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.

What to shoot?

We’ve explored how we launch projectiles at the target (Space Guns), now we will explore what we launch at the target.

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Point defense railguns firing tiny 10 gram bullets against a missile salvo.

The simplest projectile is a solid block of mass with a burning pyrotechnic tracer on it. But even a block of mass has several complexities.

The material of a bullet can also be varied to cause for differing effects upon hitting. However, for railguns and coilguns, launching the armature itself is more cost effective than using the armature as a sabot for another projectile. This means railgun and coilgun rounds are restricted to highly conductive and highly magnetic materials respectively.

The bullets need to be cylindrical because they are launched from a tube, but they need not be aerodynamic in space. This means any sort of shape is viable in space, not just a bullet or thin penetrator. Fat blocky bullets are a viable shape, as are launching thin plates, flat side to target.

Given that aerodynamics is not a concern, what is the ideal shape of a bullet in space?

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A fin stabilized Kinetic Energy Penetrator.

 

The answer is rather complicated. Certain relations are obvious though. For instance, a thinner shape applies a greater amount of pressure, as the energy is concentrated into a smaller area, so it seems like thin penetrators would be ideal.

However, there are two issues with that.

One is that Whipple Shields shatter thinner projectiles easier. Whipple Shields are often judged primarily by their critical diameter. This is the maximum diameter of projectile that they can be hit with and still successfully shatter or vaporize the projectile so that it causes no major damage. Obviously, material properties and impact velocity are both important, but projectile diameter is the main factor. Thus, thin penetrators are often not really worth the extra damage since Whipple Shields are ubiquitous.

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A single-stage and a multi-stage Whipple Shield after impact.

Conversely, high velocity projectiles can sometimes be too effective. Indeed, given a large coilgun, shooting extremely high mass, high velocity rounds, the bullets can often blast straight through the initial Whipple Shields, straight through the main bulkhead, through several filled propellant tanks, out the external bulkhead and Whipple Shield, and finally off into space again.

Because only the crew compartments are pressurized, a spacecraft can suffer complete penetration and still keep trucking at 100% effectiveness. Spacecrafts can even get blasted in half and both halves can still remain reasonably effective (assuming each half still has a functioning crew and powerplant). Or into thirds, or fourths, and so on.

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Cut in half but still threatening. Probably not for much longer though.

This is one reason why heavy redundancy in spacecrafts is necessary. A single lucky shot can immediately disable a ship if there is only one crew compartment.

On the other hand, one of my alpha testers went a different route. Rather than a single large capital ship with multiple redundancies, he preferred tons of tiny capital ships with zero redundancy. Either solution works, and has different pros and cons. More on this in a later post.

But back to projectiles being too effective. A projectile works best if it can penetrate the outer Whipple Shield and bulkhead, but is stopped there. That way, it can ricochet around, or if it shatters into plasma, it can inflict the most damage on the internals. In a sense, a larger area of effect is more important than simply raw damage.

Not only that, a projectile which passes straight through a ship fails to transfer much of its momentum to that ship, while a projectile that hits inelastically transfers all of its momentum. With very massive and very fast projectiles, inelastically hitting can cause tremendous torques on the impacted ship. Ships that spin out lose their carefully aligned targeting, and require precious seconds to reorient, which can mean life or death in a battle.

Or if the ship is particularly small, fast spins will splatter the crew against the inside of their crew compartments. This is actually one reason why I try to keep the crew modules near the center of ship mass, to reduce the torque on the crew in such a scenario.

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Of course, if you don’t have enough power behind your bullets, you won’t penetrate the main bulkheads at all.

If your projectile is too powerful, then the obvious solution is to fire large, flat, plate shaped projectiles rather than thin penetrators. This reduces the pressure, and the damage area is increased. However, this can be tricky since large bores will mess with the performance characteristics of all projectile weapons.

Another solution is to split the projectile into smaller, less massive pieces right before hitting the target. Flak bursts were developed as anti-aircraft warfare, and they remain an effective way to distribute damage over a larger area of effect. A small explosive detonates the payload into a cylinder of fragments, and the detonation speed can be determined accurately (using the Gurney Equations). This allows you to have very fine grained control as to the size of this “sparse projectile”, and you can detonate it at different proximities to yield differently sized clouds of fragments.

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A flak barrage slices through a Laser Frigate.

Other payloads possible in Children of a Dead Earth include explosive payloads and nuclear payloads. Nuclear tends to be a very powerful but expensive option, though due to the lack of atmosphere, their damage is incredible within a few meters, and then it falls of painfully fast. Explosives are similarly restricted to very small areas of effect but are much weaker, though they are fantastically cheap.

