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?

Rocketry

Maneuvering

Orbital Mechanics

Weapons

Armor

Detection

Power

Crews

Scaling Laws

 

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

Supercriticality

One of the most complex parts of a spacecraft is the power supply, which usually takes the form of a nuclear reactor.

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Cherenkov Radiation from a nuclear reactor core.

Nuclear fission is about 600,000 times more energy dense than the most energetic chemical power reactions, which very easily makes it the best power supply for space. In space, mass is a premium, so energy density is critical. Most capital ships use fission reactors to get their power.

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Schematic of a Radioistope Thermoelectric Generator.

Similarly, Radioactive Decay runs about 15,000 times more energy dense than chemical reactions, which is why Radioisotope Thermoelectric Generators (RTGs) are the next best thing. Most drones and missiles with significant power needs use RTGs. Anything with lesser power needs simply use small batteries.

Chemical energy requires tremendous amounts of mass produce reasonable amounts of power, after which this mass is ejected. This makes it not very viable for space except as rocket propellant.

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Solar panels in space. Beyond Earth, they rapidly lose effectiveness.

Solar power becomes more or less useless further away from the sun (at Jupiter, the irradiance is 30 times lower than that on Earth). They are also much harder to armor than radiators, making them a poor investment for space warships.

Beamed power suffers from the beam waist widening very quickly over distance, which limits its effectiveness to very low orbits around celestial bodies. This occurs even with a constellation of mirrors to extend the range. Beamed power similarly require receivers that can be damaged easily, and if they are to be used in close combat, they can’t be covered up, making beamed power fairly infeasible for warfare.

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A Z Machine, a device for researching Inertial Confinement Fusion (ICF).

Fusion Power is a power source that could make all other sources obsolete if it can be developed. In its current state, it is too far future of a technology. An additional consideration about Fusion Power is that it, along with Fission Power, are not limited by energy density, unlike the other aforementioned power sources. While the other power sources are limited by not producing enough power, Fission and Fusion Power both generate more power than modern systems could ever dream of using. Fission Power is not limited by how much nuclear fuel you have, it is limited primarily by how large your radiators are. This means any limitations on Fission Power currently will still remain even if Fusion Power replaces it.

All of this means that the main power supply in Children of a Dead Earth is nuclear fission.

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Cooling towers of a nuclear reactor. The actual reactor core is underground, and tiny compared to the machinery used to extract power from the core.

In Children of a Dead Earth, nuclear reactor cores are governed by the Six Factor Formula. This rather large and complex equation which determines the effective neutron multiplication factor. Given the masses of all the materials inside the core from the fuel to the moderator to the neutron poison to the coolant, the operational characteristics of the reactor can be determined. This means the rate at which a core achieves criticality or shuts down can be predicted, and it usually takes microseconds.

Given a working reactor core, the neutron flux of a reactor can be arbitrarily controlled. The neutron flux directly determines the amount of heat the reactor will produce, which means the reactor can arbitrarily control the heat produced, and by extension, the power produced.

These fission reactors are not limited by the Uranium-235 fuel (or Pu-238 or Am-242m, etc.) inside the core. In practice, the amount of nuclear fuel is minuscule compared to the rest of the reactor elements. This is why 3% enrichment of U-235 is actually quite reasonable for reactor fuel. Contrast this with nuclear weapons, where around 97% enrichment of U-235 is preferred.

The main power limit inside the core is how hot you can make it. And generally how much heat your reactor can withstand is dependent on how large you can make the reactor to reduce energy per unit area. Additionally, the design of your fuel makes somewhat of a difference, as fuels such as TRISO allow for a Pebble Bed Reactor design, which yields somewhat higher heat tolerances.

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Cutaway of a TRISO pellet, showing various ceramic layers over a U-235 pebble.

The reason temperature is the limiting factor of a nuclear reactor, rather than the actual fuel mass, is that the energy density is so high that a nuclear reactor will never come close to unlocking all of the power of even a small amount of fuel mass. Nuclear reactor design mostly boils down to how much energy can be extracted from a tiny amount of nuclear fuel without melting to slag.

Thus, how much nuclear fuel is needed has nothing to do with how much power you need, and everything to do with whether or not your reactor can achieve a critical mass.

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CANDU fuel bundles, another nuclear fuel design for heavy water reactors.

For this reason, using more or less energy dense nuclear fuels is basically pointless, as they all produce “too much”. The main difference between fuels is how they affect the neutron multiplication factor. Certain fuels, like Am-242m, can achieve a critical mass more easily, and thus, less nuclear fuel is needed. This means the reactor design might be made smaller for the same amount of power.

In practice, however, the mass of the radiators ultimately end up being the limiting factor on nuclear reactors.

Of course, simply creating an atomic pile and letting it spew neutrons and radioactive waste is not enough to produce power. Power needs to be extracted from the heat of these fast moving nuclear byproducts. A number of methods have been devised to do so.

