My little Chinese BALLS 2012 Project

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Will this project meet its stated goals? [See slide 2]

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Not a snowballs chance in hell
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Sumo310
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Re: My BALLS 2012 Project

Postby Sumo310 » Fri May 04, 2012 8:59 pm

Mike, have you looked at using Durafix to attach the fins? I'm not sure what difference it'd make, but might be worth a look?
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Re: My BALLS 2012 Project

Postby PK » Fri May 04, 2012 9:12 pm

Sumo310 wrote:Mike, have you looked at using Durafix to attach the fins? I'm not sure what difference it'd make, but might be worth a look?

I've got some of that stuff. Good for patching cavitation pits in boat props and fixing holes in beer cans. To do something like a fin can, you'd have to oven bake it, which is gonna:
a). Kill the temper.
b). Warp it.

Cut slots in thick wall tube.
Stitch the fins in.
Machine the OD.

You'll be fine

PK

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Re: My BALLS 2012 Project

Postby strud » Sat May 05, 2012 11:08 pm

Took a while to get my stuff together and take some pics but here they are :

fincan_1.jpg


fincan_bore.jpg


fincan_jig.jpg


These were made 'some time ago', probably around 2003 ish....

Note the 'ding' in one fin (image 2), this was made by the nose cone upon landing !

TIG welding aluminium was a bit tricky. I found that the best way for me was to have the current high and move fast. As you all well know, it is a highly thermally conductive metal and hence moving slow is a no no.

The tube was left with atleast 1.5mm extra wall thickness during welding which was taken off during the post weld boring operation.

CS

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Re: My BALLS 2012 Project

Postby Passaretti » Wed May 09, 2012 6:11 am

Hey guys - thanks for all the feedback. My apologies for my absence, I had my first TM run this past weekend; have been lickin' my wounds ever since completing it on Sunday.

Re: Simon - Durafix - Holy sh*t! Almost sounds to good to be true ... Just ordered a pound of rods off their website. I read some success stories on the RF that shows some promise. Nonetheless, it's worth a try - in a week or so I will try it out and post my results.

Re: Strud - Nice can's dude! That is exactly what I'm after. Your welds look like you know what you're doing. I'd love to attempt welding this can, however I don't have access to equipment. Small (stitch?) welds seem to be a great approach as to avoid overheating the base metals. For your application, what made you decide to go with all metal cans?

Re: Cryoscum. Thanks for the feedback on the structural calcs and the info on tempers. This will also help to assess what the impact is when attempting to braze the two aluminum parts together. Durafix calls out 732F (that's 388.9C in Aussie) as the target temperature for the base metal to properly braze the parts. I'm looking forward to trying this stuff, have a feeling it's going to be messy...

PK, Why do you envision oven-baking? Is this because you think it will be too difficult or impractical to locally build up enough heat to braze with a torch?

Mike
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Re: My BALLS 2012 Project

Postby OverTheTop » Wed May 09, 2012 7:46 am

I have used Durafix a few times over the years. I made some aluminium side steps for my 4x4 which stood up very well to some punishing conditions. Using Durafix is much easier than aluminium welding. There is a few hundred degrees between the melting point of the braze compared to the base metal. You just need to get enough heat in. I was using propane torches and struggling a little with the large piece I had to heat. Oxy would have been great.

Practise on some other scraps of aluminium first. There is a knack to the technique to get it right. It involves heating the area near where the joint is required and scratching the surface of the target metal using the tip of the rods. This breaks the oxide layer and makes for a strong joint. As with welding, there is some skill involved in getting both pieces of the part to around the same temperature.

The lower temperatures involved could well be beneficial to the end result!
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Re: My BALLS 2012 Project

Postby strud » Wed May 09, 2012 8:06 am

Glad you like my 'Cans' Mike :lol:

At the time it was easier for me to make them than the equivalent in composite materials. It was also perceived to be easier at the time to make a fin can accurately and with good fin symmetry in metal.

Also these fin cans were mounted directly to the motor cases as you are considering.

TIG welding is not so hard with steel and is very easy with stainless. Aluminium is quite difficult and that 'good looking can' was probably my third one, with the first couple looking much less attractive.

CS

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Re: My BALLS 2012 Project

Postby Passaretti » Thu May 10, 2012 1:57 am

cryoscum wrote:Image


Durafix specifies a base metal temperature of 732F. Assuming one can control the temperature quite well, it appears that brazing could be done with low concern for degrading the temper of T6. Does anyone else agree? Thoughts?

Last night I revisited the brilliant idear of attaching the fins DIRECTLY to the motor casing via brazing. Considering that the cost of the casing is approximately what it's (probably) going to cost to complete the fin-can itself, and not to mention the complexity... Why not consider it? The only show stopper I see is the risk of degrading the strength of the casing - KABOOM.

Would need to figure out how to remove the anodizing from the areas to be brazed, the rest of the anodizing (oxide layer) should act as a mask. I don't expect this would be too hard.

No can would mean there would be a hefty drag savings as well :D
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Re: My BALLS 2012 Project

Postby cryoscum » Thu May 10, 2012 7:16 am

Hi Mike

I suspect there will be degradation in the temper, but the annealing process takes hours to complete, and the bracing would only take minutes. Looking at it in temp terms, you'd be heating (ideally) the can to 732 whilst annealing temp is 775. That's only something like a 6% difference - a bit close for comfort, but I believe the annealing process takes place in a non-linear way, probably the time-to-annealing graph would be an inverted hyperbole.

This purely a guess, but I think an accurate & quick brazing process would give you a localized loss of maybe a quarter or third of the temper, but no more. This may be an acceptable loss, but you'd have to run some numbers. Quite possibility this would still be well below the fin can's servicibility limits...

