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Hard Science Space Warfare

Posted by Impeller Studios (Creator)
Christian Geißler posed some great questions about the underlying assumptions that drive our interpretation of space combat in the future. Zach El Hajj, our chief engineer and spacecraft designer, has the answers:

Christian makes a very strong point regarding those assumptions. Even the hardest science fiction goes in with certain assumptions that shape the universe, and one could even define science fiction by how it expands from such premises - if the assumption is taken to be true, we can expect the rest to logically follow. That is what we are trying to do with Starfighter Inc, and there’s a lot behind the scenes, as he deduced.

Out of universe, you could probably guess the reason we’re using manned fighters - Burnside’s zeroth law, people emphasize more with people than machines, and we thought it would increase immersion. We were willing to go to extreme lengths to make this happen. Early on, SFI was to be set in an alternate universe where computers developed much more slowly and so there was no alternative but for ships to be manned (this was partially in tribute to the golden age of science fiction), and for a while, we debated going the other way entirely and having pilots download themselves into their ship computers so you could pilot an “autonomous” drone and still be human. However, in both cases the technological implications caught up with us, so we decided to stick with a simpler setup. As for why manned fighters are used in our universe, there are three reasons.

Firstly, while you could just throw missile after missile, the greater the distance that the missile has to cross, the less likely it is to make it to the target. Between the target’s movement and its ability to evade, the time available for the missiles to be stopped by countermeasures or point defense systems. There’s a lot more involved than this, though; ships have a much easier time outrunning missiles because most missiles in-setting use chemical drives, whereas the ships use solid and gas core nuclear drives. What this means is that the missiles have much higher acceleration and are nearly impossible to outrun at close range, but the ships’ exponentially greater fuel efficiency lets them run for much longer and burn out the missiles in long pursuits. You could make a missile with a nuclear drive, but that’s a lot of money to throw away for an expendable munition, and at that point you’ve already got the basis of a drone or starfighter - just strap on a power system, ammo rack and gun, and now you’ve got a working platform that can be recovered. As for why manned fighters dominate over drones, that’s covered in the following two reasons.

FSC Northstar WEAVER Missile Schematic
FSC Northstar WEAVER Missile Schematic

 Secondly, the universe is dominated by corporations which don’t really place much value on human life, and between this and the starfighters lacking onboard life support (the pilot depends on his spacesuit), there’s not as much difference between manned and unmanned ships as might be expected. That’s not to deny the difference in potential performance due to limitations imposed by said sack of meat in the seat, but there comes the third reason - one which Christian already guessed.

Thirdly, EMP technology and distance-hacking is in very wide use in-setting, neither of which is a concern to a flesh and bones pilot, so at the very least said sack of meat can serve as a failsafe. Most of the time. The “human element” isn’t always reliable, either.

As for why they fight at close ranges, that's partially an artifact of the setup of the video - the gaps in the rings of Saturn are less than tens of kilometers across in some parts, half that if you consider shepherd moons and asteroids in-between. But there are strategic reasons as well. Simply put, starfighters do better at close range, because nearly all of their weapons are more likely to do damage. Missiles can get the jump on nearby ships and there’s less time for them to be taken out, kinetic weapons are unlikely to hit at a great distance due to the relative velocity of projectiles to the ships and movement lag, and lasers have difficulty focusing past a few hundred kilometers taking away their potential destructiveness.

There’s not much that can be done about the missiles, but for the rest it turns out the solution is to go bigger. Longer mass drivers can accelerate the projectile for longer and hence reach higher speeds, and larger laser lenses can focus light to greater distances. However, there’s a pretty big snag, in that the effects are not linear, but quadratic. If you assume uniform acceleration throughout the barrel (which is _not_ true - acceleration actually decreases towards the end, meaning the reality is worse than this), time and hence velocity scale to the square root of length, so you need to quadruple gun length in order to double the speed. Similarly, kinetic energy scales to the square of velocity, so power scales rapidly, too. For lasers, the focusing distance is proportional to the diameter of the lens, but mass increases to the square of diameter due to the increased area. Suffice to say, increasing range for either of these weapon types is a weighty proposition.

To support weapons suited for many thousand kilometer ranges, you need capital ships. These are also easier to fight at range due to their greater cross-sections and slower response times (between the scaling of thrust, moment of inertia and simply the high centrifugal forces in crewed vessels if they try to spin too fast), but they have a few notable caveats relative to fighters. For one, every capital ship taken out is a huge investment of resources and losing even one can be a hit to any war effort, whereas you could build a thousand starfighters for the same price and afford to lose a few.

