You cannot compare two competely different projects and tell me "Hey why not go with the old one". The SLS program was to use recycled Space Shuttle parts in order to save time and money, and asking them to add a second stage to the SLS will just break the compromise, since they have done the math where the core stage itself is already enough to get above 100 km no problem with the help of the boosters as sustainers, burning until the TWR of the core stage is high enough to become self-reliant.
And the Rs-25 is efficient and powerful enough to burn all the way to near LEO without suffering much delta v loss, they could've done the Sat V with just 2 stages but that will be horribly inefficient.
Want another example other than the SLS? Look at the Ariane 5. She also uses 2 stages with the first stage running on LH2, in my book, any rocket engine that has a specific impulse greater than 275 can pull off 2 stages without suffering much loss. The F-1 can never do that as it's Isp is only 260.
Want another example other than the SLS? Look at the Ariane 5. She also uses 2 stages with the first stage running on LH2, in my book, any rocket engine that has a specific impulse greater than 275 can pull off 2 stages without suffering much loss. The F-1 can never do that as it's Isp is only 260.