In the long run, dropping 500 kilos from Orion's design will save the future timeline only at its very beginning, in 2015. Future safety delays, future "Hydrogen Summers," will delay missions in an out year domino effect. If NASA has learned anything from the long Space Shuttle experiment, that should be obvious.
The most important reason for NASA not to abandon what appears to a clever congressional staff to be merely 500 kilos of "dead weight" is not so obvious and not so easy to communicate in budget hearings.
If only Congress bothered to read the carefully written, easy to follow reports on deep space radiation hazards so carefully and respectfully laid out by the National Academies in two recent reports commissioned by NASA.
NASA safety protocol allows an astronaut only a 3 percent probability, over the course of that individual's lifetime, of "Radiation Exposure Induced Death," or REID. Even when incorporating shielding into the design of Orion's components and hull, a nominal trip to Mars guarantees a greater than 3 percent probability, over the course of a Human lifetime, of REID.
That assessment by the National Academies made brief headlines when their latest report, in draft form, was made public at the beginning of this year's budget rounds. The news about Mars being outside the REID protocols was highlighted while similarly nominal trips to the Moon being within those same safety demands did not.
Either way, testimony blasting NASA for having any kind of manned program whatsoever, and in favor of JPL's ground-pounding vision of pure robotics, quickly absorbed the National Academies circumspect report. Apparently neither congressional staff or NASA's lobbyists had time to read their important report.
With the loss of shielding flexibility, design difficulties and safety demands will make the timeline less credible, and not more. In the end, it will make for either a more dangerous Orion, future Liberty Bell Sevens, or, more likely, the addition of another 500 kilos of aluminum inside and outside the Orion command module - without the ability to land on dry ground.