One question I have about this episode is
"Why did so many fans (and reportedly Spinrad himself) down on the design of the Planet Killer?"
I've heard many maligned the design as a "giant Hoover (vacuum cleaner)"
But I loved the design. I think it looked powerful, menacing, rugged, durable and extremely old (Spinrads early script had Spock saying it was about 3 BILLION years old).
And the glowing look it had in distant shots made me think it was glowing due to extreme radiation in the open end.
I know there were some problems with effects shots. Such as the close, forward three quarter views where you could see stars through it.
But overall I loved the design.