Should your projectile have thrusters? You can put thrusters on the projectiles you launch, and this can greatly increase their accuracy, however, the mass of the rocket engine and propellant is very costly. From what I found, barrages of thousands of small “dumb” projectiles tend to win out against tens of large “smart” projectiles, though I’d be interested if someone managed to optimize them to be competitive.

Often, a laser battery can effectively point defense tens of smart rounds, but against thousands of incoming bullets, no laser battery can keep up. Generally, against point defense, either you saturate them with a storm of tiny bullets, or you launch full blown missiles with heavy armor and large delta-v stores to get through.

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Railgun point defense against a salvo of armored missiles. The missiles and their exhaust plumes are too small and dark to see, so they are each highlighted by the green circle.

There is another interesting aspect of projectiles that is often overlooked. Muzzle velocity is often optimized to be as high as possible. It increases the range and the impact damage. And even if it is too damaging, increasing the area reduces the damage without sacrificing range. It seems velocity should always be maximized.

Yet the equations for damage on Whipple Shields and Bulkheads are very nonlinear, and they have very different damage responses between hypervelocity and hypovelocity impacts. Indeed, Whipple Shields lose effectiveness for low velocity impacts, as the projectiles suffer little to no break up at low velocities.

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Whipple Shields perform the worst between the Ballistic Range and the Shatter Range, which is approximately 3 km/s.

This is one particular case where conventional guns and their low muzzle velocity are actually desired. When the enemy is packing multiple and/or stuffed Whipple Shields optimized against high velocity railguns and coilguns, low velocity conventional guns tend to be the trump card. This is one case of many where drones, which usually carry conventional guns, tend to make short work of the enemy.

Fun with Orbital Mechanics

Children of a Dead Earth uses an N-Body Simulation to simulate its orbital mechanics. Most other games which simulate orbits use the much less accurate Patched Conic Approximation, and I’ll go into the details of why in this post.

What is the Patched Conic Approximation, first of all? The Patched Conic Approximation treats all orbits as ellipses around a celestial body. Then, when actually making thruster burns, the current ellipse is swapped out for a new elliptical trajectory. When making an exit from one orbit into the parent body’s orbit, a new ellipse around the parent body is swapped in for the child body.

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A perfectly elliptical orbit, although it rotates slightly with each period.

As you might expect, this is rather inaccurate. Indeed, when plotting actual spacecraft trajectories, NASA starts with the Patched Conic Approximation to get a “napkin estimate” of the trajectory, and then they switch to using an N-Body Simulation when they actually need to calculate it precisely.

Originally, Children of a Dead Earth was supposed to use Patched Conics. However, I discarded them when I wrote a simple N-Body Simulator to compare them against, and found the Patched Conics diverge very heavily from the N-Body Simulator. It was also the right choice: Switching to an N-Body Simulator allows a wealth of new orbital features, such as Orbital Perturbation and Lagrange Points.

So what is an N-Body Simulation? It’s simply a simulation which takes into account all gravitational forces in the entire system (in my case, the entire solar system) and applies them at each time step with a numerical integrator. Children of a Dead Earth uses a Fourth Order Symplectic Integrator by Forest and Ruth.

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An orbit around Mercury perturbed significantly by the Sun.

Orbital Perturbation is a phenomenon that only shows up in an N-Body Simulator, and it makes a world of difference. Perturbation is simply the effects of gravity from more bodies than just the primary orbited body. Patched Conics do not simulate perturbation because they are only concerned with the gravity of one body at a time.

When orbits can be perturbed, they lose their perfectly elliptical shape and become at best distorted, and at worse, completely unpredictable, losing all periodicity.

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Some orbits become perturbed so much that they become utterly unpredictable, and don’t seem to behave like orbits at all.

Why is this important? Consider the following story, which happened to one of my Alpha playtesters. He had spent a lot of delta-v injecting into orbit around Oberon in the Uranian system, and didn’t have enough delta-v to rendezvous with the destination space station around Oberon. His injection orbit was highly out-of-plane and succeeding seemed hopeless. But instead of giving up, he simply disabled stationkeeping, letting his spacecraft enter a free falling trajectory. Using the orbital perturbation of Uranus, he allowed the planet to perturb his out-of-plane orbit, gradually flattening it out into an orbit coplanar with the target space station. Eventually, by simply letting gravity do the work over several orbital periods, he had an orbit which could rendezvous with the target with minimal delta-v usage.

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Using orbital perturbation in combat around Ganymede.

