Pistons/rings

Please tell me that you have a video camera! I'm intrigued!

Is "intrigue" a euphemism for "morbid fascination"?

Ned Simmons

If he can make it work, I bet there will be some people here that want to duplicate his work! chuck

Peter Fairbrother wrote in news:BDBAA862.74DC5% snipped-for-privacy@zen.co.uk:

Some suggestions:

For improved sealing, you want the following design features.

a) As little radial waviness in the grooves as possible,

This is a fool's errand, I'm afraid. There's absolutely no way you will get a gas-tight seal at 3000 PSI with metal-metal sliding seals and no lubricant. I don't think you can use graphite even in the low pressure cylinder with pure O2. Look into Teflon, which is a great self-lubricating seal material that can withstand pure O2 exposure. You will have to arrange a very small unsupported gap between the piston, ring and cylinder, as the 3000 PSI won't need much gap to blow the seal out. You will also need to use an oxygen-safe lube on the crank because the seals will still leak, and it doesn't take much O2 leakage to turn anything bigger than a fingerprint into an inferno.

Good LUCK! You'll need it!

Jon

I want a copy of the video tape; for America's Funniest if you survive and Darwin Award if you don't.

I do know that one major company uses quote "copper alloy" sliding seals for it's HP O2 compressor cylinders, although I don't know what the copper alloy is. Those compressors are much bigger though.

They also use labyrinth seals on copper alloy pistons in cast iron or even mild steel bores sometimes.

Teflon's okay for the low pressure cylinder - graphite loaded PTFE (Teflon) is quite widely used for LP oxygen cylinders - and yes, I was surprised at the use of graphite loading. I'm told one major company thinks it's the bee's knees, some others use it sometimes, and another won't allow it anywhere near oxygen.

Another plastics possibility would be a fluorosilicone or fluorosilane; but the oxygen gas gets very hot in the hp cylinders, and I don't like to use any plastics at all at those temperatures, even though I know some are rated for it.

Besides, I've already bought the bronze :)

I could use Krytox or similar, but I'm planning to have unlubricated crossheads, conrods, crank and main bearings so there will be no lubricants or organic materials anywhere near the O2. That way there's no danger of people using the wrong lubricants, and I could scavenge the crankcase.

I probably won't bother to scavenge the crankcase though, because I want to lose about 7% of the pumped gas anyway, and that's a convenient place to do it - it allows me to get away with not-so-good seals.

It's best to throw away gas from the expander output though, so I still want the seals as good as I can get them; and the compressor may also be used for helium or nitrogen occasionally, and especially with helium any leakage is expensive.

Here's why I want to lose some of the gas. The gas from the concentrator is about 95% oxygen, the main impurities being nitrogen and argon. Any water, CO2 or hydrocarbons in the gas get removed, and the gas gets compressed and then expanded, on average three times.

Overall 80% is liquified and 20% gets thrown away, and most of the impurities get removed with the thrown away gas - nitrogen boils at about 15 K higher than oxygen, and so it gets concentrated in the gas that doesn't get liquified; and the same happens with the argon, although to a lesser extent as the boiling points are closer.

I'm not planning on using a fractionating column as such, though I have something simpler that does a similar job in mind. The final liquid should be ~99% oxygen, with the main impurity being argon.

Thank you. But I'd rather save any luck for when I start bending metal for the turbopumps!

Hey Peter,

How about a ring in the form of a helix with at least 4 turns at zero spacing between turns, and a clamp top piston?

Take care.

Brian Laws>I'm designing and building a small 3-stage high pressure compressor for

Forget the idea of using a piston compressor. Go to a metal diaphragm style compressor, one with a three element diaphragm assembly, one in contact with the product and the other side with the hydraulic working media. The one in the middle acts a conduit to actuate a leak detector switch should either of the outer diaphragms fail. The unit is a not a high volume sompressor but is capable of high pressures.

JRW

What are you going to use as the expander?

I don't think LOX is really very expensive. Have you talked to your local welding supply house? Around here, LHe is three bucks a liter.

