Index of Science Posts

Below is an index of posts about the science of the game for ease of reading.

Basic Concepts/Overviews

Why Does it Look Like That?



Orbital Mechanics






Scaling Laws


Life in the Lonely Void

A major consideration behind constructing a spacecraft that is often glossed over is the brain of the spacecraft. In most cases, this is a crew module, or a remote control module relaying orders from somewhere.

A look inside the ISS.

The reason crew compartments don’t receive the same amount of consideration, as say, the engines or the weapons, is that crew compartments have no real surprises about their design, and on larger capital ships, they are rarely a bottleneck in terms of mass, volume, power usage, or heat dissipation.

But before we discuss crews, what about alternatives? Crew provide decision making, the brains of the spacecraft, as well as providing fine grained manipulation of equipment and tools for repairs, maintenance, and so on.

The fine grained manipulation could be accomplished by minidrones, automated repair bots and the like, though handling unexpected situations is rather tricky without a human or artificial intelligence.

Though if DARPA’s work is any indicator, humanoid robots may be the ideal maintenance workers rather than minidrones.

Brains of the spacecraft can be replaced with remote control, or with an artificial intelligence.

Remote control can be spoofed or jammed, but there are countermeasures and counter-countermeasure. The main issue with remote control is the speed of light lag. Beyond high orbit of a moon, for example, the speed of light lag is too great for combat. Additionally, long term journeys have much greater potential for unexpected failure.

This means remote control is restricted to drones and missiles, remotely operated and ordered by the nearest capital ship or celestial body.

Artificial Intelligence (AI) is an interesting solution to the problem of having crews. Crews are expensive to train, take up precious mass and volume, and require power. On top of that, the heat they need to dump out can be a problem if you want to talk Stealth in Space.

However, AI is more than a series of algorithms running on a laptop. Currently, certain problems of space warfare are best solved with algorithms (see Misconceptions about Space Warfare), such as leading targets hundreds of kilometers away moving at multiple kilometers per second.

AlphaGo is a one-problem AI, but it could be the first step towards a general intelligence.

On the other hand, other classes of problems are best solved with intelligence and creativity. In particular, how to see through enemy deceptions, laying deceptions, handling unexpected scenarios and failures, and so on are all problems that algorithms would fail badly at. Anything creative or anything an algorithm is not explicitly designed for would throw it for a loop.

That means full blown Artificial General Intelligence is needed for actually commanding a military spacecraft if you want to go without crew. Additionally, it needs to be able to very carefully and precisely control minidrones to repair and maintain a spacecraft.

The field of AI today is nowhere near that sort of capability. However, even if it does progress to being usable in military scenarios, it is unclear if it would be less massive, voluminous, or require less power than humans. The first AIs will likely be extremely massive and require huge amounts of power, and it’s not clear how far they could be miniaturized.

If IBM’s Watson is any predictor of strong AI, AI may be rather massive and consume a lot of power compared to a crew.

Even when feasible AIs are developed, space militaries would be very hesitant to deploy AI-controlled spacecrafts without at least some human oversight or failsafe.

With that in mind, we are left with crews for our capital ships, and remote controls for our missiles and drones.

But just how few people can you cram into a spacecraft? Modern Supercarriers crew over 4000 people in 25 decks. In space, most of that space would be propellant tanks, and you can’t really dedicate much mass to the crew compartment. Capital ships in space would run only skeleton crews, with only small sections of the spacecraft pressurized.

Of course, you might be able to skimp on sleeping quarter space given that you can attach sleeping crew to the floor, walls, and ceiling.

In space, crew modules are somewhat massive, yet systems like radiators, armor, and weapons usually take up far more mass.

Volume is the main problem with crew modules. Crew modules are mostly empty space filled with air. Even when you pack your humans in like sardines, the majority of the crew module remains empty space. Aside from the propellant tanks, crew modules take up the most volume of any module.

This makes Modern Nuclear Submarines the closest analog to spacecrafts in terms of crew: somewhat over 100 crew for a submarine over 100 meters long.

