A blog dedicated to helping writers and worldbuilders create consistent, plausible Science Fiction.

Friday, 11 March 2016

Innovating in armor

Representations of spacecraft in fiction usually take one of two forms: an armored block bristling with turrets, or a delicate assembly of struts and solar panels straight out of NASA's blueprints.

In this post, we'll discuss how armor can becomes a component of spacecraft design as varied and interesting as the weapons and engines, and the forms it will take can affect the appearance of a spacecraft.

When we design our spacecraft, armor evokes plates of steel enclosing a spaceship much like the thick decks and armor belts of battleships.
Here is a prime example:

The Battlestar Galactica
The armor consists of metallic plates, placed on structural ribs. It is designed like it has to regularly traverse some sort of fluid environment that requires it to be streamlined. It has turrets and weapon ports and hangar bays.

It ends up following a mish-mash of WWII submarine, aircraft carrier and battleship design.

This is already 80 years outdated by today's standards, let alone in a futuristic setting. If you want to disregard this and run by the rule of cool, fine.

On the other end of the spectrum, we have this:
Gasdynamic mirror fusion spaceship.
It is very realistic. It has radiators, life support, propellant tanks. It is built around the mission, then the engine's constraints. It can have a payload of missiles or even a laser attached. However, as a combat spacecraft, it is worthless. A bucket of dust flung in its general direction would break it in two. In a game, it's defense rating would be zero. This leads to hit-first-kill-first warfare that is not fun for a reader, a player or the characters in-universe.

If you want to create tough scifi, here's what you can do with armor.

Mass shielding

This term is generally used to refer to shielding against radiation. Simply put, it consists of placing as much of your spaceship's mass as possible between the crew compartiment and the direction of danger. Unless you have incredibly powerful rocket engines (and all the associated problems), more than half of your spacecraft's mass will be propellant.

This means that you can get double duty out of your propellant reserves by placing them around your spaceship's core components.

If your propellant is liquid hydrogen, the volume for mass is a rather ridiculous 14 cubic meters per ton. Turn this volume into an advantage by placing it as a torus around your spaceship's hull! You can also use it to fill in the gaps between the plates of whipple shielding.

Many settings have ice as the propellant of choice. It is easy to handle and plentiful. As a solid, it can be shaped around the spacecraft into belts of thick 'armor'. It might perform terribly against lasers and kinetic impacts, but the sheer amount you'll have at your disposal makes it a very efficient use of your mass.

Sidonia's ice is placed around the most important sections of the ship.
Rotating armor

Maybe your propellant is a gas and cannot be used as effective mass shielding. Maybe your rocket engines are too efficient to require huge amounts of propellant. Or maybe the incoming weapon fire is too powerful to be bothered by mass shielding.

In those cases, you will have to use thick slabs of armor to protect your spacecraft from enemy fire.

Against lasers, you will want a material that has a high heat of vaporization and low thermal conductivity. This means it takes a lot of energy to vaporize a hole in it, and it is difficult to get a larger hole than what is possible through direct heating. Aluminium is bad because it transmits the energy to the area surrounding the impact site and softens it. Carbon is excellent, as it ablates away and takes the laser energy with it.

Against particle beams, you'd want something that is good at stopping charged particles and hard radiation. These are called high-Z materials - they have a lot of electrons per atom. Candidates include tantalum and lead. As they are very dense, you'll want a thin layer of these backed by a lot of lighter materials, such as plastics.

For kinetic impacts, it's not so much the materials as the configuration that is important. Thin plates of a strong material like steel, with the spaces in between filled with very light materials or even fluids works as the best Whipple Shielding.

However, each of these solutions (and most SF out there) assume that armor has to remain static.

This is especially troublesome when you are trying to estimate how long your spacecraft is likely to survive under enemy fire. An example of a calculation used to estimate this is the Time to Superimposed Impact: In a situation where a single impact of a projectile or a laser strike does not go through all of your armor layers, it gives an approximation of how long it takes on average for a second impact, on top of the previous one, to fully penetrate the armor.

Here's a rough diagram:
The first impact penetrates to a certain depth and crates a crater with a certain width. If the second impact falls within the first crater, it adds its own penetration to the first impact and fully penetrates the armor.

The calculations for determining the average duration between these superimposed impacts is rather complicated and will be explored fully in another post.

The obvious solution for increasing the spaecraft's lifetime is to increase the depth of the armor until two, three or even four superimposed craters are necessary to fully penetrate the armor. This increases the spacecraft's lifetime exponentially, giving it time to maneuver, deploy it own weapons, react to developing situations... basically, fight back.

