Is there a summary of updates somewhere from 2.0 to 3.0?
It's pretty simple really:
RS-68 main engines swapped for SSME's.
The higher efficiency of the SSME's on the Core allow for the Upper Stage to be made quite a bit smaller.
A smaller Upper Stage means that other engine choices can also be considered, like RL-10 and RL-60, not just J-2X.
While we aren't specifying any "final decision" on the Upper Stage engine selection yet, we are strongly recommending an RL-10B-2 solution be considered because that would mean that all the engines (4-seg SRB's, SSME's and RL-10B-2's) are all essentially "off the shelf".
That results in removing all of the highly expensive engine development programs which are usually the determining factor in the development schedule too.
In a nutshell, DIRECT v3.0 offers a solution needing no engine developments at all for supporting all of the ISS and Lunar objectives.
From there, the architecture is designed to fit that capability while providing nice healthy margins in all areas; performance, budget and schedule.
Ross.
