Not really that much less gravity, certainly not enough to notice, let alone have a difference to a rocket launch. Gravity varies as 1/R^2 where R in this case

Not really that much less gravity, certainly not enough to notice, let alone have a difference to a rocket launch. Gravity varies as 1/R^2 where R in this case is distance from the earth, so it's the difference between that at ~6380 km and ~6400 km (for a 20 km altitude) which is not much - you have to be well into space, at over 1000 km up, before gravity drops to less than 7 km/s^2 for instance.