12 replies to this topic

[Karl Mitchell-Shead]

Hi Guys,

Having a debate with myself and the guys on another forum....

Could cold weather cause a shell to 'underlift'?

IMO, no, this was my response, don't think i'm too far off the mark, comments from you chemists will be appreciated

[quote]Yeah but surely the ambient temperature will have little or no effect, i'm pretty sure that if you ignite a pile of thermite in a freezer it will produce as good as the same amount of energy as in an oven?

That's two extremes, outside Uk temperatures worst case scenario you will never be looking at more than 50 degrees C difference between the hot and cold extremes (-20 to + 30).

The flame propogation through a solid pyrotechnic compound is so hot and fast i can't see a puk of BP burning any slower @ -10 compared to + 25 degrees C.

If anything i reckon cooler temperatures would enhance lift as there would be less give in a mortar?!

I fully understand your point Andy, temperature affects reaction speed, but it would have little effect on a reaction as brisant as BP especially in this temperature range, the ignition temperature and energy produced will remain constant regardless of ambient temperature ie burning fuse, fuse will not burn cooler just because it's a little chilly out
[/quote

Edited by Chief, 13 September 2010 - 08:17 PM.

[Karl Mitchell-Shead]

Would somebody repair my quote? I tried!

[Potassium chlorate]

If it's good quality BP, I doubt it very much. The humidity would be worse IMHO.

[Atom Fireworks]

Heres my take on that, ime assuming we are talking about bp lift being effected by ambient temperature?

My volvo s60 t5 has a turbo, it has an intercooler to cool the hot chraged air down before it reaches the inlet manifold. It does this because cooler air carries more oxygen ( i think) and therefor to make alot of power my intercooler needs to cool the air down considerably. If i am giving it some welly and the intake temperature are through the roof and the intercooler cannot cool the air enough i loose power noticably due to lower oxygen levels and also the engine mangement injects lots of fuel to cool the combustion temps down to save the engin.

Now my thoughts on the bp is that in the cold the extra oxygen and cooler air which expands as its warmed would create a better scenario for lifting a shell, I dont think its something that will make much of a difference as bp creates its own oxygen anyway buy the cold air expanding as its heated aswell as the gasses caused by the bp would be an advantage to the lift.

If it was really warm the air trapped under the shell wouldnt expand as much as the cold air and also wouldnt contain as much oxygen so therefore would be an advantage.

Also the warmer air would make any mortar tube made of a plastic material expand a little and also will be abit more flexable and therefore its possible in extreme temperatures the pressure behind the shell could widen it slightly and loose lift pressure resulting in lower breaking shells.

Obvcourse i could be a mile of the mark and be totally wrong, ime not a chemist or have any physics knowledge but my concept sounds right to me.

Jay

[Potassium chlorate]

I think you're right. Brilliant analysis. And a good choice of car too.

[Atom Fireworks]

I think you're right. Brilliant analysis. And a good choice of car too.

Lol ohh yea theyre made sweden ha ha, i only know all this because i tune my cars for street racing, not on a serious level but when youve got nearly 300bhp it helps to put those jap cars to bed

[Algenco]

Heres my take on that, ime assuming we are talking about bp lift being effected by ambient temperature?

My volvo s60 t5 has a turbo, it has an intercooler to cool the hot chraged air down before it reaches the inlet manifold. It does this because cooler air carries more oxygen ( i think) and therefor to make alot of power my intercooler needs to cool the air down considerably. If i am giving it some welly and the intake temperature are through the roof and the intercooler cannot cool the air enough i loose power noticably due to lower oxygen levels and also the engine mangement injects lots of fuel to cool the combustion temps down to save the engin.

Now my thoughts on the bp is that in the cold the extra oxygen and cooler air which expands as its warmed would create a better scenario for lifting a shell, I dont think its something that will make much of a difference as bp creates its own oxygen anyway buy the cold air expanding as its heated aswell as the gasses caused by the bp would be an advantage to the lift.

If it was really warm the air trapped under the shell wouldnt expand as much as the cold air and also wouldnt contain as much oxygen so therefore would be an advantage.

Also the warmer air would make any mortar tube made of a plastic material expand a little and also will be abit more flexable and therefore its possible in extreme temperatures the pressure behind the shell could widen it slightly and loose lift pressure resulting in lower breaking shells.

Obvcourse i could be a mile of the mark and be totally wrong, ime not a chemist or have any physics knowledge but my concept sounds right to me.

Jay

ahh, but wouldn't any increase be offset by the the shell encoutering increased resistance of denser air?

Edited by Algenco, 14 September 2010 - 01:56 PM.

[Atom Fireworks]

That would depend on power gained minus the extra resistance, i think a spherical shell would be at an advantage in this scenareo. But again ime not well up on technical things

[digger]

Interesting analysis guys. However I guess the mass change due to density would be small and hence insignificant.

So lets put some numbers to the theory.

First find the density at the different temperatures assuming isobaric pressure (101.325KPa). So.

Density at 30C 1.1644 kg/m3
Density at 0C 1.2920 kg/m3

Now find the volume of free air under the shell. Lets assume a decent size shell here to make the numbers bigger, so taking a six in shell.

Assume that the standoff if the shell is 75mm and that one third of the space under the shell is occupied by BP, leader card etc so the effective volume can be calculated:...

(Pi*d^2*h)/4

(3.142*.15^2*.05)/4 = 0.000384 m3

Now we can calculate the difference in air mass:

(1.2920-1.1644)*0.000384 = 0.000113 kg or 0.113 grams

Given that a six inch shell would have about 90 grams of lift and about 50% of the products are gaseous then this equates to an improved lift performance of about 0.2% so it would not be noticeable. This result is being generous with the temperatures as well I doubt many displays are fired at 30C so I would guess the real world difference would be half of this value.

