Rradley

Concerning your problem space planes when no designed correctly will destabilize in low air, to fix this increase the speed of the aircraft or find a way to switch from air intake engines to rocket fuel. I do not have the mods so i cannot accuratly say what your issue may be

how fast is your takeoff speed? if its too fast the larger rockets will become unstable. for better fuel efficiency i take off at 150 - 300 ms^-1

From what I understand (i havent used any mods) but mechjeb seems to do a lot of the work for you which in my opinion defeats the objective of the game

Oh im sorry i misread your comment sorry

From the pictures it doesnt look like you put you SAS on, if you go too fast this will cause a unbalance

Send a moon base up and let them live happily ever after

I have built a rover which is for multi-planet exploration (see pictures and .craft file in link below), however i can't get it anywhere safely as when in the last stages of landing the CoM keeps flipping it over and so it crashes. If anyone can give suggestions on how to design a rocket, or build a rocket for me that'll be great! (i have no mods so only stock parts if possible please) https://www.dropbox.com/sh/ispnqhuzsygk0e7/Q1jpLJRtak