However, this also increases the fraction of the spacecraft's mass devoted to armor. To accomplish the same mission, you might have to increase the amount of propellant. If the propellant is fragile or gaseous, it will increase the surface area that you'll have to cover, and so on, snowballing into a spacecraft that is half armor and half propellant.

There is another way to do it, and it is to use rotating armor.

Armor plating can be shaped into cylinders  that cover your spacecraft's hull. These cylinders can be made to rotate during combat, gradually shifting the sites of impact relative to the incoming fire. This greatly increases the time to superimposed impact, and increases survivability with only minor mass and shape constraints.

Several shell layers can be used. If they rotate im opposite direction, you can negate any torsion effects and basically spread impacts across both the entire surfaces of the outer and inner armor cylinders.

Worldbuilding hints: Rotating cylinder armor can help distinguish warships from civilian craft, and give them a unifying shape to highlight differences between them. Also, spinning up the cylinders might signal the start of combat, and immobilizing the armor can symbolize overconfidence or defeat. Furthermore, impacts from lasers and kinetics are likely to have different penetration depths. A laser might have a small crater depth and take a very long time between superimposed impacts. Kinetics might be able to smash through several layers of armor at once, negating the rotating cylinder advantage. This might serve as an argument for keeping kinetics as a viable option in a battlefield otherwise dominated by lasers.

Projectile armor

Another assumption you have to break down is that armor sits in its place until hit by something.

This doesn't have to be the case.
Modern tank armor design has been revolutionized by ERA, or Explosive Reactive Armor. It consists of bricks on explosives that explode outwards upon being touched by a projectile. The explosive force shatters APFSDS penetrators and disrupts HEAT streams.

The latest developments of this technology is the Arena Active Protection Armor. Instead of lying in wait, passively, it uses a combination of millimetric radar and explosive plates to jump out and shoot down incoming projectiles.

Gif made of an Arena Active Protection Armor demonstration video.

The same thinking has to be applied to spacecraft armo. If you've followed any of the links to Whipple Shielding in the posts so far, you should know that the first plate smashes the projectile to pieces, the gap allows the pieces to spread out and slow down, and the second plate catches the now much weaker projectile.

This gives the two plates a much better resistance to hypervelocity projectiles than a single plate of their combined thickness.

One factor that determines how effective they are - is the distance between the two plates. The larger the gap, the more the fragments spread out and the more effective the second plate becomes.

Well... in space, volume is free.

There is no reason why, like the Arena Active Protection Armor, you can't shoot out the first plate at the incoming projectile. This would increase the gap between plates prior to impact, leading to greater effectiveness against projectiles. 

In some ways, this is point defence, but unlike trying to shoot down a projectile with another projectile, or trying to burn it down with a laser, a thick armor plate only has to be pushed in one direction, doesn't need to go fast, and can be as heavy as you like to make certain that it smashes apart the incoming projectile.

Other factors favor this solution, such as the need to track the incoming projectile in only one direction, the simplicity of the radar system to use, and the fact that the outer plate can be ejected by springs, gas canisters or even dropped from a rotating cylinder.

Particle field armor

This is the most speculative form of armor we'll look at.

All previous forms of armor emphasize the importance of mass efficiency and later, the mobility of armor. The ultimate evolution of such thinking is the particle field armor.

Particles deflected in a bubble chamber, as seen by their trails

In essence, it is a cloud of particles held in place electrostatically or magnetically, that can be moved around and configured to react to various incoming weapons fire.

Against kinetic projectiles, it has the possibility of being very effective.
Some pieces of fiction have used this type of armor (Sandcasters) as an implausibly effective solution against lasers. In our case, we set much more realistic goals.

A projectile that has had to cross the great distances between spacecraft quickly will have a great deal of kinetic energy. Trying to absorb all of this energy at once leads to the thick, multiple layers of Whipple shielding seen in conventional armor. Attempting to absorb it gradually is not possible either, as it would require slowing it down faster than it was accelerated.

A diagonal gradient in particle density can eventually cause the projectile to curve away.

One way to sidestep this problem is to try and deflect the projectile. This requires considerably much less energy, and using the particle field armor, you can use the projectile's own energy against it.

For example, say we have a coilgun-accelerated projectile incoming at 50km/s. It weighs 1kg. Our particles are 0.1-millimeter-diameter iron pellets held within a magnetic field. Our spacecraft is 10m in diameter and we want to deflect the incoming projectile by 5m so that it misses us completely.

