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Potential issue in granulating BP


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#1 Phil

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Posted 01 May 2021 - 05:01 AM

I'm trying my hand in making nozzleless "cohete" style rockets with tooling from woody's and instruction from Ned Gorski's video about such and I think I might be having an issue with the granulation process.
 
I am using 75/15/10 kno3/airfloat charcoal/sulfur all passing through a 100 mesh screen several times. To that I add 2% mineral oil suspended in 50ml of lacquer thinner. (not all at once) I work it into the powder thoroughly with my hands but when I try to compress it into a "putty ball" to push through my 20 mesh screen, it seems to break up very easily before it ever really takes "ball shape" no matter how wet or dry I have it. As soon as I press it against the screen it breaks up before I can fill the holes so what results seems to be inconsistent granule sizes and soft easy to crush granules. Do they get stronger the longer they dry? It's probably been drying for about 3-4 hours now. What am I doing wrong?

 

 

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Edited by Phil, 01 May 2021 - 05:08 AM.


#2 BasedAndRocketpilled

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Posted 01 May 2021 - 06:01 AM

I would think that soft easy to crush granules would be exactly what youre looking for in a rocket propellant. You're not going for durability but rather easy of handling, since you will be pressing them anyway.

Edited by BasedAndRocketpilled, 01 May 2021 - 06:02 AM.


#3 Phil

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Posted 01 May 2021 - 06:48 AM

I would think that soft easy to crush granules would be exactly what youre looking for in a rocket propellant. You're not going for durability but rather easy of handling, since you will be pressing them anyway.

I was under the impression hard granules were preferred because they consolidate into a more solid fuel grain. If it works that's all I'm worried about. Just so I'm understanding you, you're saying granulation of BP for rocket propellant is done pretty much solely for reasons besides performance?


Edited by Phil, 01 May 2021 - 06:54 AM.


#4 pyrokid

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Posted 01 May 2021 - 07:36 AM

Powder granulation for pressing operations is done to improve the handling properties such as flowability and dust generation. It is not the objective to create hard, durable, resilient granules, as these impede the formation of a uniform, monolithic fuel grain.
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#5 Phil

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Posted 01 May 2021 - 09:00 AM

I'm glad that's the case. Now I can go on to breaking in this tooling I just got from caleb last week. This beats trying to ram sugar rockets with a wooden dowel into hand rolled kraft tubes. That's what I was doing the last time I tried to make rockets lol.



#6 SharkWhisperer

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Posted 04 May 2021 - 01:24 PM

I'm trying my hand in making nozzleless "cohete" style rockets with tooling from woody's and instruction from Ned Gorski's video about such and I think I might be having an issue with the granulation process.
 
I am using 75/15/10 kno3/airfloat charcoal/sulfur all passing through a 100 mesh screen several times. To that I add 2% mineral oil suspended in 50ml of lacquer thinner. (not all at once) I work it into the powder thoroughly with my hands but when I try to compress it into a "putty ball" to push through my 20 mesh screen, it seems to break up very easily before it ever really takes "ball shape" no matter how wet or dry I have it. As soon as I press it against the screen it breaks up before I can fill the holes so what results seems to be inconsistent granule sizes and soft easy to crush granules. Do they get stronger the longer they dry? It's probably been drying for about 3-4 hours now. What am I doing wrong?

 

 

Most people do not granulate BP with a mineral oil/lacquer thinner mix. That would disperse your mineral oil, but probably would do less in consolidating the BP ingredients together to get not only a granule that's easier to work with, but also a BP that burns with increased speed. On a 1-10 scale of burning rate, I expect to get an increase of BP combustion speed of 1-1.5 additional units after granulation (I use 70% isopropyl for fast drying, but others use most or all water for cost savings). You can easily repowder and incorporate mineral oil afterwards. You might not be getting the burn rate increase that is a primary reason for granulating BP in the first place with your method, but it's easy to test. Burn a small pile of ungranulated BP next to the same mix after "granulation" and see if you get a speed increase. The mineral oil might complicate the comparison though, probably by slowing your burn rate... The only way to do an apples-to-apples comparison would be to do a dry incorporation of mineral oil alone vs w/solvent. You absolutely sure you even need to add mineral oil? And what's the quality of your charcoal (and thus, your BP) before even considering mineral oil addition or granulation? Is your BP satisfactory? If your charcoal is low-grade, then almost for certain your BP will be too, especially if you're relying on screen-mixing and not milling. And what is the fineness of your potassium nitrate?

