How about a 400 gallon tank with a 3k meop ? That's in the range of "pretty
small" to me.
I agree that helium offers less potential for complications. But I was
responding to "you can't pressurize LOX with nitrogen". You certainly can,
I've done it.
On Apr 21, 2016, at 5:21 AM, Ben Brockert <wikkit@xxxxxxxxx> wrote:
(I'm in Thailand and don't have my spreadsheets with me, so please
forgive approximations.)
I don't agree on solubility. Mojave, FAR, Spaceport America, PSAS's
launch site, are all in the 3000-5000 foot range. Atmospheric pressure
at 3000 feet is about 0.9 bar. Liquid oxygen at that vapor pressure is
about 88K. At 10 bar, an unusually low pressure for most pressure fed
rockets on here, nitrogen has a boiling point of about 104k. So the
nitrogen isn't just dissolving, it is liquefying, contributing Hvap to
mess up the temperature of the LOX and diluting it with nitrogen. At
30 bar, a not unreasonable pressure for a hobby liquid engine, the
boiling point of nitrogen is 123K.
If there is further dissolution on top of the liquefaction, that makes
the situation even worse. There's going to be plenty of unknowns in
the data from the tests without adding the question of dilution or
nitrogen fizz in the oxidizer.
http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html
http://encyclopedia.airliquide.com/images_encyclopedie/VaporPressureGraph/Oxygen_Vapor_Pressure.GIF
http://encyclopedia.airliquide.com/images_encyclopedie/VaporPressureGraph/nitrogen_vapor_pressure.gif
I agree that pressurizing a large tank with low pressure nitrogen to
run a pump fed engine is feasible, but this is not that case. It's
definitely not relevant to say that because it's possible to run a
very large tank (with relatively small ratio of liquid surface area to
propellant volume) that has been pre-charged with helium and then
topped with nitrogen in order to provide a relatively low positive
head that it's then reasonable to do the same thing with a much
smaller, higher pressure tank.
Even in a helium pressurized system it's good to have a diffuser on
the pressurization inlet if possible to reduce the amount of helium
needed. But I don't want to recommend to a mostly student group that
they take on an unnecessary science project in pressurizing LOX when
they already have a large project.
On Thu, Apr 21, 2016 at 11:24 AM, David Gregory
<david.c.gregory@xxxxxxxxx> wrote:
It seems we are in agreement on solubility, tho I can't tell from your post.
This is a clear area where the "new space way" is more expensive and onerous
than the "conventional aerospace way" that is so reviled. As I said below,
personal experience with no less than five test stands, many many hundreds
of tests, tens of millions of dollars of propellant burned - only a handful
pressurized lox with helium.
On Apr 20, 2016, at 8:10 PM, Randall Clague <rclague@xxxxxxxxx> wrote:
Ben says, can't pressurize LOX with nitrogen, bad bad bad, but he doesn't
say why. Solubility is why. If you pressurize your LOX with nitrogen, you
may get a LOX froth when you try to fire the engine. In the very early
days, when you don't care about that, nitrogen is fine. And boy howdy is it
cheaper.
When Henry talks about fuel in the LOX system costing lots of money, listen
to him. He was the investigating officer for an incident where that very
thing happened. It didn't do a lot of damage, but it set back the schedule
by months.
-R
On Wed, Apr 20, 2016 at 11:22 AM, David Gregory <david.c.gregory@xxxxxxxxx>
wrote:
The main issue to be avoided is the extraordinary solubility of GN2 in
liquid oxygen. I don't have my references handy but it's something like
2:1 by mass (2 lbs of gn2 disolves in 1 lb of lox). It's not condensing, it
dissolving like CO2 into soda. Tanks usually have a diffuser to slow the
pressurant velocity and avoid stirring the tank. If the tank is stirred by
the incoming n2, it will just run away as the control system tries to add
ever more gas which simply disolves faster due to more stirring.
For a flight system you have the added complexity of mechanical agitation
of the tank. But as others say, it has been done.
On Apr 20, 2016, at 5:28 AM, Ben Brockert <wikkit@xxxxxxxxx> wrote:
That is cool. Can you give more details on how to make it work, now
that you're free of the silence of the endless blue? Putting a gas
over a liquid that is significantly below the gas's boiling point has
given bad results in the straightforward attempts I've heard.
On Wed, Apr 20, 2016 at 8:18 PM, David Gregory
<david.c.gregory@xxxxxxxxx> wrote:
I've worked on five different test stands that used nitrogen
pressurization for LOx and only had an issue once. You'll want to pay
attention to to diffusion, but you don't have to use helium.
On Apr 19, 2016, at 11:31 PM, Ben Brockert <wikkit@xxxxxxxxx> wrote:
Hi Kyle,
Sounds like a fun project. Some thoughts:
* If your cryo main valve has a normal pneumatic actuator, it will
typically have a maximum actuator inlet pressure of around 150psi.