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The exposed turbine of a turboelectric nuclear reactor.

Turboelectric Fission Reactors involves letting the neutrons heat up the coolant and using the hot vaporized coolant to turn a turbine. This technique is the most common, being used in just about every nuclear reactor in the world. It requires a large turbine to effectively extract the heat, as well as plenty of coolant and turbomachinery to run it properly. Due to the size, it can take a while to warm up. Due to the mass and complexity, it generally is not used for spacecrafts.

Turboelectric Fission Reactors are heat engines, meaning they can never exceed the efficiency of a Carnot Cycle:

\eta = 1 - \frac{T_c}{T_h}

Where \eta is the efficiency, T_c is the cold temperature of the coolant prior to passing through the reactor, and T_h is the hot temperature of the coolant after passing through the reactor.

As noted in Why does it look like that? (Part 3), the cold temperature needs to stay high because this is the temperature the radiators will cool the reactor at. Cold radiators function abysmally and require huge amounts of mass. Similarly, the hot temperature needs to stay low to prevent the systems from cracking from the thermal stress.

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A thermoelectric reactor which produces 13.5 MW. Does not include radiators.

The sample reactor has a radiator temperature of 1200 K and 1688 K as the hot temperature. Using the above equation, this means the efficiency of that reactor can never exceed 29%, if it were theoretically perfect. In practice, the actual reactor runs at 22% efficiency.

On Earth, turboelectric reactors often quoted at running at 60% efficiency. This is possible because the cold temperature can be brought much lower if you are using conduction or convection to cool off the reactor. In space, using only radiators forces the cold temperature to be rather hot (> 1000 K usually) in order to keep the radiators from getting too large.

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The SNAP-10A reactor, a thermoelectric reactor developed by the USA in the 60’s.

Thermoelectric Fission Reactors are heat engines as well. However, instead of passing the hot coolant through a turbine, it instead passes against as a thermocouple heat exchanger. It involves much less turbomachinery, and thus can be produced in much smaller sizes with much lower masses. On the flip side, it is often much further from the theoretical Carnot Cycle efficiency than turboelectric reactors. In space, it is the primary go-to reactor design due to its low mass, simple design, and cheap cost.

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The TOPAZ nuclear reactor, a thermionic reactor developed by the Soviet Union and launched into space in the 80’s.

Thermionic Fission Reactors uses the concept that the flow of charge carriers (such as electrons) across a potential barrier can produce power. This flow of electrons is triggered thermally, and thus is set up very similarly to the way Thermoelectric Fission Reactors work. It is also a heat engine, and sees similar use in space as the thermoelectric fission reactor.

Fission Fragment Reactors do not use coolant at all, and instead extract power by decelerating the neutrons and radioactive waste products using a magnetohydrodynamic generator. This bypasses the Carnot Cycle entirely, allowing efficiencies estimated up to 90%, far greater than any of the previously mentioned heat engines. However, it is also the least developed of all the technologies, and to date, no working Fission Fragment Reactor has been produced.

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Another picture of the Cherenkov Radiation of a nuclear reactor. Just because.

Since Children of a Dead Earth restricts itself to functioning technologies, this means nuclear reactors are restricted to Heat Engine designs. And due to the Carnot Cycle, the efficiency is forever limited by the cold temperature, or the radiator temperature.

Due to this restriction, the most massive part of a nuclear reactor is the radiators that accompany them. The power extraction machinery tends to be the next most massive piece, with the reactor core itself generally being negligible in terms of mass. On the flip side, the tiny amount of reactor fuel tends to be one of the most expensive parts of the entire system.

Sensors and Countermeasures

There are numerous ways to guide a missile or drone to the target. But for every potential way to guide an autonomous payload to the target, there is a countermeasure, and possible counter-countermeasures, and so on.

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Close up of photodetectors of a telescope.

These techniques remain relatively unchanged from Earth, but there are a few significant differences in space. One is that there is no horizon.

Two, there is no significant intervening medium (air or water). This first makes sonar and acoustic homing irrelevant, but it also vastly reduces the effectiveness of any gaseous countermeasure like smoke. Another consequence is that exhaust plumes fade out very quickly. However, the biggest change this makes is that it reduces the noise floor heavily, making targets stand out much more against the background.

Three, there is no GPS or satellite network for additional guidance help, and if there is one, only the defender would have access to it.

Before we discuss guidance techniques, a quick primer on sensors. Sensors are built as Photodetectors for a specific wavelength likely with a telescope lens. While they can be expensive, especially if you want them diffraction limited, they require so little power that power use is assumed negligible, especially in comparison to all the high power systems of a spacecraft, missile, or drone. Sensors actually give you power, but this will be less than any rotators or computing system utilizing the sensor.

