People who've done it do seem to think you can avoid undue amounts of N2
dissolving in your LOX with a good diffuser. And also, I'd guess, not
leaving the LOX pressurized with GN2 for great lengths of time before
you use it. Something that I suspect would get a lot easier at the
price of N2 - depressurizing a LOX tank with several hundred psi worth
of He in it *hurts*, pricewise. (David G, am I close to the mark on
this procedural point - don't press LOX with N2 till right before you
plan to use it, and if there's a long hold, depress?)
Was I the investigating officer for that incident? I guess I was -
after I'd had to run TOWARD a burning LOX-kero rocket with an
extinguisher rather than away from it like a sensible person, I took
pinning down the precise cause, proving it, and making sure it never
happened again very personally. It might have even made me a wee bit
pushy on the point - a good thing in an investigating officer <grin> (I
still keep that extinguisher pin in my car door map-pocket, for luck.)
Didn't we figure the actual amount of LOX-fuel mix involved was in the
low single-digits grams? It nevertheless actually did do a fair amount
of damage - I'd describe it as maybe half a hand-grenade's worth -
straight LOX-fuel mix packs a lot more punch per gram than normal high
explosives - in that instance, the damage fortunately was (mostly)
contained, by design.
But yeah, the time it ate making sure it wouldn't happen again was
expensive. We could have had that system repaired as-was and in test
again in weeks, but understanding the problem, modifying the design, and
testing the mods to be sure we'd really fixed things so it really
wouldn't happen again took months.
Henry
On 4/20/2016 8:10 PM, Randall Clague 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 <mailto: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
<mailto: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 <mailto: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
<mailto: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 <mailto: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
>