I have a quibble with one piece of Ben's advice. I would say that
LOX-compatible plumbing that has been cleaned to LOX standard is LOX
compatible until -suspected- otherwise. The cost is LOX cleaning is low
enough, and the consequences of not LOX cleaning can be high enough, that
it's worth cleaning it again even if you're not sure it's contaminated.
Do cap it once you've cleaned it. And label it and date it, and maybe keep
a log of who/what/when/how it was cleaned. Seems like a lot of overhead,
but it will pay for it the first time you don't spend weeks trying to
figure out why the same problem happened three times.
-R
On Thursday, November 3, 2016, Ben Brockert <wikkit@xxxxxxxxx> wrote:
On Thu, Nov 3, 2016 at 9:36 PM, Graham Sortino
<dmarc-noreply@xxxxxxxxxxxxx <javascript:;>> wrote:
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(b) cut
seems to be what Robert did
http://watzlavick.com/robert/rocket/rocket1/photos/dsc_1344m.jpg) or
off the bottom of the tank and weld the dome of another tank (this iswhat
the SEDS students at U Cal San Diego did 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.
No, mild steel is not compatible with any cryogens. It becomes brittle
and could explode when pressurized or shatter when shocked. Stick with
aluminum, stainless steel, or brass. Robert's tanks are aluminum.
Insulation: Iinsulate all plumbing sections (including the tank) thathave
LOX into flowing through them. Are there any recommendations for types of
insulation?
Any will work. Silicone insulation is less likely to disappear in a
somewhat energetic cloud than plastic foam. Cryogel is great but is
mildly flammable and is expensive. Pyrogel is the best but is
expensive to buy a roll.
If you use a fuel as insulation (styrofoam, spray foam, hardware store
pipe insulation) it's important that it not cover the fittings and
that none of the fittings leak.
LOX Propellant valve - it seems like most people are using an actuatedball
valve. A hole needs to be drilled upstream in the valve to let trappedLOX
escape. I could not find any good pictures of where this hole should beso
if anyone has any references I'd appreciate it? It also seems like inmany
In the upstream face of the ball, centered in the port when the valve
is closed. It also needs to be well deburred and fillleted or it will
tear up the seal.
cases seals need to replaced in order for them to work effectively but Ihad
trouble finding best practices for this. My understanding is that thePTFE
shrinks more than the metal at cryo temperatures which causes gaps in thepeople
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
You can google cryogenic ball valve and find suppliers. Expect to
spend around $1k for a specifically cryogenic ball valve with a
pneumatic actuator.
have used in the archives except for Robert's re-built Swagelok SS-62T6,(Thursday,
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
April 23, 2015 12:21 PM) but the post suggested there were potentiallymany
problems with the valve leaking.
Other Cryo Valves (eg. Vent, purge and fill valves) - I didn't find as
references to choices people made in this area but if there are anyon
suggestions for valves to get started with I'd appreciate it.
Fittings: Do not use NPT fittings with LOX except where unavoidable (eg.
vavle fittings) because the PTFE thread tape can get into the plumbingand
create a safety/fire hazard. instead only use AN fittings without thread
tape.
Meh. I'm not as anti-NPT as some people here. I've used a lot of it
with success. Flushing out the system is a good idea. PTFE shreds in
LOX will not do anything too scary, it's mostly an annoyance of them
plugging orifices like in igniters or injectors.
Any valve with all LOX compatible materials that has been cleaned for
oxidizer service is LOX compatible until proven otherwise. It should
not be surprising that companies who have found success with non-cryo
valves in cryo use haven't shared all the details, since it does
represent hard fought experimental knowledge.
Seals and Lubricant: All seals must be PTFE and use Krytox to lubricate
valves/seals.
Don't lubricate valves at all. Krytox is LOX compatible, meaning it
won't explode, but it'll still freeze. PTFE is solid lubricant, in a
ball valve there's not really any significant metal on metal movement.
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.
You can clean it. You can also take it apart and send it to an
oxidizer cleaning shop.
Anything else that you feel is relevant?
So much. You're off to a good start, but there's a big knowledge base
to gain. As I suggested to the last person who asked on list about
this topic, you really should go to a FAR weekend on one when someone
is running a project with LOX, so you can see what people are doing
and hopefully see things you don't understand.
still
Finally, I haven't picked a chamber pressure yet for my engine as I'm
researching options but I'd say it would almost certainly be between 250and
500 psia. All work will be done on my test stand so nothing needs to bemore
flight weight.
I'm still very much in the early planning phases and I'm sure I'll have
questions as I go through but any advice of course is very muchappreciated.
Kind Regards,
Graham