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Photodetectors being installed on the HESS telescope.

Unlike lasers, sensors can be made very close to being diffraction limited because they are so low power (if kept very cold).

What can they see? As mentioned in Stealth in Space, you can detect the radiators and the exhaust plume from far away, so there’s no doubt you will see them up close. At combat ranges, the ship itself will be visible from light reflecting off the hull. Less sensitive sensors will be needed at that close of ranges, lest you burn out your sky-scanning sensors.

But what is the visual resolution of these sensors? Although you can see the exhaust plume billions of kilometers away, it will show up as a single pixel, which is not helpful. Note that you can not achieve stealth in this way. You can’t hide multiple identical ships in a single fleet via pixel resolution. Careful study of the spectrum of this single pixel over time will reveal multiple overlapped exhaust plumes.

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Data produced from a single pixel can be studied and differing emitters can be determined.

Visual resolution of a diffraction limited optic can be easily calculated using the angular diameter (follow the links for the relevant equations). Here are a few example calculations to give sense of range:

Given a detector 10 cm in diameter (reasonable for a drone, missile, or capital ship) looking for visible (550 nm, green) wavelengths, what’s the visual resolution?

At 1000 km away (orbital distance, close missile launching distance), each pixel is about 7 m in size.

At 100 km away (very long range projectile combat), each pixel is about 70 cm in size.

At 10 km away (close range projectile, drone, and laser combat), each pixel is about 7 cm in size.

At 1 km away (only missiles about to hit and maybe close ranged drones would ever get this close), each pixel is about 7 mm in size.

In Children of a Dead Earth, capital ships tend to be at least 50 m in length, often around 100 m long, or twice that for the flagships. Drones and missiles tend to be 10 m long at most, and usually much shorter.

This means that at missile launching distance, enemy ships are a tiny blob of a pixels while missiles are single pixels on screen. At very long range projectile combat, the enemy ship might be distinguishable as a shape with radiators, while missiles will be a few blurry pixels. At close projectile range, you’ll get a nice view of the enemy and their missiles as your shots tear them apart.

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Of course, taking into account enemy countermeasures, your view screen will likely look as clear as this.

Larger sensors will give you better feedback though larger sizes tends to be a problem for missiles and drones. Additionally, infrared sensors will have half the resolution stated above at best (since pixel resolution decreases as wavelength increases).

Visual resolution is rather important for missiles and drones, however, as their guidance systems need to be able to distinguish what they need to hit versus decoys. The better the visual resolution, the further away decoys need to be launched to fool the missiles.

Let’s go into the guidance techniques available in space.

Infrared (IR) Homing is a passive technique where the missile or drone chases infrared wavelengths. Often it would chase the greatest heat source, though this is easily fooled with decoys. Infrared Homing is the most important of all guidance techniques in space, principally because all ships give off tremendous amounts of heat from their engine and their radiators.

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The eye of an IR homing missile.

Ultraviolet Homing chases after the ultraviolet part of the spectrum instead of the IR portion of the spectrum. This is beneficial because UV decoys are much more difficult to produce and field. On the other hand, low heat radiators like life support radiators emit very little UV light, which means turning engines off and retracting powerplant radiators is an effective countermeasure.

Spectral Seeking is a little more sophisticated, where a specific spectra is targeted. For instance, the spectra of methane at 3000 K is unique among spectra, and only exhaust plumes of methane at 3000 K would be targeted. This means in order for decoys to throw this type of guidance off, the decoy needs to match the spectra exactly (which is difficult). On the other hand, the target needs only change the spectra of the engine or the radiators to throw off the guidance system.

Passive Radar is another passive technique where the missile chases after radio signals bouncing off of the target from third party sources (likely civilian). In space, due to distance, the signals are significantly weaker than on Earth.

Active Radar Homing is the same technique, but with an active radar system specifically illuminating the target. This is much more effective, and doesn’t rely on weak third party civilian signals. However, all radar systems can be fooled very easily by chaff. Additionally, Radar Absorbent Material is another effective countermeasure.

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A radar display affected by chaff. The left side is jammed by chaff, and produces no usable data.

Semi-active Radar Homing is also the same technique, but with the radar illuminator being separate from the radar homing device. In this case, the capital ships would have the illuminator, while the missiles would chase the illuminated targets. This yields a somewhat cheaper option, but it still has the same limitations as previously mentioned.

Laser Guidance swaps out radar for lidar, radio waves for Visible, IR, or UV light. The target is illuminated by a laser either with a separate device (Semi-Active Laser Homing) or with the homing device itself (Active Laser Homing). It can be fooled by paint absorbent to the particular wavelength, and such paint is much cheaper to produce than Radar Absorbent Materials. On Earth, this sort of countermeasure can be fooled by simply aiming near the target instead of directly at the target, but in space, no such solution is possible, since there is no nearby “terrain” to aim at.

