awesome effect

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Mar 1, 2009
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a while ago, like, probably last august, i made an EX motor out of a copper pipe and pourstone for the nozzle. this was a sugar based motor. i had never used a copper pipe before for the casing, but i did this time. well, i went outside today to grab a tool, and there's that copper pipe laying there. only, i didnt notice that it was a copper pipe at first, i actually looked like stained glass. i was completely glazed in a bright aqua blue color. now, i knew that copper when burned creates blue effects, but i didnt know that just by heating it it will have that effect over looks like corrosion, im wondering if its oxidation, like rust. also, i thought that copper turns green when it oxidizes, so was this purely from the heat created?
Could be a number of copper compounds. If you ever look at an old brass door-knob, you will find that it has the same effect you have described in places (brass contains a large percentage of copper).

It's probably copper carbonate (green) and/or copper sulfate (translucent blue). Most likely copper carbonate, through reaction with atmospheric CO2.

The heat likely caused the copper's thin oxide "tarnish" layer to be burnt off completely, exposing it to reaction.

The fact that copper compounds produce blue flame is completely unrelated. The blue light emitted comes from excited copper chloride particles which are trying to return to a normal energy level.
First of all, your heating has nothing to do with the copper pipe turning Aqua. When copper burns, it burns blue (or green) because of the way the electrons are excited. The reason your pipe turned the color it did, is because it probably rained since you left it out there, and, as we know there is sulfur in the rain (sulfuric acid). Sulfur and copper combine to form CuSO4 (copper sulfate) Copper sulfate, in its hydrated form is aqua in color, if you dried it, it would turn white.

Using copper pipe is not a smart idea at all, its a death wish. If you did your EX reading, you would see that there are only certian materials suitable for motor use, and experimenting isnt a good idea. Sure, its EX, but experimenting with cases is just not smart. When you blow up a motor and a copper pipe comes flying at you, your going to die, because it will cut you.....deep.

I could go on a rant here, but I wont, the fact that you used a copper pipe for a motor case speaks for itself.

Anyway, I will check up on your EX progress, I am interested to see how it goes.
In the future, never use copper for motor casings. It is too soft and will blow up and fragment quite easily, possibly killing or maiming you. Only use properly made Aluminum or plastic casings. Aluminum will not fragment and plastic will not sever limbs if it fragments.

Copper only has two oxides, black or red. The former is more common. The green is probably the chloride of copper, since the burning of APCP produced HCl vapors, which would have reacted with the pure metal over time. It could also be the carbonate, as a result of reacting with CO2 in the air. I'll have to check on this.
ryan, i thank you for your concern.
BTW, this is not about me using a copper pipe for a motor casing, this is about the copper pipe turning blue.
also, ryan, the pipe was in my garage, off the floor, every so often we get a shower in there, but it hasnt happened since the pipe has been there.
oh, i've left pennies outside and they turn green, so i doubt that a copper pipe in rain will turn bright blue.

again, please do not chastise me for using copper pipe for a casing, this thread is not about that. i could've said that i heated it with a torch to achieve the effect.

also, im quite sure that either the nozzle or the plug in the front would've let go before the case split. afterall, im told one of the best casing methods is by using PVC...plastics make it possible huh.
Yes, I was going to mention the stupidity of using a copper casing. This may seem a bit harsh, but the comments by others that have been posted are justified... copper is soft, and heavy.
As Ryan has mentioned, if it explodes, it would... leave quite a ... perforation in you.

Thanks for that info, Brian. I haven't used copper chloride before, so I wasnt sure what color it was so I didn't mention it :) .

And yes, r1dermon, I strongly advise you to pursue EX in a safer manner... otherwise as Ryan hinted you could become the distingushed *erm* recipient of a Darwin Award.

I still think the green color would be copper carbonate though. Both copper sulfate and copper chloride are soluble, so I would've thought the rain would simply wash them off.
ok, well, to dispell any myths that i am a darwin awards recipient waiting to happen. this was when i was first getting into EX, this was also before i knew about core burners, and this was done using UN milled sugar propellent(VERY conventional sugar propellent, i wont give formulas due to the forum guidlines, but if you would care to email me, id be glad to show you just HOW conventional it was) it was in fairly powdery form and i just wanted to static test the design. the case can always be changed. i didnt plan on launching it in a vehicle. i would agree that its not the safest idea using copper as a casing, but the amount of pressure it was exposed to wasnt enough to even create 1 lb of thrust. measured on a diet scale in 1oz increments. these were my very first EX experiments, and a lot has changed since then. so please, unless you know who i am and how i practice my experiments, dont jump to conclusions about me. i'd really appreciate it. now, back to the blueing on the copper. i dont understand how the heating of the tube and the subsequent discoloration has nothing to do with the fact that copper compounds create blue effects when burned. what does this discrepency exist? and why then, does the copper turn blue, or is it coincidental?
you dont get it, whether it is about that or not, that is DANGEROUS.
Well saying that just because copper burns blue that it's compounds will be blue is like saying that if you put purple food dye in propellant that it will produce a purple flame. They are completely unrelated.

Most colors are produced (I think with the exception of sodium yellow-orange, which is sodium D-line emission instead) are produced by excited chloride compounds.

BaCl2 for green flame.
SrCl2 for red flame.
LiCl for a reddish-violet color (not used much, Sr is nicer).
CuCl2 for blue flame.

