New ‘Problems’ after ball milling

[wingnutt]

For a few months I’ve been making and testing 3” long, core burning rocket motors in both 3/8” ID and ½” ID. The motors are hammer rammed on a mandrel, use parabolic converging angle shaped water putty nozzles with a 1/8” throat and core diameter. The tubes are made from 3” wide (76mm), fiber reinforced, water activated gummed packaging tape and wound on a custom made tube roller. The tubes are beautifully tight and have a consistent ID and OD that only varies by +/- 0.005”. For the purposes of this thread, let’s stick with the 3/8” ID motors to keep it simple.

 

My fuel is standard Teleflite mix of 63%KnO3, 27% powdered sugar and 10% dusting sulfur. The fuel was always milled for at least 2 hours at 60 rpm in a poor excuse for a tumbler, consisting of a thin walled plastic bottle on a threaded rod sticking out the chuck of an old drill. After milling the fuel still had some granularity to it and would make a hissing sound (granules) if you shook the tub that the fuel was kept in.

 

I tested the motors on a stand which uses a bar type load cell and an Arduino to send data to my laptop for recording. My build process is nailed down and I can crank out consistent motors whose thrust curves overlap with very little deviation. When testing, I use igniters that have their head touching the very top of the core. With a 2-3/16” core, the motors run for 1 second (+/- 0.05 sec), average 230gr of thrust (Isp = 2.3) and have a max thrust of 450gr. These motors contain 5 gr of fuel and have never blown out a nozzle.

 

New batch results –

This past weekend I finished and tested my new ball mill. It is well overbuilt, using a bottle made from 3” PVC which rolls on a double axel cradle at 63 rpm. To test it, I made a batch of fuel (100gr total volume as usual) and for some reason I added about 3% of finely hammered charcoal. I milled it for 2 hours and the fuel came out like talc. I’ve never made anything so finely ground before.

 

Following my usual motor building process, I built a few motors for testing the new fuel. All three of them blew the nozzles out!! I don’t mean from erosion, I mean all of the water putty was blown out with a POW, as well as a few layers of the paper tube. The blowouts all happened within 0.3 seconds of ignition and at a consistent 2000gr of thrust range.

 

I started widening the nozzle and cores and finally, at 5/32” for both the core and nozzle, I can get motors that run without blowing out. These motors now have a burn time of 0.50 seconds, have an average of 490gr of thrust (Isp = 2.4) and have a max thrust of 1350gr! So, slightly more overall impulse power, but all in half the burn time.

 

I tried an end burning type motor (using 1/8” throat and core) with a small 1” starter core. However, that only gave good initial launch thrust which quickly turned into very little thrust once the motor burned past the core.

 

From the look of it, what I have made is fuel that is VERY sensitive to internal burn pressure. By this, I mean that the graph of the Pressure vs. Burn Rate curve has a sharp knee in it making it difficult to balance throat/core diameter to core length as compared to my previous fuel.

 

Here is what I would like to know and understand…

 

1. Is this result simply due to finally ball milling the fuel to a more fine state?
2. Was it the addition of the charcoal that boosted the fuel and caused the pressure sensitivity?
3. Is it both?
4. Would further enlarging the nozzle and core bring down the max thrust and allow the motors to run longer? This means making new tooling.
5. What would be the most effective way to proceed with the goal of building motors that burn for 1 to 1.5 sec and have respectable thrust?

 

Any help here would be greatly appreciated.

 

[dagabu]

Compaction is the first thing to evaluate. Paths for fire to travel through the grain, is your first concern.

 

Second is the surface area of the fuel. Too much fuel burning at once causes a pressure spike that ruptures your tube -IF- the nozzle and cap are firmly pressed in place.

 

That's the third issue, rarely are nozzles pressed into the tube so as to deform the tube walls, once the motor pressures rise, the nozzle gets shot out. the way to tell if this is happening is to recover the tube. No Nozzle= not enough pressure when pressing the nozzle.

[Arthur]

Make only one change at a time, (mix ratio, mix time etc) then your tests develop relatable results. As you changed to a better mill consider reducing the time, weigh out say a kilo and start to mill it all then after (say) 20, 40, 60 etc minutes remove enough comp to test. When you have figures for big time intervals pick a good set of figures and do another test around that time (say 15, 20, 25 minutes) til you close in on a good process for your motor fuel.

 

If you back off on the milling time you may get to a burn pressure that lets you use the old tube and nozzle design, if that's what you want!

 

As the power and thrust increases the nozzle and the nozzle mount come under increasing stress. There are methods from incorporating steel washers to machining from graphite rod to assist the nozzle survive the heat and forces of short duration, high pressure, high velocity gas streams.

 

I'd guess that all your current problems are down to the vastly superior milling in the new mill. You could go back to the old fuel and same milling efficiency by simply milling for much less time, -OR you could experiment with better performance with better milled fuel.

[wingnutt]

First, thank you for your helpful replies. I really hope my reply below doesn’t come across as someone who asks for help, then shoots down all who reply just to do my own thing. I’m only trying to explain my understanding of the concepts, relay what I see in my tests and align it with what help has been given.