Nuclear weapons are what tilt the scale here - it is possible for a starfighter to eliminate (or at least cripple) a capital ship with a sufficiently high yield bomb, but the same mass of starfighters can disperse sufficiently to require a whole fleet of missiles. For another, the square cube law works against larger ships, at least if you assume that power consumption scales up directly with ship volume (when most power systems have slightly better specific power as they get bigger).

Atlas II-class Carrier Side View
Atlas II-class Carrier Side View

 This is because the surface area that lets the ship radiate waste heat increases more slowly than power - scale up a ship tenfold, its mass and power should increase a thousandfold, but with only a hundred times the radiative area, it’s got a tenth the relative area - requiring much larger radiators that can easily make up the greater part of the ship, presenting a huge vulnerable area. On the other hand, the reverse scaling means that starfighters can get away with very small radiators and, in some cases (actually, all the ships currently shown), if run hot enough they can suffice with using their skin in this manner. Furthermore, the much smaller scale means fighters could potentially rely on highly thermally conductive materials for a solid state heat management system, making these systems much less maintenance intensive and much more durable than a capital ships’ fluid coolant loops and mass of pumps.

But there’s a third factor that’s more pertinent in setting, which is simply the implications of owning a capital ship at all. The megacorps are not engaged in open warfare, but are mostly squabbling under the table, sabotaging each other and trying to keep their squabbles away from the public eye. Insofar as nothing is really invisible in space, all they can do is control the information. Minor skirmishes between starfighters can be excused, and a carrier can ostensibly be used for civilian purposes, but a battleship means a new scale of engagement however you look at it, and raises questions and concerns the megacorps would really rather they didn’t have to answer (besides that, they’d rather not waste themselves in a full-on campaign).

That’s our setting in a nutshell. It’s a little difficult to translate to a four minute video.

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    1. Zach El-Hajj Collaborator on

      @Ko-chin Chang: Technically speaking, space doesn’t have a temperature at all – that’s a property of matter. When they say the temperature of space is about 3 K, what they’re really saying is that the radiation is equivalent to the output of a thermal blackbody (perfectly emissive) at that temperature.

      You see, all bodies emit thermal radiation as a function of their temperature – they literally glow with heat, if not necessarily in visible wavelengths. This is generally overshadowed by conduction and convection on Earth, but without surrounding material to work with, it’s the only viable means of heat transfer in space. What makes this process distinct from others is that it is indistinct from surrounding input – if a thermal blackbody is at 300 K, it doesn’t care whether it’s in a 300 K room, a 3000 K furnace, under a 100 W lightbulb or a 3 MW laser, it will emit 459 W/m2 so long as it stays at that temperature (not that it will for long).

      As such, in space thermal equilibrium is achieved when absorption from surroundings equals emission. This amounts to:

      Energy Input = Energy Output
      Absorbed radiation + Energy generated = Emitted radiation
      Absorbance*P*A + Qgen = Emissivity*5.67e-8*A*T^4

      From this, we can identify temperature:

      T = ((Absorbance*P*A + Qgen)/(Emissivity*5.67e-8*A))^0.25

      If you have an inert object (Qgen = 0) in deep space, where the only thing to absorb is the cosmic background radiation (P = 3.129e-6 W/m2), a thermal blackbody (absorbance = emissivity = 1) will reach T = 2.725 K, just as you predicted. With different coatings, we can vary absorbance relative to emissivity or vice versa, and hence get temperature to be higher or lower, as you predicted.

      But this only works so long as the surrounding radiation is significant relative to generated heat. If generated heat is much higher, emissivity and area become the most important factorsall that matter, and emissivity is limited (from 0 to 1) which leaves only area to go higher. External input basically stops mattering. The cosmic background radiation is tiny, so it can be neglected for bodies producing essentially any heat, and while sunlight is a more definite concern (anywhere from watts to kilowatts per square meter depending on where you are), it can usually be ignored in our game, because ships produce so much more.

      Here is where, I suspect, your real question begins.

      Nearly every ship system is a power hog – if given a chance to use more energy or more mass, ships almost always go for more energy. This is even becoming a problem for current spaceships, as NASA continues to research multi-MW electric drives, but the in-game weapons take this a few steps further. Let’s take the setup of my last answer for this, a Shrike launching 5 kilogram shells at 4 km/s. Each shell requires kinetic energy of 0.5*m*v^2 = 40 MJ right out the barrel, and the mass driver must actually take more than that due to inefficiencies, so let’s expenditure of 50 MJ, with the remaining 10 MJ being wasted as heat (that’s 80% efficiency, way better than any current railgun). The ship power system produces its own heat trying to deliver that – anywhere between 0.5-3 times as much heat as usable energy (25-66% efficiency) – so let’s go with 66% efficiency, in which case that 50 MJ results in an extra 25 MJ of heat. That’s a total of 35 MJ of heat per shot, and if we’re shooting once every two seconds, that translates to 17.5 MW. The Shrike has a surface area of something like 30 m2 to radiate from, if it uses its entire skin, so what temperature would it have to be to achieve equilibrium?

