Cole, can I ask why you need an LN2 system?
And Krytox is compatible with LNG, LOx too which is it's main advantage,
but it will freeze solid at LNG temps. So best to use PTFE/Teflon bushings
and valve seats which are self lubricating at all temps.
To mitigate ignition/explosion hazards you can have an ignition source
operating during test ops like they do with Shuttle launches which use
spark generators to ignite hydrogen gas during SSME ignition sequence. Your
open test cell is a good way to preclude a buildup of gas with maybe a fan
or squirrel cage blower from a swamp cooler to dissipate gas stagnation.
BTW, nice test, like an alcohol flame.
Ken
On Mon, Jan 31, 2022 at 12:38 PM Cole Evans <coleevans@xxxxxxxx> wrote:
Thanks for the info everyone.
There are a couple of things I'm doing on this vehicle to try to mitigate
the risk of fire - notably a high flow purge of nitrogen in the engine bay
during static fires to help extinguish any fires that could originate from
leaks. In terms of pre-chilling the engine, I was kicking around the idea
of having a dedicated GSE LN2 line with a fuse valve running to the fuel
plenum. Once fuel pressure rises, the fuse valve would close and allow CH4
to flow in - preventing any buildup in the engine or elsewhere before
engine start. I have spare QD's on the vehicle, so implementation wouldn't
be too hard if it is needed. If anyone thinks this is a Very Bad Idea or
has any other recommendations, I'd love to hear them.
Few follow up questions:
* I still get a little nervous about venting methane straight overboard -
I'm trying to avoid a 'delta iv' like situation where a cloud of methane is
ignited upon engine start that could potentially damage GSE or the raceways
on the vehicle. Has this ever been a problem with similar
amateur/collegiate methane rockets or am I being too paranoid?
* I don't anticipate it being an issue, but is Krytox fine for methane
systems? I was going to go the conservative route and treat the methane
side identical to the LOX side in terms of cleaning and lubrication of
components as I haven't been able to find much information on this front.
-Cole
On Sun, Jan 30, 2022 at 10:04 AM Ben Brockert <wikkit@xxxxxxxxx> wrote:
Hi Cole,
We did methane engines at Armadillo for the Nasa Morpheus/M project. I
made a 4000 lbf engine with pintle injector and film cooling that I
uncreatively named "CH4K". All aluminum injector head, except the copper
pintle tip and stainless hardware to retain the tip, stainless film cooled
chamber and nozzle. Short video of it here:
https://www.youtube.com/watch?v=JcDSn_qIwrM
The safety protocols around methane were not hugely different from other
fuels, but I'm saying this off the top of my head and may come back with
more. Randall or one of the other ex-XCOR folks could also speak to their
methane experience, and I know Masten is doing methane now as well, a 5k
engine for their suborbital reusable rocket.
* We did not do flare stacks. It's good to have upward pointing vents so
that the cold gas has a chance to disperse and not settle to the ground.
* Everyone was wearing nomex jumpsuits, but that was our normal gear for
LOX/alcohol as well. The anti-fire things you do for normal liquid rockets
should be doubled/fastidious; no sources of ignition, no smoking nearby, no
one wearing synthetic clothes that will melt. If you don't have nomex then
everything should be cotton. Appropriate fire extinguishers positioned
well. Safety glasses, face shields, and hearing protection in case of
inadvertent ignition.
* Test only outdoors. Might seem obvious, but I've done a lot of indoor
engine tests and it would require vastly more stringent controls to test an
easily vaporized fuel indoors, to a level that is probably not worth the
hassle.
* Store LOX and fuel dewars separately. Most states have specific rules
for distances. 100+ feet.
* The propellants are miscible in a way that most other LOX propellants
aren't (aside from a few fuels like propane that are rarely used). This
means that any liquid spills once both propellants are loaded should be
assumed to be sensitive high explosives and allowed to boil off completely
until there is no sign of frost before anyone gets near it. Even with just
LOX it's good practice to never step in frost spots, and don't go walking
around in places where you can't see the ground due to cryo fog.
* That also means that horizontal testing is more dangerous than usual;
I've started an engine half full of alcohol with a LOX lead, but things
will go very badly if you try that with methane. Vertical testing is
preferred, canted downward is better than horizontal. Purges need to be
vigorous for anything horizontal.
* Pre-chilling the plumbing and injector (sometimes called priming) is
also much more dangerous. Injector architectures where it is unnecessary
are ideal; or only chilling one side. Having a somewhat ragged startup is
better than having mixed propellants on the ground. But if that goes too
far it is totally possible to hard start the engine,
https://www.youtube.com/watch?v=CRfAESmUu_c
* Think about how leaks will burn. A small leak in a mostly sealed area
can cause damage because the fire will be contained. Wires and sensors
should be fire shielded. It's not very useful to say "things will be
burnier than a non-methane engine" if this is your first engine, but, well,
things will be burnier than a non-methane engine. Presumably less burny
than a hydrogen engine.
One test anecdote: we were firing the engine Nasa had designed. At one
point the company owner John wandered downstairs, watched a test, and then
asked why the test stand was on fire. The fuel run valve had developed a
stem seal leak. On any other propellant it would have been a drip, on
methane it would get ignited by the engine and there'd be a bit of a
fireball around the valves, but it would go out when the valve was closed.
It was gradually getting worse after each run, but we wanted to finish up
the set rather than detanking and fixing it. We explained that and he
understood and we continued. You could do the same thing at FAR, but
probably not the same thing at larger, more uptight organizations that feel
like their stands shouldn't be on fire.
I know a lot of people are talking about methane and that it's the hot
new propellant, but I somewhat feel that it's been unfortunate to push it
down into student/amateur projects. It's kind of annoying to get and work
with. It makes more sense in high performance stages, possibly even a lot
of sense for things like expander cycle. But in regen you likely want to be
operating at ~1000 psi Pc, something you'd never do in a pressure fed
rocket. 90% of the process of building and testing a rocket has nothing to
do with what fuel you're using.
As I said, this is off the top of my head, so feel free to ask any
questions, and I'll see if any other thoughts come to mind.
Ben
On Sun, Jan 30, 2022 at 9:56 AM Cole Evans <coleevans@xxxxxxxx> wrote:
All,
Long time listener and first-time caller here; I'm currently in the
final stages of design for my LOX/LCH4 rocket project and am looking for
some advice on how to safely dispose of methane from GSE during and after
fill operations. I'm leaning towards a flare stack right now, but I've
found it hard to find information on the proper design of these (notably
flashback protection) and was hoping someone here might have
advice/recommendations of what the best approach would be. Additionally,
any tips on general methane system best practices and cleaning would be
greatly appreciated.
- Cole
