Slow BP Methods

[LiamPyro]

Consider that there are two ways to make a slower black powder type fuel: The fuel can be ball milled as usual and the ratio adjusted to something like 60/30/10, or the fuel can be made with the usual 75/15/10 ratio but simply screen mixed. Both of these will burn slower than regular black powder, and its conceivable that two fuels could be made that would burn at the exactly the same rate with one being screen mixed and one ball milled. In terms of making rockets, I see both methods of making a slower fuel employed. However, it seems as though adjusting ratios and ball milling would be superior seeing as this allows for a higher percentage of charcoal/sulfur fuel, whereas screen mixed black powder is simply a less efficient version of meal. The fundamental difference is whether the burn rate is controlled via surplus fuel or by making a less homogenous mixture with a larger particle size. Thoughts? Edited July 28, 2020 by LiamPyro

[SeaMonkey]

If residue would be a problem I suppose one would choose the method

which produces the least or most easily managed residue.

 

It may be possible to reduce the sulfur content as well.

 

I agree that a well milled product with very small particle sizes

will be more consistent and predictable.

Edited July 29, 2020 by SeaMonkey

[justvisiting]

Burn rate and thrust produced seem to be separate things, IMO.

[LiamPyro]

Burn rate and thrust produced seem to be separate things, IMO.

If this is the case, then two mixtures one of which is milled and the other screened could burn at the same rate, but it seems that the one with the higher fuel content (milled) would be more energy dense and thus produce more thrust.

[justvisiting]

There are so many interesting factors at play in your hypothetical scenario. You mention 60-30-10 and you mention thrust, so I guess we're talking about rockets You're not likely to use 75-15-10 in a nozzled rocket (although I have, lots of times), and you're certainly not going to use 60-30-10 in a nozzleless rocket, even if all the charcoal was fine enough to react within the motor.

 

Usually, a 60-30-10 rocket has a coarse charcoal and a fine charcoal. The fine charcoal participates in the black powder reaction in the motor. Anything that blows out the rear and produces sparks is not part of that reaction. If black powder has unsufficiently milled charcoal particles in it, those particles can't be considered as fuel.

 

When I make my screen-mixed black powder variants, I 'supermill' the charcoal. I'm not getting intimacy of incorporation by using a wheel mill like commercial manufacturers do. I am however, guaranteeing that all the charcoal has maximum fuel value. If a charcoal had a higher ash content, then more would be needed to get a similar fuel value. Also, I think something that is often overlooked is the role of water in the commercial manufacture of black powder. Commercial black powder doesn't use fast charcoals like we do. They use stuff like maple or oak. Maybe the reason willow was so popular in the 'olden days' is because manufacturing methods were less than ideal, so a 'hotter' charcoal was needed. Just a guess.

[snapper]

willow was used because military/government contracts called for it, today they use the cheapest wood they can source. if you want to slow down your burn rate there are multiple ways this is possible with the easiest being to use a different wood for your charcoal such as pine or dogwood that reacts well but slower.

[dagabu]

If this is the case, then two mixtures one of which is milled and the other screened could burn at the same rate, but it seems that the one with the higher fuel content (milled) would be more energy dense and thus produce more thrust.

 

Interesting thoughts shared here but lets get back to "Rocket Science" and look at this through a different lens since I see all sorts of words being used instead of the thing that lifts rockets... thrust.

 

Thrust is:

 

This means that you are looking at what mix and how fine of a mix would make less thrust?

 

Not 'slower' but less thrust? Or, are you burning this in the open? Is this for lift, burst, propulsion?

[dagabu]

If this is the case, then two mixtures one of which is milled and the other screened could burn at the same rate, but it seems that the one with the higher fuel content (milled) would be more energy dense and thus produce more thrust.

 

Burns at the same rate, more gas production = more usable thrust.

[LiamPyro]

 

Burns at the same rate, more gas production = more usable thrust.

I guess this is the heart of my question. If two compositions with different charcoal proportions burn at the same rate, do they in fact produce the same thrust? It seems like in one scenario, excess charcoal is wasted (burns in open air as a tail due to lack of oxygen) whereas in the other case KNO3 is wasted (oxygen isnt fully utilized due to large fuel particles /lack of homogeneity). Think about it... 75/15/10 BP can burn super quick and leave little residue if milled well, or it can burn slow and slaggy if not. To simplify the scenario, charcoal wood type is not a variable.

