question...fuels and explosives..

[forufes]

(takes deep breath)

ok. i've read some of the threads posted here..and you guys look like the real deal..in chemistry i mean..my knowledge in chemistry is average..and i'm working on a multi-disceplinary project and need the consultancy of guys like you.. i don't know were to start..but i'm sure anywhere will do, so here goes:

 

on what basis are materials with fast "burning" -or energy release-..are chosen to be explosives or fuels?? i mean both are used by the method of igniting them and using the kinetic shock-wave for destruction(warheads) or repulsive force(fuels)..right? so why isn't C4 used in jet planes and why isn't solid rocket fuel used as bombs??

 

second part of the question is:

what difference does burning the quantity of material you have all at once or continuously make?? does igniting it all at once have a positive effect in the sense of energy released (like a chain reaction).. or a negative one (like consuming a high ratio of O2 which reduces efficiency)..or is it irrelative?

[CaptainPanic]

The most obvious difference is that a fuel needs air / oxygen to combust and explode, while an explosive needs nothing at all, and will just react with itself.

 

In rocket fuel you need to distinguish between 2 types: those that are completely pre-mixed: the solid rocket fuels (which can explode, and are thus in fact also explosives, but just burn slower) and liquid rocket fuels and hybrid rocket fuels which keep the fuel and oxygen separated until it's time to burn it in the combustion chamber.

 

Then there's of course safety. C4 as jet fuel is kinda dangerous. At least normal jet fuel cannot burn unless there's a leak in the tanks. C4 can just react with itself.

 

I guess that material for bombs is chosen by its detonation speed. The higher the value, the more boom you get per weight. Rocket fuels are probably chosen for stability (because it's a rocket and not a bomb, you want it to burn in a very controlled manner - but a solid rocket cannot be turned off again once it's going) and for the most thrust kg of rocket fuel.

 

I'm not sure I understand the second part.

If you're asking if the energy released is proportional to the weight of whatever fuel / explosive you use, then the answer is: Yes. It's linearly proportional.

I'm not 100% sure how this energy translates into a blast wave or shock wave.

 

Finally - this whole field is very much "don't try this at home". If you ask the types of questions you just asked, then I believe you should study first, and only then consider to do anything more dangerous than a firecracker...

 

tell us about your multidisciplinary project. What kind of project deals with fuels and explosives?

Edited May 11, 2009 by CaptainPanic
asking for the type of project :-)

[forufes]

i'm just responsible for the design of it..but i have to take a lot of friggin things into account..almost in every science..it's not definite yet..as that will be decided after taking all factors into consideration..but it's simple objective is to clear the atmosphere and reach outer space, no matter if the method used is untraditional..so i'm almost starting all over here..

 

what i understood from wht you said is:

1-faster burning speed= bigger boom

2-fules which are premixed or react with themselves are dangerous..but USUALLY have a bigger boom, hence the tendency to use them as explosives.

 

the second part was:

the goal of the detonation of both bombs and fuel is to generate shockwaves or thrust, which are mechanically the same...so.. so does rapid detonation or slow detonation affect the amount of net energy (shockwave) released? or is that to be asked of the physics guys?

[jdurg]

The total energy really has nothing to do with the amount of thrust you get. The rate of gaseous production does.

 

In an explosive, the fuel and oxidizer are combined into either the same molecule (nitroglycerin) or the same mixture (black powder, though BP is not "technically" an explosive depending on which definition of explosive you look up).

 

When your goal is to make a "bomb" to cause damage, you want a high rate of gas to be produced at a high temperature. This almost instant production of gas and energy causes the gas that is formed to heat up INCREDIBLY fast. The energy given off by a true explosive is tremendous, and the byproducts of the reaction are generally gases. Mix a lot of energy with a lot of gas, and you get a rapid expansion of the gas. The expanding gas creates the shockwave that gives the characteristic "BOOM" and the high temperature of this gas can start fires in the general area that it goes off in.

