It's possible that if you open the vent you'll get a big burp of
liquid out, it depends on the geometry of the whole thing and what
exactly is the cause of the short filling you were seeing before.
If you can fill->cram->press->fire in short order that's your best
bet. Small rockets with cryogens inherently have less dwell time
because of the proportionally greater surface area per volume of
propellant. A one liter spherical tank has an area of 485 cm^2 per
liter, a thousand liter spherical tank has an area of 48 cm^2 per
liter.
On Tue, Sep 13, 2016 at 4:39 PM, Robert Watzlavick
<rocket@xxxxxxxxxxxxxx> wrote:
So if I close the vent first after filling to compress the ullage and squeeze
more in but then immediately open it after closing the full valve and before
pressure starts to increase significantly, I would still lose some due to
boil off. But would I end up with more usable liquid than if I had used the
normal fill process (leave the vent open the entire time)?
-Bob
On Sep 13, 2016, at 13:50, Henry Spencer <hspencer@xxxxxxxxxxxxx> wrote:
On Tue, 13 Sep 2016, Robert Watzlavick wrote:
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. ... 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?
Fast answer: no.
A BLEVE is what you'd get if you don't vent the tank. :-) Say you've got a
bad weld and a fitting breaks off at 500psi. The 500psi GOX expands out the
hole. If there was only gas in the tank, this would cause the pressure in
the tank to drop. But instead, there's LOX in equilibrium with 500psi GOX.
So some of it boils, and the GOX pressure stays at 500psi as more and more
expands out of the hole. This is why BLEVEs are special -- there is far
more gas in there than you'd think from the size of the tank, and it's all
coming out. A small pressurized tank mostly full of boiling liquid contains
the same energy as a *much* larger tank holding gas only.
With controlled venting, what you'll initially see is that the pressure
*doesn't* rapidly drop -- the liquid is boiling to keep the pressure up.
Keep venting, and the boiling will gradually chill the liquid enough to
reduce its vapor pressure, and so the pressure will start to drop. No
problem, just expect slightly slower results.
The reason why it's only slightly slower is that unless you've been stirring
the liquid vigorously, it's not *all* in equilibrium with the ullage gas.
It'll be stratified, with the warmest liquid at the top and cooler liquid
farther down, and only a thin surface layer will actually be at equilibrium
with the gas. (This happens even in non-self-pressurized cryo systems; it's
common for the liquid temperature at the engine to rise noticeably right at
the end of the burn.)
Is there any difference between allowing the tank to be pressurized with
helium vs. boiled off GOX?
Self-pressurization is generally not a good idea, because stratification
makes it difficult to control the pressure well -- for example, sloshing in
flight may mix the upper layers enough to suddenly reduce your tank
pressure! -- and you may get two-phase flow toward the end. Warmer LOX is
also noticeably less dense, which reduces tank capacity and messes up
gauging and mixture-ratio control.
Henry
