Smallest possible DD/AV bay?

I'm just researching right now for a future build... So nothing is in danger of being done (yet)...

I'm curious, if I had a 3.9"-4" OD rocket, would a 7" long DD/AV bay be too short for it?

Thanks!
Jim

Well... You might not take my word on it since I have never done one myself, but. 7-1 (for switchband) is 6" 3 " per side. I think it would be fine. Maybe a little close...

You could build the av bay into a glued-in coupler to make it shorter.

if you omit the band entirely, yes it is fine, especially if you figure in the extra 1/2" total that the bulkhead plates provide. That gives 3.75" per side. Our old couplers were half as thick and only 6" long. The new HD couplers are thicker than the airframe by a good margin.

Binder Design said:

if you omit the band entirely, yes it is fine, especially if you figure in the extra 1/2" total that the bulkhead plates provide. That gives 3.75" per side. Our old couplers were half as thick and only 6" long. The new HD couplers are thicker than the airframe by a good margin.

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So, if I read that correctly, the bulkheads fit inside the coupler, and not on the outside of the coupler, held in place by the screws and the lip of the coupler. Right?

I'm, uh, planning one that should come in at about 3.25" long... LiPo and aggressively dense mounting...

I'm working on a 4" long x 1.6" OD av bay, and it's doable (barely).

I would think what you are thinking about is possible with planning.

Greg

K'Tesh said:

So, if I read that correctly, the bulkheads fit inside the coupler, and not on the outside of the coupler, held in place by the screws and the lip of the coupler. Right?

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They are stacked plates. One fits inside to center it, the top one is a cap the same OD as the coupler tubing. Standard stuff.

Binder Design said:

They are stacked plates. One fits inside to center it, the top one is a cap the same OD as the coupler tubing. Standard stuff.

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Ok, I get it now... 4 plates total, two that fit inside the coupler, two that are the same diameter as the coupler. and the threaded rod holds them tight.

Thanks!
Jim

If you don't use the AV bay as a coupler you can make it as short as you like.

I have a glued in coupler and used a 24mm minimum diameter bay and a raven to retrofit in an avbay into an already built 4" rocket...

Zebedee said:

If you don't use the AV bay as a coupler you can make it as short as you like.

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+1

Depending on your stresses, I would consider a longer coupler, if it was going supersonic I'd be looking at 1.5 calibers to each side that is not a glued bond. Subsonic, I'd still be at least a caliber a side.

If you're just contemplating the smallest possible size, you'll have great difficulty making something smaller that a Raven in a Featherweight AV Bay. IIRC they are ~2" long and maintain full 4-output functionality, integrated battery and magnetic switch.





All the best, James

K'Tesh said:

I'm just researching right now for a future build... So nothing is in danger of being done (yet)...

I'm curious, if I had a 3.9"-4" OD rocket, would a 7" long DD/AV bay be too short for it?

Thanks!
Jim

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Should be fine. This is 6-7 inches long, I forget. I used it for my L2. I used SS screws into the payload bay and no vent band. (nylon screws for shear pins in the NC)

I did a minimum diameter 24mm F motor rocket using an ALTS2, "av bay" part was 4" -the length of the altimeter. Normally I used single deploy but a couple times it went dual. At apogee it would eject the motor casing and the sand ballast, at main deploy it has a streamer. Everything rear ejected, the NC was glued on and there was no break points in the rocket.
We were launching with no one else there so the motor case - single use- free fell, was on a playa so it could be easily found.

M

K'Tesh said:

I'm just researching right now for a future build... So nothing is in danger of being done (yet)...

I'm curious, if I had a 3.9"-4" OD rocket, would a 7" long DD/AV bay be too short for it?

Thanks!
Jim

Click to expand...

This head end DD/AV Bay for a Madcow FG 4" V2 is 4 1/8" long including the bulkheads. When I started this build, I didn't believe the 4" coupler that came with the kit it could possibly be long enough. I even ordered an 8" coupler to replace it thinking to cut it to maybe 6" or so. As I did the dry fitting, however, I realized the 4" coupler would actually work fine. This is head end DD where the nose cone is popped off the top end of the AV bay, so there is only 1 3/4" of coupler/bulkhead above the 3/8" vent band. Much longer and it would wedge too tight into the nose cone anyway. That leaves 2" of aft coupler/bulkhead which feels plenty secure to me. Smooth fit into the booster with no rocking. I do have sheer pins holding the booster and nose cone, but these are just to prevent drag separation and hold the nose cone on at apogee separation. No structural support needed from the sheer pins at all.

