Your questions answered: laser weapons
Our experts give their views on the future of directed energy weapons by answering questions from Engineer readers.
Laser weapons are about to become reality, with the US Navy recently demonstrating such a system that can shoot drones out of the sky, and announcing plans to deploy a laser onboard a ship in 2014.
We’ve seen lasers used previously as an anti-piracy measure to dazzle would-be attackers, but now so-called directed energy weapons are likely to become a more familiar sight in warfare as defence companies around the world prepare to introduce their own versions.
Directed energy weapons (DEWs) are systems that emit energy without the means of a projectile, and can use visible light, infra-red or microwave radiation, with both lethal and non-lethal effects.
The weapons are said to be particularly useful for targeting large numbers of small, low-cost targets with high precision, and some estimates put the cost of each shot of directed energy at just $1.
Companies developing DEWs include US defence firm Raytheon, which began publicly demonstrating their weapons in conjunction with the US Navy back in 2010, and Europe’s MBDA, which last year demonstrated a 40kW laser that successfully hit airborne targets at a range of over 2,000m.
We put questions from Engineer readers to two experts in the field of directed energy weapons.
- Dr Mike Cathcart, a senior research scientist at Georgia Tech Research Institute in the US who specialises in directed energy technology and recently chaired the Directed Energy Conference in London.
- Dr J Doug Beason, author of ‘The E-Bomb: How America’s new directed energy weapons will change the way future wars are fought’ and a former associate laboratory director of Los Alamos National Lab.
Cathcart and Beason (who is also chief scientist and technology adviser for the US Air Force Space Command) were keen to stress their answers represented their personal views and not those of their respective institutes or the US government.
What advantages do DEWs they have over conventional weapons, and what disadvantages?
Mike Cathcart: Directed energy weapons are often described as electromagnetic weapons as they do not employ some form of projectile motion. As such, directed energy weapons do not require the computation of ballistic trajectories to direct the beam at the target. This simplifies the aiming process for directed energy weapons. Also, re-directing the beam to new targets can be rapid.
In addition, the energy for the DEWs is ultimately derived from the conversion of electrical energy into the electromagnetic beam thus no additional shells, powder, etc are needed to make the system operational. Another advantage touted for these systems is that the output power level can be controlled so that the same system can be used for a variety of applications and/or to create a range of effects on the target (e.g. dazzling a sensor to destroying the target).
Doug Beason: DEWs are being designed for use in situations where other weapons do not work well. Since DEWs cost more than standard kinetic energy or legacy weapons (such as rifles, guns and bombs), and are more complex, it doesn’t make economic sense to use DEWs unless other weapons simply can’t do the job.
DEWs have several key attributes: they deliver energy at the speed of light (near instantaneously: 186,000 miles/second); they deliver energy at an exact “line-of-sight” (you don’t have to correct for wind, and lasers are unaffected by gravity); and they have an “infinite” magazine (you don’t have to re-load bullets). These attributes may overwhelm cost considerations if nothing else can do the job – but again, they may not.
What effects do DEWs have on their targets? How would they affect living tissue?
DB: DEWs transmit photons – packets of energy in the electromagnetic spectrum – [at wavelengths that] typically interact with a target from the “outside in”. A laser’s energy is absorbed on the target’s surface, and depending on the level of absorption and reflection, the laser burns through a target, layer-by-atomic layer. Thus, when a laser interacts with a target, there is not an instantaneous Buck Rogers-like explosion, but rather (depending on the energy) a burn through.
MC: Laser radiation, as with any radiation source, can be harmful to living tissue. Eye tissue represents the most sensitive areas relative to laser exposure. Laser safety has been an inherent part of laser development since the invention of the laser. As a result, there is a well-established set of protocols relative to operating lasers in a safe manner. That said, high-energy lasers used in a weaponeering application present a challenging operational issue. At high energy levels, terms such as “eye-safe” region have no meaning or application.
High-energy laser systems typically refer to lasers with several tens to several hundreds of kilowatts of power (or higher). At these power levels, even a very small percentage of absorption will result in a substantial amount of energy being deposited into the tissue and thus damaging the tissue, even if the laser wavelength is one traditionally deemed as eye safe. Nonetheless, contractors and government agencies have developed procedures to enable the safe and controlled application of these systems.
What are the engineering challenges involved in developing this technology for use in warfare?
