Realistic and strategic mod: Specs

[DiGuru]

For starters, almost all bodies that make up the solar system consist of ice. Just about any mass (excluding the sun and planets) you come across is made of ice. And they have existed for billions of years so far. And most are tiny: mere meters across.

yes...but they are all much further out from the sun than the earth...notice that when comets come in closer than about earth/mars they start losing not just trapped gasses, but h20 vapour in big quantities too Another thing to note is that these ice bodies do not have any direct heat (the sun is too far to do anything, or they are in shadows like on the moon (maybe) )....if heated by a fusion reactor...things are going to get pretty hot quick, and the result will be faster than the sublimation of a comet

When ice vaporizes, it takes heat away from surrounding bodies. As there is no convection in a vacuum, that heat has to come from things directly attached to it.

yes

So, yes, it would work quite well. The way you want to deploy it would depend on what you want from it. Next to your engine and very slow thermal radiation, it would be your best bet to cool something down.

Ok, it does not have to be ice per se, but that would probably be abundant and cheap.

Sure it would work well, but it would run out quick if the engine was running hot (and was travelling in habitable zone of sun)....

hmm....I'll post again later when I've thought this through properly...I'm not sure if my arguement will stand up to well

Dan.a

Agreed. And I like the argument so far.

All in all, it is a matter of scale: how much ice, how hot or cold would it be, how much heat do you want to get rid of, etc.

Btw. Water is indeed a pretty bad conductor. But that makes it quite good for cooling and heating: it will spread the heat slowly over the available volume. That's why they use it for central heating and cooling as well.

[mkruer]

i don't think the coat of ice would last long enough to be useful? It would sublime away pretty quick with the heat?

Dan.a

It depends on the temperature. When the ice is only a few Kelvins, it would last for a very, very long time.

Well....I'm not sure of this (thermdynamics == evil)...but wouldn't the fact that space is essentially vaccume almost completely annul the temperature? And ice isn't exactly a great conductor of heat, so the cooling it does do isn't likely to be too efficient (I think this was discussed in the armour thread)...

but yes, if it is plentiful enough then I think it would be used

Dan.a

yes, yes it was
http://vegastrike.sourceforge.net/forum ... php?t=1081

[mkruer]

I think everyone needs to go back and take thermo dynamics 101.

Water is a lousy conductor. What does this mean? It takes forever to warm-up and forever too cool. Metals are a much better conductor. What does this mean? Simple they can pull of heat faster, and dissipate it just as quickly.

Q: Why then do most cooling systems use water?
A: It’s cheap. Relatively speaking. In space it’s not cheap.

Q: Then why does water cooling work so much better then
A: Water is used as a medium of exchange to pull heat off the source as quickly as possible and move it to another area, not to cool by evaporation. i.e. radiation. Take a water cooling system for a CPU. Remove the radiator and the CPU will fry. The water will not dissipate heat quickly enough by itself.

Q: Are there any other materials other then water that can do this? Better?
A: Yes. Look at antifreeze. Any material that maintains viscosity and has better thermal properties will work better them water. That includes mercury

The best cooling system for space would be to run pipes containing a liquid metal (i.e. mercury) to pull it off the reactor as quickly as possible then transport the heat to another section of the ships where the pipes branch into smaller sections, and attached to each of those sections would be metal reeds. The reads would take the heat and dissipate it into space. And the now cool mercury would then send back to the beginning of the process.

[dandandaman]

I think you've found the best solution there mkruer...well, I did expect you would have, you seem to have thought heaps of this out in great detail for rylix What do you do for work btw..you seem awfully knowledgable in most of this stuff

Dan.a

[mkruer]

I think you've found the best solution there mkruer...well, I did expect you would have, you seem to have thought heaps of this out in great detail for rylix What do you do for work btw..you seem awfully knowledgable in most of this stuff

Dan.a

I think my Occupation sums it up: Web Developer, Ad-hoc programmer, and all around Tech/Science Handyman

I guess if you are interested in my though process it is very simple.

1. Form an idea.
2. Search the web to prove/disprove it.
3. Counter what you just proved/disproved
4. Let the strongest argument win. (simple argument tend to win far more often)

When it comes to competing ideas its just as simple.

Here is my thought process on ideas such as torpedoes for space combat.

Physics dictate that projectile weapons are slow compared to a laser.
Physics dictate that projectile when converted into energy tend to have more total power then a laser.
I know that a laser will almost always beat a projectile that is known.
So make a projectile stealthy. This severely handicaps a lasers advantage long range attack.
Projectiles can also be protected to resist laser to a limited degree, Again this handicaps the laser.
Projectiles can change directions. Again this handicaps the laser again. The projectile is not longer an easy target to track or hunt down and destroy.

