I'd guess the "new space way" had to do with a combination of the
relatively low cost of helium (for a while there at least) and the
relatively small size of the systems involved leading to decisions to
pay a bit extra for pressurant to keep the press systems lighter and the
procedures simpler.
On 4/20/2016 8:24 PM, David Gregory 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 <mailto: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 <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
>