DB: The primary challenge in developing a high-power laser is to produce the large amount of energy needed to efficiently power the laser. For example, a strategic laser needed to shoot down ballistic missiles has to deliver on the order of 1MW of power to the target. Because of atmospheric absorption, scattering, and other losses that occur when the laser energy propagates from the laser source to the target, the laser source may have to transmit up to 10 times that amount of power (10 MW); and since lasers are typically 10% efficient then it could take as much as 100 MW to ultimately deliver 1MW to a target. In practice, this worst-case scenario does not happen because of advances in adaptive optics, beam control, cooling technology, and frequency-tailoring techniques
MC: The early challenges related to the development of higher energy sources as well as the total size, weight, and power requirements of these systems. Many of these issues have been overcome in the last several years, though efficient thermal management remains a critical system component issue (directed energy weapons typically have low electrical efficiency). Current critical issues reside in the areas of system integration and component improvement. For example, fitting the DEW system on a particular vehicle may generate platform integration challenges. From a technology perspective, development of more efficient sources remains a high priority as well as efficient beam handling systems.
How effective would metallic/ceramic armour or specially designed surfaces that reflect certain light wavelengths be against a laser weapon? What other countermeasures could be used e.g. jamming?
DB: There are certain materials and coatings that may be used to reflect laser energy, but these are typically only good for a certain set laser wavelength. These countermeasures also add weight to the target: for example, if a missile were to be covered in a specially built metallic or ceramic case to reflect laser radiation, it would partially reflect laser weapons, but it would also dramatically reduce the range of the missile, and that would defeat why the missile might be used (because of the additional weight). Also, if a different laser wavelength is used against the target than what the armour or coating is designed to protect against, it may or may not reflect the laser as intended.
These trade-offs might work or they might backfire. Even polishing a target to have a highly reflective surface may or may not work – nothing is 100% reflective, and even if a reflectivity of 99.99% is obtained, then a 0.01% absorption of a 1 megawatt laser still results in 100 joules of absorbed energy per square centimeter per second – enough to first damage sensors, then to buckle metal if it was held onto the target for a long-enough time.
The usage cost of directed energy weapons has been projected as being as low as $1 per shot, but how do the development and manufacturing costs compare to more conventional weapons and how far will this limit their use?
DB: The costs of $1 per shot does not take into account the cost of the NRE (non-recurring engineering – or sunk costs that will not occur again), but conventional weapons also do not take NRE into account when comparing costs. It’s tough to do a one-to-one comparison, and I suspect that you can come up with a favourable cost estimate for whichever weapon system you prefer.
What range do current systems have and, if they were to miss their target, how far would the beam travel before it loses its destructive power?
MC: DEW systems have been proposed and are in development for a wide range of defence-related scenarios covering strategic scenarios such as ballistic missile defence to tactical scenarios such as ground-based air defence. The range at which the system loses its “destructive” power will be highly dependent on the actual system.
DB: Strategic lasers such as that demonstrated on the Boeing Airborne Laser testbed aircraft [which successfully] have a destructive range greater than 100 kilometres; tactical lasers, such as those projected for use on Navy ships, have less range.
What risks would laser weapons carry in terms of collateral damage of buildings or civilians by continuing through or past their targets?
DB: Because lasers are unaffected by wind and gravity, they travel in a perfectly straight path and do not spread out much – the technical term is that they undergo diffraction-limited propagation along a geodesic. Thus, laser weapons are extremely accurate, and are therefore not classed as indiscriminate area weapons (such as HG Wells’ Martian “War of the Worlds” DEW weapons) – as such, they are inherently defensive weapons. Furthermore, they are most effective when shooting up (from a plane to a missile, or from a ship to a drone), further limiting collateral damage. This is because the atmospheric density decreases in altitude, lessening the effects of scattering, absorption and refection.
Do current systems fire continuous waveform beams or single pulses?
DB: Most lasers are pulsed, and many that appear to be continuous wave (CW) lasers are actually pulsed lasers with the pulses occurring up to several million times a second. An advantage to having a pulsed laser is that it may impart more impulse (force per unit time) than a comparable CW laser.
Are particular wavelengths of light favoured and why?
DB: Typically lasers that are not absorbed by water vapor and not absorbed nor scattered by atmospheric molecules are favoured. These include lasers in the near-infrared (approximately a micron or a millionth of a meter in wavelength), among others. For example, the Airborne Laser’s COIL – Continuous Oxygen-Iodine Laser – wavelength is 1.315 microns.
How scalable, both up and down, is the technology, and how will this affect its potential usage?
MC: Inherently, the power output of a directed energy system can be controlled to a specific level. That has been one of the primary advantages touted for these systems. Several, current high-energy laser architectures allow a wide range of power outputs to be generated from the same laser weapon system. A weapon system with the ability to cover a range of applications would offer great potential in defense and security applications.
What would be the engineering challenges in developing handheld laser weapons and how likely is this to happen based on current technology?
MC: Handheld laser “weapons” have been developed in the past but proved unwieldy. Current technology developments in laser sources make the development of newer handheld systems more practical. I am not aware of any program of record that is developing such systems though.
DB: The biggest challenge to producing a handheld laser is packaging enough energy in a small enough volume so that it produces a useful weapon. Another problem is making that laser highly efficient so that it doesn’t dissipate too much waste heat.