However this does not mean that a projectile will “winâ€

[Jadel]

Hard to come up with solid numbers, when I get home I'll have a dig around.

There are three ways to transfer heat, Conduction, Convection and Radiation. Unforunately convection is out, there has been some speculation about convection, I'll see if I can find a set of thermodynamic tables for water and hydrogen. For now that leaves:

Radiation
P = ea A(Thot ^4 - Tcold ^4)

where
P = energy transfered per second (watts)
T = Temperature of hot element (Kelvin)
Tc = Temperature of surroundings (Kelvin)
e = emissivity, 0 to 1 depending on material.
a = Stefan's constant = 5.6703 x 10^-8
A = area of radiator in m^2

The main things we can affect are the area of radiator and the temperature that the radiator operates at.

according to this an ideal 1 m^2 radiator operating and 100deg C would dissipate 1083 W into the 3deg K background in space.
Upping that to 315 degrees C (equivalent to a pressurised water reactors main loop) gives a figure of 6694W.
going to 500degrees C (equivalent to liquid sodium cooled reactor) gives a figure of 19994W

Assuming a 100m long spacecraft with a spherical hull layout, the surface area would be 31,415 m^2. Using 50% efficient pressurised water radiators along 30% of it's surface, it would be able to dissipate 31,544,731 W of heat, thats 31.5 MW. For comparison the nuclear reactor on the seawolf submarine generates 52,000 horsepower or app. 38.8 MW

Edit: That figure is *shaft* horsepower, since the reactor is obviously not 100% efficient (at a stab probably closer to 30%) the total thermal power of the reactor core is probably closer to 120MW.

Of course as the background temperature rises, the efficiency of your radiator drops as well. Dumping 30MW of heat is also a good way to stand out from the 3 degree kelvin background of space.

[DiGuru]

That's more than I expected. But it is still very little. One MW laser with am efficiency of 1% would require a lot more cooling than that when fired. So, no laser or other energy weapons with radiation cooling.

And we would require a lot more power to be able to fly around at a decent speed as well.

Btw. Can you think of some mechanism (heat transfer) to 'dump' it through an engine?

[Jadel]

To get reasonable energy densities it appears that we need to use cryogenic fuesl, so I think that engine cooling will not be as much of a problem as it sounds - I understand most rocket engines circulate the fuel around to cool parts before it is used. The reactor types specified also do not require cooling when not operating.
So I would suspect radiative cooling is only required for the internal power generator and shunting off heat from internal systems and external radiation. We can make our radiators more efficient by increasing the working temperature, but as temperature increases you either need to pressurise the system to stop it from boiling or use a more exotic working fluid (sodium, lead) and after seeing what a solid lump of sodium at room temperature would do, I'm scared of what a ruptured pipe spewing 700 degree molten sodium under pressure would be like. Even more frightening, one page I read said that high pressure water systems were nasty enough that sodium cooling wasn't considered much worse.
I suspect there may be several independent systems - a refridgerative one for people and computer equipment, low temperature for mechanical parts and normal systems and a high temperature system for the reactor core.

Btw. Can you think of some mechanism (heat transfer) to 'dump' it through an engine?

Venting propellent would also work. Most of the engine types I was tossing around use copious amounts of liquid hydrogen as fuel. I think it would be too wasteful to use this sort of cooling most of the time, but in emergency conditions it would be a good thing.

[DiGuru]

Hm. Cryogenic fuel would actually increase the requirements for cooling quite a bit. Especially hydrogen. Most liquid hydrogen storages (except some on plants etc), vent the hydrogen slowly to keep it cool... And they require huge containers!

We would be better off just producing the hydrogen from water at the moment we need it.

About the high-pressure cooling: as far as I know, that is almost always the hardest part of a system when used. That's why they mostly use that for short loops (ie. to feed a turbine) or if there are really no alternatives. And the corrosion (even on stainless steel etc.) is pretty bad as well.

So, if we need it, we need it, but I agree it would be bad to handle in real life.

[FlyingAce]

how bout the excess heat is vented as a weapon?

[Jadel]

That's more than I expected. But it is still very little. One MW laser with am efficiency of 1% would require a lot more cooling than that when fired. So, no laser or other energy weapons with radiation cooling.

A quick dig around the net (Google is your friend!) shows planned efficiencies of 10% for free electron lasers - however I'm not sure what the max theoretical efficiency is. The only real world figure I could find was 4.6%, so it's not as bad as it sounds. The other thing to keep in mind is duty cycle, a 10% efficient laser delivering 1MW of power firing one second out of ten would only require 900KW of cooling for sustained firing.

Hmmm looking at fas.org http://www.fas.org/spp/starwars/program/sbl.htm This laser is in the megawatt class.

