[Herra Tohtori]

Thermodynamics doesn't work like that.

Irrelevant information.

Not entirely...

In space, there is only one way to get rid of your waste heat, and that is to radiate it.

Well, technically you can also eject hot coolant from the ship, but that won't last for long and would make you much more noticeable so it would be counterproductive in this case...

Temporarily storing energy into a heat sink is also a viable option, but limited by the thermal capacity of said heat sinks.

The interior of the craft will be at around room temperature at the very minimum, assuming that the crew inside are alive and relatively comfortable.

Yes, it would. 293 degrees Kelvin is a good approximation for that temperature.

Each of your crewmembers will be creating several watts of waste heat all the time, and therefore heating up the spacecraft.

When sitting, human body produces energy at approximately 116 W power. In vacuum and rest it would be lower, I would hazard to guess something like 40-50 W.

That's before the reactor or other power source comes into the equation.

Yes, and it has already been established that for limited period of time, a ship can either turn most of their systems off or store excess heat in heat sinks. Water tanks would be supremely good heat sink.

The skin temperature of the vessel is only defined by the difference between the rate of heat radiated from the skin, and the rate of skin heating by internal conduction.
The latter is much larger than the former at reasonable skin temperatures, so it'll be roughly the same temperature as the internals unless refrigerated.

The thermal conductivity of the wall and energy loss from the inside through a radiator also matters. If the wall is highly insulating construct and the excess heat from within the ship is transferred to radiators and radiated into space, the skin temperature of the space ship definitely won't reach the same value as within the ship.

Radiator == refrigerator. Heat transfer happens in both cases.


The skin temperature of a space ship is actually sort of difficult to determine. The heat flow through the wall of the space ship depends on the transient in temperature of course, but the surface temperature determines how much the wall is actually radiating into space, while the backflow from space has it's own effect. Things would be far easier to calculate in total vacuum without additional energy sources (close approximation would be deep space, with no nearby star around). I don't have time right now to go through the thermodynamics to estimate what the surface temperature of a ship would be, but I can say with large certainty that it would be fair bit lower than the temperature within the ship's crew quarters.


Dragon: talking about FreeSpace2 nebulas and realism together opens another can of worms...

[Herra Tohtori]

Ah screw it, I did it anyhow.

Okay, let's build a space ship with the following properties:

-shape spheroid
-radius 10 metres / diameter 20 metres (okay, sort of big but works quite well as a small asteroid as far as radar echoes go)
-surface material: Wrought iron, thermal emissivity 0.94 (black body has thermal emissivity of 1, this is a unitless value)
-main mass of the ship consists of water storage that can work as a heat sink for quite a bit of time.
-distance from sun, 1 AU
-three man crew
-low profile thermal output from systems and crew:
 *average heat output from the crew at work: 600 W, rest 150 W combined
 *life support 2 kW
 *computer systems 1 kW
 *passive sensors 500 W
 *combined power output at low profile mode: approximately 4 kW
 
Thermal equilibrium:

P_output = P_input

P_input consists of three variables:

P_space; the black body radiation of space that hits the entire surface of the spheroid
P_sun; the radiation from sun that hits one half of the ship
P_systems; the power consumed and excess heat produced by the systems of the ship.

Essentially, the black body radiation of space is the smallest of these factors:

P_space = σ * A_sphere * T⁴

σ = Stefan-Boltzmann constant = 5.6704*10^-8 W m^-2 K^-4
A_sphere = ship's surface area (4π r^2) = 1256 m^2
T = 3 K (accurate enough)

P_space = 0.00576883814 W or about 5.77 mW; small enough that it will be ignored in subsequent calculations aside from mentioning that it exists. For a significantly larger and colder object it would have more importance, but our crude stealth ship does not pay any heed to it.


At 1 AU distance rom the Sun, the sunlight is by far the biggest heating factor:

P_sun = C * A_disk

C = solar constant = 1.37 kW m^-2
A_disk = π r^2 = 314 m^2

P_sun  = 430.18 kW

For the sake of mental sanity, I am going to handle the halves of the sphere assuming that no heat transfers from the lit side to the dark side. I am also going to assume that the excess heat from systems is evenly distributed to both sides of the sphere. Radiators would just mess my equations up...


P_lit = ½P_systems + ½P_space + P_sun
P_dark = ½P_systems + ½P_systems

Hence, the output required at the lit side is about 432 kW, while at dark side it sits at about 2 kw.

From the equation of a non-black body thermal radiation we can now determine the required surface temperature for both halves:

P_lit = ε * σ * ½A_sphere * T_lit⁴
P_dark = ε * σ * ½A_sphere * T_dark⁴

where ε is the emissivity of the surface material; other variables should be clear as the sky.

