by David Weber
Weapon Effects and Armor: How Does a Weapon Damage the Target?
The most important thing from the armor designer’s perspective is what the beam does to the target. One might define a modern space-to-space weapon as a device which deliberately changes the material properties of a distant spacecraft in a way undesirable to that spacecraft’s owners. Most every anti-ship weapon damages a target by focusing some type of photon beam onto it. Beams disrupt spacecraft systems by breaking up the molecular structure of those systems so that they no longer perform as designed. When these beams consist of light of a single color, they fall under the archaic heading of “laser” beams. This term was originally an acronym standing for Light Amplification by Stimulated Emission of Radiation. Only some classes of modern beams rely upon the “stimulated emission” principle and the bomb-pumped laser is one of them. Understanding how these beams damage targets requires detailed knowledge of how the individual photons interact with atoms in the target. Only a cursory summary can be made of this rich field. Interested readers should consult an introductory radiation hydrodynamics course for more detail. Contact the author for several excellent resources.
Three factors conspire to predict beam behavior in a target: the wavelength of the beam photons, total beam energy delivered by all those photons to the target, and the rate at which that energy is deposited. Combined, these allow one to predict how deeply the beam will penetrate different materials, how much of the target’s molecular structure will be disrupted, and what kind of shock waves will result.
The author begins with the photon wavelength. One might as well use frequency or energy because they are all mathematically equivalent but weapons designers fairly consistently use wavelength. Early space energy weapons used photons in the ultraviolet, visible, infrared, and even the radio range. These wavelengths are impractical to focus at contemporary combat ranges so modern weapons use shorter wavelength photons in the X-ray to gamma ray range. Indeed, modern space weapon lasers are so commonly X-ray lasers that the term “laser” is generally synonymous with “xraser” in naval parlance. Their rarer gamma emitting cousins are called “grasers.” Both of these words have their obvious origin with the ancient “laser” though the fact that many such weapons do not operate on the principle of “stimulated emission” is generally forgotten. Confusion sometimes results because different scientific and engineering communities have different definitions of exactly what constitutes the cutoff between X and gamma rays. An astronomer’s X-ray might be a particle beam engineer’s gamma ray and so on. There appears little hope at this writing of ever clearing this up completely. This article uses the terminology of the Interstellar Association of Astronautical Engineers that a photon with a wavelength greater than one picometer (10-12 meters) is an X-ray and light with a wavelength shorter than that is a gamma ray. This value was chosen because one pm is a good cutoff point when discussing armor and weapons effects. This is because light begins to exhibit deep penetrating characteristics in common spacecraft materials for wavelengths shorter than this so that a graser cannon operating at 0.1 picometers damages a target in different ways than a laser at 10 picometers.
Weapons designers prefer shorter wavelength photons because they are (up to a point) easier to focus at the ranges of modern beam weapons. Shorter wavelength photons also tend to penetrate more deeply into armor and deposit into denser spacecraft structures like impeller nodes, fusion reactors, and weapon mounts. If preferred, the reader can imagine the shorter wavelength packets of energy in a graser beam “slipping” in between atoms in less dense materials to penetrate more deeply into the target and hitting the more closely packed atoms of more dense materials. Shorter wavelength photons also (again up to a point) deposit more energy and thus do more damage to any structure they hit. Needless to say, any characteristics which make short wavelength photons the friends of weapon designers do not to endear them to armor designers. The downside to shorter wavelength photons is that they tend to be harder to efficiently produce than their longer wavelength siblings. That means less energy on target for a given amount of input energy from the fusion reactors for ship mounted weapons or the nuclear device for bomb pumped ones. This is one fundamental physical reason why graser mounts are usually best practically mounted only on heavy cruisers or larger.
Wavelength is a microscopic property of the beam. We now turn from the microscopic to the macroscopic beam properties. Each of the countless photons possesses a tiny quantity of energy and all beam photons flow together at the speed of light to their target. Adding up all of the energy of each photon in the laser beam gives us the delivered energy to target (DETT ). Dividing that energy by the amount of time that the beam pulse lasts gives the total beam power. A full treatment of how these combine with beam spot size on target, pulse time, and a variety of other factors to do damage requires much more time and space than we have here. Two useful generalizations can be made without extensive simulation. The first and most obvious is that high DETT does more damage because it vaporizes, atomizes, and then ionizes more of the target. The second and less intuitive fact is that very high-power beams (tens of billions of gigawatts or more) begin to produce unique shock effects in solid matter. Hence, in general, it is best for a weapon designer to deposit as much energy as possible into the target in as little time as possible.
