Rodless A/V Bay

I know there have been threads on this topic, I have even partook in some. I have built one rodless A/V bay already for a LOC Norad Pro Maxx and will likely use that same bay in my PML Callisto once it is rebuilt. If it works in the Norad that is. Maybe I'll improve it or what not. These are both lightweight rockets. I've done a few in the nose cone sans all thread as well.

So why a rodless bay? Why not? I think I can save a bit of weight. Then the argument about all thread near a tracker.

For the smaller bay I epoxied 1/4" kevlar along one side in the coupler. A custom ring is glued in each end of the coupler and the sled has a plate on each end that will seal the bay. The kevlar takes all the load and the bay is just along for the ride. Not easy to explain but that isn't really why I am here. This new bay will be 4" diameter and 7" long. It is going in a PML Tethys. Which will be a bit heavier. I thought I weighed everything and built me a sim file but apparently I didn't. Their site says 62 oz but I am guessing around 70 or so. Normally for a rocket like this one that is a bit on the short side I would put a bay in the nose to handle ejection at apogee and use a JLCR. While that is still an option, blah blah blah, for reasons of my own, I want dual break dual deploy on this one.

I have the basic design for my bay figured out. The advice I need has to do with the recovery strap I will be using. The strap(s) will run through the bay, epoxied to the inside of the coupler and, like the smaller version, will take all the load which is usually handled by the rods. It won't use traditional lids. On each end, one ring will be glued to the coupler, then another one that is part of the sled will nest up against it sealing the bay. I know 1/4" kevlar is plenty strong enough for this rocket. Any reason I can't use one piece of kevlar along one side of the coupler? Now obviously two, opposite each other, would be better but for the initial shock of recovery, odds are that one strap will take most of that shock anyway. Two would come into play once the train is stretched out but be more prone to twisting. I also have the strap from the piston, which I won't be using, that can be used. It is a strap nylon and will have more surface to epoxy. Harder to tie though. A single strap works on a motor tube, it should work here.

Anything I am missing?
I am looking for another perspective. Maybe someone sees something I don't.
Thoughts?

In the picture, the yellow would be the kevlar. This is a rough-up of how the bay would look, with the coupler sliced in half just to show, having a little trouble with transparencies in FreeCAD. Those rings/plates will be permanent, glued to the coupler. #6 or maybe #4 screws will hold the bay sled in.

Did something similar.
Put a CF tube through from top to bottom with a kevlar shock cord running through it.
Used RTV to keep the kevlar from sliding.
Worked well.

FredA said:

Did something similar.
Put a CF tube through from top to bottom with a kevlar shock cord running through it.
Used RTV to keep the kevlar from sliding.
Worked well.

Click to expand...

Could you simply tie overhand knots in the kevlar to limit any travel?

There are many ways to skin a cat. Yours looks pretty strong. Using epoxy to fix the Kevlar to the inside of the bay, like you often see as an attachment to the motor tube, should be fine.

Looking forward to photos as you start construction!

FredA said:

Did something similar.
Put a CF tube through from top to bottom with a kevlar shock cord running through it.
Used RTV to keep the kevlar from sliding.
Worked well.

Click to expand...

o1d_dude said:

Could you simply tie overhand knots in the kevlar to limit any travel?

Click to expand...

I have considered both of these, the tube and knots, only with epoxy instead of RTV; but I dismissed both ideas; and haven't even thought about them again. Looking at it now, either of these would be a good option. There should be little to no stress on the A/V bay with these setups. Just a dab of epoxy, or RTV, where the strap goes through the plates to seal and hold the bay in place. Thanks for bringing it back to the surface.

DaveW6DPS said:

There are many ways to skin a cat. Yours looks pretty strong. Using epoxy to fix the Kevlar to the inside of the bay, like you often see as an attachment to the motor tube, should be fine.

Looking forward to photos as you start construction!

Click to expand...

Yeah, you and I are thinking along the same lines. Only, in this instance it is only holding the weight of the bay, not the weight of the rocket. The strap running through the coupler will take all the load.

