Space Warfare II - Stealth Reconsidered

[Clonus]

Re: Space Warfare II - Stealth Reconsidered

 

What bugs me' date=' a lot actually, is when someone says "that would be impossible because it would violate the second law blah blah blah" about some novel effect that no one has actually tried to accomplish. Take for example a membrane that is permeable in one direction by gas molecules above a certain energy, and in the other direction by gas molecules below a certain energy (I think that's how it goes, bear with me). The assertion usually made is that it's not even worth looking for such a thing, because it "violates the second". How about we actually find out what's possible?[/quote']

 

But it's the same membrane. Either it stops the low energy molecules or it doesn't.

[ajackson]

Re: Space Warfare II - Stealth Reconsidered

 

How about we actually find out what's possible?

People do work out the math on examples like that, actually. It's just time consuming and annoying, and so far hasn't produced any examples that differ from what we'd expect from the second law of thermodynamics, so unless you're specifically searching for second law violations it's generally legitimate to just assume the second law holds.

[Nyrath]

Re: Space Warfare II - Stealth Reconsidered

 

What bugs me' date=' a lot actually, is when someone says "that would be impossible because it would violate the second law blah blah blah" about some novel effect that no one has actually tried to accomplish. Take for example a membrane that is permeable in one direction by gas molecules above a certain energy, and in the other direction by gas molecules below a certain energy (I think that's how it goes, bear with me). The assertion usually made is that it's not even worth looking for such a thing, because it "violates the second". How about we actually find out what's possible?[/quote']

Well, by analogy:

The laws of mathematics state that 1 + 1 = 2. But you do not see lots of people trying to find a way to make 1 + 1 = 3, valiantly ignoring the fact that it is impossible because it would violate the law of addition.

 

In the same way, chemists calculate the results of chemical reactions using the laws of chemistry in order to tailor chemical products. They do not spend all day mixing random chemicals in the hope of violating any of the laws of chemistry. Even though it would be nifty to stumble over a way of turning lead into gold.

 

I take it that you are unfamiliar with the concept of a mathematical proof?

http://en.wikipedia.org/wiki/Mathematical_proof

Proofs are obtained from deductive reasoning, rather than from inductive or empirical arguments. That is, a proof must demonstrate that a statement is true in all cases, without a single exception.

[Yansuf]

Re: Space Warfare II - Stealth Reconsidered

 

The original heat is still there, but the heat gradient is not. The temperature of the room has risen as the heat boils the water.

 

It's like a hydroelectric power plant.

 

It converts the gravity gradient of the falling water into electricity. But the water is not annihilated by the plant, it still pours out the sluices and flows downstream.

 

In technical terms, the second law of thermodynamics says that entropy always increases. Turning heat into useful work would reduce entropy, which is forbidden. Turning a head gradient into useful work is allowed.

 

As long as the increased entropy of the working fluid (via its state change from liquid to gas) is greater than the decrease in the entropy of the heat sink, second law is not violated.

 

Using your argument, it appears to me that all existing refrigeration cycles violate the second law. The temperature is lowered by allowing (great) expansion of the working fluid, not by dumping it into another system. The only input is the (electric) power needed to recompress the working fluid.

[Nyrath]

Re: Space Warfare II - Stealth Reconsidered

 

As long as the increased entropy of the working fluid (via its state change from liquid to gas) is greater than the decrease in the entropy of the heat sink, second law is not violated.

 

Using your argument, it appears to me that all existing refrigeration cycles violate the second law. The temperature is lowered by allowing (great) expansion of the working fluid, not by dumping it into another system. The only input is the (electric) power needed to recompress the working fluid.

Right, the input of electrical power prevents violation of the second law. If there was no power input, there would be a violation.

 

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/seclaw.html

(scroll down to Second Law: Refrigerator)

http://www.phy.duke.edu/~rgb/Class/phy51/phy51/node63.html

 

Without power input, it would violate the second law. There is a name for this: perpetual motion machine (of the second kind).

