Thanks Ben- that's the kind of information I was looking for. I'll try it on my
upcoming static hot fire and see how the end of the run looks.
I ran one cold test that way so I'll listen to the audio to see if it sounds
different at the end squirting through the test orifice (the video was zoomed
in looking for leaks so you can't see the discharge).
-Bob
On Sep 13, 2016, at 12:39, Ben Brockert <wikkit@xxxxxxxxx> wrote:
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> wrote:
What does "be ready for the condensation" imply?
-Bob
On Sep 13, 2016, at 10:52, George William Herbert
<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> 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>
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