ε = 0.94 for wrought iron surface - very close to black body emissivity.

When the equations are turned around to determine temperature, we get the following:

T_lit = (P_lit / [ε * σ * ½A_sphere])^1/4
T_dark = (P_dark / [ε * σ * ½A_sphere])^1/4

T_lit = (432000 W / [0.94 * 5.6704*10^-8 W m^-2 K^-4 * 628 m²])^1/4 = 337.05 K
T_dark = (2000 W / [0.94 * 5.6704*10^-8 W m^-2 K^-4 * 628 m²])^1/4 = 87.92 K

Lit side surface temperature: 337.05 Kelvins or 63.9 degrees Celcius; dark side temperature at 87.92 Kelvins or -185.23 degrees Celcius.

Increasing ship's diameter decreases the skin temperature. Installing separate radiator to take care of the systems' excess energy output reduces the skin temperature further since the temperature doesn't need to conduct through the skin, it can go through the radiator which can be concealed and directional so that it effectively emits the radiation in narrow area of space.


Now here is the main design problem of a stealthy space ship. You can optimize the thermal equilibrium of the ship so that the ship's internal temperature stays at nice comfortable readings as long as the influx of energy stays constant. However, if moved further or closer to energy source, it would need to start using energy to actively cool itself down or heat up. Ideally, the excess heat from systems would actually keep the temperature at constant comfortable values.

It should come as no surprise that the ideal zone for a stealth ship would be at the "green zone" around any star for many reasons:

1. That is the zone where the thermal equilibrium of the ship will settle at habitable temperatures without extra heating or cooling.
2. Everything of interest most likely happens at green zone because habitable planets are there and it is far easier to work in that zone than on outer or inner regions of any system.

Discuss.

Addendum:

In reality, some heat form the lit side would conduct to the dark side. Same applies to asteroids at same region of space, so the difference between an iron asteroid and a stealth ship spheroid would be really, really hard to detect with any sort of infra-red detector and the temperature differences could just as well be addressed to the surface material's thermal emissivity, which varies from asteroid to asteroid depending on the alloy and composition and dust layer on the surface.

[Scotty]

So, can we agree that the surface temperature of a ship would not be room temperature?  Missed that part in the post.

You're assuming that

A - the ship knows where potential observers are
B - that observers don't have "all the angles covered" (or at least enough to make the tactic unviable).

And you are assuming that

A - the observers know where the ship is (which sort of precludes any reasonable attempt at stealth anyway)
B - Space is smaller than it is.  I know Herra already covered this, but I wanted to get my $0.02 in.  They don't call Space an Ocean because it has whales.  Space is ****ing HUGE!  For example, The total volume encompased by ONE AU is a little bit over 14,000,000,000,000,000,000,000,000 cubic KILOMETERS (Holy ****, that's a bigger number than I thought.)  A little over 14 HEPTILLION cubic kilometers.  For one AU. (Did I do my math wrong?)

A = (4/3)TTr³
A = (4/3)TT(149,538,000 km)³
A = (4/3)TT(3.3439 x 10²⁴ km³)
A = (4.4585 x 10²⁴ km³)TT
A = 1.4 x 10²⁵, or about 14,000,000,000,000,000,000,000,000 km³

There is a whole ****load of space to hide in.  For perspective, a Deimos would take up about 1/454,000,000,000,000,000,000th of that space.  Remember that's just one AU.

[Herra Tohtori]

Let's take another example of the size of space; angular diameter.

The entire space has angular diameter of 360 degrees for full horizontal and vertical band.

A spherical ship of one kilometre diameter would, at 1 AU distance, have angular diameter of

angle = arctan 0.5 km / 149 598 000 km = 1.91499149 × 10^-7 degrees or 0.000689396936 arc-seconds.

Now consider that the Hubble space telescope's Faint Object Camera (FOC) offers maximum resolution of 0.0072 arc-seconds at 3.6*3.6 arc-second field of view, and you're starting to realize how easy it would be to hide in space.

A kilometre diameter ship is a bigass ball and it completely disappears from optical instruments of Hubble's size at this distance. Radar would locate it, but not necessarily - it would require hitting it with a pulse and then receiving the echo...

[Akalabeth Angel]

Doesn't matter what the volume is, all that matters is that the observer has line of sight.
One would think that in any kind of war, important systems would be equipped with numerous observation stations which continually scan their full field of view for any intruders.

And the point isn't that the solar system is big, it's that it's empty. There's very little place to really hide.


On top of that all of this low-emission ship garble doesn't explain how it gets there in the first place. If FTL is undetectable, lucky for you. But if it's not, you have to jump outside or to the outer edge and approach the system using reaction drives and when you do you will be a heck of a lot more noticeable. And either you coast through the system and then you're out in the open or you have to decelerate and take up position in-cover which will of course attract more attention.