 

The earliest known example of somebody trying and failing to make a perpetual motions machine was in 1150 AD. In the almost 900 years since, people have been trying and trying to defeat the second law of thermodynamics, and they have failed every single time.

 

 

This is a good summary:

Use of the term "impossible" and perpetual motion

[ajackson]

Re: Space Warfare II - Stealth Reconsidered

 

Right' date=' the input of electrical power prevents violation of the second law. If there was no power input, there would be a violation.[/quote']

Actually, you're missing his point, though it's an artifact of clumsy wording. If you let a liquid move from a high pressure area to a low pressure area, and it transforms into a gas when doing so, its temperature goes down without external energy input, which looks like a second law violation. It's not -- modest pressure gas is higher entropy than a liquid, even if the gas is cooler than the liquid -- but using a careless definition of entropy it looks like one.

[Yansuf]

Re: Space Warfare II - Stealth Reconsidered

 

Right' date=' the input of electrical power prevents violation of the second law. If there was no power input, there would be a violation. [/quote']

 

True to my second comment.

Now please explain my first.

 

For that matter, referring to the first, refrigeration is possible inside the ship.

Now you say: "but what about the extra waste heat from generating the electric power needed for the refrigeration?" Good question.

It depends on how efficient the process is. If the efficiency is good enough, the refrigeration is more than the added waste heat, and no, that is not a perpetual motion machine.

 

BTW, remember in most of the "atomic rocket" designs, the main power plant (be it fission or fusion) is an open system, it generates thrust by expelling reaction mass. This removes a great deal of heat from the system, even if other "waste" heat is not used to pre-heat the remass.

 

Separately, another simple example; if the power for the refrigeration comes from a solar array, how much waste heat is there?

[Yansuf]

Re: Space Warfare II - Stealth Reconsidered

 

Actually' date=' you're missing his point, though it's an artifact of clumsy wording. If you let a liquid move from a high pressure area to a low pressure area, and it transforms into a gas when doing so, its temperature goes down without external energy input, which looks like a second law violation. It's not -- modest pressure gas is higher entropy than a liquid, even if the gas is cooler than the liquid -- but using a careless definition of entropy it looks like one.[/quote']

 

 

For the record, I sent my reply to Nyrath (post #32) before I saw this post.

AMR

[ajackson]

Re: Space Warfare II - Stealth Reconsidered

 

On reflection, gas moving from high to low pressure will also drop in temperature -- you can in general produce energy from a pressure differential in much the same way you can produce energy from a temperature differential. There are also some categories of chemical reaction that are endothermic but have positive entropy.

[Zeropoint]

Re: Space Warfare II - Stealth Reconsidered

 

Separately, another simple example; if the power for the refrigeration comes from a solar array, how much waste heat is there?

 

NOT zero. There will still be a net positive amount of waste heat. You've got the resistance of the wires, and even assuming a peltier junction system with no moving parts, to move heat "uphill" requires an input of energy which is released as waste heat. It happens to be exactly the same as a heat engine running in reverse. Pump X joules of energy out of the fridge, put 1.5X joules into the ship's air. Heat that needs to be removed.

[Kraven Kor]

Re: Space Warfare II - Stealth Reconsidered

 

Could you build a system that recycled as much heat / emissions / waste as possible to bring the waste heat - and thus detectable emissions - "near zero?"

 

Say, you have some type of fusion reactors, which powered a variety of drive systems - ion and/or plasma drives or the like for in-system travel, the jump or whatever FTL drives (if they exist in the setting / our imaginary future reality for this conversation), and all the other stuff. Obviously, if your engines are actually providing thrust you can't mask that - but could your other systems be so efficient that you'd be significantly harder to detect? Or to where you might be mistaken for something other than what you are? Say a cruiser coasting along or parked in an orbit, observing or whatever it is your ship(s) is there to do?

[Clonus]

Re: Space Warfare II - Stealth Reconsidered

 

 

Say, you have some type of fusion reactors, which powered a variety of drive systems - ion and/or plasma drives or the like for in-system travel, the jump or whatever FTL drives (if they exist in the setting / our imaginary future reality for this conversation), and all the other stuff. Obviously, if your engines are actually providing thrust you can't mask that - but could your other systems be so efficient that you'd be significantly harder to detect?