Each iron pellet masses 33 micrograms. When it hits the projectile, it releases 41 kilojoules of energy. We need a minimum gradient of 2:1 to deflect the projectile - 2 particles hit the top side of the projectile, and only 1 particle below. It might be possible to get much better gradients, 10 or even 100:1
If half that energy is converted into deflecting the projectile (the rest as heat), then each impact deflects the projectile by 204m/s (!).

Let's say we manage to hold 100kg of these particles per meter cube. This amounts to 3030 particles per centimeter cubed. A 1kg projectile would likely have a cross-section of about 1 cm2. It would intersect about 9510 projectiles per meter of armor. If the gradient is 'only' 5:1, this would mean it would be deflected by 132MJ/second, which translated into a sideways acceleration of 440G for a 1kg projectile.

Increasing the strength and control you have over your magnetic fields makes it possible to achieve even better gradients and particle densities, leading to greater deflections and smaller total particle mass.


  1. Armour and point defences are not so much two different defences than two ends of a spectrum, with projectile and particle armour in between. Interesting approach!

    Particle armour is more believable than sandcaster as it is generally depicted, and using it to blow projectiles off is clever.
    In addition, it can be dual-use with magnetically focused droplet radiator : once the coolant is cold enough to be manipulated with magnetism, use it as particle armour instead of bringing it back immediately.
    In some cases, you may also want to impact with hot particles (with lasers, by pushing still-hot particles with cool ones...) to prevent the projectile itself to use its own field to deflect them.

    1. Some great ideas there.

      In space, all mass is projectile.

      I should make that a quote somewhere, but the point is, every bit of volume you need can be obtained for free, and the real difference between armor and 'point' defense is time. Projected armor only reaches a short distance in the time between projectile lock and impact. Flak cannons have a range of 100m to 1000km or more, only limited by how much delay there is between targetting the incoming projectiles and impact.

      Lasers, for example, are always imulated in defensive roles are starting to shoot down incoming objects as soon as they're launched!

      I'm shaping up a post where I calculate the exact numbers for particle deflection armor, but the results I'm seeing is cones of particles instead of a uniform cloud, so it'll need active sensors.

      Dual-use is incredibly useful in a space setting, especially when your exhaust velocity is low and every kilogram you save translates into 4 or 5 kilograms less propellant required.

      Note: Some droplet radiators work with tin. Metal particles...

      The difference between the impact of hot or cold projectiles is negligible. The kinetic energy of a collision dwarfs it entirely, and the result is a slower, hotter projectile and a thin vapor cloud of particle matter that would never damage you. So, there is no need to worry about your own particles being pushed against your hull.

      A projectile using its own field.... no, I don't thik its feasible, but what CAN work is something like a tandem charge. It ejects an initial plate that clears a channel through the particle fiels, then the main body rushes through.

  2. One disadvantage of roller armor is you can't really carve ports into it. For ideal protection, you are going to have a cylinder with all external equipment mounted on one of the flat sides. That includes any radiators unless you want to somehow turn the rollers into radiators (which sounds like a maintenance nightmare, even before the enemy starts shooting.

    I'm imagining an Orion space warship with roller armor, all it's weapons are tucked under the pusher plate (which can retract to seal in the backside) the other side is a conical radiator topped with a nuclear reactor. The whole thing would fit perfectly into an aeroshell.

  3. thanks for this really nice and interesting post,
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  4. One variation of plate armour is sloped armour, used in most modern military construction. The principal is that armour is thinnest when it is perpendicular to the vector of the projectile. The more you slope the armour away from the perpendicular, the more mass a projectile has to push through in order to penetrate. An added benefit is that as the slope decreases below 45° from parallel, there is an increasing probability that the projectile will just skip off the armour. This presents a nice alternative to the rotating armour configuration: instead of rotating armour, you can mount the plate armour on a manipulable pivot. This allows you to adjust the angle of incidence so that the impact angle is largely optimised not only for maximum thickness against penetration, but maximal probability of deflection as well.

    1. My understanding is that sloped armor is only useful up to impact velocities of about 2km/s. Under that, the shape of the armor has an effect. Above that, and it becomes fluid dynamics. Modern anti-HEAT armor is flat. At space projectile velocities of 10km/s+, it's gas dynamics. Projectiles grazing at extreme angles or striking head on both produce nearly identical spherical blasts containing the projectile's energy.

      Also, as I will explain in about a week or two, when I discuss fleet configurations, armor will have to face attacks coming from extremely wide angles. It will be difficult to configure armor to face multiple attacks from multiple directions.