I do not think one could routinely have 2" of coupling on a 4" diameter AV bay, the tolerances needed are probably too tight. It happens to work here because the fit on this Madcow FG V2 kit was absolutely perfect.

View attachment 174095View attachment 174096View attachment 174097

This one is my shortest. It has two Stratologgers in it. It's 1.5" tall, but the space between the plates has be at least 2" so that there is room for the ematch wires. Allthread goes down the middle.

Jim

Raven and BRB GPS in a 38mm nosecone.

K'Tesh said:

I'm curious, if I had a 3.9"-4" OD rocket, would a 7" long DD/AV bay be too short for it?

Thanks!
Jim

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Here's a data point for you since what I think you are really meaning to ask is how short of a coupler length you can get away with. Our old couplers were 6" long, so they only had 3" inside each section and they were thin wall. So if you used one of our new 7" long HD couplers with a 1" wide band, you'd have the same overlap, but double the wall thickness as our old couplers. So yeah, it will work.

If you didn't use a band, it would give even more margin and it wouldn't split the decal in case you still wanted to fly without it. You are really putting a lot of thought into this I will say that.

When I clicked on this I first thought I'd see something about tiny 24/38mm bays. a 7" 4" OD bay is really quite huge and will be just fine.

Binder Design said:

Here's a data point for you since what I think you are really meaning to ask is how short of a coupler length you can get away with. Our old couplers were 6" long, so they only had 3" inside each section and they were thin wall. So if you used one of our new 7" long HD couplers with a 1" wide band, you'd have the same overlap, but double the wall thickness as our old couplers. So yeah, it will work.

If you didn't use a band, it would give even more margin and it wouldn't split the decal in case you still wanted to fly without it. You are really putting a lot of thought into this I will say that.

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For those who haven't figured it out yet, this thread is VERY much concerned with what Mike is saying. I AM trying to score a Velociraptor as part of a commercial build contract I'm lining up. And he's right, I'm trying to not split the decal. I'd like to build the rocket as my L1/L2 project, and as far as the DD, I don't want to lengthen the rocket (I love its lines, and it looks perfect to me as-is.

I may opt to do a 1" band, but I'd want to shorten both the drogue and the main by 1/2" each to do so, and the decals would be incorporated with the band in place (splitting them if needed). The DD/AV bay would be installed even if it was flown w/o avionics. I'm thinking about the possibility of adding a Cineroc-like fairing/periscope for a keychain camera, but that's about all that I want to do externally to my first Velociraptor (I'd like at least two) to make it different from a modern "stock" Velociraptor (as opposed to the prototype 38mm powered version, or the 75mm L3 version).

Mike and I have discussed the using a DD/AV bay as it was intended. It would be 14" long, and add 4" to the length overall. Shortening both 17" tubes by 2" each would create a problem with the main chute's space. Shortening only the drogue's space 4" IMHO would just look... Odd.

Thanks for all the responses!
Jim

DizWolf said:

When I clicked on this I first thought I'd see something about tiny 24/38mm bays. a 7" 4" OD bay is really quite huge and will be just fine.

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Same here. The concern seems to be structural, actually, not electronics volume though, so it isn't unwarranted.

CarVac said:

Same here. The concern seems to be structural, actually, not electronics volume though, so it isn't unwarranted.

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Exactly... I don't know where I heard it, but I'm sure that I read that couplers are should be at least 2 calibers long for best performance. After reading the results of this thread, I'm now sure that taking 1/7th caliber out of that won't be an issue if one side is bolted (or glued) down, but I didn't know if a DD system could fit in something so "small". Clearly it can and be safe.

Now I have to figure out what else could squeeze in there (telemetry, video, passengers/stowaways/cargo, small plastic toys... etc).

watheyak said:

Raven and BRB GPS in a 38mm nosecone.

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Do you have more info on that Kevlar shock cord strap? Looks perfect for my current build. Grommets straight through the fabric?

You can get a heck of a lot in a small space if you are forced to. Below one Raven, one BRB RF transmitter and two LiPo's in ~1.5"x2.5".