Finishing DD AV-Bay Questions

The Rocketry Forum

Help Support The Rocketry Forum:

Mike Haberer

DaHabes
TRF Lifetime Supporter
TRF Supporter
Joined
Jan 7, 2019
Messages
912
Reaction score
655
I have finished building my DD AV-Bay for my 65mm Miss Riley and have a few questions about installing it in the coupler and about switch access.

The SMT Designs AV-Bay has three screw holes on each end cap for connection of the endcap to the coupler with flat head screws. These are necessary on Steve's nose cone AV-Bays to install the bay into the coupler, but seem redundant on the DD bay as the all-threads hold everything in place. I assume no one uses them, but would like to confirm.

The switches are in the Sustainer/Main endcap and are covered by both the coupler and the sustainer. What size hole did folks drill to accommodate the arming screws?

Steve only provided round head arming screws, not flat heads. Does the round head get fully recessed when armed (I'll measure but am not at my build table at the moment)? Any reason to switch to flat head arming screws (you have to use flat head on the nose cone bays)?

Any specific position on the sustainer side of the bay to place the removeable rivets relative to the switch band? How many to use (I am assuming 3)?

My assumption on drilling the holes is mark the rivets first, insert the coupler to the switch band without the bay, drill the rivet holes, remove the coupler and measure and mark the couple for the arming screws (use a black Sharpie so you can see it through the sustainer), re-insert the coupler, install the rivets, drill the arming screw holes, remove the rivets, install the bay in the bay into the sustainer, install the rivets, test the fit of the arming screws, make the holes a bit larger if the aren't perfectly centered, insert the arming screws to bottom out, measure any excess arming screw length to remove for a flush fit, cut off excess, refit everything to verify completion.

Which size shear pins did folks use, 2-56 or 4-40, and how many? I am assuming 2-56 and 2 shear pins.

This is my first DD AV-Bay (I've only built nose cone bays up to this point), so am trying to zero in on best practices for this and future projects. TIA.
 

GAP

Member
Joined
Jul 14, 2021
Messages
17
Reaction score
13
I'm a little confused about some of your first questions. Maybe you could post a picture pointing things out.

As far as the shear pins go, most people seem to use 2-56 nylon shear pins. How many you use depends on a variety of factors. Keep in mind these shear pins have a shear strength of around 25-30lbs. So if you have two you will need at least 50lbs pushing the recovery system just to break it, quite a bit more to actually push everything out. If you're staying at a relatively low altitude you can just pick 2 or 3 shear pins and do ground ejection charge testing to see what works. Make sure to size your first ejection charge test via an online calculator. This are readily available by searching "ejection charge calculator". When flying to higher altitudes you can actually calculate the pressure differential and therefore the force pushing on the bulkhead. You would then choose enough shear pins to prevent the rocket from seperating from this force. When using less than 3 shear pins the coupler can cant when separating increasing the force required for actual separation.
 

Handeman

Lifetime Supporter
TRF Lifetime Supporter
TRF Supporter
Joined
Jan 18, 2009
Messages
7,881
Reaction score
441
Location
Stafford, VA
I've never used a pre-made av-bay like that so I can't answer specifically about your av-bay, but I usually use two counter sunk flat head screws into a pem nut instead of the removable rivits, but I put them between the switch band and the top of the av-bay about 90 deg, from the surface of the altimeter.
I also use a round head screw switch, accessed through the booster end of the coupler, in series with a pull-pin switch with the remove before flight flags, in the switch band. That allows prepping with everything turned off and no pins and flags to get in the way or damaged the av-bay. The last step before the RSO is to insert the pull-pins to keep things off, turn on the screw switches, close up the rocket, and insert the booster shear pins.
I use two 2-56 shear pins on the booster side, again, 90 deg from the surface of the altimeter board. My 55 lb. 6" L3 bird is the only one that gets three 4-40 pins on the booster.
The apogee charge only needs to open the rocket enough to get the drogue chute into the air flow and the drag of the drogue will pull everything else out. You don't want to use so large of charge it pushed the two halves all the way to the end of the apogee shock cord. That causes shock loading, requiring large hardware in the booster end and or more shear pins holding on the nosecone, that will then require larger main ejection charges, which then require larger/stronger/heavier links, anchors, and main shock cords, etc. It's a vicious circle.
 

Mike Haberer

DaHabes
TRF Lifetime Supporter
TRF Supporter
Joined
Jan 7, 2019
Messages
912
Reaction score
655
Thanks. I've got some useful tips for future builds from your response.
 

Troy3003

Well-Known Member
TRF Supporter
Joined
Feb 12, 2021
Messages
255
Reaction score
190
Location
Shelbyville, KY
The 3 screws around the outside you mentioned are not needed for the av bay. The other 2 screws for your switches can be any screw you want to use. I mark and drill holes in the body tube for those slightly larger that the thread size, 6-32 I think? I also measure the screw length needed to fully activate the screw switches and trim screws to length so once in the rocket you just snug them against the body tube and you can't overtighten them that way.
 
Top