019&start=8

That left heaps of room for the TeleMega and antenna.


It is held by an eyebolt at the bottom, and the turned tip at the top.

 

[Buckeye]

You can keep your traditional all-thread avbay for your altimeter, and then put your tracker in a separate, metal-free avbay in the nose cone. Lots of ways to do that. Check out John Coker's webpage, 3D printed nose bays from Cl(VII) and my cheapo method here:

https://www.rocketryforum.com/showt...00-nose-cone-bay-from-the-plumbing-department

Edit: Just noticed you have a TRS which is altimeter + tracker in one unit? Never mind. As mentioned above, I never had issues with my trackers (BRB900) next to all-thread.

 

[JimJarvis50]

Hey all, I recently picked up a EggTimer TRS, and in the FAQ section it mentions that you can't use it in an avbay with two allthread rods running through without the range being severely affected. As I am planning on pushing the limits on the range (RW FireFlyer to ~15k, later rockets even higher), I want to keep the range as high as possible. How have people built their avbays without the allthread? I'm not confident in the electronics sled taking the full force of rocket deployment, so I'd like some other ideas. Someone somewhere mentioned running a single length of kevlar from mmt to nosecone, and having the avbay simply attached to that so that it wasn't structural, but I'm still a little stuck on mounting the sled. Brackets on one of the bulkheads maybe?
Thanks!

One concept that I think is important for high performance rockets is to have a single "backbone" for a rocket where the entire length of the rocket is either a piece of all thread or kevlar harness. Through the bay, that means a single piece of all thread down the center with harness attached at either end. Bays with two rods down the sides violate this priciple. Maybe one rod down the center would be better for your tracker?

Jim

 

[ksaves2]

One concept that I think is important for high performance rockets is to have a single "backbone" for a rocket where the entire length of the rocket is either a piece of all thread or kevlar harness. Through the bay, that means a single piece of all thread down the center with harness attached at either end. Bays with two rods down the sides violate this priciple. Maybe one rod down the center would be better for your tracker?

Jim

Except make sure you use a large enough rocket!



If one tries to do a single all thread in a small rocket is becomes a bear to cram it all in. With this arrangement Stupidhead fortunately
realized those metal standoffs the Featherweight magnetic switch was sitting on was going to short out the danged thing. He realized
it will take nylon washers and screws to float the mag switch off the stand offs . The single all thread cuts down on the room for the electronics as it goes right down the middle. The eyenuts were custom 8-32 things from 3Dogs that are not available anymore.

Oh, that is not the final battery securing arrangement. I'm going to use servo tape and chinch it down with a zip tie once the batter is plugged in. I've cut
holes for in the G-10 for the ties. I may do a scew-in forward thrust block too but the battery chinched gently down to the tape might be secure enough.


If I had to do it again I'd use two all-thread off to the side. The current setup is working so far and I was going to fly it on a J500 but
am chickening out. I won a spacer set for the 720 case so I am going to fly it initially on a less powerful motor to keep it within sight.
If the setup works, I'll punch it with the J500. Kurt

 

[woferry]

I'll just note that I don't see why the all-thread has to be down the center if it's a continuous piece, you still get the end-to-end strength, our spines don't run down the center of our torsos. When I ordered my Go Devil 38 I asked for the av-bay bulkhead pieces to not be pre-drilled, and drilled my own off-center hole. So I have a single 1/4"-20 rod running end-to-end (two forged eye-bolts with a coupler nut to join them), but the whole deal is offset through the av-bay to ensure that my rather large LiPo would fit (was also necessary for the charge well to clear the lower eyebolt which needs to rotate to screw the lids together). Granted on this lightweight rocket I could have gone with something less than 1/4" rod, but I wanted to couple a pair of eyebolts and the smallest forged eyebolts any of my local HW stores carried was 1/4" so that's what I used.



Anyway, getting back to the OP, there's certainly always the risk of messing with the antenna by having any other metal nearby, but the farther away and less-parallel any other metal is the better off it should be. It's more complicated (resonances and the like), but one can basically think of the metal as creating a 'shadow' for the antenna, so if you had the antenna right against the rod it would have a big shadow cast on one side of it, but the farther away the smaller the shadow should be. Two rods would create two shadows, and gets nastier when it comes to the resonances. I've flown both Eggtimer TRS's and MW RTx transmitters in 3" av-bays using a central all-thread rod in a HED rocket (so there's no place in the nose for a tracker, just the av-bay), but I've been able to keep the antenna an inch or so away from the rod, and haven't had a noticeable issue with reception in any of my flights. YMMV of course.

 

[dhbarr]

Doesn't that much off-center mass have the potential to cause some wobblation ( technical term )?

 

[Moan2]

one can basically think of the metal as creating a 'shadow' for the antenna

Thanks for this analogy, I wasn't too sure on the exact reason for avoiding metal and this really helped!

I think I have things planned out in my head, I plan on having the sled have brackets on either end to hold the bulkheads on and keep the whole thing together, and a single line of kevlar running through a hole in each bulkhead. The only thing I am worried about is the kevlar catching and/or rubbing on edges of the fiberglass bulkhead holes. Would it be worth it to run a smaller tube specifically to keep the kevlar line away from the bulkhead holes, and the sled on the inside?
Thanks!

 

[OverTheTop]

Having a "steel backbone" is only one solution to the problem. Invertebrates would argue they are not necessary

It is all calculable, assuming you know what forces to expect (in off-nominal circumstances) and you provide sufficient strength to withstand them. One rod is much more simple to think about and calculate, but other solutions should be considered and not excluded by dogma.

I have a bit of a leaning towards titanium bike spokes for that sort of thing. 2mm diameter and 280kg breaking strain each . If 280kg isn't enough, use two or three! I bought some recently for $8AUD each, cut to custom length.

 

[Steve Shannon]

I'll just note that I don't see why the all-thread has to be down the center if it's a continuous piece, you still get the end-to-end strength, our spines don't run down the center of our torsos.
Snip....

Our torsos are meant to bend easily…
[emoji57]


Steve Shannon

 

[JimJarvis50]

Our torsos are meant to bend easily…
[emoji57]


Steve Shannon

So, how come they don't do that anymore?

And yes, not all rockets need backbones.

Jim

 

[Steve Shannon]

So, how come they don't do that anymore?

And yes, not all rockets need backbones.

Jim

Because we’re getting older.[emoji22]

I like the idea of a backbone.


Steve Shannon

 

[OverTheTop]

Spines are overrated. The bones themselves can only support about 10kg. It is the muscle structure around them that takes the load and keeps us upright or whatever.