Structurally sound?

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Jarhead

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Good evening all. I am working on a build and have an issue fitting everything into what is turning out to be a tight space. I have a 4-inch airframe, need DD and want a 4 grain case for best performance (my opinion of course).
I have a 6-1/2 inch Av Bay that is too long for this Configuration. I have it set up now with 2-1/2 inches inside the payload, which barely allows for the main and associated laundry. The remaining length is supposed to be in the booster but will not allow for the length of the 4 grain case.
If I shorten the Av Bay by 2 inches I will be able to shoe-horn the 4 grain in for drogue-less recovery.
The result will be 2 inches of coupler with three 2-56 shear pins securing the booster to the Av Bay. If I add a section of airframe I will lose scale and don't want that. With fiberglass components I "feel" this is enough for a solid mechanical connection; no science to rely on, just my gut. I know this works well when epoxied to the airframe but have never heard any guidelines for separation points.
What do you say my TRF siblings, is this structurally sound?
 
As a general rule of thumb, having one caliber of coupler into the airframe is generally adequate. However, it depends on things like how fast the rocket is moving and how tightly the parts fit together. There should be as little "wiggle" as possible when the airframes fit together so they don't bind when they need to separate. I would be more worried about that - IMO the fiberglass airframes are strong enough to do what you describe. Or you could hang 2" of the motor out the back of the rocket if the CG allows.
 
Thanks to all. Looking at the thread again to see how I can make this work.
 
If coupler and airframe are FG, and it's pinned... I think it'd work. Another option, if you feel safer/more comfortable with a longer coupler section is to extend the lower coupler by placing an additional couple-three inches of coupler and double-wall from the inside. Getting something like a LOC coupler and stiffy would get this done for you--but pin the existing FG.

(Merely trying to give you other options)

Or you could do a doughnut hole, as mentioned above...


Later!

--Coop
 
I think COrocket in post #4 had it right. One caliber is the accepted "norm", but less will work. Fit is going to be the biggest issue. It will have to be tight, without wobble. Also, the BT joint needs to be square and tight all 360 deg because if it isn't, you will get bending forces the coupler will have to withstand under thrust. You want to minimize that as much as possible. It will also depend on how big of motor you fly. The higher the thrust and/or closer you get to Mach and the accompanying shock waves, the more critical the strength of your joint becomes.
 
Good evening all. I am working on a build and have an issue fitting everything into what is turning out to be a tight space. I have a 4-inch airframe, need DD and want a 4 grain case for best performance (my opinion of course).
I have a 6-1/2 inch Av Bay that is too long for this Configuration. I have it set up now with 2-1/2 inches inside the payload, which barely allows for the main and associated laundry. The remaining length is supposed to be in the booster but will not allow for the length of the 4 grain case.
If I shorten the Av Bay by 2 inches I will be able to shoe-horn the 4 grain in for drogue-less recovery.
The result will be 2 inches of coupler with three 2-56 shear pins securing the booster to the Av Bay. If I add a section of airframe I will lose scale and don't want that. With fiberglass components I "feel" this is enough for a solid mechanical connection; no science to rely on, just my gut. I know this works well when epoxied to the airframe but have never heard any guidelines for separation points.
What do you say my TRF siblings, is this structurally sound?

I had the same challenge in my 3" PML Bull Pup and 4" Madcow Tembo, both relatively short rockets 40.125" and 37.125" and I wanted to use larger motors. In both cases I used a nosecone alt-bay design. There are many designs to choose from but I think one of the more simple and currently popular ones is the burrito wrap /w cable cutter for main and apogee using a hanging charge, either using tubes, vials, etc. I use the cap off a diabetic insulin pen needle tip as I have many and they work great.

You can as also use the shoulder of the nosecone as the alt-bay held in place with sheer pins and just pop the nosecone to deploy your main. I have not done this but there are several threads on it and it looks like a great design. Or as blackbranst said you can use a doughnut style again there are several designs I have viewed on TRF.
 
Have you considered rear ejection for drogue?


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