Returning to our Mk-13 anti-ship missile example for specifics we discover that precise numbers are hard to come by without a security clearance. Publically available statements indicate that the Mk-13 uses one Mk-86 general purpose fusion warhead with a yield of 15 megatons pumping six Mk-73 laser rods to produce X-ray beams. This last is unsurprising since all practical bomb-pumped laser beams are in the X-ray range. The exact wavelength of the Mk-73’s Special Laser Material output is classified. Weapon characteristics are classified to frustrate enemy countermeasures, but they also frustrate attempts to understand and predict performance. What can be stated with some certainty from known physics is that the Mk-73’s beams probably have a wavelength around ten picometers. Total energy on target and laser power figures are not even hinted at in public and useful speculation is nearly impossible. Public speculation has these weapons depositing terajoules or petajoules into their targets at powers in the petawatt to exawatt or possibly higher range. Detailed computer simulations are required to fully describe laser penetration profiles and these simulations require a great deal of information about the beam and the target for accuracy. The best such simulations are, of course, classified. The author’s speculations are enough however to give a hint of what happens when a Mk-73’s laser beam strikes a target.
1.Ten-picometer photons are energetic enough to fully and completely ionize most common spacecraft materials. This means that all of the atoms in the area illuminated by the beam have all of their chemical bonds broken as their electrons are all stripped off. This is of particular importance for two competing reasons. First, ionizations generally consume massive amounts of beam energy without doing much further damage to the target. The material is ionized. Any structure made from it is destroyed after the first ionization, but the beam goes right on stripping electrons from the ionized matter as it rapidly expands away from the rest of the ship. Hence, materials which can soak up the most beam energy in ionizations usually make excellent armor. However, continuing to pour energy into the ionized cloud of target atoms can be useful from the weapon’s perspective. It happens that, once a target is completely ionized by the first photons in a short wavelength beam, it can become largely transparent to the rest of that beam. Called “bleaching” this phenomenon means that if one puts enough short wavelength photons into the target material, one continues to burn through it.
2.Most spacecraft materials are opaque to 10 picometer photons. That means that the X-ray photons in these laser beams will be absorbed within a short distance (perhaps one millimeter) of the target’s surface.
3.The massive terajoule or petajoule DETT of the Mk-73 render the weapon cap
able of completely disintegrating huge amounts of the target. Perhaps the following reflection will supply perspective: one source close to the author mentioned test firings of Mk-73 laser heads which punched holes clear through stony iron asteroids dozens of meters across.
4.Couple the above-mentioned high energy with petawatt to exawatt power and one gets beams that convert any matter into plasma that resembles stellar core material. Nonintuitive things happen here—solid matter flows and expands like a gas, thermal radiation from the superheated matter pushes radiation shock waves through the material at the local speed of light, and mechanical shock waves travel long distances carrying tremendous energy of their own. While none of these shocks carry as much energy as the original beam they often carry more than enough to shatter the target’s structure into splinters many meters away from the original impact site.
5.Ten-picometer photons are not energetic enough to penetrate the nuclei of target atoms. This means that the target does not, in addition to worrying about being ionized or torn apart by shock waves, need to worry about having its elements transmuted into something radioactive to complicate damage control or repair efforts.
A picture of the Mk-73’s interaction with a target’s armor now emerges. The first photons in the beam atomize and then fully and completely ionize a thin outer layer of armor and bleach the resulting plasma. This takes a few billionths of a second and it clears a path for the photons that follow to repeat the process on the newly exposed material deeper inside the armor. Later photons repeat this cycle and the beam “burns through” a target very much like an incredibly high-speed cutting torch. The whole laser pulse takes roughly a tenth of a microsecond or so. The huge total energies possible in bomb pumped beams can propagate this process to tremendous depths. Thermal and mechanical shock waves race out from the nearly instantaneously ionized column or cone of material, inducing tremendous stresses in the target and often shattering it.
Armor Function: How Does Armor Work?
How does armor stop that process? The easy answer is that it typically doesn’t.
Practical starship armor systems usually cannot prevent a beam attack from doing some damage. The popular Preston of the Spaceways image has the hero’s ship charging withering enemy fire, shrugging off direct hits, and “coming right at ’em.” It is possible to equip spacecraft with such a huge mass of armor that they can “shrug off” enemy fire. These spacecraft are common enough to have their own special name. It is “fort.”
Real starship protection systems are layered affairs relying on sidewall, radiation shielding, and particle screen to defocus the incoming beam and spread the attack energy over a larger hull area so that the armor can handle the lower intensity beam that results.
Figure 2 (see end papers) shows a notational schematic of starship passive defenses: sidewall, radiation screens, outer hull armor, and core hull armor showing effect on incoming beam. Not to scale, of course.
This article will not treat these gravitic layers in detail but a brief overview helps clarify the underlying armor design. The sidewall has been mentioned in our history and consists of artificial grav waves generated between the wedges of a starship to protect its sides. These walls are less gravitically intense than an impeller wedge and suitably powerful beam weapons on opposing ships of the same class can shoot through them at ranges within energy beam range. This range is highly variable depending on the ships engaged. The sidewalls are typically generated ten kilometers or so away from the hull. The space between the sidewall and the hull is filled with particle and radiation shielding to deal with natural space hazards. These shields plow debris and radiation out of the ship’s path using a weaker gravitic field. Instead of the sidewall’s small localized region of incredibly high acceleration, these shields are more gradual. They typically work on particles for longer periods, pushing their trajectories away from the vessel’s hull. Specially mounted detection systems and the ship’s energy weapon projectors vaporize the rare piece of debris too large or fast for the particle shields to deflect by themselves. High grade systems render normal space speeds of 80% lightspeed relative safe under most conditions. These shields also provide a slight defocusing capability against incoming beams.
