Optimum range of coupler engagement for ebay/airframe break?

[SolarYellow]

What is the optimum range of coupler engagement (let's use calibers as a unit for discussion) when breaking apart an airframe/ebay/whatever that will separate at ejection? Too little, and you don't have good bending resistance/strength for the airframe structure. Too much, and maybe it doesn't have time to slide far enough to separate during the ejection pressure rise. The latter will depend on the relationship of the friction fit, length of engagement (together becoming the work that must be accomplished to separate), and ejection pressure time profile (which will be determined by powder quantity, burn rate, ejection cavity volume, leak rate through altimeter ports and the coupler joint, etc.).

Assume I'm using full-length coupler stock, longer than the rocket I'm building, so I can make it as long as I want. I tend to think 1 diameter (1D) is minimum, even for LPR. There are some nose cones I'm aware of with shoulders less than that, but that's just an empty LPR nose cone. I'm more comfortable with 1.5D, but maybe there'd be structural advantage to going to 2D? For a rocket that will reach up to Mach 1.3-1.5, either with cardboard or composite airframe, any structural advantage to going past 2D? Has anyone probed the far end and separation reliability?

@WILDMANRS
@JimJarvis50
@bad_idea

 

[Banzai88]

I've always used 1 caliber as a minimum, although CW/RW/Wildman fiberglass often comes with more than that and folks haven't seemed to have had any issues within the parameters you've stated.

 

[tfish]

1 caliber and that coupler should be stronger then the tubes it's joining.

...it can be longer to accommodate...electronics/payload or other needs..

Tony

 

[cerving]

Sometimes you don't have 1C, i.e. head-end deployment. As long as that piece is relatively short/light, it's OK. Similarly, if you have a longer/heavier piece (such as a two-stage sustainer) then you may want to make the coupler longer... I use 1.5C for mine. Beyond that I don't think there's any value in making it longer... and it takes up room that you need for other things.

 

[Alan15578]

You will have to do a loads analysis. For LP I use .5D on nose cones with good friction fit. For payload sections I use 1.0D. For Superoc I've used 4.0D. You should probably be using shear pins where length hardly matters, but you still need the loads analysis.

 

[cls]

If you are concerned about the strength of the tubing, consider gluing stringers on the inside. Just 3 or 4 thin strips of wood, nearly as long as the tubing.

 

[bad_idea]

Too much, and maybe it doesn't have time to slide far enough to separate during the ejection pressure rise.

If this were found in ground testing, loosening the fit or adding more dakka could remedy the problem. There's presumably some point at which coupler length could become absurd, but I'd think you'd have to be hunting for the edge case in order to hit a problem you couldn't fix with minor changes.

WILDMANRS
JimJarvis50
bad_idea

Honor to be mentioned in such company. My bad ideas are my own, but many of my good ideas, were stolen from Jim, so interested in his input.

Sometimes you don't have 1C, i.e. head-end deployment. As long as that piece is relatively short/light, it's OK. Similarly, if you have a longer/heavier piece (such as a two-stage sustainer) then you may want to make the coupler longer... I use 1.5C for mine. Beyond that I don't think there's any value in making it longer... and it takes up room that you need for other things.

When @SolarYellow and I discussed couplers before, I hadn't been thinking of an interstage coupler, and if using tube as a coupler I'd certainly go longer there due to the loads. Would you consider it safe to go lower than 1.5 cal. if using the sustainer motor case as the ISC?

If you are concerned about the strength of the tubing, consider gluing stringers on the inside. Just 3 or 4 thin strips of wood, nearly as long as the tubing.

Yes; to this point and that of @tfish, I've used 1 caliber in the zipperless coupler of rockets where most of the length and mass of the rocket was ahead of the coupler, but they were significantly reenforced with extra centering rings. (Loading up with extra rings was one of those aforementioned ideas I stole from Jim.)

 

[SolarYellow]

If this were found in ground testing, loosening the fit or adding more dakka could remedy the problem. There's presumably some point at which coupler length could become absurd, but I'd think you'd have to be hunting for the edge case in order to hit a problem you couldn't fix with minor changes.

Good point. Might be interesting to play with some mockups just to go searching for the edge.

When @SolarYellow and I discussed couplers before, I hadn't been thinking of an interstage coupler, and if using tube as a coupler I'd certainly go longer there due to the loads. Would you consider it safe to go lower than 1.5 cal. if using the sustainer motor case as the ISC?

I found another thread where Jim posted several photos of his rockets and their interstage details. One was favorable for scaling, so I pulled it into AutoCAD and found 1.67 diameters of coupler engagement. I figure if that works for him, between that and Cris' 1.5 RoT, I'm pretty good to go with that.

Build thread when I get there, but in the rocket I'm in the midst of working out the details on, I could go several x diameter without affecting how much space there is for everything else. Kind of along the lines of what we discussed in PM last night, but 29mm powered BT-60. It's just a matter of where I put the seam.