Doesn't matter what the Hubble can do. The Hubble's not scanning for spaceships. There are no spaceships today, but maybe a hundred years in the future. And in a hundred years into future the survelliance gear will make the Hubble look like a cheap pair of binoculars.

[Scotty]

Doesn't matter what the volume is, all that matters is that the observer has line of sight.
One would think that in any kind of war, important systems would be equipped with numerous observation stations which continually scan their full field of view for any intruders.

And the point isn't that the solar system is big, it's that it's empty. There's very little place to really hide.


On top of that all of this low-emission ship garble doesn't explain how it gets there in the first place. If FTL is undetectable, lucky for you. But if it's not, you have to jump outside or to the outer edge and approach the system using reaction drives and when you do you will be a heck of a lot more noticeable. And either you coast through the system and then you're out in the open or you have to decelerate and take up position in-cover which will of course attract more attention.

Doesn't matter what the Hubble can do. The Hubble's not scanning for spaceships. There are no spaceships today, but maybe a hundred years in the future. And in a hundred years into future the survelliance gear will make the Hubble look like a cheap pair of binoculars.

Forgive me, but I call bull**** on most of this post.

The point is that the observer isn't going to have any damn line of sight because he can't find the damn ship so far away in such a vast expanse of space.  Unless you think just having a clear line to some point in space means you can find a speck of metal in it.  If that were true, anyone in space at any given time should be able to detect everything that ever graced the emptiness of space no matter how far away it is.  You also forget that, while light is fast, it isn't instant.  An AU is about 8.3 light-minutes.  Meaning it takes 8.3 minutes, even with light-speed sensors, to pick up a footprint.  That means 8 minutes of additional maneuvering time 16.5 minutes round trip of maneuvering time, as well as 8 minutes after they get pinged for whatever ship it is to turn off the tap and go silent.

It doesn't matter that it's empty.  It matters that it's big.  You are in error here.  Look at it this way.  If a tourist gets stranded in the desert, it still takes quite a long time to find him because the desert is so huge and empty it takes forever to cover it.  Even if you wait until night and use thermal sensors, it's still going to take a while.

Have you ever heard of lying doggo?  It means to be shut down save for the least activity that can be sustained to wait for a hapless ship/vehicle/person whatever to come by.  We don't have to explain so much how it got there as we do how long it's been there.  Second, a very low acceleration will not produce a very noticable blip on sensors.  If you were to accelerate at say, 2 gravities for a long time, you would get to where you're going, albeit slowly, and still be the next best thing to undetectable.

This is my main point of contention.  "It doesn't matter what the most high-powered, highest resolution telescope in space can do.  In a hundred years things will be different."  Way to completely dodge the point.  You forget that in a hundred years, things will be different on the other end of the spectrum too.

[General Battuta]

I'm torn on this. I'm fairly certain stealth would be next to impossible in space in any reasonable realistic space warfare environment. On the other hand with modern technology we can't even keep track of all the NEOs, so who knows?

[The E]

Well, in this case, I think it's possible to write convincing fiction based on both principles. The Honorverse novels (at least the older ones) operated in a stealth is impossible universe, where the most effective tactics are based on misdirection, not on invisibility.
In Charles Stross' Singularity Sky (or better, Iron Sunrise), the space navies play by the "Space is a big place to hide in" book (especially concerning their STL second-strike bombers).

Point is: Both sets of rules can be used to make interesting drama, and that's what we're here for, isn't it? 

[Scotty]

Well, to be fair, the Honorverse novels operate in the "stealth is hard to actually acheive, but still possible" universe.  Going ballistic through a system is undetectable (See: the Argus scouting runs up to the first war), and accelerating at a very slow rate is effectively stealth, usually at anything greater than a few light minutes.  However, I will admit that sensor drones make it just about impossible to sneak anywhere effectively.  Some of the only exceptions are in unscouted or enemy held systems.

[The E]

Damn, you're right. My memory is playing tricks on me again. Yeah, ships not using impellers are considered to be effectively invisible at ranges over a few light seconds, something Honor exploited in "Echoes...", I believe.

[General Battuta]

Okay, let's be realistic here.

Any future interplanetary space warfare will end up with RKVs.

They will be unstoppable, undetectable, and completely overpowering. And then MAD will, hopefully, kick in. That or extinction.

Actually, come to think of it, MAD wouldn't work since there's no way to detect an incoming attack and retaliate. So it's just overwhelming first strike or complete trust.

[Mongoose]

In other words, reality sucks ass, so let's stick to blowing **** up in FreeSpace.

[NGTM-1R]

Actually, come to think of it, MAD wouldn't work since there's no way to detect an incoming attack and retaliate. So it's just overwhelming first strike or complete trust.