 

What make an explosive an explosive is not just the energy it gives off. It's the energy combined with the quick release of gas that causes it to break through whatever is containing it. When you "light" an explosive, it is gone just as quickly as it took to ignite. There is no continuous release of gas. When you fire a bullet out of a gun, it's because the hammer in the gun hits the priming charge (explosive) in the bullet casing which ignites the black powder which is immediately burned up and used propelling the bullet out of the barell of the gun. You'll notice that there is no continual expulsion of gas from the gun barrell because all of the "explosive" has been used up. There's nothing left to continuously produce gas.

 

In a fuel such as rocket fuel, the products are also in a gaseous state but the release of these products is very slow compared to an explosive. Yes, there is a LOT of energy given off, but it is at a slow, continous rate so as the constantly provide a release of gas, and hence pressure against the container. In addition, aside from the solid rocket fuels that are used, the oxidizer and the fuel in rocket fuel are two separate mixtures so the rocket fuel can not react unless the two are mixed. As a result, if you light a puddle of rocket fuel on fire, only the surface where the oxidizer and fuel are in contact with each other will react, thus slowing down the overall reaction rate. In an explosive, the reaction happens all throughout the entire mixture and not just at the surface. Therefore, the reaction speed is much greater and unable to be controlled.

 

If you're trying to propel something forward, you want to have a continous, slow release of gasses so that the forward propulsion can continuously happen. This is how a rocket works. The fuel/oxidizer mixture slowly react, pushing gasses out from the nozzle and creating thrust in the opposite direction of the nozzle. If you used an explosive and created a weak area in the containment, the gases would expand out through that opening propelling the container forward, but almost immediately there would be no more forward thrust and gravity would take over. Use a standard rocket fuel and the reaction only happens where the fuel and oxidizer mixes, yet still creates gasses that can propel the container forward. However, the fuel/oxidizer isn't "burned up" all at once so as long as you still have the fuel/oxidizer mixture present, the container will continue to be thrusted forward and gravity won't get a chance to pull the container back down.

 

Now what I've stated above is an INCREDIBLY simplistic explanation that I'm sure has some technical/nomenclature "errors", but again, for a simplistic explanation is is fully sound.

[Mr Skeptic]

And the reason that you don't want to use explosives to power a plane is that they are too heavy and less powerful. Explosives have their own oxidizer as part of their molecular structure, but oxidizer is heavy. In many cases, the oxidizer part is heavier than the "fuel" part. On a plane, you don't have to carry oxidizer since you use oxygen from the air as oxidizer. Fuels are chosen for their energy density and safety, but explosives for their ability to release that energy quickly (and produce gas), even if it is quite a bit less energy.

 

In any case, if you used explosives rather than fuel what you would really have is a rocket.

[jdurg]

In any case, if you used explosives rather than fuel what you would really have is a rocket.

 

I think you mean missile.

[forufes]

..detonation speed. The higher the value, the more boom you get per weight...

...thrust kg of rocket fuel.

could you please refer me to a source or table of such relations?

 

 

 

The total energy really has nothing to do with the amount of thrust you get. The rate of gaseous production does.

aagh that's the same thing i get everywhere:mad:

 

why does it have to be gaseous production? why not energy production? if you produce a lot of energy in a place full of gases, the gases will absorb the energy and expand.. why do they have to be a chemical product of the burnt fuel?

 

i always come across "exothermic" materials as the ones producing highest thrust because -if i understand correctly- have a lot of compounds which are releasd upon burning as fuel, when those gases are produced accompanied by a great amount of energy (heat) they expand and give thrust (or destruction)...

 

and so i keep asking and never got an answer; why do the gases have to be produced? the air is all around the place, it can absorb the heat and expand, right?

 

 

 

 

When your goal is to make a "bomb" to cause damage, you want a high rate of gas to be produced at a high temperature. This almost instant production of gas and energy causes the gas that is formed to heat up INCREDIBLY fast. The energy given off by a true explosive is tremendous, and the byproducts of the reaction are generally gases.

same with eplosives. why not produce the heat or energy and let the surrounding air take over the rest?

