Had the same issue all last night myself. Tried going to Mun 4 times, and every time I attempted to come back to Kerbin, somewhere in the middle the game would crash. My last left me with 300 lost science, and I had just quicksaved and full saved not 5 minutes before I hit recover. I really have no idea what we're supposed to do about this, but I hope somehow they fix it. Realistically it's the biggest thing making people lose motivation to keep playing (In fact some are already refunding their money back). As a first time player, I fell in love with this game, but I think from here on it's either sandbox mode or nothing until the issue gets resolved.