253 replies to this topic

[pyromaniac303]

Yep, your right it possibly is sodium chloride, I think there was a discussion on this a while back and it could be either. It would be a good idea to do a small test batch before you commit your whole stock of KNO3 to something that might not work. Good luck with it anyway.

[ProfHawking]

A little off topic, but not too much i hope.
Out of curiosity i had a look today at wikipedia on gunpowder
http://en.wikipedia.org/wiki/Gunpowder
Something interesting caught my eye:

"Unlike smokeless propellants, (gunpowder) acts more like an explosive since its burn rate is not affected by pressure"

Now, this is the exact opposite of my experience. If you confine it and increace the pressure the burn rate appears to massively increase.
Is this an effect of the compression (and it actually burns the same speed), or is the article incorrect?

The article later says:

"The size of the granules of powder and the confinement determine the burn rate of black powder"

Size yes, but surely the confinement here is a direct contradiction to the earlier statement. (albeit this one seems more accurate to me).

Then... it says something along the lines of the first statement again:

"A full charge of black powder seated by just a small wad of paper, with no bullet, will still burn just as quickly as if it had a full weight bullet in front of it."

Which is it? Its confused me, let alone the general public!

I thought i better check other's opinions before raising a query on the wiki.

Edited by ProfHawking, 17 July 2006 - 10:45 PM.

[FrankRizzo]

Black powder's physical burn rate (flame propagation) will vary depending on the granulation of the powder. Confinement only serves to keep the unburned powder within the flame front, ensuring maximum energy output (no unburned powder being propelled away), and the chemical reaction proceeds more or less to the same products irrespective of the pressure.

Smokeless powder's physical burn rate also depends on granulation, but the chemical reaction that occurs during smokeless powder's burn is influenced by pressure...the reaction is forced to the right toward gaseous products.

[Phoenix]

I suspect (although note here that all I'm doing is making educated guesses) that much of the increased burning rate of loose black powder is due to an increased ignition rate through the mass of grains or powder. Think quickmatch. If the powder is confined, more of the flames and dross produced will travel into the mass of powder, rather than away from it, so ignition spreads faster.

In "The Chemistry of Fireworks," M.S. Russell mentions black powder's "unique property of burning quickly at relatively low confinement." (p.9).

On p.27 Russell also says that "the burning rate laws that were discovered for the burning of gunpowder in the 19th century are equally applicable to more modern solid propellants such as cordite."

On p.28 Russell gives "Vielle's Law" - an equation for finding the burning rate of a rocket propellant, as:

RB = b p^n

where RB is burning rate, p is pressure and b and n are constants dependant upon the material being burned.

Assuming this propellant can be black powder or cordite (which the p.27 quote suggests it can) then the equation above shows that for both black powder and smokeless powder, the rate of burning will increase exponentially with pressure. However, depending on the characteristics of the material in question (b and n) the material could respond to this pressure increase to different extents.

Going back to educated guessing mode, black powder already burns pretty quickly at atmospheric pressure, whereas smokeless powder does not. If the pressure is increased, the rate at which solid propellant grains burn (not at which ignition is propagated) would not increase that much for black powder, but would increase a lot for smokeless powder.

Edited by Phoenix, 20 July 2006 - 09:44 AM.

[shadowpyro]

I've given up on the rubbish gardening website KNO3, its not used other than in delays and slow compositions.
I got some better KNO3 from a different place (this time without the red residue!).

i've noticed the discussion about the balsa wood charcoal, is balsa actually a type of tree (like corks are actually made out of "cork tree bark")? or is it just processed wood or something?

If its softwoods that we want for faster BP then has anyone ever tried using hay or something similar to turn into charcoal? (unfortuanly i dont have a farm to try this but im sure its not hard to buy hay from petstores etc)

The fire depressant in sodium chlorate weedkillers are sodium chloride, but from past experience all one would have to do to get the sodium chlorate would be to simply filter through a mesh with a size slightly larger than table salt crystals, and the outcome would be the larger crystals of sodium chlorate.