At this degree of deployment, kill times per missile will range from 1 to 10 seconds, depending on the range from the missile. Retargeting times are calculated at as low as 0.5 seconds for new targets requiring small angle changes.

So looks like unless we really up the laser efficiency it will only be effective against relatively soft targets - ICBMs are not exactly overbuilt. I don't know what sort of cooling system this laser is using, but it is chemical based so it may be venting it's reactants.

[DiGuru]

That's more than I expected. But it is still very little. One MW laser with am efficiency of 1% would require a lot more cooling than that when fired. So, no laser or other energy weapons with radiation cooling.

A quick dig around the net (Google is your friend!) shows planned efficiencies of 10% for free electron lasers - however I'm not sure what the max theoretical efficiency is. The only real world figure I could find was 4.6%, so it's not as bad as it sounds. The other thing to keep in mind is duty cycle, a 10% efficient laser delivering 1MW of power firing one second out of ten would only require 900KW of cooling for sustained firing.

Overall, yes. But the heat is produced. And that's only one laser. So, yes, the amount of heat we can dump over time would be less, but it has to be stored/transferred as well. And that would require a huge amount of heat transport systems.

We probably have to devise three separate modes: stationed, travelling and combat, which would require multiple systems of which only the first two states could be done by passive thermal radiation.

Hmmm looking at fas.org http://www.fas.org/spp/starwars/program/sbl.htm This laser is in the megawatt class.

At this degree of deployment, kill times per missile will range from 1 to 10 seconds, depending on the range from the missile. Retargeting times are calculated at as low as 0.5 seconds for new targets requiring small angle changes.

So looks like unless we really up the laser efficiency it will only be effective against relatively soft targets - ICBMs are not exactly overbuilt. I don't know what sort of cooling system this laser is using, but it is chemical based so it may be venting it's reactants.

That depends. Look at this:

http://yarchive.net/mil/laser_weapons.html

There is some more very good stuff in that archive. (I send Paul F Austin a mail if he would want to pacipitate, but it was returned as unknown adress....)

The first post in that link describes very high-power chemical lasers. Ie.: bomb-pumped (non-nucleair), low frequency lasers. The weapons some of our laser missiles would carry.

Why bomb-pumped? It is very hard to make those lasers continuous. And why low frequency and/or non-nucleair? A nucleair explosion would produce very high frequency electromagnetic waves and very high-energy particles (Gamma- Rontgen- or X-ray), that are really hard to focus.

[DiGuru]

About hydrogen fuel, there are some *very* important distinctions: do we use it as chemical propellant, fusion fuel or reaction mass?

To use hydrogen as a chemical propellant, we want it cold and liquid. And that's hard. It consumes a very big volume, and needs to be kept REAL cold. That's why not much rocket engines that are stored for any amount of time use it, although it holds the best energy for any chemical fuel. And, no, I don't think thermal radiation would keep it cool enough (but I might be wrong), and it would not be a good idea to mount your fuel outside the hull and particle armor.

For fusion fuel, we probably want He3 as well. Mix to match energy consumption. The hydrogen would have to be a plasma, so any form of hydrogen would do.

As reaction mass, we need a plasma as well.

So, if we want to use chemical propellant, we need cryogenic hydrogen. Which would be very bad. But if we need it to fuel a fusion plant and for reaction mass, we have other options.

The energy needed to produce hydrogen from water would be just a fraction of the energy the fusion plant would require to operate at all.

And we can dump the oxygen (or carbondyoxide after breating) as reaction mass as well.

So, I vote for bringing plenty of water or ice and breaking it into atoms when needed. And having a large capacitor or something to bring the fusion plant online if needed. And using multiple power plants.

[Jadel]

Hm. Cryogenic fuel would actually increase the requirements for cooling quite a bit. Especially hydrogen. Most liquid hydrogen storages (except some on plants etc), vent the hydrogen slowly to keep it cool... And they require huge containers!

If we are using a thermal engine, almost any fluid is usable. The advantages of hydrogen is that it's available in large quantities (it is the most common element in the universe after all.)
I wouldn't worry about the handling too much either, From what I understand it's far less nasty than some of the storable liquid propellants (UDMH and red fuming nitric acid anyone?)
Likewise the heat problem will have to be taken care of, but we can take a leaf out of the SR-71's book of tricks. If we have a number of inlets placed throughout the tank, we select the one with the highest average temperature to feed from. The rest will just have to be taken care of via radiation.

[DiGuru]

Hm. Cryogenic fuel would actually increase the requirements for cooling quite a bit. Especially hydrogen. Most liquid hydrogen storages (except some on plants etc), vent the hydrogen slowly to keep it cool... And they require huge containers!

