Nylon all-thread for an av-bay?

[emckee]

Has anyone tried using nylon all-thread to secure end-plates on an av-bay?

I'm considering a pair of 1/4"x20 nylon rods for a 2" dia., 7" long bay on a ~2kg rocket (Darkstar Jr). I'm trying to avoid steel all-thread due to potential interference with a 900MHz GPS system.

Back of the envelope calculations suggest that a pair of 1/4"x20 nylon rods (6/6, with yield strength of 45MPa, UTS ~75 MPa) will take ~2850N in tension or roughly double that in shear force on the threads, which should be able to handle full-force accelerations of over 100 G's before yielding (not breaking).

Anyone have data showing typical accelerations higher than this?

Has anyone tried this before? Does anyone have other ideas or considerations I may have missed?

Thanks!
Erik

 

[mikec]

I'm trying to avoid steel all-thread due to potential interference with a 900MHz GPS system.

I'd be more concerned about interference with the altimeter. I like putting transmitters in the NC far away from the avbay for this reason.

The nylon would probably work, but I wouldn't mess with it personally. If you have any kind of off-nominal deployment, the nylon will likely rip apart.

 

[Random Flying Object]

I use a 900MHz radio for my Telemetry system and haven't experienced any issues with surrounding Ti and SS parts. It is also separated from my backup altimeter. You might want to double check your numbers, that seems like a whole lot of force on nylon threads, a pull test would be a good idea.


Sent from my iPad using Rocketry Forum

 

[CarVac]

With appropriate connections for the shock cords, you can use the av-bay itself to bear the tension. Not only is it neater, you save SO MUCH SPACE by not having the big thick 1/4" rods going through the avionics bay. Especially in a 2" rocket.

We did this on Bare Necessities, and our 3/8" G10 sled easily snapped the 2000+ (I don't remember exactly what it was) pound test Kevlar in the middle (not at any of the knots) when it separated at Mach 4. It let us fit a BRB 70cm GPS, AIM XTRA, Raven, and MiniTimer4 all with substantial batteries in around 9" of tapered av-bay ranging from 1.5" to 2.5".

And it's RF transparent.

 

[blackbrandt]

Can I get a few picture, carvac? That sounds like an interesting method.

 

[CarVac]

You can see the Y-harness of the Kevlar emerging on either side of the ARRD.

The Kevlar itself is a loop routed through holes in the bulkplate and the sled itself, and then tied with the main shock cord in a figure eight knot with the main cord backfed between the other two cords.

 

[warnerr]

Thanks for the great idea, Carvac! This relieves a lot of stress from two tiny aluminum rods in a tiny nosecone I am putting together. Nice setup!

 

[CarVac]

Oh yeah: another benefit was that with the thicker sled, we could just screw the altimeters directly into the fiberglass without needing nuts or standoffs; just spacers. It's very nice because that means you can use both sides more effectively (as we did).

 

[patelldp]

I've been using aluminum allthread almost exclusively. If you're going to try Nylon, get the finest thread count as possible (1/4-28 vs. 1/4-20, etc.).