Why Mars Can Be Affordable And Achievable, Joe Cassady/AeroJet, Huffington Post
"On the other hand, the Space Launch System (SLS) is the key to placing large blocks of payload into Earth orbit. An analogy here on Earth would be how intermodal shipping of supplies and manufactured goods is accomplished. Short haul is accomplished by trucks or rail, while long haul is accomplished by massive ships. In this same way, SLS accomplishes the short haul mission to orbit, whereas the cargo is transported over the much larger distance to Mars by solar electric transport ships. The two different propulsion systems, each of which is appropriate for its particular type of mission, are complementary. And to transport the astronauts, we add in a third element: the Orion multi-purpose crew vehicle as well as a deep-space habitat module, which together provide living space and life support systems."