      Finally, movable idea is great idea, but suffers from the fact that in realistic situations, it must react to incoming projectiles. Either the warning time is very short, requiring you to move heavy plates quickly, or very long, with a corresponding likelihood of ultravelocity impacts that do not care about sloping. Projectiles will devastate it if attacking unannounced, as was proved possible in stealth in space.

      If movable, reactive sloping armor is widespread, the any two-stage attack will defeat it.a

    2. Yes, and no. In practice, no armour is going to survive a 2km/s impact by anything massing over a kg... Unless it is several meters thick. The deflection capability drops off considerably at such speeds, so there is that point. However, sloped armour ALWAYS presents greater armour thickness to penetrate. Again, though, this is not going to be of very much help if the projectile can penetrate 10 or 20 meters of armour.
      Sloped armour IS flat. All modern anti-HEAT designs are sloped, relative to the incoming AOA. The M1A is a prime example. The sloped hulls of mosern warships have two functions: they deflect incoming radio signals away from the sender, but they are also sloped armour, which increases the effective armour thickness. This is especially true for modern armoured turret housings. OTOH. HEAT rounds very seldom travel at 2km/s. They rely on hot gas to burn through the armour. This means there is low probability of deflection, but the slope of the armour still means there is more thickness of armour that the gas has to burn through.
      Yes, movable armour can be defeated through using multiple, near simultaneous, vectors of attack; and it will not always be possible to move it quckly enough into optimal position. However, fixed armour is even more easily defeatable. It is easier to reposition a movable shield than an entire vessel, and there is nothing to prevent you from having multiple movable shield panels. It is not perfect, but it gives you a little more advantage.
      In the end, the choice of armour configuration wil always depend upon the strategic and tactical environments that a vessel or craft are built for. No armour is going to survive hypervelocity impacts by large masses. No armour is going to be perfectly oriented against multiple pronged attacks. No armour is going to be optimal in all situations. Sometimes it is better not to have ANY armour. But sometimes a configuration will give you a significant advantage.
      BTW, the actual utility of sloped armour against high velocity projectiles is largely dependent upon the thickness of the armour. If you have meter thick plates, there is still the possiblity of deflecting 2km/s projectiles, although you are still going to cut a significant gouge out of the plate. Don't forget that even thin atmosphere is enough to deflect a capsule approaching at 5 or 10 km/s if the AOA is close enough to parallel.

    3. In the last paragraph, I meant "meterS thick plates. A single meter might not be sufficient, but there is a point at which the thickness will sap the energy from the projectile, and deflection again becomes an issue... HOWEVER, the armour plate is going to be severely scarred.

  5. The space environment offers some interesting variations for plate armour. In very low g, there is a natural tendency for liquids and dust to collect on the nearest available surface. The problem with solid armour is that once you are able to split a wedge in that armour, the wedge tends to stay there. Also, virtually all of the impact energy remains at the site of that wedge, aolwing it to split even deeper. On the other hand, projectiles tend to have very poor performance when striking liquids or sand. The energy is dispersed over a larger area, and the gaps tend to fill in. An excellent armour would be a thick layer of highly viscuous liquid or gel, especially one impregnated with sand. The liquid flows back into place, especially if it is highly viscuous, and the sand adds mass to defeat the impact energy. If you replace the sand with loose metal pellets or filings, you can send an electric charge through the layer, or switch on a magnetic field, which will further protect the layer, and reduce any particulate loss of shield mass, even if this is the outermost layer.
    Note: if the liquid is internal, you don't want the space between solid masses to be completely filled with the liquid. The liquid will transmit a compression wave upon impact, and if the water has no voided space to move through, it could split the solid layers itself.

    1. The is a very simple solution to the problem you mentioned: segmented armor. Cracks and fissures don't expand over gaps.

      Also, spacecraft armor will likely be some sort of whipple shielding, meaning that cracks will never be able to travel very deep.

      As for liquid/fluid armor, it suffers from the fundamental problem that impacts vaporize material and push armor out of the way when trying to drill a hole through it,both are translated into extreme pressures.

      A solid material has very tensile strength: it cannot easily be ripped apart by internal forces. A liquid has much lower tensile strength (usually not stronger than the surface tension of water). A small impact might generate pressures that blow away very large amounts of fluid, making it disproportionately devastating.

      In most realistic cases (no extremes in energies, low or high), armor divided into sections both horizontally and vertically reacts better to damage than any fluid. Also, it is unlikely that a magnetic or electrostatic force could ever match the pressures generated by vaporized materials.

