The last half of the atmosphere is an easy flight; drag is reduced to the point that it barely matters and most of the bigger engines have reached almost peak e

The last half of the atmosphere is an easy flight; drag is reduced to the point that it barely matters and most of the bigger engines have reached almost peak efficiency. Due to lack of drag you will also barely loose any velocity anymore. The first half is hard because drag is so high but if you can sustain atleast 150m/s (or Mach 0.5) until you reach about 15km altitude and then sustain atleast 1.5G until 35km you can make it easily into space. If necessary you can try to gain some vertical velocity and abuse body-lift to drag you up higher despite not having sufficient lifting power in your boosters (if you have between 1.02 and 1.1 TWR) Most rockets burn 80% of their fuel to get out of the atmosphere, the remaining 20% are usually good enough to get into orbit, insert into any transfer orbit you like and fly off into empty space.