Ok, I've got the 48 mm dia bored cast iron first stage cylinder, and the 22
mm dia drilled-and-reamed cast iron second cylinder for my hp oxygen pump.
Haven't done the third cylinder yet, but it's 9.2 mm dia from memory.
Thy are fairly accurate, but I haven't done the bronze pistons yet, so I can
still remove metal.
The problem is that the inside surfaces aren't good enough. My newbie
question is: What do people do after a pretty good boring or reaming to
improve the surface finish of the inside of a cylinder?
Have you checked the compatability of bronze and other copper alloys with
oxygen at high pressure and concentration? I have a niggling worry at the
back of my mind concerning spontaneous compustion, but I can't find a
reference to it in my old chemistry books.
I have used lox, but not for 30 years or so. I have boned up on more recent
I wouldn't at all mind taking a refresher course in cryogenic and lox
handling, but I don't know of one - BP and AP do courses, but they are
theory courses for managers, not hands-on handling courses.
I expect if I succeed with the rocket engine there will be a few more people
who will also want to start handling lox, so if anyone knows of a suitable
course I'd dearly like to know about it.
I've used HPG (high pressure gaseous) oxygen equipment made from brass, and
I'm told that BP use copper alloys for the pistons in their HP oxygen
Copper will oxidise under those conditions (it also gets pretty hot in the
cylinders), but it isn't a "burning" reaction, just surface oxidisation.
Further, this is bronze, not copper, and the tin oxides will form a
protective film on the metal.
If there is copper dust and it deflagrates, the cylinders are not sealed,
there are high flow volume valve outlets (for when the output pressure is
low, the hp cylinders don't do any pumping, the gas just flows through
them), and there is overspecified high pressure piping in the cooling coils
between the cylinders with enough volume for the hot oxygen to expand into.
I don't know whether it is possible to detonate copper dust in HP oxygen,
but I can't imagine any likely circumstances under which such a mixture
could come about. There would have to be a significant quantity - unlikely,
there isn't that much wear - of unoxidised dust - unlikely again, any dust
will oxidise almost almost as soon as it is made.
Not having the inclination to spend the cash, or more importantly the
patience to wait for one to be delivered, I didn't bother getting a hone. I
superglued some wet-and-dry to some round metal pipes and rods a little
smaller than the bores, leaving the ends of the wet-n'-dry loose so I could
put folded cloth in between the layers, and chucked them in a drill press on
highish speed, holding the cylinders parallel by feel and rotating them
Took a lot of careful doings on the coarsest wet-n'-dry to get some
scratches out, but it seems to have worked well, they are as round and
straight as I can measure. I didn't know cast iron could get that shiny!
Pistons 2 and 3 should run at about 150 C, and will get to that temperature
quite quickly. I don't think they are likely to get above 200 C under any
conditions. I don't really mind if they leak a bit on startup, so perhaps
slightly undersize at room temperature bronze pistons would do.
But yes, a very good gas seal is required, there is 3,000 psi on one side of
the smallest piston, and atmospheric pressure on the other. However the
better seals are only needed on the smaller cylinders, so expansion wouldn't
be so much of a problem there.
Can anybody put numbers on the accuracy required for 3,000 psi on one side
of a ~10 mm dia piston with very little leakage?
Looking at the possibility of using bronze rings on cast iron pistons, I
don't think it will be possible to use rings with gaps in them. This kind of
limits me to one ring, which is somehow fixed to the cast iron piston.
Anybody got any other ideas?
If the ring has no gap then I don't see how it will help you because it will
still either leak when cold or jam when it gets hot. People use normal type
gapped rings on miniature IC engines so I don't see it being a problem. There
is plenty of material on the web about how to make small ones.
First lets put some numbers on the expansion though. Assuming a CoE of 17 x
10-6 per degree C for bronze and 12 for cast iron in a 9.2mm cylinder.
The piston will expand 0.62 thou per 100C and the cylinder 0.43 thou. Worst
case with a stone cold cylinder and 150C piston would be 0.92 thou relative
expansion. Clearly this is unlikely but without knowing how hot the cylinder
will get it's hard to be more precise.
If the cylinder reaches 100C and the piston 200C the relative expansion will be
0.80 thou. You might take a first guess at making the piston 0.6 to 0.7 thou
undersize and see how it goes. If it jams polish a tad more off it. If it
doesn't then make another piston a bit bigger.
Sorry to mix and match the numbers but I can't deal with metric when it comes
to very small numbers. 0.015mm means bugger all to me.
I would think the leakage at 3000psi through a ~1 thou gap would be significant
How close you can run the piston will depend on how perfect the bore is. I
think it's unlikely to be within less than a couple of tenths of dead straight
and true even with best machining. One way of checking would be to make a very
close fitting go gauge and scrape any high spots where it sticks.
The longer the piston is the more likely it will jam as it heats up if there is
any bend in the cylinder. A technique to minimise that would be to only have
the piston at full diameter at the top and bottom and waist it slightly in the
centre. It will then follow any bend.
No, but diesel injector pumps work in this sort of region, they rely
entirely on fit with no rings, but a) it is a liquid they're working
with and b) they are lubricated (by the diesel).
I suppose it might be worth you looking at diesel pump elements, don't
know much about modern car ones but the older in-line types use
elements in the region of 5 to 8mm bore.
BTW - hardened steel works very well in cast iron and should have a similar
CoE. Starting with 3/8" silver steel, rough turning to leave a few thou on the
o/d, heat and quench and then grind to size wouldn't be an expensive way of
making a piston.
Having further considered your device in some detail and with readily available
information on Google I've come to the following conclusions.
High pressure gas pumps won't work without elastomeric seals. Compression of
oxygen to high pressures can generate temperatures that will ignite elastomeric
seals and even the metal surfaces of the device and particulate contaminants
can provide a fuel source for high temperature oxidisation and ignition.
Choice of materials is critical. Brass, which will melt but not burn, is
favoured and anodised aluminium (aluminium oxide) is inert to a considerable
extent. Ferrous materials are not.
What you are proposing is potentially dangerous to a degree that I had not
really considered previously. I'm not saying it can't be done but I suggest it
can possibly only be done with considerable testing and expertise in similar
systems. Most of the devices I've found detail about online are noted as not to
be used with high pressure hydrogen or oxygen.
What you are currently trying to build might very well kill you and I think it
best that myself and anyone else not experienced in high pressure oxygen
systems bow out from giving further advice.