What is the primary reason for SpaceX motion to have astronauts board Dragon before fueling up the rocket?
From what I understand though, it shouldn't be necessary - Falcon 9 and Dragon 2 have enough payload capacity and delta-V to reach ISS with nominal payload, on non-supercooled oxygen.
For better performance they are switching to supercooled fuel. To maintain its temperature it needs to be loaded as late as possible while preflight checking requires the crew to be aboard well before launch. Rather than risking loading them without a safe means of escape they are loading the crew prior to oxidizer fueling. If there is a problem it is easier to escape from the top of the rocket than to escape the elevator or crew access arm. Installation photos at: "Crew Access Arm Installed for Starliner Missions.
See: "Letter raises questions about SpaceX fueling plans and committee roles" (Nov 8 2016), by Jeff Foust:
"The break in precedent is required because of changes to the Falcon 9. The upgraded version of the rocket, introduced less than two weeks after Stafford’s original letter, uses “supercooled” liquid oxygen that is denser and provides greater performance. Maintaining those low temperatures requires fueling the rocket much closer to the scheduled launch time than is the case for other vehicles, which means that, for crewed missions, astronauts would have to board the rocket prior to fueling rather than after it is complete.
Tweet from Elon Musk and video showing a successful abort procedure.
Letter (.PDF) from Lt. Gen. Tom Stafford (Ret.) to Bill Gerstenmaier, who heads NASA’s human spaceflight operation:
December 9, 2015
Mr. William Gerstenmaier Associate Administrator for Human
Exploration and Operations National Aeronautics 8nd Space
Administration 300 E Street SW Washington, D.C. 20546
Dear Mr. Gerstenmaier,
We sincerely appreciated the briefing on the Commercial Crew Program
from Kathy Lueders and Bill Jordan to our U.S. committee members.
Thank you for making the briefing available to the committee. As is
nornal when the committee begins reviewing a topic, the briefing
raised about as many questions as it answered. I will not list all
the topics we will continue to follow, but there is one major issue
that I believe deserves your careful attention.
There is a unanimous, and strong, feeling by the committee that
scheduling the crew to be on board the Dragon spacecraft prior to
loading oxidizer into the rocket is contrary to booster safety
criteria that has been in place for over 50 years, both in this
country and internationally. Historically, neither the crew nor any
other personnel have ever been allowed in or near the booster during
fueling. Only after the booster is fully fueled and stabilized are
the few essential people allowed near it.
Furthermore, in addition to the personnel risk, there is the risk of
operating the engines outside their design input conditions. As an
experienced "Prop" guy you know the problem here as well as anyone.
Pump fed chemical engines require a sufficient and consistent input
pressure to reduce the likelihood of cavitation or unsteady flow
operations. We are concerned that there may be insufficient precooling
of the tank and plumbing with the current planned oxidizer fill
scenario, and without recirculatiOn there may be stratification of
oxidizer temperature that will cause a variation in the input
conditions to the oxidizer pump.
In summary, we are deeply concerned about introducing the practice of
fueling with the crew onboard, and about the lack of even a
recirculation pump for oxidizer conditioning on Falcon 9.
Sincerely, Thomas P. Stafford Lt. Gen., USAF (Ret.) Chairman NASA
International Space Station Advisory Committee