Cutaway of a modern nuclear submarine.

However, nuclear submarines are fully pressurized, while spacecrafts would not. This means spacecrafts would have even less space for people, and so crew requirements were estimated at roughly half that of a modern nuclear submarine. Of course, some jobs you can’t simply halve, and larger ships with more systems require more crew.

It should be noted that crew in a spacecraft is certainly not a novel topic. Winchell Chung’s Atomic Rockets website has a great break down on all of the considerations of crew.

In Children of a Dead Earth, most capital ships run between 40 to 80 crew, and are based heavily on modern nuclear submarine crews.

Crew complement of a very large capital ship massing about 10 kt.

screen-shot-2016-09-21-at-2-59-05-pmThese numbers are based on a tally of all the jobs needed, which scales based not by mass of the ship, but on the number of subsystems, type of subsystems, and several other factors. Thus, an enormous 10+ kiloton methane tanker can run on a tiny crew, while a small, 1 kiloton fast attack craft may require a much larger crew.

screen-shot-2016-09-21-at-2-59-10-pmWith such small crews, they would have to be highly trained to take over multiple jobs in case of injury or death of other crew members. Similar to modern nuclear submarines, crew members live 18 hour days, 6 hours on watch, and 12 hours off watch. Meals between each watch, with the enlisted men and women hot bunking to save on the precious space.

While a pure oxygen atmosphere (as seen on Skylab) is less massive and requires less pressurization than a 22% oxygen, 78% nitrogen atmosphere (as seen on the ISS), it is a fire hazard. And in combat, fire hazards are never fun.

screen-shot-2016-09-21-at-2-59-06-pmWater can be easily recycled as on the ISS. However, recycling food from human waste is a lot trickier, requiring a small ecosystem, likely using algae, to photosynthesize food from nuclear reactor grow lights. The technology to do this is much closer than AI is, and is very easily foreseeable as a staple in modern space travel.

A complete algae ecosystem able to supply nearly infinite food would be excellent for long voyages with lots of crew, such as for a colony ship or space liner. However, the dumb solution is far simpler, cheaper, and less error prone. Store the food, just like how modern nuclear submarines work, and restock at every spaceport. And in combat, getting your provisions shot up is far less of a concern than getting your algae beds destroyed.

In Children of a Dead Earth, ships by default carry provisions for 6 months, which is greater than most campaign mission in game. Only a few missions exceed 6 months, and most are one month or less.

screen-shot-2016-09-21-at-2-59-09-pmCrew modules produce a small amount of heat primarily from the lighting system, the galley cooking system (unless you’re forcing your crew to only eat Soylent), and the heat emitted by each crew member into the air. While the heat produced is minor (kilowatts) compared to the main reactor (megawatts), the low temperature (room temperature, 293 K) that the coolant runs at forces the radiators to be only somewhat smaller than the main reactor radiators.

As mentioned in prior posts, radiation is a concern for crew, which is one reason why the cylindrical shape is preferred. Getting your crew module far away from the reactors is a free way to reduce radiation below the 50 milli-Sieverts annual limit. Additionally, radiation shielding, while not negligible, is cheap and low mass enough to not be too much of a concern. It tends to only be a mass or cost problem if you absolutely want your crew module next door to your reactor.

Radiation types and how easy they are to block.

Children of a Dead Earth simulates all types of radiation, from Alpha DecayBeta Decay, and Gamma Decay from Radioisotope Thermoelectric Generators, to Neutron Radiation (both fast and thermal) from Nuclear Fission Reactors. However, in practice, Alpha Decay and Beta Decay are more or less irrelevant to humans due to their low penetration, and Gamma Decay is rarely an issue.

Neutron radiation, on the other hand, is the bulk of the radiation problem, and it is often the main reason why your spacecraft will need radiation shielding. It generally is far worse of a problem than even the Cosmic Rays from space.

