What's Really Involved in an Explosion?

[Rockazella]

What creates an explosion? I used to think it had to do with large amounts of heat energy being released very quickly. The heat would then cause air to expand very rapidly and thus you get your explosion and shock wave.

This explanation would mean that explosions couldn't really exist in space (no air or any gas).
However I read an article on nuclear explosion propelled space craft, and now I'm not so sure I have explosions correct. The article said the spacecraft could simply harness the explosions with the use of a large parathute type thing. If this is true, what's really involved in an explosion?

 

[russ_watters]

Your interpretation is exactly correct. In space though, the blast of the explosion is the explosive and bomb fragments themselves. That probably means the explosive force dissipates quickly, though shrapnal pretty much goes forever.

For nuclear bombs propelling a spacecraft , its an action-reaction engine just like a regular rocket, but it pulses. The nuclear matter is sent in one direction VERY quickly by the explosion and the spacecraft goes the other way.

 

[marcus]

Originally posted by Rockazella
What creates an explosion? I used to think it had to do with large amounts of heat energy being released very quickly. The heat would then cause air to expand very rapidly and thus you get your explosion and shock wave.

This explanation would mean that explosions couldn't really exist in space (no air or any gas).
However I read an article on nuclear explosion propelled space craft, and now I'm not so sure I have explosions correct. The article said the spacecraft could simply harness the explosions with the use of a large parathute type thing. If this is true, what's really involved in an explosion?

with several ordinary chemical explosives like trinitrotoluene and black powder the explosive material is formulated to release a large volume of gas (as well as heat)

TNT is C7 H5 N3 O6 (this proportional formula does not give structure)

You can imagine it yielding a lot of CO, and N2 and stuff.
one big molecule breaking up and yielding a great many small molecules of gas occupying a large volume---big increase of entropy

The TNT explosion is not caused by heating surrounding air so much as by suddenly producing a large volume of hot gas
consisting of the chemical results of the molecule breaking up.

Black powder does something analogous. It produces CO and CO2 and N2 and stuff like that by a reaction between
KNO3 potassium nitrate
C charcoal
S sulfur

The sulfur is there to give the potassium something to do
after it gives up its NO3 nitrate.
The C is there to take some of the oxygen from the freed-up
NO3 and make gas molecules (CO and CO2)
and the N is there to make N2 and other gas molecules.
These things are mixed in correct proportion to make the most
amount of gas. So that is oldfashioned gunpowder.

Again, it is not heating the surrounding air so much as making gas, which will be hot because of the reaction heat (as in the case of TNT).

The volume a gas occupies depends on the number of molecules---so an explosive designer should try to have the results be a lot of small gaseous molecules

Explosives should work fine in space for many of the same purposes. You do not need heated air for many purposes that
explosives are used for.

NUCLEAR fireballs are plasma. The plasma takes the place of the hot expanding gas from a chemical explosion. To a first approximation the fireball can be treated as a "gas" of electrons.
I believe that thermodynamically the electrons are more important than the nuclei in describing the fireball. Even the metal casing becomes plasma. The result can be thought of as an expanding ionized gas. There have been schemes to propel rockets by a series of nuclear explosions at least since the 1950s (Freeman Dyson was an early proponent).

This is only a very partial and un-expert answer, but maybe more information will come in this thread.

Personally I do not like that nuclear explosion space propulsion idea because it is scary. I believe cleaner more controlled schemes can be developed.

 

[Rockazella]

Thanks russ and marcus.

with several ordinary chemical explosives like trinitrotoluene and black powder the explosive material is formulated to release a large volume of gas (as well as heat)

So does the expansion rating of different explosives have to do with the rate they create gas?

 

[marcus]

Originally posted by Rockazella
Thanks russ and marcus.



So does the expansion rating of different explosives have to do with the rate they create gas?

I must be cautious since not an expert in explosives. Russ or some of the others may supply an answer. I would check the Encyclopedia Britannica "Explosives" article. I just looked in my old (1970) Britannica and the article on explosives is fascinating. But I cannot summarize it for you. It seems that several things effect the rate at which the chemical reactions proceed, and
influence the explosive properties.

An interesting figure quoted for various high explosives is the
speed with which the detonation wave passes thru the material.
For TNT this speed is a few thousand meters per second depending on the density. For one type of blasting jelly about half again as dense as water it was given as 8000 meters per second. So the gas and heat-producing reaction travels very quickly through the charge and the speed it travels seems to influence the effectiveness. For some applications a slower detonation may work better etc etc. too much to summarize.
Interesting question though.

 

[russ_watters]

Originally posted by marcus
I must be cautious since not an expert in explosives. Russ or some of the others may supply an answer.

Heh. Not likely. I didn't get far enough with chemistry to know much about reaction rates.

 

[Alexander]

Rate of chemical (or nuclear) reaction is basicly set by probability of penetration via barrier separating two local minima of potential energy (first one U1 for the reactant(s), second U2 for the product(s) of reaction). If a barrier is big and fat (thick), reaction is exponentially slow (exact speed can be found by integrating barrier's fatness and multiplying obtained probability by rate of bounce of reactant particles inside potential well (this rate as well as barrier heigth depend on temperature, by the way).

In case of very transparent barrier (shallow and slim = unstable reactant) the rate becomes simply sqrt(2(U1-U2)/m), where m is average mass of product of reaction (molecule or atom in case of chemical reaction and atomic parts (whatever product is) in case of nuclear reaction. Usually this last number is used to estimate final speed of product of reactions, because as soon as reaction starts, temperature rized and barrier becomes quite transparent for most explosives. Thus a reactant with highest energy content U decomposing into final product(s) with lowest mass gives you most speed of your bang.

Note here: not most energy, but most speed.

Higheest energy (per say, kg of original reactant) is different requirement - and usually high-speed-optimized explozives are far not most energy efficient. Slow burning fuels (oils) are usually most energy efficient, even when you add mass of oxygen required to decompose them.