 

Only if you lower your internal temperature to slightly above absolute zero.

[Kristopher]

Re: Space Warfare II - Stealth Reconsidered

 

Well, by analogy:

The laws of mathematics state that 1 + 1 = 2. But you do not see lots of people trying to find a way to make 1 + 1 = 3, valiantly ignoring the fact that it is impossible because it would violate the law of addition.

 

In the same way, chemists calculate the results of chemical reactions using the laws of chemistry in order to tailor chemical products. They do not spend all day mixing random chemicals in the hope of violating any of the laws of chemistry. Even though it would be nifty to stumble over a way of turning lead into gold.

 

I take it that you are unfamiliar with the concept of a mathematical proof?

http://en.wikipedia.org/wiki/Mathematical_proof

Proofs are obtained from deductive reasoning, rather than from inductive or empirical arguments. That is, a proof must demonstrate that a statement is true in all cases, without a single exception.

 

Yes, I'm familiar. But then you're claiming that the Second is a mathematical proof?

 

I'm not talking about wacko ideas like perpeutal motion machines. I'm just trying to leave the door open for things we might not have discovered yet, and constantly bashing even the most novel and unusual idea with "the second law says no!" before it's even tried strikes me as counterproductive and silly.

[Clonus]

Re: Space Warfare II - Stealth Reconsidered

 

Yes, I'm familiar. But then you're claiming that the Second is a mathematical proof?

 

I'm not talking about wacko ideas like perpeutal motion machines.

 

Yeah, you are. Any device that doesn't produce any waste heat is a perpetual motion machine.

[Kraven Kor]

Re: Space Warfare II - Stealth Reconsidered

 

Only if you lower your internal temperature to slightly above absolute zero.

 

So no amount of insulation / heat recycling could let you have heated interiors but still mask your heat signature?

 

Again, not talking zero waste heat, just talking maximum efficiency.

[ajackson]

Re: Space Warfare II - Stealth Reconsidered

 

Yes' date=' I'm familiar. But then you're claiming that the Second is a mathematical proof?[/quote']

Actually, it is. The second law is dependent on the information content of the system being finite, but it works for a wide variety of systems with little resemblance to our universe.

[ajackson]

Re: Space Warfare II - Stealth Reconsidered

 

So no amount of insulation / heat recycling could let you have heated interiors but still mask your heat signature?

Depends how much you want to mask it. There's no way you can over the long term reduce your heat signature below the biological heat output of your crew (on the order of 100W per person) but that's mostly irrelevant in real applications -- for non-accelerating vehicles, reflected signature generally exceeds heat signature anyway.

[Clonus]

Re: Space Warfare II - Stealth Reconsidered

 

So no amount of insulation / heat recycling could let you have heated interiors but still mask your heat signature?

 

Again, not talking zero waste heat, just talking maximum efficiency.

 

No amount would let you have heated interiors but still mask your heat signature for very long. Insulation just delays the time it takes for heat to get out and has the unpleasant side effect of causing heat to build up inside as long as it is being produced. "Heat recycling" runs up against the fundamental reality that energy is a conserved quantity, meaning that sooner or later all your heat will radiate out. All you can ever do is delay it. This of course means the logical method for launching a surprise attack would be using an entirely automated attack, or and better, pose as legitimate space traffic.

[Nyrath]

Re: Space Warfare II - Stealth Reconsidered

 

Actually' date=' you're missing his point, though it's an artifact of clumsy wording. If you let a liquid move from a high pressure area to a low pressure area, and it transforms into a gas when doing so, its temperature goes down without external energy input, which looks like a second law violation. It's not -- modest pressure gas is higher entropy than a liquid, even if the gas is cooler than the liquid -- but using a careless definition of entropy it looks like one.[/quote']

 

Well, perhaps, but not according to the original post.