 

Mix a lot of energy with a lot of gas, and you get a rapid expansion of the gas.

i know i'm repeating myself, but to mix those two they don't have to be both produced from the explosion, am i right?

 

 

 

If you're trying to propel something forward, you want to have a continous, slow release of gasses so that the forward propulsion can continuously happen.This is how a rocket works.

 

but theoretically, burning it all at once or slowly will give off the same amount of energy, right? like how a bullet works, so why not rockets be the same? why the difference between rocket fuel and a bullet's priming charge?

 

If you used an explosive and created a weak area in the containment, the gases would expand out through that opening propelling the container forward, but almost immediately there would be no more forward thrust and gravity would take over.

not if the system used to "extract" energy from the explosive was as efficient as the one used with the slowly burned fuel, they do contain the same amount of potential energy after all don't they?

 

isn't it like giving off 1 mega joule per second for five seconds, and giving off five mega joules the first second and zero the other four?

 

...the container will continue to be thrusted forward and gravity won't get a chance to pull the container back down.

but gravity will pull it down no matter what, it's the same in both methods, right?

Now what I've stated above is an INCREDIBLY simplistic explanation that I'm sure has some technical/nomenclature "errors", but again, for a simplistic explanation is is fully sound.

it was great, certainly learned much, but i'd really appreciate a more detailed analysis:-)

 

And the reason that you don't want to use explosives to power a plane is that they are too heavy and less powerful.

all of them?

no matter what?

 

Explosives have their own oxidizer as part of their molecular structure, but oxidizer is heavy.

it makes it heavier even at the molecular level?

 

aren't there any "explosives" which interact with the surroundig air as oxidizer....

 

In many cases, the oxidizer part is heavier than the "fuel" part. On a plane, you don't have to carry oxidizer since you use oxygen from the air as oxidizer.

EXACTLY, just like this ^^^

 

 

 

In any case, if you used explosives rather than fuel what you would really have is a rocket.

what does that make rocket fuel???

explosive or fuel?

 

i'm really really confused here, learned many new things, but haven't sorted them out yet..

I think you mean missile.

?

what's the difference?

 

(if there's one)

[Skye]

In reality, whether you have an explosion or a slower combustion, there is a contribution to the work done from both the production of gases and the expansion of gases due to the production of heat. However often one is ignored to simplify calculations or comparisons between systems. In explosives the expansion of gases due to heat production is often ignored. This is because much of the useful work done is by the rapid production of gases. The rate at which explosives react is such that heat transfer can't keep up with the expanding pressure wave and has a small enough effect to be ignored. The rate of reaction is also so fast that it quickly exhausts the oxygen in the surrounding air, so an oxidiser must be included. In slower combustions, such as in a jet engine or a car engine, the productions of gases can be ignored at a simple level. The combustion is then considered to be a 'heat addition' and the chemical reaction is ignored. However this only provides a basic level of information and chemical reactions are included in a lot of analyses.

 

As far as your opening questions, the basis for choosing fuels comes down to a range of factors. The first internal combustion engine was powered by gun powder. The Diesel engine was envisaged to run on coal. However liquid fuels, distilled from oil, have many advantages which have ended up in them dominating internal combustion and jet engines. The are easy to transport and transfer and contain lots of energy by weight and volume. Given that internal combustion and jet engines are used for transport these are very useful advantages.