 

dagabu –

1. Voids due to poor compaction is not an issue. I am nothing if not methodical in my ramming doctrine. After pouring each small scoop of fuel, I vibrate the casing/ramming base to settle the fuel and rotate the ramming tool, 120 degrees or so, 3 times during ramming before the next scoop. The casings are not buckled at all.

1. True that more surface area can cause pressure spikes, however that pressure can be regulated by throat diameter. I have no issue at all with rupturing casings, only blowing out nozzles. My nozzles are not pressed into place, but rather injected into the base as a thick paste around a nozzle shaped forming rod. The moist putty paste glues itself to the inside of the casing using its own inherent bonding agents, as well as the glue that is on the tape that the casings are made from.

1. I have been able to recover both the tubes and the nozzles. Like I stated earlier, when the nozzles blow, they take a few layers of the tube with it. So, I don’t think nozzle holding strength is an issue either. There is simply way more pressure involved now than ever necessary for a 3” long, 3/8” ID motor.

 

Overall, all three initial nozzle blowing tests blew their nozzles out within 5% of each other – right at 2200gr of thrust, +/- 100gr. All three thrust curves overlay one another dang near perfectly. I’ll take your advice on surface area and start backing off on core length (about 1/8” at a time) to see if I can find what length works best.

 

Arthur –

“Only change one thing at a time.”

 

You are sooo right. I knew I shouldn’t have added the charcoal. That is why I stated “…and for some reason I added about 3% of finely hammered charcoal.” I’ll mix another batch using my original formula, mill it for 2hrs and retest. If it really is down to superior milling, then I’ll test, redesign, then retool for whatever I find that works.

 

Having to be cautious about timing the milling seems like it would introduce more variance than could easily be controlled. I’d prefer to take it to the point of diminishing returns for the sake of consistency, if not safety as well.

 

As far as incorporating steel washers, graphite rods, etc., I’d like to keep this as simple as possible. This is a cheap pastime for me and maximizing the ‘enjoyment/dollar’ ratio is part of the fun.

 

I’m not afraid of change, I only want to be able to control what I can easily reproduce in order to yield the desired result. However, knowing when the desired result is unobtainable with the current reproducible products is the key.

 

To your last point, for now I will go down the path of experimenting with better performance with the better milled fuel. I will keep everyone well informed of my progress.

[justvisiting]

Wingnutt, why don't you try hand ramming a clay nozzle? The nozzle expands somewhat and is harder to blow out.

[Arthur]

My personal hunch, I can't see your work from here! The improved milling has made the same mix a much faster fuel, the easy way to slow it is to reduce the mill time. However you've no idea what the quantified improvement is in terms of minutes and seconds which is why I suggested doing some trial times and testing them. As a pure guess your mix will be more like the old mix you like at about 30 to 60 minutes of milling. If you can reduce the burn rate hence the pressure then your established nozzle method should be good.

 

Early military nozzles were graphite in an asbestos filled bakelite mix. The asbestos is nasty but not half as nasty as a military warhead. in the world of hobby, you should replace the asbestos with some tough heat resistant fibre.

[wingnutt]

Pyromaniac -

I would if I had the tooling to do so. Currently, I don't know someone with a lathe and I'm not comfortable in dropping $$$ on tooling, grog, etc..

 

Arthur -

Currently, I think I am too new to the forum to be able to post pics. I wish I could, it would really help.

Heat resistant fiber you say. What about some ground up or milled vermiculite added to the water putty mix? It's good up to 1100C and oddly enough, I have plenty of it.

[justvisiting]

You say you want a longer burn time? You already had that. Do you have some objection to hand ramming a clay nozzle? Too simple/cheap/easy?

[wingnutt]

You say you want a longer burn time? You already had that. Do you have some objection to hand ramming a clay nozzle? Too simple/cheap/easy?

 

Reverting back to barely milled fuel now would feel like leaving money on the table. Kind of like driving a high powered sports car at idle all the time. Now that I know that the potential is there, and I have the ability to easily obtain it, why not learn to control it? Also, the more powerful fuel ignites easier and burns cleaner. I would like to learn to control the new fuel or to determine if it is at all possible.

 

About the clay nozzle questions, I don't know what to say. I get the feeling that you didn't read my response to the first time you asked about them. But to reiterate... I would hand ram clay nozzles if I had the tools to do so. I currently do not have that tooling on hand, nor do I have the means to easily obtain them. There is no objection to it, only a barrier.

[justvisiting]

Wingnutt, I didn't realize it was me (justvisiting) you were addressing when you called me 'pyromaniac' I guess I just don't get what your problem is. You say you are making nozzled coreburners "hammer rammed on a mandrel". Maybe I don't know what that means. Are you not packing propellant around a spindle by using a hammer and a cored drift? For such small rockets, I don't see why you couldn't make tooling with simple hand tools. I guess I misunderstand the problem. The solution seems so obvious to me

 

If you can't post a picture here (I've had problems too), you can upload to Pyrobin and link to that picture here.