  6. Hmmmmm... It looks like my previous response never ot posted. Short summary, because I put too much time into it yesterday:

    There are pros and cons with segmented armour. Yes, the gaps stop the cracking. OTOH, the gaps themselves do the job of the projectile: to make a gap that can be exploited. Still, as opposed to cracks in armour, which are essentially permanent damage; the gaps are generally kept sealed by the pressure from surrounding armour plate... so it is better to have the gaps... and staggered laminated armour underneath to prevent exploitation of those gaps. Since projectiles are essentially wedges intended to spread armour plate mass apart, the smaller the segments of armour, the better. As the armour is reduced to near the size of the projectile, the projectile stops functioning like a wedge, and starts to function like a hammer... which is not very effecient at penetration. This is the point of the sand or filing armour: it is essentially segmented, laminated, armour ith extremely small plate size. OTOH, segmented plates are vulnerable to embedded explosives. If the penetrator is effective enough to get under the surface layer, the explosive will blow out the armour plates from within.

    Liquids and segmented armour have extremely low tensile strength. However, under many circumstances, tensile strength is actually not as important as compression strength. As I said, the intent of a projectile is to force armoured mass apart. Tensile strength helps resist a wedge, but does nothing once the armour is split. Compression strength, OTOH, pushes the ends of the armour plate back together, preventing further exploitation. Since the renaissance, military installations (bunkers and forts, etc... things that don't move) have favoured sand and water as an armoured defense. In a fortification, concrete, stone, and metal are only used as a barrier for retaining the sand (or a pool of water). The actual effective armour is the large layer of sand and/or water (or other fluid). Naval vessels actually have fairly thin armour plate for the potential damage they are built to absorb. They make up for this by building nested hull skins with thick voids between them. These thick voids are generally fillled with water (distal voids) and fuel/oil or water (proximal voids). These are actually the first lines of armour. The armour plate itself is on the interior, just outside the innermost hull skin, which is itself used to catch spalling.
    Yes, pressure can blow out water and sand... but this is also true of any (other) ablative armour. It takes more energy to blow out the water than it does to penetrate solid armour, so the ejection of water or sand is preferable... and protective (it abates by design). The benefit with sand and water is that it is remarkably easy to replenish the armoured layers that are blown out, especially in space, where the retaining wall is much more effective at restraining the water in the 0g environment. Although this is counter to your expectations that segmented and laminated armour respond react better to damage, this has been proven not to be the case in actual military experience and testing/analysis.
    That said, yes, you generally need thicker layers of sand or water to provide the same protection against individual hits (the metalic filings with electric current would provide equivalent or superior protection in thickness), but this is made up for in its ability to close gaps (every hit is against virtually the same armour strength, so the armour survives longer in combat).

    1. (Okay, now I understand... even my shortened version was too long) Continuing my previos post:

      Finally, a note on laminated armour: research prior to WWII actually demonstrated that lamination decreases the actual penetration resistence of the armour. If you have multiple layers, instead of the thickness of each layer being added together, the effective strength is equal to the actual thickness of the thicker layer, plus about 2/3 to 3/4 the actual thickness of the additional layers without spacing, and/or 1/2 the actual thickness of layers that are spaced. If you have a round capable of penetrating a single plate of armour with the total combine thickness of the laminated armour, then it will penetrate all the layers of laminated armour, and do so much more easily.
      Of course, the obvious question is, if this were true, why laminate and/or space armour?
      First, it is not always technologically possible to manufacture armour in the required thickness. Second, different armours have superior performance to others under different circumstances (compression strength is more important than tensile strength against solid penetrators, but the tensile strength is more important against explsives... for example), so armourors use different armours to counter the various different warheads (or other weapons). Third, if the penetrator is NOT strong enough to puncture the full combined thickness, the barrier between layers will stop any deepening of cracking, and will prevent secondary impacts from using the gap in the penetrated layer from applying leverage to force open the remaining layers (the new shell will have to face a "pristeen" armour wall). Forth, again, if the penetrator is not sufficient to penetrate the entire thickness, but sufficient to penetrate the outer layer(s) of armour, the gap acts as a shock absorber (well, actually it redirects the force laterally throughout the damaged plate, obsorbing all the extra force, and preventing it from damaging the inner layers. Fifth, if the armour is spaced, it can trap explosive rounds outside the inner layers of armour, preventing the armour from being ripped out from within. Likewise, spaced armour can trap spalling, so that the fragments do not do as much damage to fragile interior components (even if the shell penetrates all the way in, the spaced armour makes sure that it is just the original shell that it doing the damage, and not a widening spray of debris). Etc.