During my cold water/LN2 tests this last weekend, I wanted to see how
long the LOX tank could sit with the vent closed prior to the run. The
idea was to reduce the amount of helium needed by allowing it to
self-pressurize for a bit. After filling, I closed the vent valve on
the tank and then closed the fill valve per the modified procedure to
reduce the ullage in the tank. While sitting there, the tank pressure
increased about 70 psi per minute and after 5 minutes, it reached 370
psi which was right around the regulator setpoint of 390 psi. This
seems like a good idea to reduce the helium demand but what will happen
if the test gets delayed and I need to vent the tank? Is there danger
of a BLEVE due to the rapid pressure drop?
I suppose I could modify the fill procedure to: 1) close the vent, 2)
let it sit for a few seconds to compress the ullage, 3) close the fill,
then 4) reopen the vent. Then a few minutes prior to launch, close the
vent to allow the self-pressurization. Is there any difference between
allowing the tank to be pressurized with helium vs. boiled off GOX?
-Bob
