Don't use a steel tank as most carbon steels aren't compatible with
LOX. Use an aluminum or stainless steel tank instead. The aluminum
version will also have a thicker heel to drill into for a fitting. For
stainless you'll have to weld a fitting onto it.
For your test stand, find a surplus 1/2 or 3/4 inch Worcester cryogenic
ball valve and add a rotary or linear actuator to it.
NPT fittings and PTFE are fine for LOX - I just got tired of stripping
out the threads. AN fittings in a boss port will likely leak with LOX
if you're relying on the o-ring to seal. K-Seals are the solution but
seem to be unavailable in small quantities. If you weld the fitting on
and avoid the o-ring, it would work better. If you want to use AN
fittings you could always use an AN to NPT adapter with the NPT end
going into the bottom of the propellant tank.
In theory, the LN2 should be as clean as the LOX but I have to relate
the story again about finding a glob of grease in my LOX tank after
filling it with LN2. Since then I've added a filter which should
prevent solids from getting into my tank.
-Bob
On 11/03/2016 08:36 PM, Graham Sortino (Redacted sender gnsortino for
DMARC) wrote:
Dear Arocket -
I was wondering if I could ask a few questions about LOX and just in general cryogenic handling.
Just to give a bit of background on myself... I am fairly comfortable using GOX, which I've been using on my test stand for about 3 years and I'm thinking about slowly moving up to LOX. I want to start first with many N2 cold flow tests until I'm comfortable before actually testing LOX.
Unfortunately, I live in Northern New Jersey and I don't believe there is anyone else on this list that lives realistically close enough and also has LOX/rocketry experience to possibly discuss in person best practices (although please let me know if I am mistaken) so I wanted solicit this list for some safety tips.
I've been reading the archives and trying to gather some best practices and I was wondering if I could ask for some pointers on a few questions I'm not clear on.
1. *Tanks - *For a simple LOX compatible tank I've seen 2 approaches
(a) drill a hole in the bottom of a steel gas's supplier tank and
add fittings (this seems to be what Robert did
http://watzlavick.com/robert/rocket/rocket1/photos/dsc_1344m.jpg)
or (b) cut off the bottom of the tank and weld the dome of another
tank (this is what the SEDS students at U Cal San Diego did
http://www.sedsucsd.org/files/ static_fire_system_final_
report.pdf
<http://www.sedsucsd.org/files/static_fire_system_final_report.pdf>).
I'm guessing (b) is better for draining the LOX out of the bottom
of the tank since its rounded although I'd prefer to use (a)
because I do not have welding experience.
2. *Insulation:* Iinsulate all plumbing sections (including the tank)
that have LOX into flowing through them. Are there any
recommendations for types of insulation?
3. *LOX Propellant valve -* it seems like most people are using an
actuated ball valve. A hole needs to be drilled upstream in the
valve to let trapped LOX escape. I could not find any good
pictures of where this hole should be so if anyone has any
references I'd appreciate it? It also seems like in many cases
seals need to replaced in order for them to work effectively but I
had trouble finding best practices for this. My understanding is
that the PTFE shrinks more than the metal at cryo temperatures
which causes gaps in the seals and possibly a spring energized
seals can help here. Also, I was hoping to start with off the
shelf cryo valves to minimize the chances of accidents but I've
been having trouble finding references of valves people have used
in the archives except for Robert's re-built Swagelok SS-62T6,
which is probably more than I need because it doesn't have to be
flight weight. I did find one reference to the SDSU students using
this valve http://www.mcmaster.com/#4632k144/=wvcozg from a
posting dated (Thursday, April 23, 2015 12:21 PM) but the post
suggested there were potentially problems with the valve leaking.
4. *Other Cryo Valves *(eg. Vent, purge and fill valves) - I didn't
find as many references to choices people made in this area but if
there are any suggestions for valves to get started with I'd
appreciate it.
5. *Fittings: D*o not use NPT fittings with LOX except where
unavoidable (eg. on vavle fittings) because the PTFE thread tape
can get into the plumbing and create a safety/fire hazard. instead
only use AN fittings without thread tape.
6. *Seals and Lubricant:* All seals must be PTFE and use Krytox to
lubricate valves/seals.
7. *N2 vs LOX: *if I start w/ N2 and switch to LOX do I need to buy
all new plumbing or can I clean the N2 plumbing and re-use it for
LOX assuming of course it is LOX compatible.
8. Anything else that you feel is relevant?
Finally, I haven't picked a chamber pressure yet for my engine as I'm still researching options but I'd say it would almost certainly be between 250 and 500 psia. All work will be done on my test stand so nothing needs to be flight weight.
I'm still very much in the early planning phases and I'm sure I'll have more questions as I go through but any advice of course is very much appreciated.
Kind Regards,
Graham