Another consideration mentioned a few times in previous posts is that crew modules are put close to the center of mass in case of fast rotations. Spinning a multi-kiloton spacecraft around fast enough to produce 9 g’s or more, enough to cause fatal damage to the crew, is rare, but it does happen in game. Keeping the crew near the center of mass reduces the centripetal acceleration on the crew in such cases.

Although it seems you can withstand up to 46 g’s like John Stapp did, it’s not really recommended without extreme harnessing, especially in an uncontrolled or unpredictable manner.

It is very difficult to knock a multi-kiloton spacecraft into a fast spin, and if you have enough firepower to do so, you generally don’t need to slush the crew in this manner.

On the other hand, for smaller spacecraft, under a kiloton fast attack spacecraft, knocking them into a tailspin is actually rather common. To exacerbate this, small spacecraft with enormous projectile weapons can often knock themselves into unpleasant spins through recoil alone. As such, keeping the crew near the center of mass is most important on smaller spacecraft.

So that is what you need to keep your capital ships running smoothly over the months, and able to react to unexpected situations in combat! And with a crew, the brains of your ship, you have the final piece needed to assemble a spacecraft and go to war.

Go Small or Go Home

Just how big can you feasibly make a spacecraft? The size of an aircraft carrier? The size of an asteroid? How about the size of a small moon? Today we will look at scalability of spacecrafts and the systems within.

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A large (190 m) capital ship compared against the Space Shuttle Orbiter (37 m).

When designing a spacecraft, certain questions inevitably arise concerning how it should be sized. Crewed spacecrafts very obviously have a lower size bound, since you can’t really miniaturize people like you can lasers or rocket engines. At the very least, your spacecraft needs to be able to fit people. However, there is no clear upper size bound. With missiles and drones, there is no obvious lower size bound either.

Let’s take a look at size limits of subsystems.

Power usage is more or less the primary way to increase effectiveness of systems, and size is generally the way to reduce thermal and mechanical stresses caused by this power use. But these laws are almost never linear, and often hit ultimate limits.

Take lasers, for instance. As outlined in The Photon Lance, scaling a laser up or down in size produces very little difference in power output. However, scaling it up in size reduces the power per volume and power per area so it won’t melt when activated.

This means you often want to keep your weapons and subsystems as small as possible, but it’s physical limits that force them to grow larger.

Screen Shot 2016-09-02 at 5.50.22 PM.png
A Nuclear Thermal Rocket (NTR) next to a Magnetoplasmadynamic (MPD) Thruster at the bottom of the image. The NTR is roughly 60 times as long, and masses 6000 times as much. Much of that difference is in the NTR’s enormous nozzle.

A trend with sizing of subsystems is that systems tend to work more efficiently when larger. A single 200 kN rocket thruster, for example, will perform more efficiently and be less massive than ten 20 kN thrusters. Larger singular systems distribute mass better and require fewer complex parts than many smaller systems.

On the other hand, those ten lower efficiency thrusters would probably be preferred in combat to the single high efficiency thruster because of redundancy. Compare a stray shot taking out all of your thrust versus taking out only one-tenth of your thrust. Clearly, there is a balance to be struck, between redundancy and efficiency.

The Saturn V used five rockets for its first stage. If one rocket failed unexpectedly, they still had 80% of their thrust remaining. This actually happened on the Apollo 13 mission, and they were able to continue their mission (until later failures).

Similarly, crew modules come with significant overhead, such as the plumbing for the sewage and air recirculators. As a crew module expands in size, this overhead reduces proportionally to the number of people within. However, bunching all of your crew together in a single module is a major liability in combat.

Alternatively, rather than making a single large spacecraft with highly redundant systems, some playtesters went the route of smaller spacecraft with no redundant systems. In that case, the redundancy is with the spacecrafts themselves, rather than with the subsystems.

Another consideration is that smaller subsystems can be manufactured more cheaply on assembly lines compared to single large subsystems. In the era of widespread, highly advanced Additive Manufacturing, these benefits are less pronounced, however.

