Game Development Reference
In-Depth Information
Some Explosion Terminology
Before we burrow into the physics of explosions, it's helpful to spend a little time defining some
explosion terminology.
•
Autoignition temperature
: The temperature at which an explosive will explode without
an external ignition source. As an example, the autoignition temperature of kerosene is
463
K
(374
o
F
).
•
Blast wave
: A region of high pressure that expands radially away from the point of explosion.
•
Flash point
: The minimum temperature at which the gas above a liquid fuel will ignite in
the presence of a flame source. The flash point temperature is lower than the autoignition
temperature.
•
Heat of explosion
: The theoretical amount of energy that is released during an explosion.
Under the SI system of units, heat of explosion will be in units of
J
/
kg
.
•
Overpressure
: The difference between the pressure inside the blast wave and the ambient
air pressure.
Explosion Basics
An explosion as defined in this chapter is a sudden release of energy. The energy released
might be the result of a chemical reaction, nuclear fission, or some other process. Explosions
happen in very short time durations, typically in thousandths of a second (milliseconds). Gas
generated by the explosion expands rapidly in every direction from the
point of explosion
. The
rapidly expanding gas pushes into the stationary gas in front of it, causing a region of high pressure
known as a
blast wave
. The blast wave expands outwards at a very high velocity, oftentimes
greater than the speed of sound. The blast wave loses energy quickly as its distance increases
from the point of explosion. The difference between the blast wave pressure and the ambient
air pressure is called the
overpressure
of the blast wave.
Because the blast wave expands outwards so rapidly, behind the blast wave is a region of
low air pressure. This low-pressure region “sucks” the air along with it, causing a wind that
initially follows the blast wave. As the blast wave continues outward, the relative pressure in
front of and behind the blast wave changes such that the direction of the wind can reverse
direction, and for a time it can blow in towards the point of explosion.
Explosions can also produce light, noise, and heat. Depending on the type of explosive,
there may also be a fireball associated with the explosion. Most of the damage by an explosion
to people, vehicles, and structures, however, is done by the overpressure in the blast wave.
Recall from Chapter 3 that the force exerted by a pressure is equal to the pressure multiplied
by the surface area on which the force acts. A blast wave with a relatively small overpressure
(small compared to the atmospheric pressure) can exert a crushingly large force on an object it
slams into.
When modeling the effects of a blast wave, as shown in Figure 13-1, keep in mind that the
force vector from the explosion acts radially from the point of explosion. If the explosion is at