[LiamPyro]

 

Usually, a 60-30-10 rocket has a coarse charcoal and a fine charcoal. The fine charcoal participates in the black powder reaction in the motor. Anything that blows out the rear and produces sparks is not part of that reaction. If black powder has unsufficiently milled charcoal particles in it, those particles can't be considered as fuel.

 

When I make my screen-mixed black powder variants, I 'supermill' the charcoal. I'm not getting intimacy of incorporation by using a wheel mill like commercial manufacturers do. I am however, guaranteeing that all the charcoal has maximum fuel value.

My mill is simply a modified harbor freight rock tumbler with 5/16 brass media so I also supermill my willow charcoal separately before milling all the BP components together to create a faster end product that works great in my pressed 4oz end burners (and as lift). Fuel value of the charcoal is a big part of this scenario Ive described... maximum fuel value achieved through milling still leaves unburned charcoal in a 60/30/10 mix because of a stoichiometric lack of oxygen. However, when screened 75/15/10 burns slow it must be because the charcoal isnt fine enough to combust fully in situ (in the rocket) despite plenty of oxygen. So in both cases there is inefficiency. Edited August 13, 2020 by LiamPyro

[lawrence]

My interest in this is in making 30mm comets with a quiet lift.

Mine comet effects are fine but lift is always very loud, reducing the amount of lift always looses too much height.

Is this a matter of tube length, lift powder speed or what.

Help!

[shockie]

Consider that there are two ways to make a slower black powder type fuel: The fuel can be ball milled as usual and the ratio adjusted to something like 60/30/10, or the fuel can be made with the usual 75/15/10 ratio but simply screen mixed. Both of these will burn slower than regular black powder, and its conceivable that two fuels could be made that would burn at the exactly the same rate with one being screen mixed and one ball milled. In terms of making rockets, I see both methods of making a slower fuel employed. However, it seems as though adjusting ratios and ball milling would be superior seeing as this allows for a higher percentage of charcoal/sulfur fuel, whereas screen mixed black powder is simply a less efficient version of meal. The fundamental difference is whether the burn rate is controlled via surplus fuel or by making a less homogenous mixture with a larger particle size. Thoughts?

 

The reason the 75/15/10 will burn slower than it's normal burn rate, is because of granulation size..... for example, as you go from larger to smaller granulations BP , the burn rate increases....that's why there is commercial 1F,2F,3F,4F slower to faster burning

 

Another factor to consider is amount of moisture.....as moisture content in the BP goes from say 2% upwards, that will result in slower BP

 

Ball-milling the black powder ingredients ensures a very small, fine particle size.

This maximizes the surface area of the ingredients, and the resulting burn speed.

After milling, the ingredients are also extremely intimately mixed.

This is also necessary to maximize the speed and power of the BP.

 

So this can be done to either 6:3:1 ot 75/15/10 BP....it's good for both of them....

 

ball millin vs screen mixing:

 

Ball milling does what I said above..but you can still screen it with a screen mesh to make sure your particle sizes are more or less uniform.... the smaller the particle size of the KNO3/C/S, and more intimately it is mixed the faster the burn....

Screen mixing will usually result in slower burn rate powder no matter if it's 6:3:1 or 75/15/10....why the particle size is greater and it's not as intimately mixed as ball milling....

 

so basically in order of fastest to slowest:

 

1, ball milled BP 75/15/10

2. screen mixed BP 75/15/10

3. Ball milled BP 63/30/10

4. screen mixed BP 63/30/10

 

another factor is you press pressure....if you were to go from say 1.3 gm/3 to 1.9gm/3 while pressing that would decrease burn rate

 

ANother thing to consider when BP burns it produces 40% gaseous by-products and 60% particulates in the gas flow....it's really dirty,hence the switch to smokeless powder because it doesn't produce ah much residue....

 

The reason you see slower ie 6:3:1 used sky/stick BP rocket motors, is in most Cored BP rocket motors it reduces the chamber pressure which can burst the paper tubes.... where as typical cored-end burner Estes engine can use a faster BP like 75/15/10 and thinner paper tubes as thechamber pressure generated is much less.

 

If you ever looked closely at thrust-time curves from cored to cored-endburners to medium cored end burners you will see the MAx thrust in a short period of time,in a cored BP rocket motor only lasts a second at most...

while conversley in a cored-end burner you see a high thrust spike that lasts for maybe .2 sec with a longer lower sustainer thrust.....

 

Now you hear of SUPER BP....these are jusr cored-endburners with a better nozzle shape so they are much more efficient burning the BP than the typical cylindrical shaped older nozzle designs were.....

Edited November 28, 2020 by shockie