Oxidizer Question

[uncrichie]

I've noticed several uses of BP requiring up to 5% of dextrin. I'm just curious if I/we should be using more oxidizer than the usual 75% (75-15-10+5)? With my limited knowledge I can surmise that varying any of the 3 main ingredients will make a significant change in burn characteristics. So why does the addition of 1%-5% dextrin not change the burn rate to the point of having to add more KnO3? I know some have mentioned that dextrin will act as a fuel some others are not sure. So maybe you chemistry guys (Mumbles) out there can shed some light on this? Thanks. Kurt

[Seymour]

It does certainly have an effect, which is why using such a high a % as 5 is common, but not universal.

 

Essentially it slows the burn rate, and shifts the carbon oxide mix away from high in dioxide towards high in monoxide. Depending on your use for the black powder, these effects may not be all that bad at all.

 

I'd say the main area that using 5% might be common is in the use of Black Powder primes. Here the burn speed is not as critical as strength. In fact accounting for the air that the star travels through, it's beneficial to have a higher fuel value, and extra charcoal is sometimes added intentional to more correctly ballance oxygen in the full situation.

 

For the situations that burn speed is important and which is a binder is desired (pretty much just making burst, and lift, if you don't corn that), I would recommend that you don't use 5%, since 2% of decent dextrin or some other alternative is plenty. However many people use 5%, perhaps so that they can add dextrin to all their powder and know it can be used for anything? (though same goes with 4%). While for sure it slows the powder, it might do less so than you expect. If you have decent charcoal and mill it properly, it will still be burning more than well enough for anything you want to do. Dextrin is loaded with Oxygen and Hydrogen, so the fuel value is not great, and the 75/15/10 formula is somewhat highly oxidised.

 

You might be surprise by how much you can delicately mess with the ratios with BP and have it perform well. There are many alternative formulas, often lower in sulfur and higher in charcoal. Usually, but not always they have a higher fuel content than 75/15/10. But for most people, the common formula is a neater set of numbers, and in firework use as long as those two critical factors are met (charcoal and milling) it all works well, so there is little incentive to try the fancy formulas.

[psyco_1322]

When building cylinder shells, you use granulated bp. It needs to be strong and durable powder, it's what's filling the gaps in the shells and often the entire center of the shell. If the powder was to use less dextrin, it would be weaker, consequently making the shell weaker at the same time. The shell undergoes a lot of compacting and crushing forces, and with weak grains, this would turn them to fines and dust. That then can cause the powder to settle more, and form gaps inside the shell, and also alters the burn rates of the burst. While 5% may not be 100% necessary for lift, it is quite necessary for filling shells. Then again, if you use a good charcoal and mill it properly, you will not really be gaining much from using a few less percent dextrin.

[Jonathan]

uncrichie,

 

If your goal is to make rockets, you may not want to granulate your B.P., inasmuch as you're going to ram the fuel into the motor tube and crush all your granules.

 

That aside, I've found red gum-bound B.P. granules burn slightly faster than dextrin-bound granules. I can't use either in a 75:15:10 mix for nozzled rockets, however; the rockets explode (CATO).

 

Footnote: I've found that dry dextrin powder makes a dandy fuel with certain oxidizers; e.g., sodium chlorate.

[Mumbles]

5% is just sort of the universal standard. Don't know why, maybe just for ease of calculation. As others have said, in most cases, 5% is more than enough. I've used in the ball park of 3% with stars, and they come out nice and hard. The one application where I know a lot is intentionally used is for things like spider stars and some comets, where they're going to be broken hard with a flash bag or lifted hard. Even then, it's only 6 or 7%.

 

Dextrin is basically small chains of sugar, so it's not surprising that it works well with chlorates. I wonder if it'd be any safer than actual sugars with chlorates, which are surprisingly sensitive.

[uncrichie]

Wow, thanks everyone. What thorough and very understandable information, you guys never cease to amaze me. Being very new to all this I was concerned that 5% might be too much for lift and coating rice hulls. The charcoal is black willow and I am milling. However I'm double milling at the moment (all KnO3+1/3 C in one jar, 2/3 C and all S and dextrin in another jar) then #100 mesh screen mixing together several times. The results are what you would expect using this method, not great but not terribly bad either. If I can get away with using less dextrin it certainly couldn't hurt burn performance. Thanks again. Kurt

[nater]

Jonathan,

 

Have you ever rammed a rocket with ungranulated BP?

[uncrichie]

Oops disregard. Edited September 19, 2013 by uncrichie

[dagabu]

Jonathan,

 

Have you ever rammed a rocket with ungranulated BP?

 

Most of us only do that once then we granulate it with water to cut the dust. I find compaction impossible to achieve with raw powder, it has to be granulated to make a solid grain IMHO.