Jim

Yanno, I'm reading through this whole thread, and thinking "Why doesn't he (the OP) just call up Praxair or Linde or Airgas, and have them drop off a big dewar tank, and have them send by the tank truck every week or two to fill it with Liquid Oxygen? Bada-Bing. Simpler, safer, and probably cheaper in the long run. ;-)

Sanity Check time... This stuff is Very Dangerous to play with - but you already know that, or you wouldn't be messing with liquid fueled rocketry in the first place... :-) And building your own liquefaction plant probably triples the dangers involved - are you /sure/ you know what you're doing??? Not that I'd deny you your fun, but you aren't gonna catch me casually messing around with it.

I don't see even big users like Rocketdyne making their own LOX and LH2 on site, even though they go through a whole lot of it when they test-fire big consumers like the Space Shuttle Main Engines - but when they have tests planned I see a whole lot of cryogenic tank trucks turning off Topanga Canyon Bl. onto Plummer St. and heading up the hill to the Santa Susana Test Lab...

Liquifiers are a 'big deal.' Basically you need one full time staff member to watch over one. There's a fair amount of anicilary equipment, you need to be able to helium leak check, and there's a lot to go wrong.

The big LHe ones I saw at BNL used rotary screw compressors. I think the motors were 750 hp.

Jim

My father was an engineer for Rocketdyne from the mid fifties to the mid eighties, and worked on both coasts. I was allowed to watch a number of test runs and was present at a demo on the dangers of LOX. It involved a person behind a blast shield in armor with very long poles with ~10cc of LOX and kero, poured into a small pit in the ground. They expected it to go off just by pouring it together. It did not. The demo guy picked up a small rock about 1/2" dia. and threw it near the hole.... LARGE KA-BANG... most impressive.

Union Carbide had a cracking plant about 30 miles from the cape. Do not know about now, as I now longer live in that area. But during the Atlas, Gemini and Apollo projects there would be many trucks going to the cape.

Scott

Yep. But I'm in the UK and there is no Praxair, Linde or Airgas lox delivery service here afaik. It is possible to get lox here if you have a doctor's prescription, and it's obtainable generally, though less easily - I personally can get it, but it took some doing - but if you tell your supplier that you are using it for rockety you likely won't get any more.

Lox is becoming hard to obtain. Something similar has happened in the US with HTP (high test peroxide), and HTP is almost unavailable in the US (and the UK). Nitrous oxide is becoming harder to get in both countries too.

Even compressed gaseous oxygen in cylinders is getting harder to get here. But air is still free.

Have you been to a high power model rocketry launch? They take place in the middle of big empty fields or deserts, and can you get a lox delivery there?

There are lots of UK regulations about moving lox by car or train too, and I'm not trying to do that, I'm avoiding it, it's a hassle and it's expensive.

The kind of equipment I am talking about will normally be left working overnight in the middle of nowhere. In the morning you come back to it, and the dewar is hopefully full enough to fill your rocket. Remotely.

Besides which, the entire compressor/expander only contains about 4 stp litres of gaseous oxygen in total, and less than 10 ml of high pressure oxygen; and even if it all burned some metal part at once it still wouldn't cause an explosion, just a brief flame if it managed to burn through the metal. Which would be fairly surprising, as the metal is quite thick in order to contain the pressure, or it's copper alloys that won't burn at that pressure.

Sure it's dangerous, but in this size it's not _that_ dangerous, and it's very unlikely to kill anyone otherwise than in a fire it causes, or do more than cause a bad burn.

To give you some idea, there have been several fires in aluminium bodied oxygen pressure regulators in the US recently, in which some people got burned by 3-foot flames, but afaik no-one was killed.

Any fire in this equipment would be much less energetic - there is no aluminium in the construction - and wouldn't last more than a second at that pitch; only about 1/100th as much oxygen gas is available.

Yes, though I don't actually see the necessary connection there. I presume we are talking about the lox now.

That is very dangerous too, although in a slightly different way, and there is much more of it.

A third more at most I'd say, and probably much less. It could even be safer doing things that way.

Yes and no. I do know a lot about some parts of it, and not enough about others, but I know that I don't know it all, and I am searching the literature: I'm also posting here and elsewhere for comments, and I am seeking advice.

