how much do you overlap the air frame over the coupler?

[chipotle mg]

When you are building a coupler / air-frame separation point on your rockets , how much do you generally overlap? For instance is it 1 body tube diameter the coupler will fit inside the airframe? It may also depend on the use of friction or shear pins I understand, but I would appreciate any opinions for best practice for either scenario.

I am trying to build a 38mm minimum diameter coupler and am trying to decide if I want to go with a shear pin or 2 or just use friction fit. I have electronic charge with black powder as my intended method for separation and also intend to do dual deploy with the main parachute above the coupler and drogue below. This is somewhat of a performance rocket in terms of altitude and very well likely will go about mach 1 or a bit over that, but I will try to select longer burn motors to keep the mad acceleration relatively lower. So drag separation maybe an issue.

Let me know what you think! thank you

 

[rharshberger]

My usual is 1.5 calibers or so, its also dependent on coupler fit or at least it is for me. A 38mm would get something close to 2" but if its a 6" coupler then I would use half the length of the coupler (3").

 

[chipotle mg]

I have an 8 inch coupler in this case. Also the coupler is the avionics bay. I was thinking 2 inches overlap on either end and a 4 inch switch band vent band if you will in the middle. Perhaps one small sheer pin on each separation point as well?

 

[rharshberger]

I have an 8 inch coupler in this case. Also the coupler is the avionics bay. I was thinking 2 inches overlap on either end and a 4 inch switch band vent band if you will in the middle. Perhaps one small sheer pin on each separation point as well?

I dont use switch bands as my switches are inside the bay. But you can do it the way you suggested.

 

[chipotle mg]

I dont use switch bands as my switches are inside the bay. But you can do it the way you suggested.

So do you use a magnet switch? How do you access the switch? Do you need vent holes for altimeters? Forgive me I am new and learning. Thank you

 

[teepot]

I go for 1 body diameter at least. I have a 38mm MD. My av-bay is 8". No switch band and 4" up and 4" down. At the speed this is supposed to go I want the break point as rigid as possible.

 

[Steve Shannon]

So do you use a magnet switch? How do you access the switch? Do you need vent holes for altimeters? Forgive me I am new and learning. Thank you

If you use a barometric altimeter, you must have a vent hole.

 

[rharshberger]

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So do you use a magnet switch? How do you access the switch? Do you need vent holes for altimeters? Forgive me I am new and learning. Thank you

Through the static ports for the altimeter, one is drilled oversized to allow access to the switch (I use mostly Perfectflight/Missleworks screw switches).

 

[cerving]

If you use a barometric altimeter, you must have a vent hole.

If you don't use a switch band, drill three vent holes through the airframe and the coupler, after you're fitted the shear pins. For a 38mm MD rocket, three 1/16" holes will be adequate. If you're using a screw switch, you can probably get away with just that one hole, since it's probably going to be a lot larger.

 

[teepot]

I do have vent holes through the BT and the av-bay. 3 holes at 1/8th IIRC.

 

[3stoogesrocketry]

I do have vent holes through the BT and the av-bay. 3 holes at 1/8th IIRC.

You need to have your altimeter bay ( your coupler ) vented to the outside of you are using a barometric style altimeter. The altimeter must be sealed from either charge as well