No, of course I agree with you that the kinetic energy that you need is GmM/R and if you hold the mass of the payload (m) constant then it will be larger for a more massive planet or a planet with a smaller radius. But, you've completely missed the point of the argument I was trying to make. If humans were to grow up on Mars for example (which is what I understand the question to be asking) they would quite likely be taller since your height will depend on the stress that your bones can take, with lower surface gravity you can grow bigger, if the planet were more massive you wouldn't grow as big (this is a common theme in many sci-fi stories). To keep the maximum stress on your bones constant a person's mass would be m=m0*R^2/GM where m0 is some constant of proportionality. This is just a rough estimate though - I wouldn't know over what range this holds, it probably wouldn't hold all the way down to 10% of the surface gravity. Now assuming the mass of the ship that you want to launch is proportional to the mass of the creatures (say mship=k*m) then the kinetic energy that you need to launch the ship into orbit would be KE=k*m0*R or just directly proportional to the radius. <div class="Discussion_UserSignature"> </div>