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|
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This is already 80 years outdated by today's standards, let alone in a futuristic setting.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.|
If you want to create tough scifi, here's what you can do with armor.
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.|
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.
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.