      1791 K, or 1518 C.

      That’s only 220 C below the melting point of iron on Earth. Actually, since the vapor pressure at this temperature is less than 10 Pa, it’s more than hot enough to boil iron in space.

      This is not actually as bad as it sounds. There are metals and ceramics that can go up to 2000 and even 3000 K in vacuum, so some ships can survive getting that hot, provided the cockpit is insulated from the rest. It is also assuming constant firing and thermal equilibrium, when actually the ship probably does so intermittently, and needs time to reach this temperature anyway. But on the other hand, that is using one gun, with all systems at pretty high efficiencies, and without any external inputs as from bombs or enemy laser fire.

      The problem isn’t that space is cold, it’s that you can’t dump heat fast enough.

    2. Zach El-Hajj Collaborator on

      BambooCrawler: You’re welcome! No worries about forgetting, there’s a lot of detail and it can get quite counterintuitive at times.

      Whether mass drivers or missiles outperform the other is a good question. Your points are definitely massive advantages of the former: there’s no propellant cost (actually, there’s a slight propellant cost from the ship to counter gun recoil), the launched projectiles are up to speed the moment they’re out the barrel (where missiles have to accelerate), they’re immune to most missile counters, and you can fit a lot more of them and launch them much more quickly. But in missiles’ favor, they can follow their targets reducing the impact of movement and need to aim precisely, smart missiles can dodge some of their counters, a big enough bomb can destroy essentially any target in a single blow without even impacting it, and a single explosive can take out multiple nearby ships at once. Each definitely has its place in combat.

      Of course, you could also bring the two together, with a very large mass driver that could pre-boost the missile, or just launches the warheads as ultra-heavy explosive rounds. However, that would be a massive power hog and cause incredible recoil, more than I’d try to mount on a Shrike. On a bigger ship like the Hyperion, it would be another story.

    3. Missing avatar

      Ko-chin Chang on

      Isn't space like 3 degrees Kelvin? Why would you need radiators at all? If anything that excess heat is needed to keep the crew warm. If you have a stellar object nearby that emits radiation you would need a shiny coating to reflect the radiation away to keep the ship from overheating but in the deep darkness of space it is very cold.

      How much excess heat are we generating that we need a radiator to radiate that?

    4. Missing avatar

      BambooCrawler on

      Zach, thank you very much for your amazing answer.

      I do agree on the part of the nukes and temperature stuff, I really forgot that part.

      On debris part I think it would be more rational just to launch big "Bullets" with mass drivers because they don't need propelant and they have a bigger fire rate.

    5. Zach El-Hajj Collaborator on

      Brent D. Schultz: Thank you! We will!

      Max Everingham: Ouch, you're correct. Kickstarter won't let us edit past the first half hour so there's nothing we can do about it now, but good catch there.

      BambooCrawler: Yes and no. You’re right that there’s no shockwave, at least not at any useful range. The exploded object’s own vaporized material does provides a means to propagate it, but it all too quickly becomes too diffuse to do anything. Explosions in space put out the same energy they would on Earth, they just do damage via different mechanisms – via collision of accelerated debris and heating from emitted radiation. At high enough temperatures, additional damage is done via impulsive shock, as the target vaporizes so quickly that the vapour moves faster than the material’s speed of sound, bouncing about, ripping out chunks and splitting the surface apart (that's certainly not the bulkhead you want to be stationed on, but if it's any consolation, you probably won't survive long enough to feel much pain). These effects do fall off far more quickly than a shockwave, which combined with the greater distances means that even nuclear explosions are far less potent than would be expected, but that doesn’t mean they can’t annihilate you if you’re on the receiving end. Nukes may also do damage more insidiously through the effects of higher radiation - embrittling materials, irradiating poorly protected crew – which can hurt at even low intensities and incredible ranges, but that’s another topic entirely.

      Which of these effects dominates depends on the nature of the bomb in question. Chemical bombs tend towards more debris and nukes tend towards more thermal radiation, but there are variations that reverse the trend, such as the Casaba Howitzer which uses a nuke to accelerate a narrow jet of plasma.

    6. Missing avatar

      BambooCrawler on

      But explosions don't work in space, because there's no air to support shockwave. How are you gonna make this work?

    7. Missing avatar

      Max Everingham on

      I think you mean 'empathise' (para 3)

    8. Brent D. Schultz on

      This is AMAZINGLY well tailored and the lore/thinking behind it is awesome. Keep up the great work!

    9. Missing avatar

      jack mamais Collaborator on

      And this, ladies and gents, is why we have a physicist on our team :)