Edited by shadowpyro, 15 August 2006 - 03:27 PM.

[BrightStar]

Having given the BP burn rate questions above some thought, I think I might have at least a partial explanation.

According to Davis in COPE, whilst the BP burn rate has a low pressure exponent and is essentially constant, it is very sensitive to temperature.

If you consider a closed, well-confined (ie constant volume), thermally insulated container filled with gas which experiences a rapid rise in pressure, the temperature will rise in proportion to the pressure. This comes from the ideal gas law from which is derived:

Pressure1 * Volume1 / Temperature1 = Pressure2 * Volume2 / Temperature2

Imagine a BP salute at room temperature of 293 kelvin. The ignition temperature of BP is taken to be 300 centigrade or 573 kelvin. This implies that a rapid 96% increase in pressure (only 14.1 PSI) will be sufficient to ignite all of the BP in the salute, regardless of the actual flame front. In this scenario, it is the temperature, not the pressure causing the rapid ignition.

The fun comes when you consider that home made, riced BP (not pressed), is porous and compressible and itself contains a fair bit of air. In this case it is speculated that the compression of the soft BP grains can cause the gas temperature to rise inside the porous grain, causing it to start to burn from the inside out as well as the outside in. If true, this would make riced BP appear to be more pressure sensitive when confined, faster than commercial pressed powder and indeed more dangerous to use in a musket or canon.

As an interesting aside, I have been told that the value of the pressure exponent of whistle mix is very close to one. This is what allows it to whistle when pressed into a tube ? if it had a pressure exponent higher than one it would explode, less than one it would burn at a constant rate. The oscillation can only work with a value of very nearly one.

Does my BP burn rate explanation seem plausible to others here? I'm now just trying to work out how it applies to my experiments with BP rocket motors and nozzle sizes...

Edited by BrightStar, 08 August 2006 - 12:02 PM.

[pyromaniac303]

i've noticed the discussion about the balsa wood charcoal, is balsa actually a type of tree (like corks are actually made out of "cork tree bark")? or is it just processed wood or something?

Balsa is a tree that grows in south america, so there is not much chance of you just being able to go and chop one down somewhere. I get my balsa from model shops and you can also pick up some offcuts very cheaply from places that use it. It is the lightest wood I have ever used and it is not very strong in normal form, the charcoal is extremely light and if you do not like working with normal air float charcoal then this is 10x worse.

If its softwoods that we want for faster BP then has anyone ever tried using hay or something similar to turn into charcoal? (unfortuanly i dont have a farm to try this but im sure its not hard to buy hay from petstores etc)

People use rye charcoal for extremely fast BP but I have never tried it, and it is said to be more sensitive than normal charcoal. Also I think I read somewhere it is made without sulfur.

[shadowpyro]

Yeah i remeber reading the rye charcoal in some breaking compositions...though where someone could get hold of rye im not so sure. Theres paulowna charcoal as well which is used too.

Now for anyone who are in the UK and might be running outta white pine or willow, then try turning those kindling wood pieces which you can get from most petrol stations used for barbcues into charcoal. i've just tried it today and BOY DID I GET A BIG SURPRISE! (btw the charcoal was very soft and light after cooking)
I'm not sure what type of wood it is but the package says that the wood is from farmed renewable sources. I think they use mainly pine in these "tree farms" right?

[Andrew]

The simple Pressure Volume and Temperature model is not applicable in this situation. You neglected to consider the specific heat capacity of BP is far higher than that of air. Several orders of magnitude higher in fact.

You also have to consider the drivers. In a closed system you cannot just increase pressure. To double the pressure you need to keep the volume constant and double the temperature, keep the temperature constant and half the volume, or, a combination of the two. In real life terms the pressure is the resultant of a function of the volume and the temperature. The pressure is not something you can change directly in a closed system. You also have to consider the compressibility of the gaseous element.

