## STARFIGHTER INC

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1. ### Zach El-HajjCollaborator on March 21, 2017

@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
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-HajjCollaborator on March 21, 2017

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. ### Ko-chin Chang on March 21, 2017

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. ### BambooCrawler on March 20, 2017

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-HajjCollaborator on March 19, 2017

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. ### BambooCrawler on March 19, 2017

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

7. ### Max Everingham on March 18, 2017

I think you mean 'empathise' (para 3)

8. ### Brent D. Schultz on March 18, 2017

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

9. ### jack mamaisCollaborator on March 18, 2017

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