In short, I'd risk it with the fin can, but not the motor case, unless it can be demonstrated that the case is made from a high end alloy with virtually no temper. Anybody know what alloy the cases are generally made from?
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Re: My BALLS 2012 Project

Postby OverTheTop » Thu May 10, 2012 7:53 am

If you do go down the brazing route there are some nifty temperature indicating crayons that let you know when you are at the right temperature. You just mark them on the metal and they change color at the designated temperature.

Based on the temperatures you have quoted I would be a little nervous about changing the temper of the aluminium with such a small temperature difference. Having said that, a small change in temper shouldn't change the temper all that much as Nick said. 6061-T6 has a UTS of around 300MPa and 6061-T4 around 207MPa. About 31% less. Given that there are many large safety margins designed into these components normally (factors of 2-5 or more are not uncommon) it is probably quite within the realms of possibility.

Another thing to consider is the low pressures inside the motor casing. I believe they are a couple of hundred psi. You might need to confirm that with the manufacturers or other sources.

Considering that the critical parts of the motor assembly is the ends where the threads are, if the fins are brazed on they should probably be better kept away from the area where the threads are. That means the threads would still have the correct strength.

Another positive of brazing the fins on is that although the UTS is reduced, the fins effectively provide radial ribs to support the casing at that point, helping with pressure containment!

I hope that's given you something to chew on :D
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Re: My BALLS 2012 Project

Postby Passaretti » Thu May 10, 2012 9:23 am

You guys both made some really good points ...

The CTI site says the 98mm casings are 6061-T6.

Starting to sound like a BALLS project if you ask me!!
Mike Passaretti, TRA 5369

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Re: My BALLS 2012 Project

Postby Passaretti » Thu May 10, 2012 10:37 am

Ok, here's one piece of the puzzle ... What kind of preassures are we dealing with? The geometry of the casing is well known, and is approx OD=3.85", ID=3.61". This would leave a wall thickness (t) of approx. 0.125".

The pressure rating (PR) of an aluminum pipe is as follows:

Image

Allowable Strength or stress of a 6061-T6 Al pipe is given by: S = (1.17 x Minimum Yield Strength) / 1.65
Min. Yield Strength of a 3.85" OD, 0.125" wall thickness pipe is 37KSI
and so S = 26.2KSI
Do = 3.85 inches
t = 0.125 inches

So PR = 1704 PSI.

Based on the above, I would expect the pressure rating of a 98mm motor casing to be somewhere in this neighborhood. Assuming a factor safety of at least 2, chamber pressures would have to be less than 800 PSI. That sound about right to anyone?

If any of the above nonsense holds water then my next question would be what will brazing due to the min. yield strength specifically?
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Re: My BALLS 2012 Project

Postby OverTheTop » Thu May 10, 2012 11:05 am

Need to find the internal pressures of the motor with more certainty from the manufacturer. I seem to remember a surprisingly low figure of around 200psi. Not sure what motor this related to. Don't take my word for it, get some information from the "horse's mouth".
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Re: My BALLS 2012 Project

Postby Passaretti » Thu May 10, 2012 11:12 am

Using PROPEL20 (provided with McCreary's ECP book), I took a crack at the chamber pressure. I had to make a number of assumptions about the grains, but I suspect the chamber pressure is in the ball park.

I assumed:

a grain length based on an aspect ratio of 1.6 (grain diameter is pretty standard)
core diameter of 1" (guesstimate)
dia of nozzle throat of 0.8"

Propellant density was derived from thrustcurve data and assumed grain geometry. Isp also came from thrustcurve.org

Chamber pressure comes out to approx. 1100PSI...
Attachments
PROPEL20 outputs.png
Mike Passaretti, TRA 5369

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Re: My BALLS 2012 Project

Postby Passaretti » Thu May 10, 2012 11:15 am

OverTheTop wrote:Need to find the internal pressures of the motor with more certainty from the manufacturer. I seem to remember a surprisingly low figure of around 200psi. Not sure what motor this related to. Don't take my word for it, get some information from the "horse's mouth".


I will drop CTI a line tomorrow, see what they can offer. Worst case, I'm sure a little more research will help. There are a ton of guys doing EX around these days that should have a good sense. Also, knowing the grain geometry would also help to tweak the PROPEL20 eq's.
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Re: My BALLS 2012 Project

Postby cryoscum » Thu May 10, 2012 12:36 pm

Metric please, gees!!

In looking for an operating pressure I could only find this from a CAR news feed:
1.JPG
1.JPG (40.44 KiB) Viewed 903 times

Seems to indicate something like 800 Psi for a 3 grain 38mm motor case.
Once you do have the actual operating pressure for an N5800, as a worst case scenario, assume you will lose most if not all of the temper and see if the pressure is then too high.
If you lose all the temper, the yield strength would be something like 8Ksi, making S=5.67Ksi and thus PR=368Psi.
I suspect that the operating pressure will be higher than that, but may be wrong.

Realistically speaking though, I'm guessing you'll be able to bargain on something like a T4 outcome after brazing and then your yield strength would be 16Ksi, S=11.3Ksi and thus PR=736Psi. This is in the ballpark of what you'll want, I think, but you've lost all of your safety factor...

A bit of a risk I think, but no better place to try it than at Balls! It'll either be cool or REALLY cool! :lol:
AMRS L3 | NAR L3 | QRS 089 | MDRA 224
AMRS Technical Advisory Group

Total impulse for 2016: 32,458 Ns (thus far)
Total impulse for 2015: 84,231 Ns
Total impulse for 2014: 40,757 Ns
Total impulse for 2013: 62,927 Ns


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