Life in the Lonely Void

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

A look inside the ISS.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Cutaway of a modern nuclear submarine.

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

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

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

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

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

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

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

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

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

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

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

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

Radiation types and how easy they are to block.

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

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

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

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

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

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

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


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.

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.

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.

plume spectra.png
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.

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.

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.

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.

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.

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.

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.

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.

decoys 2.png
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.

decoys 3.png
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.

100 missiles.png
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.