Because the assumptions made for one class of payload don't apply to another class of payload with vastly different mission specifications. A great place to st

Because the assumptions made for one class of payload don't apply to another class of payload with vastly different mission specifications. A great place to start is this amazing video by The Everyday Astronaut: https://www.youtube.com/watch?v=LbH1ZDImaI8 https://www.youtube.com/watch?v=LbH1ZDImaI8 Notice how the type of fuel used itself is a consideration for the type of rockets we may send to Mars if we're considering In-Situ refueling.