*From:* "Jeff Findley"
*Date:* Tue, 16 Dec 2008 09:32:55 -0500

Please be aware that this is the "bait" Ares I of the "bait and
switch" which Griffin pulled on all of us. For Ares I, ESAS
assumed a four segment SRB topped with an air started SSME powered
upper stage. This has been replaced with a five segment SRB topped
with a much less efficient J-2X powered upper stage. Performance
of Ares I has suffered as its development has progressed.

Yes, section 6 starts off by recommending the 4 segment / SSME
configuration, but they compare three different Ares I configurations, the
four segment / SSME, five segment / J-28 and five segment / 4 LR–85.

The predicted payload performance of the five segment / J-28 version was
slightly better than the four segment / SSME one, and the five segment one
was slightly cheaper per flight. Perhaps the latter is why it was chosen?

Obviously in the course of development weights are likely to grow and
payload to slip, but wouldn't that also be true of the four segment
version?