There are certain minimum size limits that show up with drones and missiles, too. For instance, nuclear warheads have a minimum size. The smallest nuclear device ever made was the W54 at about 20 kg and the size of a large suitcase. This lower limit is due to Critical Mass needed for fission. Thus, for missiles, their warhead tends to determine just how small you can make the missile. If your missile has no warhead, their lower size limit is based on the rocket motor generally.

For drones, it is similarly the mass and volume of the weapons on that drone which limit the size of them.

Screen Shot 2016-09-02 at 5.38.09 PM.png
A small drone compared to the size of a human. The majority of the drone is reaction mass to offset the mass of the weapon and radiators.

But these are all lower limits. What about upper limits?

Generally, lower limits are all the rage because you want to make everything small, compact, and low mass. The smaller (volumetrically) you can make everything, the less armor you’ll need. The less massive you make everything, the greater the delta-v and thrust you’ll have.

There is actually very little stopping you from making enormous lasers or railguns, but simply making them bigger doesn’t actually improve their effectiveness or power, it only makes them deal with thermal and mechanical stress better. Essentially, you make things big because you have to, not because you want to.

But suppose you don’t care about making the most effective spacecraft, you just want to go big.

Screen Shot 2016-09-02 at 6.09.35 PM.png
A large civilian craft compared to a human. The human is the small black dot in the lower right.

At this point the Square-Cube Law begins to rear its head, and that is that volume scales cubically, while surface area scales quadratically. This is why large animals are built very differently than small ones. Without gravity, these issues are not quite as severe, but they still appear.

For instance, on the positive side, larger spacecraft are more efficient about their armor-to-everything-else ratio, because armor scales by surface area, and everything else scales by volume. Large capital ships tend to be armored like tanks while smaller ships run much lighter.

But on the negative side, acceleration suffers badly. Attaching thrusters to a spacecraft scales by surface area, and the mass of spacecraft scales by volume. Thus, the larger a spacecraft becomes, the lower and lower its acceleration inevitably becomes. As found in Burn Rockets Burn, thrust is hugely important, which is why only Nuclear Thermal Rockets and Combustion Rockets see major use in combat.

A ship that can’t dodge is a sitting duck to all manner of weapons. Most capital ships in Children of a Dead Earth range from hundreds of milli-g’s to full g’s of acceleration, and even that affords only partial dodging usually. Dropping that acceleration further is often fatal in combat.

biggest smallest.png
The smallest and largest capital ships in game. Roughly a 8x difference in mass, but only a roughly 4x difference in length (219 m vs 60 m).

Another negative aspect of growing in size is that the the cross sectional area of the spacecraft grows accordingly. And having a fat targetable cross section vastly increases enemy projectile ranges against you.

For combat spacecrafts, then, miniaturizing your spacecrafts is often the most ideal choice. But what about civilian crafts? Civilian crafts make a lot more sense to balloon up in size, especially for the sanity of the passengers.

The acceleration is still a problem, as if it’s too low, the spacecraft will have difficulty getting anywhere taking enormous amounts of time. But the other issues are gone. If travel time is not an issue, such as with a multi-generational colony ship, then you could try scaling up to truly enormous sizes.

Maybe you could hollow out a small moon to make a colony ship.

When you start hitting small moon or asteroid sizes, though, then you begin to have to worry about gravitational stresses collapsing your ship into itself! But that’s far beyond the scope of what you’ll find in Children of a Dead Earth.

The Wheels on the Spacecraft Go Round and Round

Spinning a ten kiloton spacecraft around is no easy feat. Even more impressive if you want to be able to turn on a dime. This article covers the issues of Attitude Control of spacecrafts.

A capital ship with four main engines gimbaled off to the side to provide torque.

On land, sea, or in air, rotation is easily done by pushing off the nearest medium, with anything from a tractor tread to an impeller to a rudder. In space, no such thing is possible, which means rotation can only be accomplished in one of two ways: by expelling mass via a rocket engine, or by storing rotational momentum internally. Incidentally, the second technique is only really viable in space due to the lack of any major medium, as friction would quickly degrade stored internal momentum.