I will lay out the parts to the kit to double check, but it is looking like room in the booster will be in short supply, so dual break may not even be possible. I will try to get some build pics in any case. Documenting builds is not my strong suit.

Yea, did a knot on the main side to keep it from sliding through while under drogue.
Movement of the bay after the main is out is a don't care.
Used RTV on the Kevlar as I've had Kevlar fail at an epoxy joint - to me, Kevlar and Epoxy don't mix.

FredA said:

Yea, did a knot on the main side to keep it from sliding through while under drogue.
Movement of the bay after the main is out is a don't care.
Used RTV on the Kevlar as I've had Kevlar fail at an epoxy joint - to me, Kevlar and Epoxy don't mix.

Click to expand...

I've seen folks post saying epoxy in kevlar will create a weak spot. But it has always worked well on attaching the harness to the MMT. Gorilla glue maybe, the poly foam stuff.

BTW - I used a CF tube that ran top to bottom so that there wasn't anyway for ejection gasses to get to my electronics.
I originally considered just the Kevlar, but was concerned about sealing at the end-caps only.

Right, the exact reason I was looking at using tubing. That design relies on end-caps/lids to seal. My option is about the same only I am removing the stepped lids and instead using two plates, one permanently attached to the coupler, the other attached to the sled. It should be roughly the same surface area as a stepped lid or stiffy/plate combo, just without a step. If I have the pieces printed, I may implement O-rings, just for the heck of it.

I printed caps like this:


The near cap is the aft cap and other the forward one.
The aft cap has a deeper inset and has a spot to hold a DPDT switch for power.

The large holes are where the CF tube goes.
The aft cap has a stop so the tube cannot go all the way through the cap -- the CF tube gets epoxied in place on this end.
The forward cap has a very tight clearance hole for the CF to pass through during assembly.

The slots are where wire ties are used to hold the caps in place once assembled.
There are holes for the ematch wires to penetrate the caps.
There is also a hole for the BRB antenna on the aft cap.

All holes for wires, antenna and wire ties get a light coating of RTV to provide a seal against ejection charges.

o1d_dude said:

Could you simply tie overhand knots in the kevlar to limit any travel?

Click to expand...

I would stay away from overhand knots and securing epoxy with Kevlar. Both weaken it considerably. Overhand knots are probably the weakest of all knots. And using epoxy to secure Kevlar can create a zone of weakness right at the interface of the bare Kevlar and epoxy. (Or so I remember reading in my research.)


Tony

I did rodless by inverting the usual eye-bolts on the bulkheads, opening them up with a propane torch, then feeding the steel through holes at each end of the G10 sled, closed the eye-bolts and welded them shut. The sled is now the tension devise.

In truth, this is probably gross overkill since rods do attenuate tracker signal, but there is some directions away from the rod(s) that are less affected, and the wild motion of a descending rocket ensure the signal gets sprayed far and wide. More rockets probably get lost due to solder joint failure than signal failure.

I did this for a no-metal rocket.
Only metal was the electronics and the aluminum in the fuel.
The pass-through shock cord took all the tension so simple, small wire ties kept the EBay closed.
Very lightweight too.

FredA said:

I printed caps like this:
View attachment 409945

The near cap is the aft cap and other the forward one.
The aft cap has a deeper inset and has a spot to hold a DPDT switch for power.

The large holes are where the CF tube goes.
The aft cap has a stop so the tube cannot go all the way through the cap -- the CF tube gets epoxied in place on this end.
The forward cap has a very tight clearance hole for the CF to pass through during assembly.

The slots are where wire ties are used to hold the caps in place once assembled.
There are holes for the ematch wires to penetrate the caps.
There is also a hole for the BRB antenna on the aft cap.

All holes for wires, antenna and wire ties get a light coating of RTV to provide a seal against ejection charges.

Click to expand...

Nice. Man I need to get a printer. There is always a kit or motor hardware or some shiny new homebrew tool or firearm that takes my money.