When taking offit is desirable to getas much accelerationas possible to work upthe necessary speedwithout burning up. A man in a spacesuitwill have to carry hisown breathing air insome kind of container. He will need a lot of itand also some meansof communicationwith other membersof the crew. Weightlessness mayproduce some effectswe do not know aboutand human bodiesmay take it pretty badlyor not stand the strain. Theexpense of trainingis also a problem,along with the factthat wings are uselessin space.