[forufes]

In reality, whether you have an explosion or a slower combustion, there is a contribution to the work done from both the production of gases and the expansion of gases due to the production of heat. However often one is ignored to simplify calculations or comparisons between systems. In explosives the expansion of gases due to heat production is often ignored. This is because much of the useful work done is by the rapid production of gases. The rate at which explosives react is such that heat transfer can't keep up with the expanding pressure wave and has a small enough effect to be ignored. The rate of reaction is also so fast that it quickly exhausts the oxygen in the surrounding air, so an oxidiser must be included. In slower combustions, such as in a jet engine or a car engine, the productions of gases can be ignored at a simple level. The combustion is then considered to be a 'heat addition' and the chemical reaction is ignored. However this only provides a basic level of information and chemical reactions are included in a lot of analyses.

oohhh:eek:

 

i seeee..

 

so simply, explosives burn air too quickly for the heat to expand it, so, it has to produce it's own gas to expand..

 

fuels on the other side don't swallow up air so quickly, which has time to absorb the heat and expand on its leisure, giving off thrust.

 

but in reality as you said both expanding of existing gasses and production of new ones is present, the speed or rate of the chemical reaction is what determines which to be neglected.

 

have i gotten it right?

 

(um, what if the surrounding gasses were not ones needed for the chemical reacton i.e not burnable, but expandable?)

 

As far as your opening questions, the basis for choosing fuels comes down to a range of factors. The first internal combustion engine was powered by gun powder. The Diesel engine was envisaged to run on coal. However liquid fuels, distilled from oil, have many advantages which have ended up in them dominating internal combustion and jet engines. The are easy to transport and transfer and contain lots of energy by weight and volume. Given that internal combustion and jet engines are used for transport these are very useful advantages.

but why did gun powder stay used in explosives?

if the new liquid fuels were better?

 

i mean, if one method was discovered to be better than the old one and replaced it as a fuel, why not as an explosive too?

---

Merged post follows:

Consecutive posts merged

 

In any case, if you used explosives rather than fuel what you would really have is a rocket.

 

 

 

 

what does that make rocket fuel???

explosive or fuel?

 

 

i guess that was kinda answered before..

In rocket fuel you need to distinguish between 2 types: those that are completely pre-mixed: the solid rocket fuels (which can explode, and are thus in fact also explosives, but just burn slower) and liquid rocket fuels and hybrid rocket fuels which keep the fuel and oxygen separated until it's time to burn it in the combustion chamber.

 

but still

i'm really really confused here, learned many new things, but haven't sorted them out yet..

[Skye]

oohhh:eek:

 

i seeee..

 

so simply, explosives burn air too quickly for the heat to expand it, so, it has to produce it's own gas to expand..

 

 

fuels on the other side don't swallow up air so quickly, which has time to absorb the heat and expand on its leisure, giving off thrust.

 

but in reality as you said both expanding of existing gasses and production of new ones is present, the speed or rate of the chemical reaction is what determines which to be neglected.

 

have i gotten it right?

Pretty much.

(um, what if the surrounding gasses were not ones needed for the chemical reacton i.e not burnable, but expandable?)

If it's just a fuel, and doesn't include an oxidiser, then it won't combust and nothing will happen. If there is a fuel and an oxidiser then it will combust and expand due to the production of gases (and also due to the production of heat to a lesser extent). As it expands into the surrounding gas it will transfer the pressure (and heat) into that gas quite like it transfers pressure into surrounding air.

 

but why did gun powder stay used in explosives?

if the new liquid fuels were better?

 

i mean, if one method was discovered to be better than the old one and replaced it as a fuel, why not as an explosive too?

Liquid fuels are used extensively in explosives. The most common industrial explosive, used for blasting, is the mixture of the fertiliser ammonium nitrate and a liquid explosive, such as fuel oil or diesel. It is cheap and highly energetic. However for military use it isn't ideal. While it is releases a lot of energy, it does so fairly slowly. The effect of an explosive on materials depends to a large extent on the speed at which the blast expands. In most cases the desired result is for the material to break in a brittle kind of way, and a higher velocity explosive causes brittle fracturing more readily. In industrial blasting, most often rock is being broken. Rock is quite brittle and so it fractures easily. In military weapons, most often metal is being broken, which is much less brittle and requires higher blast velocities to fracture.