That would be an unusually low pressure to do all your engine tests,
so you may want to add another regulator for pneumatic pressure. It
shouldn't be very expensive.
* The vent valves have their own symbol, neither manual or electronic.
What kind of valve are they? Vent absolutely must be remote operated,
and almost always should be normally open (so that if power is lost to
the stand the vents will open). The vent lines themselves out of those
valves need to point in directions that won't spray at people.
* You have to use helium to pressurize LOX, it doesn't matter how much
it costs. You may find it's more cost effective over the long run to
have nitrogen for the fuel and helium for the LOX. It means a little
bit more plumbing but it reduces operating cost and eliminates any
possibility of cross contamination from the fuel side to the LOX side.
* The gauges shown by the engine should probably be transducers.
* It's a fairly minor thing, but the ball on the symbol for a check
valve indicates the inlet end. So two of yours are backwards.
* The helium and possible nitrogen bottles have hand valves on them,
so the hand valve on your high pressure line probably isn't necessary.
High pressure regulators also often have filters on their inlet
already, so you may not have to put that filter in. If the regulators
don't have filters, they're a good idea though.
* Basically every design document says to not relieve pressure from a
line by cracking open a fitting, but... I wouldn't bother with the
vent valve on the high pressure side. Unless the line between the
bottle and the regulator is particularly long, it's not unreasonable
to let that pressure off when unscrewing the CGA fitting from the
bottle. CGA fittings take a big wrench and have a lot of straight
threads, so there's little risk of it coming all the way undone and
whipping around.
* I've used non-cryo rated valves for cryo vent before and gotten away
with it. Needs to have suitable materials (brass or stainless body,
PTFE seats and seals) and it may require experimentation, but it can
be done.
* For remote LOX fill you hook the dewar up to a normally closed cryo
fill valve via a cryo hose (McMaster sells a -8 or -10 one), open the
globe valve on the dewar, and run away. Operate the fill valve
remotely until liquid shoots out the vent. (Have a dip tube on the
vent line inside the tank to ensure some ullage). For remote fuel you
could do a similar setup with an electric fuel pump. I generally
wouldn't bother with either, I prefer to have the LOX dewar and fuel
drum away from the test stand at test time.
* For LOX system drying, the optimum is to not ever put water in the
system after it has been oxidizer cleaned and component level and
assembled. Do liquid nitrogen if you want to do a cold flow before
comitting to LOX, which is a good idea. If you must load water in it,
all ball valves have to be opened half way and blown through with dry
gas as you suggest. One option is to hook up a bottle of nitrogen and
slowly blow the entire bottle through the system. Another is to get an
oilless compressor, put a high quality water and oil filter on its
outlet, and blow clean air through. Any ball valves have to be 45
degrees open though, if full open any water trapped in the cavity
between ball and body will stay there. And operate the ball valve a
dozen or so times with gas flowing.
* You're going to want purges to clear out the lines to the engine
before and after running. Typically they come from above the tank
press valve, through an orifice and a solenoid valve and a check valve
or two, and are injected on the downstream side of the engine run
valve.
What's your plan for propellant tanks?
An important next step is to make another P&ID that goes all the way
to component level, making sure that fitting types sizes are correct
on everything. Then from that you can create a realistic BOM and
figure out how much everything is going to cost.
No mild steel parts anywhere in the LOX system, and if you bolt the
LOX tank or downstream parts to a steel structure they need thermal
isolation.
Ben
On Wed, Apr 20, 2016 at 9:15 AM, Kyle Meeuwsen
<meeuwsen.engr@xxxxxxxxx> wrote:
Hello All,
This is my first post and I hope I am doing it correctly...
I am with the Portland State Aerospace Society and we are developing
our
first liquid engine test stand. This is a LOX and Ethanol system
pressurized
by either Nitrogen or Helium depending on the quote back from
Praxair.
Below is a link to the Test Stand PID on Imgur. I am looking for
either
approval or to discuss design concerns. Please note that the valve
below the
LOX tank is cryo rated with a pneumatic actuator that is pressurized
by two
solenoids which are fed from the LOX pressurant line.
The senior design team before me also selected relief valves and
solenoids
for venting on the top of the LOX tank that are not cryo rated. I
have
concerns that heat transfer from the tank to the LOX might freeze
these
components that are attached to the top of the LOX tank. I could
attempt a
very shaky heat transfer analysis unless people know right away that
these
components will freeze very quickly.
I am also still searching for filters and burst discs, and a way to
safely
fill the LOX and Ethanol tank from a great distance away.
Also, any suggestions on how to purge the system of moisture prior to
testing would be greatly appreciated. I have been told to run hot
pressurant
gas through the system for an extending period of time but I am
currently
not in possession of a heat exchanger or an ample amount of
pressurant gas.
https://imgur.com/gallery/wVbtgUz