Sure it would, same way it did when the first ballistic missiles were designed. Many platforms, only one needs to launch.

[General Battuta]

Come again?

[NGTM-1R]

You make MAD work in a world like that by making it impossible to kill all weapons platforms simulatanously, and each one is enough to ensure the destruction of your opponent.

It would be complex, as a math problem, but certainly doable.

[General Battuta]

Ahh, I gotcha. Yeah, automated (or manned) offworld RKV launchers would do that. Second-strike capability.

[Akalabeth Angel]

The point is that the observer isn't going to have any damn line of sight because he can't find the damn ship so far away in such a vast expanse of space.  Unless you think just having a clear line to some point in space means you can find a speck of metal in it.  If that were true, anyone in space at any given time should be able to detect everything that ever graced the emptiness of space no matter how far away it is.

We have no trouble detecting stars thousands of light years away, it should be equally not troubling to detect a vessel radiating a couple AU away.

It doesn't matter that it's empty.  It matters that it's big.  You are in error here.  Look at it this way.  If a tourist gets stranded in the desert, it still takes quite a long time to find him because the desert is so huge and empty it takes forever to cover it.  Even if you wait until night and use thermal sensors, it's still going to take a while.

Space doesn't have a horizon.

Let me put it this way. The Ocean is a vast place and turbulent place, but it's possible to give an orbital satellite enough resolution to pick up the algae thrown up in the wake of a submarine and to track it's movement (despite being underwater).

Have you ever heard of lying doggo?  It means to be shut down save for the least activity that can be sustained to wait for a hapless ship/vehicle/person whatever to come by.  We don't have to explain so much how it got there as we do how long it's been there.  Second, a very low acceleration will not produce a very noticable blip on sensors.  If you were to accelerate at say, 2 gravities for a long time, you would get to where you're going, albeit slowly, and still be the next best thing to undetectable.

The longer you accelerate the longer the interval of time that the enemy has to detect you. Also you forget that going someplace in space requires you to stop as well which means you will be decelerating for an equally long time. Or maybe you just want to pass through the system, but if you're scouting that means you don't know what's there so you're throwing  yourself across a system hoping that you don't fly right into the enemy. (ie usually you're tracking his fleet movements and his fleet could be off doing who knows what. Patrols, wargames, logistic transport, etcetera). Of course space is big, you're unlikely to fly right through his formation but you don't necessarily have to.

This is my main point of contention.  "It doesn't matter what the most high-powered, highest resolution telescope in space can do.  In a hundred years things will be different."  Way to completely dodge the point.  You forget that in a hundred years, things will be different on the other end of the spectrum too.

In a hundread years the Laws of Thermodynamics will not change.
Ships will still radiate excess heat and they will still use reaction drives.

[Flipside]

Thing is, telescopes can only scan a tiny portion of the sky, Radio Telescopes, however, can scan quite large areas of sky, and are particularly good at noticing energy signatures that weren't there the last time it looked. Our fascination with Supernovae have somewhat encouraged that.

To be honest, I'm more inclined to use the Honorverse approach, the technique described by Herra is designed to be centred on misdirection, that the approaching vessel would be assumed to be a piece of space-debris, of course, the moment it changes direction or velocity, that's going to set alarm bells ringing, so you'd have to drift into the system, and that's a trade of between speed and thrust signature outside the system, get that wrong, and they'll already know you are coming.

I believe the argument is that, no matter what you do, a race at our level of technology or higher would detect something, so the answer lay in convincing whatever is watching that what they are seeing isn't anything to worry about, and the options for that are severely limited in space, especially when you add weapons to the equation, because they add extra power/cooling requirement and would probably lessen any camouflaging abilities of the vessel.

[Spoon]

We have no trouble detecting stars thousands of light years away, it should be equally not troubling to detect a vessel radiating a couple AU away.

Because a star that is a huge object, giving of 'endless' amounts of energy, light and radiation is completely on the same scale as a 1km long starship right?

That's like saying that because you can see a mountain at a few clicks away, you will also be able to see a mouse at the same distance with the naked eye.

[Herra Tohtori]

Main advantage for a ship on a clandestine mission: They know exactly where they will be looking at, while counter-intelligence needs to keep looking at everything and everywhere in the system.

Main disadvantage for the same ship: They can only use passive sensors in order to not give their position away, and have also limited means of communication. On the other hand the other side can use active sensors (radar mainly) to locate objects in space.

It all boils down to how good active sensors are available for the static observation posts in the system. If they are good enough to notice a ship beyond their effective passive sensor range, then stealth is not very useful.

However, as was said earlier, stealth is always a relative term and I still say it would be possible to operate secretly within hostile space, with certain limitations, because no one is ever going to have arbitrarily good sensors.