Not that I want to break your dreams, but you'll run into some serious issues way before you reach 1 megaton. Once ago, I made a launcher that was around 100 ktons, it didn't even leave the pad, the game crashed:
Launching from saturn
I only have 4 gb of ram on my phone, but 8 gb is "only" twice that value, and I'm still really far from the megaton mark.