Tig on dewar

if you're dying to find out exact comp of material, there are those machines that you put a few filings in that will exactly identify the compound (metals)

--Yeah, but I'm welding it now! :-)

would you stand next to it after it is filled with lox and pressurized ?

there is your answer

williamhenry wrote: : would you stand next to it after it is filled with lox and pressurized ? --No way! The good news is that neither is my pal Roger; it's going to be pressurized while in a bunker a loooong way from people. Got it all done just a while ago; I practiced on two fittings that are going on the outer wrapper, so by the time I started work on the inner ones that'll see pressure I had a better feel for what I was doing. Roger's going to be videotaping this thing with 3 or 4 cameras, so we'll have a good idea of what breaks first, heh.

For the length of time that you need to store the liquid oxygen and liquid propane - a short time? - is it necessary to use a dewar flask? Is that adding a complication you don't need??? Would two buckets or cans wrapped in polystyrene foam do? Ordinary mild steel will do just fine - if it is more brittle, that is a largely irrelevant issue in this case of thin elastically flexible walls and no concentrated stress. What I am thinking is - is this concern about welding to a flask still with a vacuum inside the walls an unnecessary complication? Could you do better making your own containers for the liquid propane and liquid oxygen which are simpler to make than modifying a dewar flask and more ideal in shape, size or whatever for your application?

And another insulating suggestion - builders "ramfoam" is a very good cryogenic insulant. Smear it over the surface with a home-made "shoe" connected to the deliver tube and the thin layer "blows" for a jacket - like the yellow stuff you often see on hot water cylinders. You can see an example at

Richard Smith

--He plans to pressurize the tanks to something like 150psi, so some sort of pressure vessel is necessary. Also, figure these will be in a sand pit in the middle of a desert, probably on a hot day, etc. Foam-only containers would be a little scary, too, for fear of fracturing one.

Yes. If you don't, the oxygen is cold enough to condense water vapour and even CO2 out of the air. A layer of frost all over your machinery is a complication you can do without.

I've never welded a Dewar, but I have silver soldered an old copper one. I imagine anyone who's a competent TIG welder (which I'm not) would have no trouble,

Any liquid air flowing through uninsulated pipes will cause all kinds of havoc. Valves made for the stuff will jam, and it doesn't matter whether they're manual or electric. Fought that battle before. I never understood why; it just happens. Jamming open is a very bad thing!

Generally you can avoid a lot of trouble by using valves made for the purpose (right sort of rubbers, no external moving parts) and purging the lines beforehand with a non-freezing gas (argon, helium, or nitrogen (which you probably have to hand and won't freeze solid).

It's the external water frost that's the real problem. There's just so much water vapour around that unless you're working in a desert the icicles will build up to a unbreakable huge layer.

steamer wrote in news:WdRDe.5070$p% snipped-for-privacy@typhoon.sonic.net:

--Re: valves: he's done this before and he tells me this time he's got a pair of brand new store-bought solenoid valves that are intended for cryo temps; cost him a bundle apparently. --Welding all done now and went pretty easily, altho I did burn thru in a few spots, easily filled (my favorite quote from a welding book sez that burn-thru means you're getting good penetration, so if you learn to fill the holes you're in the groove, so to speak.). This was my first time working with stainless; I've used the SS rod before, but not to bond stainless to stainless. The less power I used the better it went. --Can't wait to see the video of the rocket firing; what Roger does is spin one up until it blows apart from centripetal force, to get an idea of what that particular engine design can tolerate. He then rigs another engine of similar configuration,backs off on the fuel flow to about 90% and gets the real data on thrust, etc.

What a cool friend to have.. makes me a little jealous.

John

Has everyone seen on his web page the "exploded" view of one of his rocket engines?!!! He doesn't do things by halves - looks like it flew into more pieces than that!!! The picture is of an rocket motor which literally exploded!

--Roger tells me that several things can cause an engine to explode. Excessive rpm is the preferred method, because it means everything's working very well. But sometimes he has something called, IIRC, a "hard start" where the gasses are in an incorrect ratio as they enter the combustion chamber. That's when things get ...interesting, heh.

One of the most interesting failures in rocket engines is resonance. They vibrate at a particular frequency with sufficient amplitude to shear them clean in half. It's one of the few failure modes that leaves a large fragment behind - most of the others make shrapnel. If you have a combustion chamber that's even thinking about resonating, and you have something like a turbine wheel that acts as a siren anywhere near that frequency, or even just pressure-sensitive "chuffing" in the combustion, then you lose the engine. Parts have to be designed to be non-resonant (or at least with very poor Q) and even then their frequencies have to be measured and deliberately avoided by components like pumps. That's why turbines generally have near-prime numbers of blades.

Read Stumpf's "Titan II" book for some descriptions.

Mild steel might be okay, but I am not sure. I do know that lots of steels have very low charpy numbers when cold. Stainless steels are usually good for low temps. Those stories of warming up axe heads on cold days to prevent the head from shattering are true. Dan

snipped-for-privacy@ntlworld.com wrote:

--What's a "charpy number"?

It is a test done to determine the ability of a metal to adsorb a shock. Al though it is for a shock it is a good indicator of how the metal will be if stressed with a nick or crack in it. It is a easy test to set up. I googled and found lots of information that said that

I am not a mechanical eng> --What's a "charpy number"?

If the Dewars were built for LOX the inner shell will most likely be

Note that LOX Dewars are quite different to the more common LN2 Dewars, which can be dangerous to use with LOX.

Also note that the outer shells are sometimes made from low carbon or even ordinary mild steel. This should not come in contact with LOX as it can burn, and it also gets brittle at the low temperatures involved.

Assuming it's stainless, an austenitic stainless rod (eg 316L) should be okay for both the chemical effects and the cold. If it's ally you'd probably be wise to etch clean and anodise it after welding.

I don't know what the high nickel rods you mention are, but in general high nickel-containing alloys are hard to ignite and don't burn well, and have good cryogenic properties, so it will probably be okay.

Check to see if the finished weld is austenitic (a magnet won't stick to it), when it will almost certainly take the cold (there are about two rather uncommon exceptions to this though, so beware. LOX can be efffff'n dangerous).

Many magnetic nickel-containing alloys will take cryo cold too, but I'd need to know the rod composition to tell for sure. If you have some liquid nitrogen handy, dip a test piece in and hit it with a hammer - if it doesn't go brittle with cold it's probably okay.

Another suitable method is to braze or silver solder the joint, but you have to get the finished joint very clean before it comes in contact with LOX. Both brazing metal and silver solder are good with LOX, but fluxes can be a problem.