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View Full Version : My understanding of detonation...


rjche
May 17th, 2003, 10:14 PM
There are many explanations of detonation, most of which are more confusing than they are useful. Here is the way I learned about detonation. If it is not entirely compatible with other theories, it sure works well in figuring out what to do to speed things up.


Chemical Reactions require an activation energy before they can occur.

Activation energy refers to the kinetic energy of motion of the molecules, and is in the form of varying velocity among the molecules of each reactant.

The requirement is that enough differential collision speed exist to cause breaking of bonds, thus freeing the bonded elements or radicals to impact and bond to other elements in the resulting "soup". Good explosives are compounds which release much energy in these swaps of bonding buddies. That energy is released as heat, radiation, and pressure.

Temperature measures the average velocity of any-thing's molecules. In reactions.

Higher temperature makes almost all things expand. This is because as the average kinetic motion of a reactant rises it is vibrating (colliding) more violently and thus recoils result more frequently and the average is thus increased (temp) due to more happening per second. That greater temperature shows up as an expansion of the material.

The vast majority of volume taken up by all compounds on the molecular level is empty space. It has been compared to the matter to space ratio of our solar system or universe.

Space can be reduced by great pressures which can resist the violent impacts of even hot reactants.

All materials can be compressed enormously with ENOUGH pressure.

A way to visualize this (which was given to me by an antique professor) was to view all the people in the football stadium at a big game. If they sit still in their seats they can form a rather compact crowd. If they all get into a fist swinging riot, the crowd will expand. If they start shooting at each other it will expand even more.

Since chemical reactions are controlled by temperature it is necessary that the initial reaction cause a large energy transfer and very fast. Thus heavy metal compounds ten to make good initiators, because metallic gasses (aka vapors) carry much larger heat loads than do other gasses.

If the starting temp is too low even high explosives will not explode with the usual initiators. (Even dynamite will not go off with a sizable booster at liquid nitrogen temperatures. LN temperature requires way too far for temperature to go to get to minimum activation temperature.)


Catalysts only help things react at a lower temperature and when they are present a different reaction is going on than when they are not. Thus they do not contradict the concept that all reactions are related to thermal, (kinetic) energy levels of reactants. Catyalized reaction rates also increase rapidly with temperature.

A rough rule of thumb for gut feels is that reaction doubles for every ten degrees C rise in temp. Raise some explosive from 20 C to 2000 C and the reaction rate increases by 2 to the hundredth power.

Therefore detonation is a thermal reaction. The brisance of a detonation is related to how fast the heat caused by the initial reaction can spread through the medium. That heat energy can propagate via pressure waves, by hot gas traveling, or by radiant radiation (due to high temperatures) shineing through some explosives which are transparent to the wavelengths radiated by the reaction.

Radiation travels with the speed of light.

Pressure travels at the velocity of sound in the material AT THE TEMPERATURE of the pressure wavefront. Things get fuzzy here for the pressure is entering cold uncompressed explosive, and temp does not rise until pressure compresses the cold explosive enough to raise temperature above the minimum reaction value. Yet the velocity of sound in such detonations is very large due to the high temperature.

Because pressure can transfer much energy, which turns to head when it compresses things, many lower explosives with air bubbles in them tend to have more brisance than solid ones.

The pressure compresses the air bubbles raising their temperature to light emitting temperatures. Thus around every bubble both radiation and conduction heat transfer occurs, and all material close to that bubble begins exploding.

It takes a large booster to have enough energy to sustain the pressure wave throughout the mass of such explosives, and small initiators just cause some reaction close around them.

Radiation is not all that important in chemical explosives but it is a major transfer method in nuclear explosives, where it exhibits its ability to cause great pressure same as vibrating molecules do in much lower temperature explosives. It also travels at the speed of light, so nearly instantaneous initiation of a whole mass can be accomplished.

A pressure wave can compress even solids at the pressures of high explosives, and any compression of a material causes a temperature rise. As the colliding molecules or atoms are forced into a smaller space their average number of impacts rises (temperature) because they don't have to go as far to hit something next to them.

We have seen this when we let a locomotive run over a penny to make the oval pendant kids used to make. The penny was hot enough to burn fingers if the engine alone ran over it so you could get it seconds after it was squashed.

(If the train was long and many wheels rolled over the penny, all you got was a small sheet of copper foil unless it fell off the rail early. This hot penny thing was in spite of the enormous heat conductivity between the copper penny and the steel rail at the contact pressure existing).

Thus some high explosives which are entirely absent any air cavities in them can explode violently due to high temperatures caused by the pressure wave alone passing at very high velocity because of the very high temp and pressure of the initiating explosion. If the temperature rise is above the activation energy of the explosive then the whole thing detonates a the detonation velocity of the pressure wave that exists. This is typical of cast and liquid explosives.

Rolling dynamite cartridges was done by miners because it speeded up their explosion (brisance) and made smaller lumps of coal. Rolling the sticks caused lots of air bubbles to form in the gooey explosive. This was done just before they were loaded into the holes.

Low explosives don't detonate because they never get to the pressures or temperatures which will cause their reactants to rise in temperature enough to detonate, from pressure waves or radiation, or air compression.

Gunpowder ground very fine, (not grained so flame can pass quickly throughout all of it) will burn rather slowly as in rocket propellant. Although it is fine, the flame cannot penetrate very far before it is all absorbed so things tend to go slowly.

Hope that helps non theoretical types to have a better understanding of the great mystery of detonation fronts in high explosives. Detonation is entirely different than burning fronts that exist in low explosives.

As an example a line of grained black powder laid on the ground and lit will cause the reaction zone to travel a few inches a second.

A line of petn laid on the ground and initiated will cause the reaction zone to travel about 240,000 inches a second.

As actor Candy said when that bald headed grizzly bear chased him into the cabin, BIG BIG REAL BIG.

That also is a good description of the difference between deflagration and detonation.

(Deflagration is another word for burning)

Gas vapors in your car deflagrate, unless you put too cheap a gas in your high compression motor, then under heavy load the burning changes to detonation, and raises hell with the engine if allowed to go on long.