The pipe is staying that color. Thus, it isn't blue because these compounds (the x Cl's) are making it blue from burning... to do that it would have to be combustion temp constatly. It's simply a copper compound that happens to be an aqua color.
And as I said earlier, the fact that it has turned blue MAY be because it's been heated, making the blue-compound-forming reaction to proceed faster than it would otherwise.
Ryan, he got the point...enough already. the question is about discoloration, so let's get back on topic.

so basically anything with a chlorine in its compound will produce a specific color when heated?

BTW, thank you CTulanko
Not *every* chloride compound... it just happens to be that these specfic chloride compounds release energy in the form of light in a useful wavelength. However, im guessing that chloride compounds in general do this. The majority probably produce light in the ultraviolet, infrared or boring color ranges though.
As far as I know its mostly metal-type chlorides that do interesting things in respect to color. I haven't heard of any non-metals doing anything, for example HCl doesn't do anything.
Originally posted by bjmcder
In the future, never use copper for motor casings. It is too soft and will blow up and fragment quite easily, possibly killing or maiming you. Only use properly made Aluminum or plastic casings. Aluminum will not fragment and plastic will not sever limbs if it fragments.

Copper only has two oxides, black or red. The former is more common. The green is probably the chloride of copper, since the burning of APCP produced HCl vapors, which would have reacted with the pure metal over time. It could also be the carbonate, as a result of reacting with CO2 in the air. I'll have to check on this.

AL will fragment, this is a common myth, however AL tubing is MUCH safer to use then copper tubing.

I've heard it can fragment, but I've seen some radically split cases that didn't seem to have fragmented (much).
If your aluminum is fragmenting and creating schrapnel, your are getting pressure spikes that are approaching detonation levels. The pressure rise time in a typical CATO is fairly long (multiple 1/100th's of a second) when compared to a detonation (fractions of ms).
I agree. With a CATO, as soon as the casing wall is ruptured, the pressurised gases are free to exit the casing. The gases are not being produced at a rate high enough to continue pressurization with the sudden opening avaliable for escape. Thus, no more casing will have to be ruptured to release the pressure, and any further damage to the casing sustained (ie. twisting, peeling) is from the momentum of the initial rupture itself and the gas flow out of the rupture. I hope I'm making sence here, getting across this idea is pretty hard :) .

I'm guessing that steel fragments because of it's crystalline formation or something. And that would be why heat-treated steel differs from non-heat treated steel... Crystal formation.

The only time this condition (gas production rate > pressure release rate) is satisfied is, as Brian said, a detonation situation. The gas production rate is pretty much instantaneous in that case.

If your propellants are detonating, you're making propellant with some pretty strange constituents :p .
Does anyone have any hard data on the strength of each material instead of a bunch of educated guesses?

Strength is not as much of an issue as are failure modes, methinks. But no, I have to just ape common wisdom. We need a metaurgist like Drake Damerau!
There’s a few things here to which I would comment. The first is CATOs and fragmentation of the case. Fragmentation can be characterized by fracture toughness and other properties such as ductility or even impact toughness.

All metals in any form or regardless of heat treatment consist of crystals. That’s one of the things that make an element a metal. The size and shape of the crystals and even the material between the crystals can greatly effect fragmentation. But the easiest way to determine the fragmentation characteristics of a material is the ductility. The more ductile a material is, the less prone to fragmentation it generally is.

Some materials completely loose their ductility when subjected to a shock. (yes, even aluminum.) Take silly putty and make a snake out of it. The stuff is very ductile. But when you pull on it very quickly, it will snap before it has a chance to stretch. This can also be true for some metals.

(You guys didn’t think I was going to tell you to start making motors out of copper pipe did you?)

The blue/green copper thing… It is oxidation. Copper will form compounds with many things. Oxides are very common. Many of the copper compounds are blue or green. There’s no way of knowing what they may be without knowing the composition of the EX motor but if it was a sugar motor I doubt there were any chlorates in it. The compound could be a sulfate or just an oxide.
Originally posted by rstaff3
Hey Doc, glad to have you onboard!

I'll second that! I know the folks in Advanced Topics appreciate you're popping in there. Make it regular. We need a materials guy over there.
Just my .02, however if Doc talks about shrapnel, listen. He is the chief metallurgist in a place that makes artillery shells. You know, things that CATO on purpose.

Thanks for dropping in. Now stay where you are welcome.
His office discussion must be like Bizzaro-World's rocket forum. :D
Yeah, discussions at the water cooler can be interesting to say the least. Here's the latest discussion. I have invented a new steel alloy for the attached (picture) weapon system. No bunker is safe any more! Those are reinforced concrete blocks and for scale, the "bullet" is over 4 feet long.

If you want actual numbers for material strengths and a quick calculation of hoop stress and bursting pressures for common casing materials, Richard Nakka has an excellent MS EXCEL application on his website ( under the rocketry software section.

I'm impressed by Nakka's talent and generousity in providing such a wealth of knowledge to the experimental community. His description of the application is below;


Motor Casing Design

This is an MS EXCEL spreadsheet that is used to determine the Design Pressure and Burst Pressure of a solid rocket motor casing. Also determines the elastic deformation of the casing under pressure (important for case-bonding consideration). Strength and mechanical properties are supplied for many casing materials such as steels, aluminum alloys, PVC, etc.