The goal of these systems is to rotate enormous spacecrafts at reasonable or high speeds in combat, while being both cheap and non-massive.

The first technique is simple. Firing a thruster off center of your spacecraft will cause it to torque. Since rotation is the goal, and rotational acceleration is not, a second thruster must be fired to decelerate it at the end. And because such a thrust would send the center of mass off center, often two thrusters on opposite sides are used to start rotating, and two different opposing thrusters are fired to stop rotating.

A capital ship fires two vernier thrusters at opposite ends and directions to rotate. The rockets are water resistojets (hence the transparent plumes) and they tap into the main NTR propellant tanks.

These are called Vernier Thrusters, and see heavy use in space travel.

There are significant disadvantages to this method, however, chiefly that additional reaction mass is needed. Not only that, such thrusters usually can’t be Nuclear Thermal Rockets (NTRs), due to radiation concerns (recall that most crew modules are placed as far away from the main engine NTRs as possible). Cold gas thrusters don’t provide near enough thrust to be useful in combat, which means combustion rockets and resistojets are in.

Combustion rockets suffer from the issue that they require propellant(s) that are almost guaranteed to be different than NTR propellants, so additional propellant tanks must be added in, which takes up space and mass. This leaves resistojets as the prime method of providing torque to your spacecraft, since they can use the same propellant as your NTR.

Alternatively, instead of a system of multiple Vernier Thrusters spread out across your hull, simply putting a gimbal on your main thruster will do the trick instead. This allows your main engine to turn, producing off center thrust on your ship, rotating it. In this way, your main engines can serve the dual purpose of getting you places as well as orienting you in combat. This is one of the cheapest forms of thrust vectoring.

A few more complex thrust vectoring designs for achieving reverse thrust.

The primary disadvantage of it is that propellant is still expended when turning, however, no additional propellant tanks are needed as your main thrusters are doing the turning. For capital ships, the amount of delta-v spent turning is negligible, though for smaller crafts like missiles, the delta-v spent can raise a few eyebrows.

A more subtle disadvantage of gimbaled thrusters is that the size of the opening that the engines need to have can balloon significantly. This can yield much larger aft sections of the ship, and increase the targetable cross section of the spacecraft heavily.

Gimbaled thrust tends to be the cheapest and simplest solution to turning in space, and many capital ships and all drones and missiles in Children of a Dead Earth use them. Some capital ships opt for resistojet Vernier Thrusters, primarily for getting a smaller targetable cross section.

Screen Shot 2016-08-18 at 11.05.38 PM.png
A missile’s rocket nozzle is slightly gimbaled to the side, only a few degrees off center, generating tremendous torque. Screen distortion is from the camera being inside the rocket plume.

But suppose you don’t want to spend precious propellant turning? Then you need to invest in the second technique towards turning in space: exploiting Conservation of Angular Momentum.

Given a system without little or no external medium (such as space) and assuming you don’t want to expend propellant, the only way to rotate is by using conservation of angular momentum.

A quick example. Consider a pair of identical masses loosely attached to one another, floating in space. If one of the masses begins to spin in one direction, the other mass must spin in the opposite direction at an equal speed, otherwise it would violate Newton’s Laws of Motion.

This is the basic principle of operation for a Reaction Wheel, which is a flywheel with a motor attached. A flywheel is a mass with a high Moment of Inertia, or ability to resist rotational changes, along a single axis. When you spin a reaction wheel inside your ship using the motor, your spacecraft must spin in the opposite direction. Three reaction wheels must be used to get a full range of motion (one for each axis: pitch, yaw and roll). The Kepler Spacecraft uses reaction wheels.

A simple reaction wheel.

A Momentum Wheel is a reaction wheel which is constantly spinning at a very high speed. If a momentum wheel is spinning inside a spacecraft, simply braking it can cause a huge change in rotational momentum, causing a significant torque on your ship. Six momentum wheels are needed instead of three, two for each axis in opposite directions.

