Launch Party

• WWizards of the Coast2012-10-01
  Official Ruling
  If you sacrifice an attacking or blocking creature during the declare blockers step, it won’t deal combat damage. If you wait until the combat damage step, but that creature is dealt lethal damage, it’ll be destroyed before you get a chance to sacrifice it.
• WWizards of the Coast2012-10-01
  Official Ruling
  If the target creature is an illegal target when Launch Party tries to resolve, it won’t resolve and none of its effects will happen. No player will lose 2 life.
• WWizards of the Coast2013-04-15
  Official Ruling
  You must sacrifice exactly one creature to cast this spell; you cannot cast it without sacrificing a creature, and you cannot sacrifice additional creatures.
• WWizards of the Coast2013-04-15
  Official Ruling
  Players can only respond once this spell has been cast and all its costs have been paid. No one can try to destroy the creature you sacrificed to prevent you from casting this spell.

Rulings via Scryfall