It is reasonable to question use of physical armor given the existence of gravitic protective fields. The reason that armor still has a place in modern warship design lies mostly in the power, reliability, and maintenance costs of the sidewall. It is true that a sidewall generator takes up less mass to provide the same protection than a given mass of armor. However, it consumes the ship’s power and may even require mounting more massive reactors. Sidewall generators also consume too much power to run off a warship’s distributed storage banks for any useful length of time. Combat experience from the last several centuries of space warfare has shown that ships often lose sidewalls in combat either due to generator failure or loss of supporting power system components. Armor, while incapable of completely preventing damage, can at least preserve vital systems long enough to get a ship out of danger. Finally sidewall generators require more maintenance than armor. Some navies have the resources to do that maintenance and some do not. For all these reasons navies have historically relied on heavy gravitic shielding backed up by carefully designed material armor systems.
The actual process of stopping a beam once it has been defocused by the sidewalls and rad screens has been described as “a glorified radiation shielding problem” by one shipbuilder of the author’s acquaintance. In theory it is quite simple: place enough matter between the protected system and the attacking beam that the beam’s intensity is reduced to a safe level by the time it reaches the back face of the armor. Modern weapon characteristics make it in some ways simple to choose armor materials. The many solid matter properties such as strength, heat capacity, thermal conductivity, and toughness that play an important role in our everyday lives depend on chemical bonds between atoms. Modern beam weapons tend to break all these bonds as they ionize the armor. Simply put, the armor’s strengths in its solid form are irrelevant when the beam converts it to plasma. Recall that a beam can lose a great deal of energy ionizing a target without large increases in damage. Hence, armor designers look for materials with as many electrons per nucleus as possible to maximize the ionization energy pulled out of the opposing beam. The more mundane solid material properties like strength and thermal conductivity do play an important secondary role, however, because the job is not over once the armor has absorbed the beam. It must still dissipate the huge thermal and mechanical energies of the strike so that as little as possible gets through to the protected structures. This is where heat capacity, thermal conductivity, toughness, strength, and elasticity play their secondary but no less crucial role in an armor system.
A single homogeneous layer of material typically cannot accomplish all of these tasks. Hence composites of many materials are frequently used. Armor, like many other familiar structures in modern life, is formed, worked, and machined at the scale of individual atoms and molecules and so these are most properly called nanocomposites. Cross sections often reveal microscopic and macroscopic variations, some gradual and some sharp, within an apparently homogenous armor plate. Each of these layers is designed with a different function in mind, though all layers absorb beam energy in addition to their other functions. Refer to the right-hand side of Figure 2. Boundary layers at the surface and inside the plate provide structural support, the density to quickly absorb large amounts of energy, and also a path for heat and shock to quickly dissipate from nearby impacts thus limiting the extent of damage to surrounding armor. Boundary layers that are directly exposed to vacuum at the skin of the ship frequently receive special surface treatments to improve their ability to radiate the heat from nearby beam impacts into space quickly. Sandwiched between boundary layers, one finds scattering layers. Also called “refractory” layers, these typically have lower densities and absorb less energy directly than boundary layers. Their primary purpose is
to further spread and diffuse an incoming beam. These layers tend to occupy the majority of the volume in a cross section of armor while thinner but denser boundary layers contain most of the mass. Finally, while all layers contribute to the structural strength and kinetic impact resistance of the armor, one sometimes sees dedicated structural layers. These layers stiffen and reinforce the armor and also attach it to the underlying battlesteel of the ship’s primary load-bearing structures. While these interfaces usually do not figure directly in protecting against an incoming beam they play an important role in the armor’s tertiary function by providing outer hull structural integrity under impeller drive. A ship whose outer hull armor is badly mangled must be very careful with its acceleration.
Literally hundreds of different armor nanoconstruction techniques can be used to form and machine the different layers, but one deserves particular mention. Designers sometimes use special structures within the armor surface to attempt to reflect a portion of the incident beam back out towards space. Most often used near the surface and called “face-mirroring” these tiny X-ray mirrors are formed by spacing highly absorbing layers of boundary material separated by precisely machined transparent spacers that are one beam wavelength in thickness. That this is only possible when the weapon wavelength is known precisely is the major reason that wavelength and any other information useful in deriving it (such as laser rod composition) is classified. The technique becomes progressively less useful as wavelengths get shorter and even under the best of circumstances does not reflect much beam energy. The precision machining required is time consuming and the materials used are typically expensive. However, it still finds application, particularly where there is risk of direct strike against bare hull as on hammerheads. Every reflected photon helps in these situations. Face-mirrored armor designed for the wrong wavelength means that the design wastes mass and construction effort for no improvement in protection.
