normally, GPU is used.
But from the power bus, battery can provide power to DC2DC converter and also DC2AC converter to provide power to other devices.
Could you try in a new sortie and fire the SD-10 under the same situation (enemy pos, self conditions, like alt, att, speed etc) as when it failed in your trk?
only thing that can be simulated is that if upper broken, then you need to switch to lower
but normally in game if upper is broken, most likely the whole aircraft is malfunc