Using your argument, it appears to me that all existing refrigeration cycles violate the second law. The temperature is lowered by allowing (great) expansion of the working fluid, not by dumping it into another system. The only input is the (electric) power needed to recompress the working fluid.

The confusion seems to be due to the original poster conflating "temperature" with "entropy." The second law of thermodynamics states that in a closed system entropy increases. Therefore all existing refrigeration cycles do not violate the second law.

 

Refrigerators can pump heat from one place to another, but they increase entropy while doing so. Thus the second law is maintained.

 

In any event, the heat is not destroyed, just moved. That's why on a chilly day you always find the cat curled up near the bottom vents on the refrigerator, where all the heat pumped from the interior is jettisoned.

[CorpCommander]

Re: Space Warfare II - Stealth Reconsidered

 

My first thought is you don't have to get rid of all your waste heat. Just enough to satisfy 1/r^2 such that your level of heat less than the level that can be detected at that range. The more efficient you are the closer you can get to your less stealthy opponent.

 

Stealth is simply a defense and a weak one at that. Once you are seen all of its value goes away. So perhaps a better use of time, energy and equipment space on your ship is to find a way to use that heat defensively. Turn a problem into an asset. Especially since it is pretty damn hard to mask anyway.

 

That said here is my Perpetual Motion Story. Totally True.

 

Back around 1960 or so my Dad was called out to his Uncle's house to help with something. When he got there his other Uncles were there too and so was a big truck which had a large iron wheel. They needed to get this down into the basement. Everyone was breaking their back trying to move this incredibly heavy wheel when my Dad asked Sal, "so are you going to use this as the foundation for a waterheater or something?"

 

Sal's eyes lit up and said "No." He became professor for a moment and explained, "this wheel will change history for I have figured out a way to make limitless energy." All work came to an instant stop. The sweaty men and the perplexed truck driver all looked on. "You see, we shall mount this wheel on a special bracket I have set up in the basement. We shall spin the wheel. It shall turn so fast that eventually it will be impossible to stop and we will draw energy from it."

 

"Oh for Christ's Sake!" said the 5 Italians and the truck driver. What the truck driver didn't know was the long standing rivalry between Sal and his brothers. You see they were all in the heating oil business. Sal was environmentally conscious long before it became fasionable. He wanted to create perpetual motion and put the Oil Industry out of business.

 

My father and his brother were both chemical engineers. Sal was the smartest of the old school Italians, having gone all the way through the 6th grade before getting a job (back in the old days school was looked down upon as a luxury and a waste of time.) He and my Uncle tried to explain to Sal a concept called friction and offered to take him to either Northeastern University or MIT to discuss his theory with engineers of repute.

 

Sal is still looking to put the Oil Industry out of business. He later won awards for improving the efficiency of heaters for homes. Ultimately we are all ignorant, but the difference between us is degrees of ignorance. I think for me the lesson is that enthusiasm for the subject matter and a willingness to accept the truth as you discover it will go a long way in helping build your understanding and perhaps lead you to find an application of knowlege no one else has developed.

[Nyrath]

Re: Space Warfare II - Stealth Reconsidered

 

True to my second comment.

Now please explain my first.

Explained in the post above this. Heat is moved by the refrigeration cycle, but entropy is increased in doing so. If I am not explaining this adequately, it is because I'm not a professional teacher. You might be better off finding a high school physics teacher and asking them to explain.

 

True For that matter, referring to the first, refrigeration is possible inside the ship.

Now you say: "but what about the extra waste heat from generating the electric power needed for the refrigeration?" Good question.

It depends on how efficient the process is. If the efficiency is good enough, the refrigeration is more than the added waste heat, and no, that is not a perpetual motion machine.

Again, the refrigeration pumps heat from whatever is being refrigerated to somewhere else. The heat is still there, just relocated. Add to that the heat from the power source attached to the refrigerator. The heat just gets larger and larger.

