HEL THREATS ARE HERE: ARE WE READY?
By Kyle Brady
Seemingly mere moments after the invention of the acronym “L-A-S-E-R” by Gordon Gould in 1957 and the demonstration of a working ruby laser by Theodore Maiman in 1960, U.S. and Soviet weapon technologists began exploring the idea of using lasers as a weapon to combat missiles and other aerial threats. Between 1965 and the end of the Cold War, Soviet scientists pursued both gas and solid-state laser technologies with the explicit aim of engaging ballistic missiles in their terminal stage. Parallel U.S. research efforts over the same period resulted in the development of numerous prototype chemical laser systems with progressively higher output powers, culminating in systems such as the megawatt (MW)-class Mid-Infrared Advanced Chemical (MIRACL) laser and other powerful systems. (For more information on these and other laser weapon development programs, readers are encouraged to consult Cook  or a similar comprehensive history.)
Though the threat of nuclear conflict—and the commensurate need for antiballistic missile defense—waned with the fall of the Berlin Wall and the subsequent dissolution of the Soviet Union in 1991, laser weapon development has not abated. Indeed, with the more recent proliferation of missile technologies in various conflict zones, as well as the genesis of cheap and capable commercial-off-the-shelf (COTS) drones, laser weapons have garnered substantial and increasing interest in the defense community.
Recent attacks such as those on the Abqaiq refinery  have also highlighted the limitations of traditional air defense tools against such asymmetric warfare tactics as drone and rocket or missile attacks—namely, the low cost of implementing such attacks relative to the high cost of the antiair/missile systems currently employed to stop them puts the defender at a significant disadvantage.
But laser weapons offer the potential to flip that equation, with estimates of the cost to employ such systems as low as single-digit dollars per shot.
In recent years, there has been significant interest in directed-energy weapons (DEWs) in general and high-energy lasers (HELs) in particular. Systems such as the (estimated) 30-kW Laser Weapon System (LaWS) (shown in Figure 1) have been tested against small boat and drone targets and integrated on Navy vessels such as the USS Ponce for point defense . Follow-on efforts have also been deployed, including the Solid State Laser Technology Maturation (SSL-TM) laser, which is reported to be in the 150-kW range and currently in use on the USS Portland .
Other planned projects—such as the High-Energy Laser with Integrated Optical-dazzler and Surveillance (HELIOS), which is intended to be deployed on an Arleigh Burke-class destroyer—are also being prepared for integration into shipboard systems in short order . Still others, such as the Self-Protect High-Energy Laser Demonstrator (SHiELD) (shown in Figure 2), aim to develop aircraft-mounted HEL systems as an active missile defense system . As these integration activities proceed, laser power is only expected to increase, thus increasing the effective range and/or limiting the time required to generate damage.
The United States is, of course, not alone in noticing the potential of laser weapons. Though much of what is known about ongoing foreign development is classified, media reports indicate a high level of interest from near-peer competitors in the area of HELs for multiple applications [5, 6]. These weapons are purportedly of similar powers as current or developmental U.S. systems and are expected to be employed in a variety of critical military roles, including anti-access/area-denial (A2/AD), terminal defense, counter-unmanned aerial vehicle (UAV), counterspace, and gray-zone warfare.
Concern over the growing laser threat from adversaries is not just a hypothetical exercise. As evidenced by low-energy laser attacks conducted in Djibouti in 2018 and similar follow-on attacks in the East China Sea , laser systems are already being deployed against U.S. forces to harass and deny access. In the event of a wider conflict, more powerful laser threats can be expected to form an important part of integrated air defense systems, adding a completely new dimension to missions in highly contested airspace.
THE NATURE OF THE THREAT
As a group, DEWs—including both high-power microwave (HPM) and HEL weapons—pose fundamentally different survivability problems compared to traditional kinetic energy (KE) weapons, such as missiles, antiaircraft artillery (AAA), and other projectile-based threats. The underlying physics associated with a KE threat are primarily mechanical in origin: severance, mechanical deformation, buckling, shearing/mechanical material removal, etc. With HELs, however, damage is caused instead by thermal mechanisms, including melting, sublimation, and combustion/ oxidation. As a result, the damage prediction tools developed for modeling KE threats have little to no direct applicability to HEL weapons.
Moreover, with high-quality electro-optical/infrared (EO/IR) and beam director systems typical of HEL weapons, these systems are trainable—that is, they can be directed at specific areas on a target to generate the desired end effect—and redirectable in real-time during lasing. In contrast, KE-based weapon effects are largely stochastic in nature; once the weapon is fired, there is little to no ability to control how the weapon interacts with the target. The result is that the nature of the analysis changes from one of probabilities—identifying the likelihood that a bullet or fragment trajectory of sufficient mass/velocity will intersect with a given component—to one of time-to-effect.