So the next thing to consider is the effect on drag, Interesting one. I have put together a shell altitude calculator in the past that takes into account drag including air density. So assuming that the drag acts equally on the way up and the way down (over simplification, but should allow a basic comparison to be done) we can do a calculation.

I won't go through it here, but you can download the spreadsheet HERE

So changing the density of air to those above then the difference in height attained is a mere 20cm

So to sum up the effects of temperature are negligible on both drag and the expansion of the gas below the shell. Just the way the shell happens to spin when leaving the mortar would have more effect.

Although interesting, the charge cooler analogy whilst somewhat correct lacks in similarity due to the fact you are talking about charge cooling on the whole mass with a much higher possible temperature difference due to the compression (and higher pressures than atmospheric at max boost) etc etc.

Edited by digger, 15 September 2010 - 05:14 PM.

[martyn]

Interesting analysis guys. However I guess the mass change due to density would be small and hence insignificant.

So lets put some numbers to the theory.

First find the density at the different temperatures assuming isobaric pressure (101.325KPa). So.

Density at 30C 1.1644 kg/m3
Density at 0C 1.2920 kg/m3

Now find the volume of free air under the shell. Lets assume a decent size shell here to make the numbers bigger, so taking a six in shell.

Assume that the standoff if the shell is 75mm and that one third of the space under the shell is occupied by BP, leader card etc so the effective volume can be calculated:...

(Pi*d^2*h)/4

(3.142*.15^2*.05)/4 = 0.000384 m3

Now we can calculate the difference in air mass:

(1.2920-1.1644)*0.000384 = 0.000113 kg or 0.113 grams

Given that a six inch shell would have about 90 grams of lift and about 50% of the products are gaseous then this equates to an improved lift performance of about 0.2% so it would not be noticeable. This result is being generous with the temperatures as well I doubt many displays are fired at 30C so I would guess the real world difference would be half of this value.

So the next thing to consider is the effect on drag, Interesting one. I have put together a shell altitude calculator in the past that takes into account drag including air density. So assuming that the drag acts equally on the way up and the way down (over simplification, but should allow a basic comparison to be done) we can do a calculation.

I won't go through it here, but you can download the spreadsheet HERE

So changing the density of air to those above then the difference in height attained is a mere 20cm

So to sum up the effects of temperature are negligible on both drag and the expansion of the gas below the shell. Just the way the shell happens to spin when leaving the mortar would have more effect.

Although interesting, the charge cooler analogy whilst somewhat correct lacks in similarity due to the fact you are talking about charge cooling on the whole mass with a much higher possible temperature difference due to the compression (and higher pressures than atmospheric at max boost) etc etc.

Yeah, wot he said - that's wot I thought

Edited by martyn, 15 September 2010 - 05:19 PM.

[Potassium chlorate]

Speaking of the number 0.2%; I read somewhere that 0,2% of water, but not more than that, will increase the burning rate of BP.

[Atom Fireworks]

Interesting analysis guys. However I guess the mass change due to density would be small and hence insignificant.

So lets put some numbers to the theory.

First find the density at the different temperatures assuming isobaric pressure (101.325KPa). So.

Density at 30C 1.1644 kg/m3
Density at 0C 1.2920 kg/m3

Now find the volume of free air under the shell. Lets assume a decent size shell here to make the numbers bigger, so taking a six in shell.

Assume that the standoff if the shell is 75mm and that one third of the space under the shell is occupied by BP, leader card etc so the effective volume can be calculated:...

(Pi*d^2*h)/4

(3.142*.15^2*.05)/4 = 0.000384 m3

Now we can calculate the difference in air mass:

(1.2920-1.1644)*0.000384 = 0.000113 kg or 0.113 grams

Given that a six inch shell would have about 90 grams of lift and about 50% of the products are gaseous then this equates to an improved lift performance of about 0.2% so it would not be noticeable. This result is being generous with the temperatures as well I doubt many displays are fired at 30C so I would guess the real world difference would be half of this value.

So the next thing to consider is the effect on drag, Interesting one. I have put together a shell altitude calculator in the past that takes into account drag including air density. So assuming that the drag acts equally on the way up and the way down (over simplification, but should allow a basic comparison to be done) we can do a calculation.

I won't go through it here, but you can download the spreadsheet HERE

So changing the density of air to those above then the difference in height attained is a mere 20cm

So to sum up the effects of temperature are negligible on both drag and the expansion of the gas below the shell. Just the way the shell happens to spin when leaving the mortar would have more effect.

Although interesting, the charge cooler analogy whilst somewhat correct lacks in similarity due to the fact you are talking about charge cooling on the whole mass with a much higher possible temperature difference due to the compression (and higher pressures than atmospheric at max boost) etc etc.

Theres the maths behind the theory, i didnt think it would make much difference but to answer chief's question there is a difference all be it extremely small , You amaze me with your maths digger you did the spread sheet about shells hights on here for me aswell. Clever guy certainly glad to have you mooching around the forum.

Jay

[digger]

Glad you liked it.

Not to difficult on the maths front though. Maths at this level is pretty simple, just think about what is going on and describe it with numbers and you are there.

The shell height calculator uses a bit more complicated maths, but even this is not rocket science. The hardest part is writing the functions for the iteration as there is a couple of variables to solve for in one go.

To use the shell height calculator go to pyroguide.com and look at the bottom of the how high does my shell go article. Basically fire a dummy and time it from firing to impact, put this time into the calculator along with the weight and size and you can get the fusing time so that it goes off at apogee. Or you can use it to optimise your lift quantity to get it to reach the height and fusing time you want.