Comment Re: Why does it take so long? (Score 4, Interesting) 211
While the "Shuttle Derived" messaging was used to sell the program, it's hardly anything but. The first few flights will use left over RS-25Ds from the shuttle program, but they are far too expensive for new ones to be built and throw away each flight, so the RS-25E and RS-25F engines needed to be developed. The 4 segment SRBs from the shuttle aren't powerful enough for SLS so they've had to develop a 5 segment SRB with a new type of solid fuel with a completely new grain. The casings are also being redesigned to be expendable. While the tank is shuttle derived, it needs a completely redesigned aft section to support the engines, plumbing is completely different, and the a new interstate to support the upper stage and payloads. It would have been cheaper and faster to start from scratch, but that doesn't keep the trough filled.