Momentum Wheels are often used for spin stabilization, as the huge amount of stored rotational energy will resist external torques. The Hubble Space Telescope uses Momentum Wheels.

Control Moment Gyroscope (CMG) is a single momentum wheel on a dual-axis gyroscope. By rotating the momentum wheel about the two gimbal axis, the angular momentum balance of the spacecraft can be altered at a whim. A single CMG can rotate a spacecraft along any axis by simply rotating the gyroscope to be in line with that axis. They are the most expensive and complex of the above systems, and are used on the ISS.

A Control Moment Gyroscope. The two axis of the gimbal allow the wheel to be oriented in any direction.

These systems promise the ability to rotate your spacecraft without expending propellant. Not only that, they require no exposed external systems, so they can’t be damaged unless the main bulkhead armor is penetrated. Sounds like a win-win, right?

Unfortunately, their effectiveness is very poor for combat operations. Modern CMGs are often used to very slowly change orientations over the course of minutes and hours. I originally had never intended to implement Vernier Thrusters or Gimbaled Thrusters in game, and was only going to use Momentum Wheels and CMGs. That rapidly fell apart when I did the math on them.

Consider the above example with the two masses on a string. Because they are identical, the spins will be equal and opposite. But if one mass has twice the moment of inertia, it will spin at half the (reverse) speed as the other. The greater the moment of inertia, the slower the spin.

If one of these masses is the spacecraft and the other is the reaction wheel, you want the spacecraft to have a lower moment of inertia. Thus, if your spacecraft has a moment of inertia 100 times that of your reaction wheel, it will spin 100 times slower than that reaction wheel.

Moment of Inertia is proportional to mass and the square of distance from the rotation axis. Roughly speaking then, very voluminous and very massive objects will have the greatest moment of inertia.

When headspinning, rotational speed can be increased by pulling ones’ legs in. This reduces the distance from the rotation axis and thus reduces the moment of inertia.

Spacecrafts by nature will have a greater volume than your reaction wheels, since they envelope these wheels. And they are guaranteed to have a greater mass than your reaction wheels, unless you are okay with an abysmal mass ratio (less than 2). Thus, you are guaranteed a moment of inertia far tinier than your spacecraft’s moment of inertia. And as I discovered, even spinning your wheels at enormous speeds yields rotations that take minutes or even hours.

In short, these techniques were not viable for combat rotations, barring some sort of future technology.

This leaves thrusters as the only viable method of spinning about in combat. How fast can they spin?

Because thrusters affect acceleration rather than velocity, the answer is that it varies. For instance, the time to spin 90 degrees is not going to be twice the time it takes to spin 45 degrees. And it varies based on which axis of rotation is used.

A simple metric is the Full Turnabout Time, which is the time it takes to spin 180 degrees about the slowest axis. This is essentially the “slowest” possible turning time for the ship, and most turns will be much faster, a fraction of this time.

When the enemy targets parts of your ship away from your center of mass, you can dodge simply by spinning along one axis. This particular ship is spinning with a single gimbaled NTR.

For medium sized capital ships with gimbaled thrusters in game, 20-30 seconds is a common value. Capital ships with vernier thrusters tend in the 10-20 second range, as do small sized capital ships. Very large capital ships can take up to a minute to do a full turnabout. Gimbaled drones and missiles tend to take 5 seconds or less for a full turnabout, with some being able to do a 180 in under a second.

Much faster turnabouts are possible by simply adding more and more vernier thrusters or gimbaled thrusters. However, this is often fast enough to deal with the rapidly changing nature of space combat. It is rare for a capital ships to ever need to flip a 180. Most of their turns are much smaller angle shifts, small dodges and broadsides.

Raw Steel

Armor is one of the simplest things to manufacture, yet it holds some of the most complex results.

Capital ship with armor visualizer enabled, demonstrating which main armor tiles are damaged or destroyed.

A 5 cm plate of steel is rather easy to produce, but analyzing how well it will perform under a wide variety of types of damage in different orders is rather difficult.