 

A common theme with folks newer to the hobby is frustration with rockets and other devices because they haven't learned how to reliably make good hot BP. And not realizing when the BP batches they worked so hard on actually suck, and why, and how to fix this. This includes 100% of people who try to make functional BP from (overpriced) commercial BP "kits". And don't forget that Ned, although providing some decent tutorials and tips, has a vested financial interest in selling his info/website and promoting Skylighter's extortionately-priced chems/kits.

 

Good luck & have fun. Folks here are happy to help troubleshoot if you hit any snags.



#7 BetICouldMake1

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Posted 04 May 2021 - 04:51 PM

As stated earlier the purpose of granulating rocket fuel is not to increase power. The only reason you would granulate your rocket fuel is to reduce dust and make it easier to measure and load increments. Using oil in the fuel is something Ned (and some others) do (or did) when building nozzle-less rockets. It's not necessary but it may help temper the relatively hot fuel and/or aid in consolidation. I personally think adding oil to fuel doesn't make any sense since nozzle-less rockets will handle the hottest BP possible if you make them correctly, and if you want to temper your fuel you can do that by adjusting the ratio of charcoal to KNO3 rather than working with stinky solvents and slimy oils. You can just as easily granulate your fuel using straight water, or press them using your milled or screen mixed fuel slightly dampened with water (my preferred method). 

 

Making hot BP is not necessary for making good rockets. I would wager that more people have had problems from using too hot a fuel than too weak. Nozzle-less rockets don't require hot BP fuel, but they can handle hot BP fuel and will obviously have more power if you're using hotter BP. 

 

The cohete rockets in particular are very forgiving since they have a relatively short and fat spindle for a BP rocket. I bet you get them flying in no time.


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#8 justvisiting

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Posted 05 May 2021 - 01:51 AM

BetICouldMake1, that was very well said, I gotta say  :)

 

I'd just add that while nozzleless rockets don't require hot BP fuel, the lift suffers more than a little when they don't have it.


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#9 SharkWhisperer

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Posted 05 May 2021 - 02:04 PM

As stated earlier the purpose of granulating rocket fuel is not to increase power. The only reason you would granulate your rocket fuel is to reduce dust and make it easier to measure and load increments.

I think you'll find a lot of folks that disagree with this blanket statement, and enjoy the very obvious burn rate increase from better chem incorporation during BP (moist) granulation, in addition to it being less messy to handle.
 



#10 justvisiting

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Posted 06 May 2021 - 10:50 AM

SharkWhisper, honestly, I have never heard anybody say what you just said. I've tested over 200 rocket motors with my Acme test stand, and a "very obvious" burn rate increase did not stand out to me. Sometimes I tested mill dust, sometimes alcohol-granulated, water-granulated, sometimes 70% alcohol-granulated. I'm not saying I made actual comparisons, but I'd like to think I'd have noticed an increase in performance, since that's my main reason to test motors. I would say that if powder is to be burned 'loose', the point would certainly be valid. On the other hand, if powder is pressed moist into a rocket motor, there will be an increase in impulse (over dry-pressed powder) because the improvement in consolidation allows more powder to be squeezed into the same space. The way I understand it, 'loose' granulated powder burns faster than mill dust because the flame travels more easily through the mixture. 

 

I would say that the type and particle size of the charcoal used would have a greater effect on impulse in a rocket than whether or not the powder was granulated.


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#11 FrankRizzo

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Posted 07 May 2021 - 11:59 AM

Cohete and other nozzless motor fuel should ideally be the hottest BP that you can make. Granulate with 70% isopropyl alcohol if you need to reduce the dust, but don't add mineral oil to slow it down. Additionally, adding about 3% moisture to the fuel before ramming/pressing will greatly increase the consolidation.


Edited by FrankRizzo, 07 May 2021 - 12:01 PM.





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