If we are using a thermal engine, almost any fluid is usable. The advantages of hydrogen is that it's available in large quantities (it is the most common element in the universe after all.)
I wouldn't worry about the handling too much either, From what I understand it's far less nasty than some of the storable liquid propellants (UDMH and red fuming nitric acid anyone?)
Likewise the heat problem will have to be taken care of, but we can take a leaf out of the SR-71's book of tricks. If we have a number of inlets placed throughout the tank, we select the one with the highest average temperature to feed from. The rest will just have to be taken care of via radiation.

Agreed, as long as we are talking about chemical propellants. They would be used as thrusters for the most part anyway, when we have a large enough amount of power coming out of the power plant. Which we can use (as you said as well) to heat and accelerate almost any reaction mass, as long as we can break it into plasma and/or charge it.

For power plants, I'm not positive about using cryogenic hydrogen. It would probably take as much energy to split the water as to keep the hydrogen cold enough, I reckon.

[DiGuru]

I don't have exact specs, but for any reaction drive, the things that matter are the amount of mass expelled times the speed at which that is done.

So, while chemical boosters try to increase the reaction by increasing the amount of mass (as the speed is dependent on the heat, which depends on the fuel and nozzle used), a magnetic accelerator (like a railgun for atoms and molecules) would want to increase the speed as much as it could.

And a spacecraft is mostly limited by the amount of 'fuel' it can take, as long as we want to limit the time a journey takes to acceptable levels.

So, if we can get a bigger punch by using the 'fuel' for generating huge amounts of energy and expelling the remainer at speeds as close to the speed of light we can manage, we should.

We go faster and further with less 'fuel'. And that's what engines are all about.

[Jadel]

We go faster and further with less 'fuel'. And that's what engines are all about.

Yep, Isp or specific impulse is the standard measure of the efficiency of an engine, it is basically the exhaust velocity divided by the acceleration due to gravity on earth ("one gee"). this means it's also equal to the amound of seconds that one unit weight of fuel can produce one unit of thrust.
Now the engines we are using for the main drives are of the nuclear thermal type, basically propellent is heated by a nuclear reactor (it doesn't matter exactly how) this causes it to expand and escape under pressure producing thrust.
I'm trying to come up with some hard numbers on this, but it's getting late and I think I'm getting it wrong.....

[DiGuru]

We go faster and further with less 'fuel'. And that's what engines are all about.

Yep, Isp or specific impulse is the standard measure of the efficiency of an engine, it is basically the exhaust velocity divided by the acceleration due to gravity on earth ("one gee"). this means it's also equal to the amound of seconds that one unit weight of fuel can produce one unit of thrust.
Now the engines we are using for the main drives are of the nuclear thermal type, basically propellent is heated by a nuclear reactor (it doesn't matter exactly how) this causes it to expand and escape under pressure producing thrust.
I'm trying to come up with some hard numbers on this, but it's getting late and I think I'm getting it wrong.....

How come? You seem to be on the spot real nice!

[Jadel]

How come? You seem to be on the spot real nice!

Thanks, I am trying to keep the math correct and state whats based on fact, wild speculation or just plain making it up as I go along. I had another half a page of calculations to follow but it was late (my time) and I had that fuzzy feeling that meant things weren't working quite as well as they should. Once I track down a decent calculator and get my math checked, I'll post the rest...

[martin]

about water cooling: water is used for cooling because it has a high heat capacity (i.e. carries a lot of heat energy per volume and kelvin). The removal of heat is achieved by letting the water flow, i.e. not by heat conduction but by moving hot mass. Yes, metals have a higher heat conductivity but you cannot easily run them through pipes. The heat conduction itself is actually done by the electrons (that's the reason why insulators are generally bad heat conductors).

about range of lasers: the perfect parallel laser beam does not exist in reality. all beams diverge, i.e. get wider with the distance (due to diffraction). You can focus them however at a certain distance. If you want to reduce divergence (i.e. make it as parallel as possible), you have to use a wide beam. However, for weapons wide beams are bad because that will decrease the power density at the target (the amount of dumped energy per time and irradiated area). In the worst case, if power density is too small, you only illuminate the area or blind the pilot, but do no damage to the ship (to do damage, you need to overcome the so called "damage threshold", where you heat up the target above melting temperature). In other words: it is a difference whether you use a laser and focus it on 10 square micrometers or use the same laser and widen it to illuminate 1 square meter.

OK, enough for now, maybe next time I'll say something about cw vs. pulsed lasers.

[The_Tick]

On plausible FTL drives :

the Alcubierre Warp Drive (look here : http://www.astro.cf.ac.uk/groups/relati ... el94a.html)
It relies on spatial distortion and is, in my opinion, more plausible than the other three described at the beginning of this thread