Adhesive for graphite/aluminum nozzles

[hybridfreak]

Hello,

What is everyone using to glue graphite inserts into aluminum carriers?
I recently broke a nozzle on the convergent section because it stuck to the ABS grain I used in a 38mm hybrid. So now I want to make the next one in a way that the area that broke is also aluminum and has a little draft angle.



Thanks!
Roman

 

[jsdemar]

High temp red silicone rtv.

Graphite with sharp edges is prone to cracking. A straight cylinder will work as well as a tapered insert.

It’s better to have a phenolic carrier and not have direct contact with aluminum.

 

[kramer714]

yeah, so that happened. When I have nozzles that are 'melt sealed' to the grain, I never try and remove the nozzle cold. I get the hot enough to pull the soft grain off then let the nozzle cool. The little bit of grain that stays with the nozzle comes off easily when it is cold. Skyripper style nozzles are very prone to this.

I use Harbor Freight welding gloves and a blow torch or just heat the nozzle / grain in the BBQ until the grain is soft.

Mike K

 

[MClark]

High temp red silicone rtv.

Graphite with sharp edges is prone to cracking. A straight cylinder will work as well as a tapered insert.

It’s better to have a phenolic carrier and not have direct contact with aluminum.

Yeah, what he said.
I do an aluminum carrier with a phenolic insulation tube between Al and graphite.
I use silicone to seal but it takes forever for it to cure in middle of nozzle. I had some two part silicones I was using..had no leaks with either one so it didn’t matter.

 

[hybridfreak]

yeah, so that happened. When I have nozzles that are 'melt sealed' to the grain, I never try and remove the nozzle cold. I get the hot enough to pull the soft grain off then let the nozzle cool. The little bit of grain that stays with the nozzle comes off easily when it is cold. Skyripper style nozzles are very prone to this.

I use Harbor Freight welding gloves and a blow torch or just heat the nozzle / grain in the BBQ until the grain is soft.

Mike K

I saw Rattworks even provided single-use graphite nozzles on their 29mm motors... Thanks for the tip!

And thank you for all the other input, very much appreciated! I'll go with high-temp. RTV for now since I also have time to wait for it to cure until the next launch/test. I'll also make the part that broke off aluminum and taper it a little to add a draft angle like in a mold for injection molding so it releases more easily.

I've only ever seen phenolic used on very large motors, eh? I'm working on 38/54mm hybrids that burn 5s at most so Al/Graphite should suffice in regards to heat transfer but contact corrosion Al/C might be a thing.

Cheers!
Roman

 

[lr64]

I think some RTVs cure by humidity, so it may help to keep the joint in a humid place. I'd say mix in a microscopic amount of water, if it wasn't such a critical application. I could just see that tiny amount remaining, and boiling at the wrong time, or leaving a few voids.

 

[wclaybaugh2]

High temp red silicone rtv.

Graphite with sharp edges is prone to cracking. A straight cylinder will work as well as a tapered insert.

It’s better to have a phenolic carrier and not have direct contact with aluminum.

What John said.

The different CTE’s mean stress cracking is certain even if one uses a taper. Before 1975 the professional solution was a layer of cork between the graphite and aluminum: this needs to be thick enough to absorb the thermal expansion.

For longer burns the RTV will cook off (as will the cork). Cotronics makes a nice 5000F carbon based adhesive and a 4000F Zirconia based glue that work nicely in this application (I use the Zirconia system to bond graphite to a Zirconia coated Titanium carrier shell).

Bill