In combat, using orbital perturbation to your advantage is another effective but difficult technique to use. Dropping into a low orbit is often a useful tactical choice, as it forces your opponent to expend copious amounts of delta-v. However, it reduces orbital perturbation of nearby bodies.

On the other hand, entering very high orbits causes combat speeds to slow heavily, yet it costs much less delta-v, and it greatly enhances orbital perturbation. A perturbed orbit is much harder for the enemy to intercept because of its unpredictability, and the enemy will have to expend much more delta-v to reach you while you let gravity pull you along.

Furthermore, techniques like Orbit Phasing, which is a very effective way to intercept enemies, get thrown out the window when your target stops stationkeeping and enters free fall. Intercepting the enemy requires much more care than using simple maneuvers. After all, Orbit Phasing and Hohmann Transfers were developed for the Patched Conics Approximation, and are much harder to pull off in an N-Body Simulation.

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A propellant depot in a tight orbit around the Jupiter-Europa L4 Lagrange Point.

Lagrange Points are also simulated only through N-Body Simulations; Patched Conics fail to correctly simulate these. Lagrange Points are five or fewer points around each celestial body and its parent where an orbit that remains stable relative to both bodies. Children of a Dead Earth simulates all manner of Lagrange Point orbits, from simple circular ones to more exotic Tadpole Orbits.

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A figure 8 orbit around Mercury’s fourth Lagrange Point, Sun-Mercury L4. This orbit is a tadpole orbit that is twisted on itself.

That’s all for the N-Body Simulator! There is one additional aspect of Children of a Dead Earth’s orbital mechanics that is unique, and that is the way it handles multiple frames of reference, however, that will be another full post in and of itself.

Burn Rockets Burn

We’re long overdue for a post about the rocketry of the game itself, so here it is finally.

Reaction engines are the cornerstone of any exploration of space warfare. Zero-propellant drives such as solar sails, laser sails, electromagnetic tethers, and the like, are not explored by Children of a Dead Earth due to certain limitations, particularly thrust. As you’ll see soon enough, thrust ends up being a heavily limiting factor for space travel.

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Design for a Nuclear Thermal Rocket, which is more or less a nuclear reactor jammed into the thrust chamber of a thermal rocket engine.

The primary drive of Children of a Dead Earth is the Solid Core Nuclear Thermal Rocket, though a number of other technologies are supported. Many futuristic and experimental technologies were not included because a full treatment of these technology’s limitations has been published in scientific literature. Implementing the basic equations for a technology’s abilities, without fully implementing the mechanical and thermal stresses of that technology, would be disingenuous towards the end goal of the game. From my perspective, only exploring what a technology can do without keeping tabs on what it can’t is no better than inventing fictitious technologies altogether.

But anyways, why is the Nuclear Thermal Rocket (NTR) the go to drive in use? If you’ve been following the blog, you’ll find that it’s constantly brought up that delta-v is the limiting factor on just about everything. And due to the rocket equation, the easiest way to get more delta-v is to get a better drive with a better exhaust velocity. Well designed Solid Core Nuclear Thermal Rockets achieve 4 – 9 km/s, better than chemical propulsion, but mediocre in comparison, for instance, to ion thrusters, which can achieve 100 km/s or more. Or if you go for laser propulsion, or fission sails, or many more options, you can achieve orders of magnitude better exhaust velocity.

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Hey look, those Magnetoplasmadynamic Thrusters have great exhaust velocities. Why don’t we use them instead?

You can also scale up the thrust of a rocket, but not the exhaust velocity. If you stick two identical engines together, your thrust doubles, but your exhaust velocity stays constant. So isn’t exhaust velocity the most important attribute of an engine?

Not exactly. You can work with a mediocre exhaust velocity with a greater mass ratio (though this maxes out too, this will be discussed in future posts) and staging. Counterintuitively, trying to get around mediocre acceleration is actually far more difficult.

After implementing the Nuclear Thermal Rocket, I looked into ion thrusters, and settled on the Magnetoplasmadynamic (MPD) Thruster, because it had some of the highest thrust out of all of them, and thrust is quite nice for dodging in combat. I built a few thrusters in the megawatt range, tried them out on a few missions, and their limitations became immediately apparent.

Thrust, and by extension, acceleration, is not simply important for dodging in combat. Low accelerations not only prevent you from using standard orbital maneuvers like Hohmann Transfers or Orbit Phasing, they vastly increase burn time and ultimately travel time. Getting between planets, for instance, might require a longer burn time than the actual period of the planets themselves, years, or even decades! Getting cargo anywhere in the solar system was prohibitively slow. NTRs and chemical propulsion turned out to be far superior.

