My First Donut AV Bay - 5 inch Jart

A couple of years ago I bought a 5” Black Jart - I just had to have one (thanks again Kit).

I decided to build it dual deploy, so purchased the additional parts from Wildman.
When I simmed it out, I wanted to stay with 54mm, but it was pretty obvious that I wanted to fly longer 54mm motors, and to do that I’d have to use a donut AV Bay.

I usually take a lot of time with my builds. This one has been on the bench for awhile – I may only have a few minutes here and there – so it has taken me some time to finally pull this together.

Here's my first photo. I prefer the simple, clean look, rather than using a switch band.

This first photo shows the cut-outs for the first switch.






And this shows the second. D'oh! (Didn't measure twice, but it will still work. I might mount an LED here to make it look like I meant to do it...)






And here you can see the coupler with switches in relation to a 54mm 6XL case, thus why I need to build a donut AV bay.

Ok, so here's how I thought I would do this.

This set of photos shows the core of the AV bay, with one end permanently attached.

It includes:
>A length of stainless all-thread that will carry the load of the recovery system.
>BlastCaps for ejection charges.
>Modular electrical connectors.
>AeroPack on the aft end ("why?" you ask. More on that later...)

Worsaer said:

>AeroPack on the aft end ("why?" you ask. More on that later...)

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I have been wanting to try this for years but don't fly enough electronics to justify building a new rocket. Glad to see someone else has the idea. The though of a slip in and screw on altimeter sled is the ultimate in simplicity.

grouch said:

I have been wanting to try this for years but don't fly enough electronics to justify building a new rocket. Glad to see someone else has the idea. The though of a slip in and screw on altimeter sled is the ultimate in simplicity.

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Almost. That IS a great idea - but not what I had in mind for this build. For this build the longer motor will extend through the center of the AV bay, with the electronics in the space between.

More photos in a bit.

The half-baked idea was to allow the greatest flexibility by leaving the center of the AV bay open from top to bottom. That is accomplished by using a CR instead of a bulkhead as the lid.

For that to work, there needs to be a way to seal against ejection gasses around the inner circumference of the CR.
As this pic shows, there is a small CR to serve as a backstop for the lid, using a Viton o-ring as the sealing surface.





These two pics show the lid in place, without the sleds and electronics. (You can also see the color-coded ejection charge connection leads as a reminder of which one goes where when assembling things.)

Hooked and subscribed. I can't wait to figure out what you are doing with the aeropack.

escape tower motor.

DizWolf said:

escape tower motor.

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Now that could be fun! I was thinking about landing the upper section on a floating barge, but can't afford the barge...

I opted to use two sleds. For electronics, it has a brand new Marsa54L for the primary system, and a Missile Works RRC2+ for the backup system.




Batteries are on the second sled.

Nice work on your "thinking outside the box" altimeter bay.

Glad you're having fun with the Black Jart.

Thanks Kit. I enjoy building about as much as flying. It's fun tinkering with high-speed flying machines. The icing on the cake is seeing 'em jump off the pad.

Here's where it starts coming together...

Here's the mounted altimeter sled.




And the mounted battery sled.




And a couple of pics with the lid in place showing how it fits together (outside the coupler, of course.)

So, why use an Aeropack retainer on the AV Bay? Here's one way to illustrate the problem I was trying to solve. The donut bay makes sense when flying long motors.

As this first Rocksim pic shows, a 6xl case extends well into the bay.




Now, what if I want to fly a 4 grain case? Here's how it would look.





Because the AV bay is open through the center, a shorter motor presents a problem, as the ejection charge would pressurize the entire airframe, not just the lower section. The idea was to use the Aeropack and a solid closure to seal off the center when flying shorter motors.






This is simple, and should work great when flying shorter motors - so, what was I missing? One evening, out of nowhere I had one of those "Oh Cr@p!" moments, realizing I had missed something simple that would have resulted in a ballistic recovery when flying full length motors.

Who's already figured it out?

Worsaer said:

Who's already figured it out?

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?????

Got me, I fly short-medium-large motors with out any issues, just the way it came.

Please enlighten us, what you figured out.

I'm guessing you have to address the problem you created, by leaving off the vent band. I.E. driving the coupler into the NC at apogee.

Let's see when flying long motors without the cap, when apogee charge goes off if you get deployment you now have a hole through the avbay and can now not get enough pressure to deploy the main chute.

......let's think this one through. So, with out the "cap" and with a long motor, the appogee charge goes off without a hitch right. Now, when you are looking to have the main charge fire......how do you pressurize the forward section with a hole all the way through the av bay?????

I have 4 donut av bays and I don't see it working without the forward end sealed off from the ejection charge.

-Jason
Curious and subscribed.


......never mind. Post 6 addresses this. Seems like a lot of work for something that could have just been a bulk plate instead of a centering ring. That's the fun part of this hobby though. Always a new idea out there to be thought of.

Dwatkins said:

Let's see when flying long motors without the cap, when apogee charge goes off if you get deployment you now have a hole through the avbay and can now not get enough pressure to deploy the main chute.

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Yep, exactly. If I had fully thought it through in advance and used a simple end bulkplate, I could have avoided the complexity of the o-ring seal, and never needed the Aeropack.

I was able to retro fit it using a 54mm coupler and bulkplate, so it's not a total disaster; just a waste of time and parts.


Live and learn.

Worsaer said:

Yep, exactly. If I had fully thought it through in advance and used a simple end bulkplate, I could have avoided the complexity of the o-ring seal, and never needed the Aeropack.

I was able to retro fit it using a 54mm coupler and bulkplate, so it's not a total disaster; just a waste of time and parts.


Live and learn.

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And that is why we all appreciate build threads on here! We all learn something together!

First flight was a success!

3,860 on a 4 grain CTI K940 White Thunder. Redundant dual deploy worked perfectly.

Not only a success but it looks great in person! I was glad to check it out, and sorry I missed the flight.