How big is the bang?
Bombs haven’t gone away. They remain a weapon of choice for terrorists and other criminals.
When protecting people, assets and events from bombs, one of the biggest challenges is developing an accurate understanding of ‘just how big the bang will be’.
Preparing for and managing a blast requires that one first and foremost develop an understanding of explosive effects, both in terms of blast and fragmentation. This enables the prediction factors such as:
How far the blast might travel
Which areas around the blast will be most vulnerable
Which critical assets might be at risk
How the blast might impact the surrounding areas.
What, if any, physical strengthening may be required and so on.
Suppose your analysis of the available information indicates that protection is not possible. In that case, one’s planning must inevitably move to the things which can be controlled, such as post-blast planning, business continuity and resilience, emergency repairs, HR support and other related issues.
To understand blasts, one must first know that an explosion is a complex mix of gas and fluid dynamics, reflections and immense pressure waves travelling at great speed. Traditionally, the only people who truly appreciated the effects of blasts were users of explosives (and their supporting scientists), who sometimes utter the phrase “well, that was too close!” The inability to understand actual explosive effects in a ‘real-time’ and ‘real-world’ environment has always been a problem.
The requirement for security, safety and emergency managers to have a realistic understanding of explosive effect is manyfold. For example, where are people able to enter your site or event while carrying items? With that information in mind, consider the following: for an average person, a weight of approximately 5 kg can be carried in an outstretched arm. 10 kg can be carried by the side of the body and 20 kg can usually be carried in two arms. Anything heavier will usually be transported on wheels. If there is a critical part of the site, what happens if 5 kg of explosives detonates near that location? What about 10 kg? Is there a need for a policy that states nothing bigger than a briefcase or handbag (say 5 kg) can be brought into the area?
If there are public access areas where literally anything can be brought in, what might be the effects on the surrounding work areas if 20 kg of explosives were to detonate?
The ability to test the effects of vehicle bombs on the structure around a potential target site may lead to changes in where public or employee parking is allocated.
The ability to test emergency procedures, evacuation routes and assembly areas against realistic explosive events is of obvious benefit. For example, through testing, one might discover that an evacuation route, although out of sight from the bomb, is not the safest path. Practising bomb threat assessment, including search capability and the responses to unidentified items, or finding a bomb on-site with a realistic indication of the effects of a detonation, will improve safety.
Understanding explosive effects also help identify areas where protection and prevention are not possible and where there needs to be a reliance on engineering solutions and post-blast measures.
Some years ago, the concept was conceived of using radio energy to indicate blast effects. Despite being two very different forms of physics: electromagnetic energy verses the hydro-dynamics of blast, there are similarities that could be explored as well as differences:
- Electromagnetic waves travel in straight lines and unlike blasts, will not flow over objects.
- Some frequencies will pass through thin materials, reflect from solid ones and reflect around corners as does blast.
- Electromagnetic waves dissipate based on a square root basis whereas blast decreases in accordance with a cube root rule as the moving wall of compressed air slows and returns to ambient pressure.
- The effects of an explosion are dependent upon the amount, type and confinement of the explosive material. The effects of a radio transmission are dependent on the strength and frequency of the signal.
- Electromagnetic signals reflect immediately and do not build up an increased pressure against a surface in the manner of a blast wave. As a result, there is no indication of reflected pressure.
- Electromagnetic signals travel at the speed of light, blast travels at a greatly reduced speed. The differences will not be observable.
- There is no replication with an electromagnetic wave of the blast wave’s shock front.
- The ability for an electromagnetic wave to penetrate a wall does not represent structural damage, which means buildings do not collapse.
- An electromagnetic signal does not generate fragmentation.
- An electromagnetic signal has no propulsive effect and does not simulate the manner in which a human body can be accelerated and projected resulting in impact-related injuries.
- An electromagnetic signal does not simulate the negative pressure phase of an explosion.
Despite the differences between the types of energy, there was confidence that it might be possible to emulate one using the other.
The criteria for a ‘blast emulation system’ was identified as:
- indicate blast injury and structural damage at various distances related to selected explosive type and weight;
- be Omni-directional;
- penetrate thin walls and materials;
- be reflected by strong walls and materials;
- be reflected around corners;
- flow over and around items using the principles of hydrodynamics;
- operate out to at least a distance equivalent to the injury distances for (say) 100 kilograms of TNT (trinitrotoluene);
- be simple to operate;
- be non-hazardous to store, transport and operate;
- be deployable in a wide range of physical environments;
- be easily transportable across jurisdictions; and
- have a low cost of procurement and maintenance.