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It doesn’t matter that you have tons of delta-v, if it takes you years or decades to use it all.

With combat spacecraft, it only got worse. The accelerations are so poor that orbital evasion maneuvers are impossible to execute against high-g missiles or anything else really. In range of weapons fire, enemy projectile range is limited primarily by the target’s areal cross section, and its acceleration. With accelerations as low as the MPD Thruster was yielding (micro-g’s at best, nano-g’s at worst), my warships were basically immobile, sitting ducks for the enemy. And I should emphasize: The MPD Thruster yields one of the highest thrusts of any of the ion drive designs.

Okay, though we can just crank up the power consumption, and get higher thrusts, right? You can actually, as long as you keep an eye on the various stresses of the design. In particular, Onset Phenomenon of MPD Thrusters gets rather nasty at megawatt levels of power.

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Designing a MPD Thruster. Compare it to the NTR above. This MPD Thruster has 4 times the exhaust velocity, yet the NTR has over 10,000 times the thrust.

But more power requires more nuclear reactors, and more reactors require more heat radiators. The amount of heat radiators by mass became overwhelming, and reduced the spacecraft’s overall acceleration faster than adding more MPD Thrusters could increase it.

At this point, the Rocket Power equation (it’s further down in the link) should be pointed out. For a given amount of power, the thrust is inversely proportional to the exhaust velocity. This equation became very evident with MPD Thrusters. One could increase the mass flow of the engine (and thus, the thrust) at the cost of plasma excitation (and thus, exhaust velocity). The two quantities were directly opposed.

I designed a MPD Thruster with exhaust velocities comparable to NTRs, and found the thrust still lacking. NTRs had a rocket power in the multi-gigawatt range, and my MPD Thrusters were in the tens or hundreds of megawatts range. They’re both run via nuclear reactors, so why were MPD Thrusters so much worse in this regard?

It wasn’t until I designed resistojets powered by a nuclear reactor (Nuclear Electric Propulsion) that it hit me. The additional step between the nuclear power generation and actually utilizing this power is the problem. NTRs and Combustion Rockets expel most of their waste heat through the exhaust itself, while nuclear powered resistojets and MPD Thrusters must expel most of their their waste heat through heat radiators unconnected to the drive. Making a MPD Thruster or resistojet with the same power as NTRs requires a staggering amount of radiators while NTRs do not. Counting the mass of the radiators needed for such a high powered MPD Thruster into the drive’s total thrust-to-mass ratio yields abysmal ratios.

In fact, most drives suffer from this same limitation that NTRs and Combustion Rockets avoid. Only a few drives, like Nuclear Pulse Propulsion manage to sidestep the issue of requiring enormous amounts of radiators to have comparable power. But ion drives, and nearly all other high-exhaust-velocity counterparts, fall flat. As it turns out, thrust is hugely important to spacecraft.

It was at this point when it finally clicked for me why aerospace engineer Robert Zubrin, inventor of the Nuclear Salt Water Rocket, has long since called ion thrusters (in particular, VASIMR) a hoax. I personally wouldn’t call them hoaxes, but in order for them to be the future of space travel would require a hypothetical advance in technology to fix their glaring flaws. As it stands, ion thrusters don’t appear to be viable for any sort of bulk space transportation. For scouts and tiny probe spacecrafts, ion thrusters are great. But for moving cargo, passengers, and military ordnance around the solar systems, ion thrusters and electric propulsion simply aren’t going to cut it.

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Leave it to Zubrin to conceive of an engine that requires continuously detonating nukes inside your rocket’s thrust chamber.

Extremely high thrust propulsion looks to be the way to go, unless the radiator limitations can be solved somehow. Discounting far future drives like antimatter engines, this cleanly kills off a huge portion of potential drives for bulk space travel: ion thrusters, most sails and tethers, electric thrusters, photon thrusters, fission fragment thrusters. All you are left with besides chemical propulsion is nuclear, nuclear, nuclear. Maybe I should’ve listened to Robert Zubrin from the start.

That’s a quick run through on the rocketry of the game, next time we’ll explore one of the most overlooked, yet extremely important parts about space travel: the propellant tanks themselves, as well as the pros and cons of different propellants to use. As it turns out, neither of these are as simply as you might imagine.

How Realistic Is It Actually?

Children of a Dead Earth was developed primarily to answer the question: What would space warfare actually be like? Various hard science fiction novels and other media have attempted to answer that question in the past, but these works have always come up short, at best relying on rampant speculation, and at worst, inventing fictitious technologies to support their conclusions.