[Maserface]

Most of us only do that once then we granulate it with water to cut the dust. I find compaction impossible to achieve with raw powder, it has to be granulated to make a solid grain IMHO.

 

I agree completely, I tried and tried to ram meal powder but eventually settled on 2% dextrin granulation. Now I just wet it with alcohol and granulate with no binder, keeps the dust down, and makes increments more consistent.

Edited September 19, 2013 by Maserface

[Arthur]

Most things will work with 2% dextrin if your locality is dry enough for things to naturally dry and stay dry Edited September 19, 2013 by Arthur

[oldspark]

I make all my rockets with meal powder with 2% mineral oil added, hand ramed and pressed, no dust and packs very well.

[dagabu]

I make all my rockets with meal powder with 2% mineral oil added, hand ramed and pressed, no dust and packs very well.

 

Wow! 2% is a whole lot of oil, even 1% in my endburners makes them so slow that they will not even lift. I would love to see some video of your tails as well.

[Jonathan]

nater and dagabu,

 

I have hand-rammed one (1) 3/8" I.D. bottle rocket with a 60:30:10 mix straight from the ball mill. The rocket was nozzled, cored, and top-fused. It rose to perhaps 300-400 feet. My prior attempts to use granulated (even just hand-mixed) 75:15:10 powder in such a rocket produced CATOs. I have to say that I hand ram, because I don't have a press. The fine powdery-ness of the ungranulated 60:30:10 didn't bother me; but I was working in somewhat humid conditions and with a small amount of fuel.

[Bobosan]

All the rockets I've done have been hand rammed with 75-15-10 meal and (knocking on wood) haven't had a CATO yet. All the meal used usually has 2-3% dextrin added as it's milled for use as lift powder rather than rocket fuel. I do suspect that size matters (that's what she said anyway ) since I have only worked with 4oz maximum tubes so far.

 

If I'm careful, I don't find working with meal to be a messy undertaking.

[psyco_1322]

Wow! 2% is a whole lot of oil, even 1% in my endburners makes them so slow that they will not even lift. I would love to see some video of your tails as well.

 

I would think it would impact end burners quite a bit, but with core burners, you don't really notice it as much. Once you hit 5-6% they will really start to loose some power but you will start seeing the chuffing effect.

[FlaMtnBkr]

I saw a chlorate blue formula that had around 17 % dextrine in it as a fuel. Haven't tried it yet but I imagine it can be used as a low temp auxiliary fuel in certain situations.

 

On the other hand when ramming, even pressing, rockets I would want a soft granulate that will compact easily. I don't use dextrine at all when making BP for rockets. Just wetting and pressing through a screen will make granules durable enough for use in rockets. Adding dextrine would just increase the risk that there is a crack in the grain and a resulting CATO, IMO.

[Mumbles]

I don't use dextrin when granulating most things for ramming or pressing. The lone exception is for spolettes, because I just use the fines from making lift and burst. On the hobbyist level, there really isn't any reason to need to add dextrin. Commercially where granulates can be stored for years and even a little dust can cause problems, dextrin is preferred. They can easily overcome the additional issues by just using more pressure. This isn't quite as simple for us without industrial tools.

[hindsight]

It is of more than just theoretical interest to me that dextrin affects the compaction of BP, leading potentially to cracks in the grain, and commercial BP contains dextrin, requiring higher pressure on the BP during pressing, because I am about to press my first 3 pound nozzleless coreburner using commercial BP. I now have access to significant quantities of various larger meshes of commercial BP (and the press/tube/holder being used can effectively turn charcoal into diamond if need be), so I would like an opinion on how many psi the commercial BP should subjected to, since having CATO's on the maiden voyage is frustrating. I am assuming the pressure required is greater than usual. Thanks. Edited September 26, 2013 by hindsight

[Mumbles]

Commercial BP does NOT contain dextrin to my knowledge.

[Col]

I press mine to 6500psi, cant remember the last time one cato`d. 6500psi equates to around 5100 lbs on a 1" solid rammer.

[hindsight]

Okay, commercial BP does NOT contain dextrin. Sorry, Mumbles, I was basing my statement upon what I mistakenly thought you were suggesting.

Commercially where granulates can be stored for years and even a little dust can cause problems, dextrin is preferred. They can easily overcome the additional issues by just using more pressure.

. Appreciate the correction. Edited September 27, 2013 by hindsight

[hindsight]

Col, Thanks. I assume you mean 6500psi for commercial BP, since that was the specific question. Edited September 27, 2013 by hindsight

[Col]

The 6500psi is a good starting point. You could always press a short test section (2") and cut the tube off carefully to inspect the grain. If you press a clay bulkhead above it, you may get 4 sections in the tube at different pressures to compare. Edited September 27, 2013 by Col

[hindsight]

Will do, Col. Thanks again.