Dramatically reduce blast impulse and heat release

• Works in enclosed spaces such as buildings, vehicles and ships.
• Takes up little space.
• Made from readily available materials.
• No sophisticated manufacturing processes required.
• Easy to install with just hand tools.
• Low cost.
• Proven effective by multiple tests at an independent research institute.

Blast impulse is:

• momentum transfer per unit area from blast waves into targets (bar-ms, Pa-s, psi-sec and so on);
• the area under the pressure-time curve of a blast wave;
• the mechanism that knocks down barriers, destroys vehicles and hurls people against walls.

Blast effect mitigation means:

• reducing impulse and peak pressure;
• reducing and suppressing heat release from detonations and deflagrations, which can prevent post-blast fires (“keep cool and carry on”).

In the US, regulations set separation distances (D) from explosive hazards (Q for quantity). Q is often called net TNT-equivalent charge weight (NEW) or NEQ.

D is based upon allowable pressures suitable for the situation: separation from public highways and communities, between explosives manufacturing facilities, between facilities on military bases and so on.

Q/D constraints limit where magazines, aircraft “bombing up” operations and even military units disembarking in ports (in peacetime) can be near urban areas.

Impulse is the proper criterion, but these regulations assume a linkage between peak pressure and impulse. It is crude, but pressure is easier to measure.

Bottom line: blast mitigation reduces the net effective Q (NEQ, NEW), which under Q/D regulations would greatly expand usable military base areas and allow safe operation of energetic material-handling operations near highways, ports and resources.

Bonus: blast-mitigating containers may allow Hazard Class 1.1 (“detonating”) materials to be reclassified as 1.4 (“insensitive”), substantially reducing limits on public roadway and railway transportation use.

Reducing peak pressure is often easy. Reducing blast impulse is difficult.

Sir Isaac Newton was right: total energy and total momentum are conserved (reversible processes) when two masses collide. Pressure is reduced and velocity increases as fluids pass through nozzles and screens. Pressure recovers and velocity drops as the fluid passes into the surroundings (ambient conditions).

But friction, viscous drag and heat transfer (irreversible energy and momentum processes) extract energy and momentum from the fluid. Blast impulse reduction is possible through the artful use of every possible combination of irreversible processes and mechanisms.

Mechanisms for rapidly dissipating blast energy and momentum in granular media:

• Multitudinous collisions between granules disperse momentum.
• Phase change materials (PCMs) absorb energy (kJ) through the enthalpy of fusion or decomposition (ΔHm, kJ/kg).
• Aerodynamic drag [proportional to (velocity)²] due to huge specific surface area.
• Heat transfer from hot gas to granular media due to huge specific surface area.
• Interference heating (in gas flow velocity above Mach 8).*

*Interference heating is the breakdown of the boundary layer due to turbulent flow and shock wave interactions. Heat transfer across the boundary layer increases about tenfold.

Modeling blast mitigation requires a much greater experimental database. Parametric relationships of particle and shock wave velocities, impulse and temperature reductions, as functions of blast intensity, media density and thickness, total media mass, specific surface and granular media characteristic dimensions (and dimension range), cannot be established without more experimental data.

The optimum thickness or placement of PCMs requires much more experimentation, since packed beds of any kind (sand, powdered PCMs, granular two-phase media) increase impulse and reflected shock parameters.

Experiments by Halsey and Berry* showed substantial reductions of fragment kinetic energy when cased munitions (bombs and projectiles) packed in granular pumice (another volcanic foamed glass) detonate. Experiments must be performed to quantify kinetic energy reductions and obtain quantified parametric data to model and design protection means.

*Halsey, C. C. and Berry, S. L., Expanding Pumice Technology to the Protection of Missile Warheads from Sympathetic Detonation. Paper presented at the DOD Explosives Safety Board Explosives Safety Seminar, Las Vegas, NV, 20 to 22 August 1996.