Children of a Dead Earth solves these issues by first being a complete and utter simulation of space warfare, and second by relying only on technology that has been explicitly demonstrated to work. This is critically important: there is absolutely minimal guesswork in this game, instead everything is necessarily derived from equations, and the mathematical results of these equations.

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In case it is not obvious by now, Children of a Dead Earth was designed with no vision in mind, unlike just about every other videogame, novel, or movie attempting to do the same. This is because I never wanted to corrupt the ultimate goal of this project, which was to discover what space warfare would be like, rather than to say what space warfare would be like. To have an initial vision when building this game would have been starting with a conclusion, and then twisting reality to support that vision. By starting with no vision whatsoever, the conclusion would be generated by implementing the equations, and observing how they interact. In this way, the end result of Children of a Dead Earth was little like I had ever imagined actual space warfare would be like, and this will probably be true for you as well.

To reiterate: Children of a Dead Earth is a simulation first, and a game second. No amount of realism was compromised to make things more fun, or to make things prettier. It is science first, everything else second. Despite this, the game still remains fun, but you’ll find it plays very differently than any space warfare game you’ve ever played.

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Some assertions in making this game. All technologies implemented in Children of a Dead Earth have all been demonstrated to be feasible, and successful prototypes of all of them have been created in real life. For engines, this includes Nuclear Thermal Rockets, Combustion Rockets, Resistojets, and Magnetoplasmadynamic Thrusters. For weaponry, this includes Conventional Cannons, Railguns, Coilguns, Linear Induction Motor Launchers, and Arclamp Pumped Solid State Lasers. For powerplants, this includes Radioisotope Thermoelectric Generators and Thermoelectric Solid Core Fission Reactors. And of course, radiation shielding, monolithic armor plating, Whipple Shields, and heat radiators are all fully implemented in game as well.

Another assertion: stealth in space is not feasible. It’s a topic large enough for its own post, so I’ll summarize: if you actually want stealthy engines, you need to either to move so slowly that getting between planets will take centuries, or you need engines that are insanely efficient, more efficient than any technology ever imagined.engine.png

A final assertion. Spacecrafts will be crewed, but any concept of ‘fighters’ will not be a thing. The mass of having people is significant, and an issue. The spaceflight analogue to fighter jets do not need a pilot, and instead these drones can be remotely piloted by the nearest capital ship. Such a thing exists today already: UAVs. On the other hand, capital ships do need a crew, because of speed of light lag. Trying to command a spacecraft from across the solar system is not just highly prone to jamming and spoofing, but the seconds or even minutes of lag in command would prove fatal in combat. On the other hand, drones will be close enough to their carrier ship that speed of light lag is not a significant issue.

How granular is the simulation? Extremely granular. Let’s take railguns, for instance.science-railgun.pngTo determine the muzzle velocity of a railgun, an equation for the force applied on the armature was used. This equation was numerically integrated to determine the acceleration of the projectile over time steps sometimes as granular as nanoseconds across the entire rails. The equation itself requires knowledge of the inductance of the rails, as well as the resistance across the rails. Inductance is generated from another equation depending on the relations between the rail dimensions, and the resistance is calculated from the material properties and dimensions of the armature and rails. This also informs the size and mass of the railgun, which is critical for ship design later on.

But that’s just the beginning. A weapon is more than a projectile launched. It also generates heat, which can melt the rails or the armature itself. The ablation of the rails and of the armature are calculated separately from additional equations utilizing the material specific heat, conductivity, density, dimensions, and vacuum permeability. If the ablation is too significant, the railgun will not operate properly, and you will be unable to operate the railgun. Even if you do not ablate your rails, if the bulk temperature of the rails becomes too high, the materials will be unusable until they cool, and you must wait for the railgun to expel its excess heat via radiation.

All projectiles have minor imperfections. A tiny off centering of the armature can produce significant inaccuracies in firing, and this too is calculated. The recoil of the railgun can become imbalanced, and cause significant cantilever beam deflection with the railgun barrel. Based on the tensile strength of the rail material and the Elastic Modulus, this instability is another factor that one must work around when designing railguns in Children of a Dead Earth, lest they shatter when you fire them.

And finally, don’t forget about the turrets! The more massive you make your railgun, the greater the moment of inertia of the weapon, and the harder it is to rotate with the reaction wheels inside the mantlet housing. You will find that you have to make tradeoffs with your weapon, as a railgun which takes ten minutes to aim at a target is a railgun which is not usable for combat.

That’s a taste of the depth of simulation within Children of a Dead Earth. Next up, I’ll be tackling how space warfare actually ended up, based on what has sprung forth from these equations.