How far apart should I space apple trees in my orchard.

A 30 ft grid is a good minimum distance, and it gives you shooting lanes. On a decent site with full sun the trees will ultimately be creating a canopy that will keep other vegetation from competing with your orchard site. Pear trees can be planted on a 20′ grid as they have more upright branching. Planting clover around your fruit trees is a great choice.