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Schematic of a laser guidance system. The laser is “painted” on the moving target, and the missile chases the dot like a cat.

Beam Riding is a technique which uses either a laser beam or a radar beam to illuminate the target. Then, the missile “rides” the beam down, using the beam as a guide. On Earth, this restricts the missile to line-of-sight attacks, which is problematic, but in space, this is not a problem at all. Unlike laser guidance, it is immune to absorbent materials. The main issue, however, is diffraction. Any laser or radio beam will diffract, meaning the beam gets increasingly inaccurate as range increases. This restricts missile usage to very close combat, similar to lasers themselves.

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Diagram of a missile riding a beam to the target.

Neutron Homing is a guidance technique that does not currently exist on Earth, but is likely to gain prominence in space. The nuclear reactors on a capital ship dump out an extraordinary amount of neutron radiation (both fast and thermal) in every direction, and these neutrons more or less pass through any material short of radiation shielding. Trying to shield this radiation in every direction requires a huge amount of mass, so weak unidirectional shielding in a single direction (towards the crew compartment) is used instead in addition to long distance.

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Shielding against neutron radiation is rather costly in terms of mass.

This means that the neutron radiator of the reactor will be dumped out in a sphere shape, attenuated with the inverse square law. The background neutron of space is essentially zero in the absence of an atmosphere. This means very even trace quantities of neutrons can be picked up and used by homing missiles. The main countermeasure besides tons of expensive shielding would be launching neutron decoys: cheap, highly radioactive waste materials spewing off plenty of neutron radiation. Alternatively, a highly directional beam of neutrons can be generated as a way to throw off such missiles.

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Schematic of a Neutron Generator.

Command Guidance dispenses with all those sophisticated tracking methods, and relies solely on the launching ship to guide the missile with manual controls. It is immune to all countermeasures except for communications jamming and spoofing. This is a common technique for drones, since they do not need to collide with the target, they only need to aim at the target. In Children of a Dead Earth, this is the go-to method for aiming missiles if the enemy has countermeasures for every other technique tried.

Inertial Guidance is the ultimate “dumb” missile method, which rejects all attempts at actual guidance and homing, and simply follows a preset trajectory based on the target’s initial velocity and position. It is the least accurate of all homing techniques, but it is completely immune to all countermeasures. It can be defeated by simple acceleration or dodging, however.

Whew! That’s a lot of different guidance techniques and countermeasures and counter-countermeasures. So what is employed? All are employed to a small degree, but the primary homing technique used is one of the simplest: Infrared Homing.

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A salvo of missiles and a fleet of three ships in the background dropping numerous decoy flares.

This is because of the heat radiators on ships, and the extremely bright exhaust plumes. Countermeasures can be developed for every other homing technique, but for IR Homing, countermeasures are much more expensive.

First off, smoke (including thermal smoke) dissipates rapidly in space. Without an atmosphere, smoke expands at a constant velocity, required a huge amount of mass to provide a smokescreen for any extended period of time.

Second, deploying thermal decoys is expensive in terms of mass. Concealing hundreds of megawatts of radiator heat against a black background requires a similar amount of power emitted by the decoys. And these decoys have to be burning for a decent amount of time (10 seconds seems to be the minimum). This means the decoys will be rather massive pyrotechnic payloads.

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These three ships really don’t want to get hit.

Extremely massive payloads are difficult to launch at high velocities away from ones own spacecraft. If they aren’t far enough away by the time the missiles hit, significant damage can still result even if the missiles hit the decoys. Launching very massive decoys at high speeds requires significant amounts of power, which requires even larger decoys, which requires even larger launchers, and so on.

Third, using a high powered IR laser to blind the incoming missiles works great, except these missiles are insensitive to all but the brightest signals. This means the laser needs to produce comparable power to the radiators they decoy, which is costly.

This is exacerbated by the fact that missiles are launched in salvos, not one by one. As a result, the laser needs to widen its beam in encompass an entire formation of missiles, vastly reducing the power. And missile salvo formations can easily reach hundreds of meters wide, which means your beam needs to be hundreds of meters wide too, which is pathetically weak. Either that, or you need hundreds of high power lasers, each focused on tracking and blinding each individual missile, which is prohibitively expensive.

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A salvo of 100 missiles on its way to the enemy defense force around Ceres. Enemy force is roughly 40 km away.

Generally speaking, IR homing is the most effective guidance system, and IR decoys are the most commonly used counter-measure. In particular, dropping radiators and shutting off the engine while launching decoy flares is a common survival technique against missiles, though it is not foolproof. Things are especially difficult for large laser crafts with enormous amounts of waste heat.

In the end, no exact technique trumps all other techniques, and most electronic warfare focuses around IR homing and counter-measures. And when counter-measures are effective, Command Guidance is the usual response.