Shrink fit thrust ring on aluminum case?

[SolarYellow]

Thinking about "case as coupler" where the aft end of the sustainer is a male slip fit into the top of the interstage coupler. Photos of other successful rockets show about 1 2/3 calibers of engagement in such a scenario. In an MD rocket, one would want the sustainer airframe or fin can to start above that, and it would need a thrust ring. Most commercial cases don't have any provision for thrust rings a caliber or more ahead of the aft end of the tube.

Has anyone had success with a simple machined ring being shrink fitted (where the ring is heated and the case cooled and then they are allowed to reach equilibrium after assembly, resulting in an interference fit)? I don't think much of an interference would be required to make the junction more than adequate to transfer the maximum thrust of the 29mm or perhaps long-burn 38mm motors I'd use it with.

Technically, since the ring could be removed without damaging the case, I think it should even be non-EX, as long as the flush aft closure was certified.

 

[lr64]

Appropriate degrees of press fit are probably covered in a book called Machinery's Handbook. Then you only have to figure out the required temperature differences, given aluminum's coefficient of thermal expansion.

 

[pugachu]

you would need to make sure that the hot ring doesn't locally anneal the case that you are putting it onto. 6061-T6 aluminum will recrystalize at surprisingly low temperatures

 

[Adrian A]

Thinking about "case as coupler" where the aft end of the sustainer is a male slip fit into the top of the interstage coupler. Photos of other successful rockets show about 1 2/3 calibers of engagement in such a scenario. In an MD rocket, one would want the sustainer airframe or fin can to start above that, and it would need a thrust ring. Most commercial cases don't have any provision for thrust rings a caliber or more ahead of the aft end of the tube.

Has anyone had success with a simple machined ring being shrink fitted (where the ring is heated and the case cooled and then they are allowed to reach equilibrium after assembly, resulting in an interference fit)? I don't think much of an interference would be required to make the junction more than adequate to transfer the maximum thrust of the 29mm or perhaps long-burn 38mm motors I'd use it with.

Technically, since the ring could be removed without damaging the case, I think it should even be non-EX, as long as the flush aft closure was certified.

On my min diameter rockets with the case as the coupler, I have a thrust ring inside the sustainer at the top, or I just friction fit.

 

[OzHybrid]

If you heatstrink 2 components together and they are the same material, it's going to require cutting them to separate. Once heatshrunk together, you cannot get a temperature differential. If they are 2 different materials like a steel bearing in an aluminium bush, then the different thermal expansion ratios will allow separation again.
A split clamp would be a better solution.

 

[SolarYellow]

I don't know how to make a split clamp that is the thickness to fair to an MD fin can. I already figured that removal would require cutting, but that's still doable without damaging the case, if you can work with care.

@Adrian A,
One potential configuration is sub-MD with a fin can sleeve, but the functional parts of the rocket on top of the case at the same OD as the case. Kinda like stuff you build. Still want the fin can mechanically supported, would probably friction fit it to keep it from moving forward.

 

[OzHybrid]

What motor are you using? What is the OD of your fin can. Sketch?

 

[Blast it Tom!]

I've done quite a few shrink fit calcs in my day. In addition to @pugachu's concern, a real problem, especially at these diameters, is tolerances. How accurate can you get each piece? Then you have to do your calcs for the two extremes of your tolerance range. The minimum interference has to be enough, and the maximum can't be too much. And of course, if you anneal the aluminum, you've lost a lot of strength and you could yield the weaker piece...

@OzHybrid's concern is likely correct, though with heavier pieces it is possible to pop the outer one loose. It's a transient calc, and you have to have enough heat and get it at exactly the right time. Likely impossible in this case.