What is the rule of thumb for AV bay length? I am designing a 7.5" L3 cert rocket and I am trying to keep it as short as possible. How long should I make the AV bay (minimum)?
AV Bay Length
- Thread starter davdue
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GRIFFIN
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The coupler should slide into the body 7.5" on each end (forward and aft) If you decide to add a switch band, add that length to the total.
So, 15" with no switch band OR 17" with a 2" switch band.
That's the "rule of thumb" I was taught.
blackjack2564
Crazy Jim's Gone Banana's
+1 !!
woferry
Well-Known Member
I'd think "as big as you need to carry your electronics and make your electrical connections" is probably the minimum answer. 
Anything else likely depends more on the mechanics of where it is in the rocket, as if it's a coupler it needs to be a minimum length to function as a coupler (I've always seen 1 caliber as the rule of thumb, so that would be 7.5" at least, probably x2 since it would presumably be coupling two tubes together), and other details such as where you would put vent holes, how you will arm it, etc. The Excel DD I've finished building has a 14" long e-bay, which is way longer than I needed, but it's made from 2 7" couplers that are joined together by a 4" vent band (both the upper and lower tubes are meant to separate from the e-bay), so that leaves 5" per end for the coupler mate on a 4" diameter rocket. My actual sled inside that e-bay is less than half that length and has enough space for my two altimeters, 2 batteries and two magnetic switches, so I could have easily functioned with a 6-7" e-bay on this rocket, but it would have required things go together differently as well. My first attempt at a DD rocket was a Firestorm 54 that used a single coupler on a 54mm rocket, it was perhaps 5-6" long (again, half of it epoxied in the upper tube, and the lower half exposed as the coupler to the lower tube) and obviously less than 54mm inner diameter, I had difficulty fitting a StratoLogger SL100 and a 9V battery in it so I could have used something longer (with the newer SL CF it would have worked great though).
Once you're up to a diameter of 7.5", I'd think you even have the option of laying the electronics down instead of having to orient everything vertically, which could make for a very short e-bay so long as its not doubling as a coupler. Just keep in mind that some altimeters need to face a certain way, either because they have a single-axis accelerometer or other requirements that they be upright before arming.
As I'm all about flying electronics, my next build (5.5" LOC) is single-deploy but will still have some sort of e-bay. I'm bouncing between two plans right now, one would simply screw/strap the altimeter and battery to a CR/bulk plate in the nosecone (laying flat, not vertical), the other would have a BT-55 tube running vertical inside the main body that would function as the e-bay, leaving the rest of the body for other things (chute and more electronics for illumination).
Anything else likely depends more on the mechanics of where it is in the rocket, as if it's a coupler it needs to be a minimum length to function as a coupler (I've always seen 1 caliber as the rule of thumb, so that would be 7.5" at least, probably x2 since it would presumably be coupling two tubes together), and other details such as where you would put vent holes, how you will arm it, etc. The Excel DD I've finished building has a 14" long e-bay, which is way longer than I needed, but it's made from 2 7" couplers that are joined together by a 4" vent band (both the upper and lower tubes are meant to separate from the e-bay), so that leaves 5" per end for the coupler mate on a 4" diameter rocket. My actual sled inside that e-bay is less than half that length and has enough space for my two altimeters, 2 batteries and two magnetic switches, so I could have easily functioned with a 6-7" e-bay on this rocket, but it would have required things go together differently as well. My first attempt at a DD rocket was a Firestorm 54 that used a single coupler on a 54mm rocket, it was perhaps 5-6" long (again, half of it epoxied in the upper tube, and the lower half exposed as the coupler to the lower tube) and obviously less than 54mm inner diameter, I had difficulty fitting a StratoLogger SL100 and a 9V battery in it so I could have used something longer (with the newer SL CF it would have worked great though).Once you're up to a diameter of 7.5", I'd think you even have the option of laying the electronics down instead of having to orient everything vertically, which could make for a very short e-bay so long as its not doubling as a coupler. Just keep in mind that some altimeters need to face a certain way, either because they have a single-axis accelerometer or other requirements that they be upright before arming.
As I'm all about flying electronics, my next build (5.5" LOC) is single-deploy but will still have some sort of e-bay. I'm bouncing between two plans right now, one would simply screw/strap the altimeter and battery to a CR/bulk plate in the nosecone (laying flat, not vertical), the other would have a BT-55 tube running vertical inside the main body that would function as the e-bay, leaving the rest of the body for other things (chute and more electronics for illumination).
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