Self pressurizing LOX is more difficult than it seems. The pressure of a
LOX tank is the vapor pressure of the warmest liquid in the tank. You can
easily have a situation where a tank seems self-pressurized to say 400 psi,
but as the liquid flows out the pressure drops to 100 psi as the warm
portion boils off. If you want it to maintain pressure then the liquid has
to be stirred to get consistent temperature, and even then it will
still drop in pressure as it flows because heat is removed to vaporize some
of the liquid. Paul Breed has posted the math here before.
Armadillo did a lot of testing with self pressurized propellants but went
back to helium pressurization. The two phase flow or cavitation in the
plumbing and injector makes controlling mixture ratio over the burn more
difficult. The density of LOX also drops really quickly as it gets warmer,
so you get less performance out of a stage. It took a lot of different
injector tests to find one that would play nice.
The only BLEVE concern is if there's too little ullage and the tank bursts
when full of warm propellant. Vents still vent, though it takes much longer
for a self pressurized tank of propellant to vent down to atmospheric than
the same tank and vent geometry with helium pressurization. So it is less
safe in the situation where venting the tank is the last ditch effort for
cutting off propellant flow to the engine.
In your specific situation here you have a helium pressurization system as
well, if you let it self pressurize somewhat, then helium pressurization,
then fire, you should expect the end of the burn to be lower
performance and possibly unstable as the warmest LOX at the top of the tank
runs through the engine.
Ben
On Tuesday, September 13, 2016, Robert Watzlavick <rocket@xxxxxxxxxxxxxx>
wrote:
Just a note- I've been using LOX for many years without incident but only
within a narrow set of use cases I just want to be aware of any corner
cases I hadn't considered as I switch to flight weight tanks and
potentially different procedures
-Bob
On Sep 13, 2016, at 11:36, Robert Watzlavick <rocket@xxxxxxxxxxxxxx
<javascript:_e(%7B%7D,'cvml','rocket@xxxxxxxxxxxxxx');>> wrote:
What does "be ready for the condensation" imply?
-Bob
On Sep 13, 2016, at 10:52, George William Herbert <
george.herbert@xxxxxxxxx
<javascript:_e(%7B%7D,'cvml','george.herbert@xxxxxxxxx');>> wrote:
A constant regulated flow of same-material gas (GOX, nitrogen gas etc) can
hold a constant pressure above its liquid but will be condensing at the
interface. If it's the same propellant its harmless and may be taking less
density / heat out that way.
You just need to be ready for the condensation.
Sent from my iPhone
On Sep 13, 2016, at 8:32 AM, Evan Daniel <evanbd@xxxxxxxxx
<javascript:_e(%7B%7D,'cvml','evanbd@xxxxxxxxx');>> wrote:
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
<javascript:_e(%7B%7D,'cvml','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
