As the tank warms up, you lose density. So you need more ullage if you use
warmer lox. Also, your mix ratio will shift slightly, so double check that.
If it gets too warm, you'll start to see two phase flow in your injector
and/or plumbing, which might be undesirable.
That said, plenty of people run self pressurized nitrous oxide, and
occasionally self pressurized LOX. No reason it can't work. Adding a little
helium might help, but might not be worth it.
Evan Daniel
On Sep 13, 2016 9:30 AM, "Robert Watzlavick" <rocket@xxxxxxxxxxxxxx> wrote:
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
