I doubt I'll build a 38mm monocopter to fly on the newly announced I49N or I59WN long burn, end burning motors but was contemplating it nevertheless.
Late last night, I thought back on my experiences with the G69N end burner. On the first flight, I noticed the liner was completely burned away at the nozzle end and and the anodized finish inside looked discolored. I asked about this on TRF 1.0 and the general opinion was this didn't sound good and should report it, which I did. AeroTech responded that there was no problem so I threw caution to the wind and decided I'd keep and fly the other two loads that I had. On each of these flights, the liner burned through in places. There's no apparent damage to the case and this load is just so cool in my Hat of Death.
But, if the liner burns through on a 2 second burn, what will happen on a 7+ second burn?
The other factor is what I've seen on various monocopters - the motors burn through on the outside edge just above the nozzle. This has happened on C6s, D12s, E9s and D5s and is more prevalent on the long burn motors. The worst I've seen is two E9s in a Art Applewhite beta-test Double Helix. Toasty (the photos are here).
To summarize, I'm concerned about a propellant/grain type that I've seen burn through its liner in a rocket that tends to burn through its casings. I know relating the G69N to the Ix9N may not be appropriate, but it is something to think about.
I'm less worried about my Whirlygig-38 as the rotation around the motor axis is symmetric. However, I'm sure this all will make its maiden flight all the more exciting.