Mounting Electronics Between TTW Fins

[jqavins]

This may not be the right subforum for AV bay design, I don't hang here much, so please bear with me.

I had a thought that one could use the space between TTW fins, between the MMT and airframe tube as AV bays. One could even attach components directly to the fins tabs, employing them as sleds. Then it occurred to me that this might be something lots of people already do, since there's really nothing new under the sun. Or that, if people are not doing this, there might be a very good reason.

Here's a pretty near scale cross section of a 38 mm MMT inside a 4" tube, with four ¼" TTW fins, and a Featherweight tracker viewed end-on. It looks like making a quadrant of the booster into an AV bay would be pretty easy. Altimeter, tracker, air start electronics, or whatever could go into the same quadrant or multiple quadrants. Either the MMT and fin subassembly or just the aft centering ring would have to be removable from the airframe. A layer of fiberglass wool insulation between the MMT and the electronics would be a good idea.

Thoughts?

 

[tsmith1315]

Joe, check out the T-LOC, with just that type of e-bay:

https://locprecision.com/products/7-5-t-loc

 

[cerving]

I have a few issues with that... although I have seen it done.

1. You have to cut a pretty big hole in the body tube for access to the electronics, so you have to make sure that you shore up the body tube around the hole so it's not a structurally weak spot.
2. Assuming that you're using a reloadable motor, putting a tracker there may be problematic due the presence of a large piece of metal right next to the antenna. You're almost always best off putting a tracker in the nose cone.
3. Depending on the motor type, you may generate some extra heat, which is probably not going to be too great for either the electronics or the batteries. Some kind of foam insulation between the motor mount and the sled may be necessary, or at least recommended.

 

[jqavins]

1. You have to cut a pretty big hole in the body tube for access to the electronics, so you have to make sure that you shore up the body tube around the hole so it's not a structurally weak spot.

Since

Either the MMT and fin subassembly or just the aft centering ring would have to be removable from the airframe.

why would another large opening be needed? A small one for arming would be needed, but not necessarily an bigger than it has to be when in a dedicated AV bay farther up. Or am I missing something?

3. Depending on the motor type, you may generate some extra heat, which is probably not going to be too great for either the electronics or the batteries. Some kind of foam insulation between the motor mount and the sled may be necessary, or at least recommended.

I did say

A layer of fiberglass wool insulation between the MMT and the electronics would be a good idea.

Maybe, under high acceleration, the glass wool is not the best choice of insulating material.

 

[DEmery]

why would another large opening be needed? A small one for arming would be needed, but not necessarily an bigger than it has to be when in a dedicated AV bay farther up. Or am I missing something?

You need to be able to wire the altimeter and charge the battery. I did this once before. I covered the big hole with a door or panel that had a small arming hole. There is still a big hole there, though I didn’t have any structural issues with it.