I will have to satisfy several safety committees before I can fly freely with insurance.

Then I have to go through lots more regulatory hoops before I can sell the equipment with CE marking (which is my intention).

Then there's my own safety filters, the built-in reflex ones, and the "stop, that's stupid" and the "that's too dangerous" and the "no, people could get hurt" ones. And the "don't assume that anyone else knows what seems obvious to you" one. And the "people can be very stupid, design with that in mind" one too.

I'm quite glad about all of these, as I don't know everything about the safe use of oxygen. Though I wish the legal regulations were more about safety and less about paperwork.

Nor me. Not casually. That teeshirt is the rags I use to wipe the metal of today's projects.

Is that a mixed metaphor?

First the hot high pressure oxygen goes from the HP cylinder through a fan air cooled copper cooling coil.

There is a stainless check valve/hand valve/connector, then it goes into the upper end of a vertical coil of stainless steel tube, which is surrounded by two straight vertical concentric stainless tubes. The space between the two outer tubes is evacuated for insulation.

At the lower end of the small coiled stainless tube is a double nozzle, and a slower/seperator/fractionating gizmo. The oxygen is expanded to atmospheric pressure in two stages, and partly turns to liquid. The gizmo slows the droplets, partly concentrates the impurities in the gas phase, and seperates the liquid from the gas. The liquid falls into a dewar. The gas is very cold, and it passes back up along the double tube, precooling the compressed gas coming in.

The exhaust gas is then partly discarded to the atmosphere, fresh 95% oxygen is added at atmospheric pressure plus a few inches wg, and this goes to the compressor input filter.

It isn't, it's fairly cheap. I can get lox at an affordable price. See my other post for why I am making my own.

It's a lot more than that around here!

OK, then the expander uses no moving parts then. It's not a turbine brake or anything. That's good.

Jim

Why am I not surprised? "We're from The Government, and we're here to help..."

Okay, I know you have to go out in a big open field to set off a rocket. The thing that worried me would be setting up and running the LOX distilling equipment for a week before your rocketry weekend to fill up the dewar - in the closet of your third-floor flat with 100 people living in the same apartment building...

Great. As long as you are keenly aware of what you /don't/ know, and go find the answers first, that should keep you from doing anything too deadly. I hope. ;-)

It's the people who charge ahead (in the face of warning bells that would stop sensible people) who earn a posthumous Darwin Award.

-->--

I don't think you're quite Wright on that one, Gunner... ^_^

They did lots of experiments and went as far as they could to perfect it before they (literally) went off on a wing and a prayer.

Orville & Wilbur found out that the aerodynamic tables that everyone else was taking as 100% Gospel were really utter crap, and built their own wind tunnel to get better data to work with. Not perfect data, but at least close enough to get off the ground. ;-P

Wing warping to turn, the canard elevator, they invented a lot of revolutionary stuff that wasn't well documented that the recreators are only figuring out again now how far ahead they were.

Nowadays, systems for making bulk LOX safely been done and perfected

- and those who don't learn from the past are doomed to have stuff blow up in their faces.

I'd find out who builds the equipment for the production plants, and ask them if they can divulge how they would build a "pilot plant" or "demonstration" sized unit.

Hey, they have to build pilot plants themselves to make sure their ideas for changes on new models will work... They might even have a roll of blueprints sitting on their shelf with all the details already solved - including selling you the "fiddly bits" like that pressure reduction liquefaction column pre-made for a reasonable cost, and a layout that will fit on a large pallet or a small trailer and hold up to the rigors of transport.

Including all the myriad of safety switches and automatic shutdown sequences, valves and pop-offs needed to let it run unattended and self-shutdown in case of problems. You often can't just 'cut the power' alone to shut down something complex like that, the high pressure O2 will go places it shouldn't.

I'm still not too thrilled with Peter's idea to start up his small LOX plant in the middle of an open field, go back to his tent or trailer (caravan?) and go to bed, and let the plant run all night - if he doesn't have all failure modes covered by safety switches and something leaks he might get one hell of a 3AM "Wakeup Call"... :-0

And Guy Fawkes Day was last week, so Peter can't use that excuse for something going for another 360-odd days.