While this distinction may seem minor, from a modeling and simulation (M&S) perspective the difference entails a fundamentally different solution approach. KE weapon damage vulnerabilities may be modeled as a discrete, probabilistic event that is essentially independent of time, with damage determined from the spatial overlap of a threat function and the target geometry. However, HEL-induced damage accumulates over time and is thus subject to any number of time-dependent processes, such as heat transfer, relative motion between weapon and target, changing transmission medium or beam properties, changing target absorptive properties, and more, requiring a solution method that estimates damage at every time-step. Damage potential is therefore limited not by a static relationship between the threat function and target but by the overall engagement time, a quantity that changes dynamically based upon the specific nature of an engagement. Put another way, the addition of time dependence blurs the line between the traditional definitions of susceptibility—the inability of a platform to avoid a particular threat—and vulnerability—the inability to withstand a threat.
THE NEED FOR PRACTICAL M&S TOOLS
Given the tight threat-target coupling involved in determining HEL vulnerability, there is a manifest need for end-to-end simulation tools that can accurately and efficiently model an entire engagement, from detection though endgame. This need is felt at all levels of the defense community. Research and development organizations must understand the nature of the threat and the impact of survivability and lethality technologies for their efforts to be effective. The acquisition community must understand the functional effects of technologies and have methods to generate a cost-benefit analysis to make informed decisions on requirements for current and future platforms. The operational community must understand how these weapons and platforms interact under real-world conditions to make decisions on how best to mitigate their effects and/or most effectively employ their own systems.
Answering these kinds of questions is, of course, predicated on the ability to efficiently and confidently model HEL threats at engineering, engagement, and mission levels with rigorously test-validated codes. The generation of such overarching M&S tools is predicated on the development of HEL damage models capable of accurately and efficiently predicting component and platform-level effects over time from a given laser engagement. In and of itself, the breadth of the damage mechanisms and underlying physics involved makes this a challenging problem, and one in which much of the physics relevant to traditional KE threats does not apply.
At its most basic level, HEL damage is a thermal effect, with the type and rate of damage defined by how the laser energy interacts and couples with a material surface. For example, upon illuminating an outer mold line (OML) surface (illustrated in Figure 3), the power of a beam of a given wavelength will be absorbed at, transmitted through, or reflected off the surface based upon the optical properties of the surface materials in question. These properties are not necessarily static; the relative proportions of transmitted, absorbed, and reflected energy may change dramatically as a surface is heated due to changes in the laser wavelength absorption coefficient.
As absorbed energy raises the temperature of the material substrate, heat transfer mechanisms—including conduction, convection, and radiation—become important in different proportions based upon the underlying material properties, physical dimensions, and other conditions around the illuminating spot. Erosion of the material—ultimately resulting in penetration—may be dictated by any number of processes, including melting, oxidation, sublimation, mechanical failure, and often a complex combination of several processes acting in parallel. Once a surface is penetrated—or even prior to penetration depending on the heat transfer modes—the laser then interacts in complex ways with any of a number of internal components, generating a multitude of potential failure modes, ranging from electrical shorts and signal loss to dry bay fires.
Outcomes are ultimately dependent upon the components illuminated, the power and energy deposited on the components, the materials and design of the components, and much more. As should be evident from the foregoing discussion, predicting the outcome of an HEL engagement rapidly becomes a highly complex, multiscale, multiphysics, time-based problem that must be approached differently than traditional KE threats and their more “immediate” damage mechanisms and effects.
STATE OF THE ART IN HEL M&S
Addressing this simulation challenge is further constrained by the computational demands associated with defining platform-level vulnerabilities. Defining system vulnerabilities typically requires thousands of individual laser “shots” at various aimpoints, approach angles, standoffs, and mission engagement scenarios. As a result, despite the physical complexities associated with any individual shot, the runtime must be severely constrained to make the overall computation tractable. Optimizing a balance between the competing needs of accuracy and speed therefore becomes of paramount importance to successful modeling approaches for HEL survivability.
Several codes of varying detail and fidelity exist for the modeling of HEL interactions with materials. At the highest level of fidelity, finite element codes such as ALE-3D are capable of solving complex, multiphysics problems, including heat flow, multiphase flow, chemical kinetics, and more. However, two limitations exist for such detailed modeling efforts.
First and foremost, modeling the underlying physics—and using a computational mesh sufficiently fine to solve such problems—is inherently time-consuming. For all but one-off specialized uses, or as a basis upon which to build engineering models, such approaches cannot meet the historical runtime criteria for platform-level survivability modeling. Second, these approaches rely upon an accurate subgrid-scale model of the important physics, and a detailed understanding of boundary conditions to be reasonably predictive. In practice, much survivability-relevant testing must, of necessity, be performed under conditions that do not conform to idealized laboratory conditions, making the experimental validation of these models challenging.