[wingnutt]

justvisiting - My deepest apologies. Wow, I really made a mess of things here. Called you by the wrong name, used the wrong terminology and didn't fully think my questions through before asking just to name a few. Again, I am sorry for all of it.

 

To clarify, after rolling the tubes and the putty for the nozzles have cured, I drill the hole for the throat. The tube/nozzle combo is then slid down over a spindle mounted in a block of wood and the fuel is rammed around the spindle using a hollowed ram (is this called a drift?).

 

I would love to learn how to build the tooling needed to ram my own nozzles if it can be made with simple tools. if you wouldn't mind providing a link or giving suggestions where to begin looking, I would greatly appreciate it. In the meantime, I'll begin searching on my own.

 

I guess I should have approached my ask in a different way. I have read many times where ball milling KnO3/sugar/sulfur fuel produces a much better performing product. This is why I took the time to build a mill. My assumption was that the 'better performing fuel' would be more in line with the performance I am trying to accomplish in my motor builds. This is why I was asking for help in trying to tame it.

 

With your and Arthur's suggestions to back off on the milling to get back to where I was, I take it that the well milled fuel isn't suited for my needs of good thrust over a longer time. Since I have a decent amount of fuel still on hand, I'll see what I can get from larger nozzles and/or shorter cores.

[justvisiting]

Wingnutt, no offense taken

 

I'll offer some thoughts, and see what you like. You could take a small cylinder of aluminum, hard wood or plastic with a hole in it, and slip it snugly down over your spindle all the way to the bottom. It could be maybe 1/4" thick, more or less. Then when you mount the tube on it you can take your cored drift/rammer and use it to pack a little dry powdered clay in the bottom. It will form a nozzle. If your tube is good and your ramming technique is good, it will bulge the tube slightly in the nozzle area. This is what locks the nozzle in place. Then, you can ram your increments in as usual. The little 'stand-off' is important because the tube doesn't bulge in that area, so it's hard to push out when the propellant burns. This is how many folks form nozzles. Some people use cat litter, and some like certain brands. For your purposes, powdered cat litter should work. The nozzle area likes to grip the spindle. If you wipe a little wax onto the spindle at the nozzle area before ramming, it will release easier.

 

The little standoff may stick in the end of the rocket if it slips off the spindle. If it works to your satisfaction, you can come up with something more permanent. I'm just suggesting something easy to try that doesn't cost money. Again, this is how most rocket tooling is made, with a base to slip the tube onto. YouTube has lots of videos about hand ramming rockets too. Ned Gorski has a good one for small rockets, I think.

 

People don't ball mill nozzled rocket fuel usually. When you ball mill it, you get more power, yes- but shorter burn time. If you were to ball mill standard black powder (75-15-10) IN A SAFE LOCATION, you could make your little rockets without a nozzle, and they would fly fine. The 15 is charcoal though, not sugar. I don't know your level of pyro experience, so I don't want to suggest things outside your comfort zone.

 

Lots of knowledgeable, helpful folks here. You'll get this problem solved.

 

https://www.youtube.com/watch?v=dq9zgLINgp4

Edited August 30, 2019 by justvisiting

[wingnutt]

justvisiting -

Thank you for taking the time to explain what is easily doable. I guess I over thought the situation and seeing it laid out so well puts a different light on it. I started making some new tooling out of aluminum and will give ramming nozzles a try.

 

 

Posting Pics -

I think I found my problem in posting pics. I didn't see the "Attach This File" button the first time. I have posted a pic of my original 3/8" motor tests that set my baseline using the slightly milled fuel. The data has been normalized to show only the thrust data above the 5% max thrust point for calculating impulse power.

 

 

- UPDATE -

I made a new batch of fuel holding fast to my previous formula - no charcoal added. Ball milled it for 2 hours and made a couple of test runs using my original tubes, nozzles and throat diameter. Essentially everything is the same as it was before the last batch of fuel with the only difference being that the fuel was much better milled.

 

The new fuel falls right in the middle between the old standard fuel and the batch with the added charcoal. Burn time increased from 0.35 seconds to just over 0.5 seconds and max thrust is down from 1300gr to about 880gr. Even using the original smaller throat size of 1/8", I never blew one out. I still think that there is room for increasing the burn time and lowering the thrust a tad, via a larger throat, to achieve what I am after.

 

The results are easily seen in the graphs and I will post the comparison between the three different batches when I get home this evening.

 

From what I can see, properly milling the fuel did unlock more of the fuel's power potential. Adding the charcoal however, made the fuel far too pressure sensitive for my purpose and setup.

[Arthur]

Great. While you have a tube and nozzle system that works, keep the powder just inside what the casing will hold. Then if you need to improve the casing you will have more room to improve the fuel without CATO.

[wingnutt]

Arthur-

Will do.

 

Attached is a graph showing thrust curves of all three fuel types. The three that are to the lower right are the original fuel. The two to the upper left are the ones with added charcoal. Finally, the orange one in the middle is the latest batch of original formula, but well milled.