I imagine that scout class titans could be dropped from orbit and use rockets to be able to descend safely, but the larger titan classes would probably cause catastrophic damage o the planet they were dropped on, if you used some kind of drop pod assembly.  I imagine the Imperium might own one or two sets of booster rockets to orbit drop the largest titans, but only for when they just don't care in the slightest what the planet being invaded ends up looking like in the end.<br/><br/>Naturally, I assume that any story of a terminator getting stepped on by a Titan and surviving is talking about a scout class titan, which is basically equivelent to a ridiculously heavy tank, but not quite a completely unique class of war machine.<br/><br/><br/>(http://images.wikia.com/warhammer40k/images/1/16/Godengines.jpg)<br/><br/><br/>There's the relative sizes of the different Titan classes, though the smallest walker there is the Knight titan, which is a PDF equivelent of titans that strikes me as a somewhat insane idea.