Twas a nice though though

[BrightStar]

Andrew, Thanks for your thoughts on this. I suppose I have been trying to explore only the initial ignition phase of the BP grain combustion where the PV=NKT efffects in the surrounding air seem relevant.

The simple Pressure Volume and Temperature model is not applicable in this situation..

Agreed - I had been thinking of an ideal gas adiabatic compression scenario only as a starting point for discussion. It is still interesting that if you were to very rapidly halve the air volume in the hypothetical salute (eg by attaching an air piston and hitting it hard) the resultant rise in air temperature would be sufficient to ignite the BP. I might not volunteer to demonstrate this though

You neglected to consider the specific heat capacity of BP is far higher than that of air.

Yes, I had been considering only the rapid temperature rise in the air, igniting the exposed surface of the BP grains simultaneously, by contrast to the slower mechanism of a flame front propagating from the starting point progressively throughout the powder. In this sense, the ignition rate of the well confined BP system has become pressure dependent via the temperature rise the initial pressure wave causes. We are far from equilibrium here and the heat capacity of the grains and casing have been ignored as longer term factors.

You also have to consider the drivers. In a closed system you cannot just increase pressure. To double the pressure you need to keep the volume constant and double the temperature, keep the temperature constant and half the volume, or, a combination of the two. In real life terms the pressure is the resultant of a function of the volume and the temperature. The pressure is not something you can change directly in a closed system. You also have to consider the compressibility of the gaseous element..

There will be a non-linear function linking the drivers to the pressure and temperature as the burn progresses. I had been considering only the rate of the initial ignition of the BP grains.

Here's an interesting reference I found to the adiabatic compression of the air in soft home made BP grains increasing the burn rate, as applied to early muskets:

Ulrich Bretscher's Black Powder Page - Handgonnes

Does anyone have any references to the real thermodynamics of confined BP systems? (I'm still thinking of my BP rocket motors...)

Edited by BrightStar, 11 August 2006 - 08:24 PM.

[EnigmaticBiker]

Yeah i remeber reading the rye charcoal in some breaking compositions...though where someone could get hold of rye im not so sure. Theres paulowna charcoal as well which is used too.

Now for anyone who are in the UK and might be running outta white pine or willow, then try turning those kindling wood pieces which you can get from most petrol stations used for barbcues into charcoal. i've just tried it today and BOY DID I GET A BIG SURPRISE! (btw the charcoal was very soft and light after cooking)
I'm not sure what type of wood it is but the package says that the wood is from farmed renewable sources. I think they use mainly pine in these "tree farms" right?

Yes it will be pine, which type I can't remember.
In your last post you mentioned cork in passing, this also makes fast, easily powdered charcoal.
(The low density means you need a lot of corks )

[adamw]

Yes, most probably pine which has a lot of 'sap' resin - that is why it is good for kindling.

[Pretty green flames]

Wood for kindling is usually a mix of different woods such as pine, spruce, larch.... so this may not be the best option if your aim is consistency as different BP batches will differ in burn speed because you will not always have the same charcoal.

I speak from experience (and from the other side of the EU , so some things may be different) but if it works for you by all means use it, everyone has their secrets for making fast BP .

[minalth]

Here's an interesting reference I found to the adiabatic compression of the air in soft home made BP grains increasing the burn rate, as applied to early muskets:

Ulrich Bretscher's Black Powder Page - Handgonnes

Ulrich says that you shouldn't use large limbs of a tree [EDIT: for charcoal], only small (young) ones. I assume that has something to do with sap content or cell structure, can anyone confirm this or have any ideas about why?

Edited by minalth, 24 August 2006 - 11:56 AM.

[Frozentech]

Ulrich says that you shouldn't use large limbs of a tree [EDIT: for charcoal], only small (young) ones. I assume that has something to do with sap content or cell structure, can anyone confirm this or have any ideas about why?

I've wondered why also, but I can verify that with Cottonwood at least, charcoal from young saplings, about 2" diameter makes faster BP by far than the trunk wood of large trees. I am just guessing, but it may have to do with the ratio of lignin to cellulose in the wood ?