rocket launchers are way better against packs of 24 motherships than THELs: the AoE dmg takes down packs slowly, but all at once. the real advantage is in the limited resources in this mission: high lvl THELs require an enormous battery plant to maintain, while rocket launchers just need a bit of minerals to keep firing. I upgraded to high lvl RL, but since upgrading doesn't increase missile damage, simply building more RLs might be more cost effective.