There are three main methods of damage: projectile, photonic, and plasma. Generally speaking, given equal amounts of energy, plasma damage is the least effective in many cases, and projectile damage tends to be the most effective. These differences vary significantly and may even reverse depending on the type of armor, however.

When a projectile hits armor, there are numerous ways that it may affect the armor. If the projectile does not penetrate, it can fracture the armor anyways, either where the bullet hit or on the other side of the armor. It could spall fragments off the inside of the armor, which can cause damage to internal compartments. And all of these scenarios are possible too if the projectile does fully penetrate.

Different ways in which a projectile can affect armor.

Of course, with hypervelocity projectiles, in many cases, the projectile will shatter into plasma upon hitting. Not only that, the armor may shatter into plasma too. An interesting effect of this (which is modeled in Children of a Dead Earth) is that in certain cases, more armor can detrimental. Too much armor can get shocked into plasma or spallations and inflict even more damage than a thinner armor plate.

Laser damage is somewhat simpler. The primary method by which photons inflict damage is by ablating away the armor, causing material to melt, evaporate, or sublimate away.

Alternatively, a pulsed laser could be used to trigger shockwaves from the rapid expansion of the affected armor material. In this way, lasers can be used to inflict mechanical damage on the armor. A very thorough discussion on laser effects on armor can be found here.

Determining a model for how well a piece of armor reacts to every different scenario (or combination of scenarios) is rather difficult. On top of the different types of damage, there are many different kinds of armor materials and design choices.

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The armor visualizer clearly demonstrates the effectiveness of a drone projectile barrage.

Material properties that are important to armor materials follow.

The Ultimate Tensile Strength of a material is the capacity of that material to withstand tension, or being pulled apart. The Yield Strength, or the stress point at which plastic deformation of the material begins, is another metric which very important to determining the effect of mechanical damage.

The Density of a material plays a key role in resisting damage, particularly projectile impacts. One interesting point of note is that high density Whipple Shields perform worse due to a greater amount of Whipple Shield material being shocked into plasma, and hitting the main armor.

Against melt ablation by lasers, very different properties are used. Often, material strength is more or less irrelevant against laser damage. Instead, high Melting PointsBoiling Points, and Specific Heat Capacities are of crucial importance for armor to resist heating. All of these quantities allow the material to absorb more energy without failing.

Alternatively, some materials can get away with very poor heat resistance by instead having a very high Thermal Conductivity. If the material conducts heat away fast enough, a laser can dump energy into the material all day and never heat it up, as the material will conduct the heat away to surrounding armor tiles.

And of course, a number of other properties play a minor role in determining damage, like the Elastic Modulus and Shear Modulus. But generally when choosing armor, your main concern should be a high Ultimate Tensile Strength and a high Melting Point.

Aluminum Whipple Shields are easily cut through by a laser beam.

So with those properties in mind, what material is best to use for armor?

Consider the material used for a single monolithic plate. Common choices are metals like lightweight Aluminum and Titanium, or the very dense Depleted Uranium.

Alternatively, metal alloys like Steel, particularly high strength steels like Maraging Steel make for great armor. However, on Earth, high density is valuable for lower volume, but in space, low density armor is more important, as armor can and will drastically affect your spacecrafts mass ratio.

This means ceramics like Reinforced Carbon-Carbon or Boron Nitride see use everywhere in space. They tend to be somewhat weaker than many high strength alloys, but their low density and resistance against laser damage more than makes up for it. On top of that, they are extremely cheap. In practice, ceramic armor tends to be the most common spacecraft armor.

Synthetic fibers like Aramid, which includes Kevlar, Technora, and Twaron have exceptional strength against projectile damage while having very low density as well. On the other hand, they suffer badly against laser damage, having rather low melting points.