On the opposite end of the spectrum, engineering-level HEL penetration tools currently in use are able to reasonably match experimental material penetration data under some conditions, reducing the problem to one-dimensional heat transfer or explicitly setting the relationships between irradiance and material erosion rate. These kinds of reduced-order approaches yield fast, readily scalable models appropriate for platform evaluations. However, the former operation mode relies upon accurate prediction of material temperature—which may be influenced heavily by the degree and nature of power coupling at the material surface—while the latter mode is wholly dependent upon experimentally derived material erosion rates—which may be overly deterministic and unable to account for conditions outside of those tested in the experimental validation dataset. Both cases ultimately rely on extensive, high-quality, detailed test data to produce models with reasonable fidelity, and it is an ongoing challenge for these models to capture changing erosion behavior as a function of changing boundary conditions.
Left unaddressed in the preceding discussion, however, is the reality that material penetration itself is not generally a primary driver of system failure; system damage ultimately results from loss of function of internal components. While penetration of materials is often a prerequisite, failures of interest to lethality or vulnerability of a platform result from functional losses that adversely impact the ability to continue a mission or flight. As a result, in most cases the primary items of concern can be internal components, such as flight controls, fuel systems, avionics, sensors, etc., whose failure modes and damage mechanisms involve a great deal more physical complexity than solely material erosion.
An important example includes the ignition of an onboard fire due to HEL engagement of a fuel storage or conveyance component. In addition to the aforementioned complexities associated with material penetration, simulating ignition and fire requires a model that captures—at some fidelity level—the physics of the fluid release (leakage, rupture, atomization, etc.), fuel vaporization, heat and mass transport, and chemical kinetics. Accurate modeling of the combustion process is in itself an active and ongoing research topic that does not easily lend itself to rapid solutions; beyond its inherently multi-phase nature, prediction of limit phenomena such as ignition relies on highly coupled, “computationally stiff” chemical kinetic models that far exceed the computing limits imposed on a practical platform-level survivability tool.
Moreover, relevant experimental validation datasets for HEL-generated fires are few and far between, and they often focus on platform-level end effects rather than the underlying damage mechanisms, phenomenology, and limits. While these datasets are useful qualitatively, more detailed, research-oriented data are needed to support model development for survivability purposes.
Another informative example is the response of electronic components, whose HEL damage modes can include electrical shorting, thermal overload, and erroneous feedback in addition to complete output failure. While a penetration model may determine when a housing material becomes compromised, it is far from obvious how to determine which failure mode is likely, and at what laser powers and over what time periods. As with ignition and fire model development, detailed testing focused on actual—and functioning, wherever possible—components is critical to evaluating the various potential damage mechanisms and assigning reasonable, probability-based damage prediction curves to support survivability analysis.
One ongoing effort to begin addressing these modeling and testing deficiencies is the Test Resource Management Center-funded and PEO-STRI-managed Modeling and Simulation of Blue Aircraft Survivability to HEL Irradiation (MSAS) program. MSAS is a multiyear testing and model development effort, led by the SURVICE Engineering Company, aimed at generating fast-running and experimentally validated engineering-level modeling tools to support survivability analysis of HEL engagements on U.S. aerial systems (such as shown in the example in Figure 4). An overview of the data and modeling tools generated by this effort will be shared in a future article.
FAILING TO PLAN IS PLANNING TO FAIL
The risk to U.S. weapons systems from HEL threats is real. In fact, for some applications it is already here, with existing lasers powerful enough to dazzle and blind already deployed against military personnel and materiel worldwide. Though more powerful weapons capable of more substantive damage remain largely in the testing and development stage, the overt progress of both domestic and adversary weapon programs indicates that the day is rapidly approaching where such weapons will form a real and ubiquitous threat to aerial systems. Moreover, laser weapons are a fundamentally different threat than traditional ballistic threats, and the technologies and design strategies developed to protect against KE weapons are unlikely to have significant relevance when applied to a speed-of-light, thermal-damage threat. Accordingly, HEL vulnerability requirements and reduction methods cannot be considered a “nice-to-have”; they must be an integral part of any military system expected to function in contested environments. As new acquisition programs are developed, HEL vulnerability needs to be considered an integral part of system survivability from the start and built in at the initial design stage. Specific requirements will thus be needed for platforms and critical subsystems, and M&S tools must be built to ensure that research, acquisition, and operations in this area are informed by the best available data.
After decades of development, practical laser weapons are now upon us. The only remaining question is whether we are ready to respond.
ABOUT THE AUTHOR
Dr. Kyle Brady is currently a subject-matter specialist with the SURVICE Engineering Company, primarily focusing on HEL survivability and fire prediction and serving as the test lead for the MSAS program. Previously, he served as a research engineer supporting fundamental and applied combustion research at the Air Force Research Laboratory, where he focused on novel combustion concepts for gas turbine main combustors, inter-turbine burners, and augmentors. Dr. Brady holds a Ph.D. from the University of Connecticut.
Special thanks are extended to Messrs. Ron Dexter, Matt Perini, Bob Anderson, and Jim Tucker for their contributions to this article.
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