A prototype system was developed against the criteria. The only criterion not met was the ability to “flow over and around items using the principles of hydrodynamics”. In addition to the stated criteria, the developed system provides an indication of fragmentation injuries and the maximum charge weight was increased to 20,000 kg.
The system consisted of a transmitter and a number of receivers. The transmitter is positioned where the exercise controller determines an explosive device could be placed. The appropriate explosive type and charge weight is selected from the onboard menus of 25 explosives and weights from 0.1 to 22,000 kg.
The receivers are issued to participants and positioned at critical points or areas of concern.
Use of the globally available WiFi frequency of 2.4 GHz results in a signal which can pass through thin materials, be reflected by solid ones and reflect around corners thereby providing a reasonable indication of blast and fragmentation effects. This effect is recognised by everyone who loses their WiFi signal when passing behind solid objects. This frequency also means the system can be used wherever WiFi is permitted.
Safety was a critical element of the design including:
- The system transmits at low power (~40 milliwatts) on the globally available WiFi frequency of 2.4 GHz.
- The system cannot be used as a firing system.
- The installed Sealed Lead Acid (SLA) and Lithium Polymer (LiPo) batteries are approved for carriage on aircraft. Users should confirm local requirements.
- The system can be used wherever WiFi devices are permitted.
The details of the event and which receivers indicated which levels of damage is backloaded to the transmitter for later downloading and analysis by the scenario controllers.
The Home screen shows the calculated maximum distances for blast lethality (Red LED = lung damage), blast injury (Orange LED = ear damage) and fragmentation strike of greeter than 79 joules (Blue LED). Variations between the calculated injury distances and the distances shown by the receivers reflect the reality of the built and natural environments. Barriers and reflecting surfaces will change both blast effects and RF signals. If a person is facing away from the seat of the explosion and whether the person is breathing in or out at the time will alter the blast injuries received particularly ear, lung and other soft tissue damage. Similarly, if the person wearing a receiver is facing away from the transmitter and thereby shielding the receiver the indicated damage may vary compared to a person facing the transmitter.
The system can be used to indicate damage to structures from an explosion. If the structure does not fail immediately when it is struck by the initial peak incident pressure the pressure will continue to build until either the structure fails or the reflected pressure is reached. The Orange LED indicates at ~ 35 kPa peak incident pressure which roughly equates, depending on a range of variables and assumptions, to a reflected pressure of >70 kPa which exceeds the point at which structural failure will occur.
Some consideration of the scenario is required to determine the probable effects of an explosion at the nominated site. A small charge may cause significant damage to surrounding fixtures and damage nearby walls and façade but is unlikely to cause the destruction of an entire building. A large explosive device, if it impacts enough structural elements may cause the demolition of the building. Additional engineering and blast modelling will be required to determine the exact failure modes based on the results of the scenarios.
It is stressed that a system using RF to emulate blast effects is not intended or designed to indicate minimum safe evacuation distances from potential explosive effects. The evacuation distances must align with the site’s emergency instructions. If the site’s bomb safety evacuation distances are not known, as a minimum it is recommended that an evacuation distance of 100 m and behind something solid should be considered with greater distances applied as required.
Because the system uses radio frequency to indicate hydrodynamic effects it is not an exact replication, hence the use of the term “emulation”.
The lack of realistic indication of what a bomb actually limits training, planning, emergency practices and structural and security assessments. The design and development of a system to emulate explosive effects in a real-time and ‘real world’ environment provides responsible managers with a capability that enhances training, operational planning, emergency procedures and practices, blast assessments and hence safety.
Additional information including technical details can be obtained from Services@layer3servcies.net.au or from the author.
Donald S. Williams CPP. Don Williams served as an ammunition technical officer for 20 years. He holds qualifications in Security Management, Security Risk Management as well as Project and Resource Management and is a Certified Protection Professional. He is a long-term member of the Institute of Explosive Engineers, ASIS-International, the International Association of Protective Structures, and is a Distinguished Life Member of the International Association of Bomb Technicians and Investigators. He is the author of over 130 publications relating to bomb safety and security, emergency management and related issues. He is a Director of Layer 3 Services. dsw57@internode.on.net .
