Tanks

O2 side has 50 psi in tank - increase flow thru ducts in start transient. Lose 5 psi in the 
tank when flow starts. Using the EMA and slowing the oxidizer control valve decreased 
pressure loss in the tank.

With less pressure loss - vehicle requires less Helium. The Helium itself isn't heavy, but 
1lb of He requires 10lb of hardware. Saving He saves weight. The expert did relate that 
they have a fixed He bottle size.

Some hot H2 is used to pressurize the hydrogen tank. The possibility of taking gaseous O2 
to pressureze oxidizer tank was considered but never actually done. Residual He at end of 
the mission is lighter than residual O2 so they continued to pressurize the tanks with He.

50 psi in tanks. If pressure loss to 42 psi - the pumps don't have enough pressure to work 
properly.

Expert speculated:
LO2 at bottom heavier propellant at bottom probably not the reason - (shuttle O2 is on top).

Preflight predictions of needs are compared with known tankage capacities.