Then there are the more exotic materials. UHMWPE, or Ultra-High Molecular Weight Polyethylene, is a plastic which has enormous strength and incredible low density. Similar to Aramids, though, its melting point is rather low. Nanocomposites tend to very strong as well, but are difficult and expensive to manufacture. And of course, you can use Aerogel in game, though its usefulness as armor is questionable. Same with Metallic Microlattice.

A few materials are omitted, most prominently, Carbon Nanotubes (CNTs) and Carbyne. CNTs promise incredible strength, but with today’s technology, they are limited to bulk use. This entails essentially crushing a pile of CNTs together and hoping for the best, which performs very poorly compared to the strength of a single CNT. It is unclear if achieving CNT strength on a macro scale is possible or just a pipe dream. In all likelihood, they will likely be used as a way to bolster existing material strengths.

This particular capital ship uses Reinforced Carbon-Carbon and Silicon Carbide as its main armor layers.

But armor is more than just a single plate of the strongest stuff you can find.

Whipple Shields are simple, cheap, and very effective. They are a specialized case of Spaced Armor. A thin plate of low density material spaced out far off from the main armor can shock incoming hypervelocity projectiles into plasma. The purpose of the spacing is to allow time for the plasma to expand into a greater area, and thus spread its damage thin.

In practice, Whipple Shields delay the inevitable. When the enemy is dumping a thousand bullets per minute at you, your Whipple Shield is going to get demolished very quickly. However, they buy you a small amount of time (less than a minute), and those tens of seconds might be exactly what you need to get in close with more powerful, closer range weapons.

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Using a laser at long ranges to etch away an enemy’s Whipple Shield is not the fastest method of dealing with it, but it works.

Composite Armor is a fancy term for multiple layers of very different materials. As shown above, different materials perform better under different situations, so having composite armor is a way to have the best armor for each situations. In Children of a Dead Earth, each layer can be spaced out as well, combining Composite Armor with Spaced Armor/Whipple Shields.

Sloped Armor is simple, cheap, and absolutely crucial. Simply angling the armor plates can have a tremendous effect on survivability. This is not just because the effective armor thickness increases with the angle, but also because it causes projectiles to ricochet, deform, or deflect. Sloped armor is arguably the most effective defensive measure.

In practice, missiles with extremely pointy faces survived far longer against enemy point defenses than missiles with flat faces. I discovered this when one of my alpha testers pointed out that between two of the smallest missiles, one with a tiny flak payload and one with a tiny nuclear payload, the flak missiles were surviving at 10x the rate of the nuclear missiles. As it turned out, the flak payload with long and thin, and convex armor hull wrapped it into a point, while the nuclear payload with short and fat, leading to a flat face.

All missile designs were given a sharp nose after that.

Ever since that incident, missile noses were deliberately elongated into points, regardless of payload.

There are a few more design choices not in Children of a Dead Earth but should be mentioned for completeness.

Slat Armor uses a cheap grid around the armor to deform explosive projectiles. In space, however, most payloads are detonated before colliding, and explosive projectiles are already weak in space. Slat armor is easily defeated by Tandem Charges.

Reactive Armor layers the armor with explosives which detonate outwards when hit. While effective against incoming explosive payloads, again, explosive payloads are not used nearly as much as nuclear payloads and simple projectiles. Additionally, this armor needs to be outside of any Whipple Shield, lest the Whipple Shield blasts out numerous tiles with each expanding burst of plasma. Not a very effective armor if the enemy is carpeting you with bullets. Also defeated by Tandem Charges.

Electric Armor is two plates of armor separated by an electrical insulator, and one side charged. When a bullet hits and closes the circuit between the two plates, the electrical discharge will vaporize the bullet or even turn it into plasma. This would require an additional armor layer beyond the Electric Armor to shrug off the plasma, likely spaced out to let the plasma spread thin. While promising, this technology does not exist in any current form, and its limitations are unclear.

Radiators dark, crew dead. A swath of holes have been cut longitudinally through the majority of the craft’s hull.

That’s all for armor! Armoring your spacecraft designs is very simple, but the ramifications end up being very complex. And as with many designs, optimal solutions are never as obvious as you may think.