Quick Sciency question

[PercyShelley]

Does anyone here know the energy density of a modern smokeless powder? I was under the impression that compared to, say, gasoline, it's rather low on account of it carrying around its own oxidizer.

Still, a solid mj/kg is better than idle speculation.


Edit:


http://www.z-hat.com/Efficiency of the 300 Hawk.htm


That site is giving 1,550,000 foot-pounds per pound, which is .9 mj/kg if my math isn't completely screwed up.

 

[Sindawe]

I think your math may be all screwed up. But don't quote me, its been seven years or so since I've set up such calculations, and I'm going to ignore the number of significant figures.

Problem: Determine the energy in joules in one kilogram of smokeless powder, given that one pound of smokeless powder has 1,550,000 foot-pounds of energy and that one kilogram is 2.2 pounds.

The problem is one of unit conversion. First, determine the energy in joules in one pound of powder:

(1,550,000 fp/pound) * ( 1 joule/0.738 fp)

= 2100271.00 J/pound

Now convert pounds to kilograms:

(2100271.00 J/pound) * ( 2.2 pound/kilogram)

= 4620596.21 J / kilogram or 4.62 MJ/kg

Still much less than that of gasoline, which is around 45 MJ/kg per http://hypertextbook.com/facts/2003/ArthurGolnik.shtml

 

[Hemicuda]

Isn't it also going to depend on the grade and ratings of the powder?

there are only approximately 350,000 varinats of modern smokeless powder... made by 500 different companies... (numbers exagerated to clarify! ) and all have different burn characteristics...

 

[Mal H]

Isn't it also going to depend on the grade and ratings of the powder?

In general, yes, but not to the extent that it will make a difference of more than a relatively small factor.

One pound of smokeless will contain roughly the same amount of acual propellent in the form of nitrocellulose, nitroglycerin or a combination of both. The energy under discussion is not a function of time, and the various powders available are different principally in their burn rate. The burn rate is achieved by various means - different shape of grains, different grain coverings (inhibitors), different density, as well as different proportions of either or both nitro compounds.

So, the bottom line is that there will certainly be measurable differences in the megajoules per kilogram of one type of powder versus a kg of another powder, but it won't be anywhere near the factor or 10 when comparing smokeless powder to the same weight of gasoline.

 

[PercyShelley]

Thanks Sindawe, yes, I did screw up (divided when I should have multiplied). I should also have been able to tell by looking up other comperable compounds; .9 mj/kg is rather low. 4 is pretty close to other explosives.

Now, all we need to do is make a gun that runs on gasoline! It's got ten times the energy density that nitrocellulose does.

I wish I could give a prize to the first person who can spot the fallacy in the above statement.

 

[Sylvan-Forge]

Now, all we need to do is make a gun that runs on gasoline! It's got ten times the energy density that nitrocellulose does.

I wish I could give a prize to the first person who can spot the fallacy in the above statement.

You'd need an awful large cartridge to give ample air-space for a near stoichiometric combustion.

 

[PercyShelley]

You're good at this.

Although IIRC there are airsoft guns that run on some sortof gaseous combustion.

 

[Gbro]

And the "Impulse" Nail gun that burns propane/butane. a very impressive tool!