 

BTW' date=' remember in most of the "atomic rocket" designs, the main power plant (be it fission or fusion) is an open system, it generates thrust by expelling reaction mass. This removes a great deal of heat from the system, even if other "waste" heat is not used to pre-heat the remass.[/quote']

Yes, I know about open cycle cooling, you might have read about it on my website. But the main thing that open cycle cooling is not is "stealthy". Which, if you recall, was the point of this entire thread.

Blasting out a sixty gigawatt plume of hot plasma is not the way to try and avoid being noticed.

 

 

Separately' date=' another simple example; if the power for the refrigeration comes from a solar array, how much waste heat is there?[/quote']

The current state-of-the-art solar cell arrays on the International Space Station are about 14.5% efficient. This means (in Earth orbit) every square meter of solar cell produces abotu 198 watts of electricity, and 1167 watts of waste heat.

[CorpCommander]

Re: Space Warfare II - Stealth Reconsidered

 

Blasting out a sixty gigawatt plume of hot plasma is not the way to try and avoid being noticed.

 

.sig worthy!

[Nyrath]

Re: Space Warfare II - Stealth Reconsidered

 

Could you build a system that recycled as much heat / emissions / waste as possible to bring the waste heat - and thus detectable emissions - "near zero?"

Forgive me but you are apparently not listening. It is impossible to recycle waste heat. The best you can do is pump the heat from one spot to another. If you use a radiator to throw the heat overboard, you are creating a huge thermal emission that will be visible for Astronomical Units away. If you do not throw the heat overboard, the temperature inside the spacecraft will grow until the crew dies and things start to melt.

 

Keep in mind that the background of deep space is at a chilly three degrees above absolute zero (-454.27 degrees Fahrenheit). Anything hotter than that will show up like a signal flare at midnight.

 

Say' date=' you have some type of fusion reactors, which powered a variety of drive systems - ion and/or plasma drives or the like for in-system travel, the jump or whatever FTL drives (if they exist in the setting / our imaginary future reality for this conversation), and all the other stuff. Obviously, if your engines are actually providing thrust you can't mask that - but could your other systems be so efficient that you'd be [i']significantly harder[/i] to detect?

Obviously if your engines providing thrust, you will be visible from Pluto's orbit.

Typical fusion reactor designs I've seen are hard pressed to have efficiencies of 50%. For a typical ion drive, you'd need about one gigawatt of electricity. With a 50% efficient fusion plant, it would put out 1 gigawatt of electricity and one gigawatt of waste heat.

 

Or to where you might be mistaken for something other than what you are? Say a cruiser coasting along or parked in an orbit' date=' observing or whatever it is your ship(s) is there to do?[/quote']

Again this is covered in my website. Coasting along with the power plant off and the crew shivering in the dark, the detection radius will be reduced. Reduced from Pluto's orbit to a mere 150 times the distance from the Earth to the Moon (about 38,800,000 kilometers).

 

The trouble is, everybody will see you when you use your engines to insert yourself into your coasting trajectory. After that, your position can be calculated exactly according to the laws of orbital mechanics. If you light up your engine again to change your trajectory, again everybody will see it.

[Nyrath]

Re: Space Warfare II - Stealth Reconsidered

 

Yes, I'm familiar. But then you're claiming that the Second is a mathematical proof?

 

I'm not talking about wacko ideas like perpeutal motion machines. I'm just trying to leave the door open for things we might not have discovered yet, and constantly bashing even the most novel and unusual idea with "the second law says no!" before it's even tried strikes me as counterproductive and silly.

http://en.wikipedia.org/wiki/Perpetual_motion#Use_of_the_term_.22impossible.22_and_perpetual_motion

 

The quote is in a page about perpetual motion, but it applies to any attempt to invest effort in something forbidden by the second law.

 

Yes, it is remotely possible that some flaw may be found. But after almost one thousand years of experiments failing to find a flaw with the second law, most scientists have better things to do with their time.

[Nyrath]

Re: Space Warfare II - Stealth Reconsidered

 

My first thought is you don't have to get rid of all your waste heat. Just enough to satisfy 1/r^2 such that your level of heat less than the level that can be detected at that range. The more efficient you are the closer you can get to your less stealthy opponent.

 

True, but with current technology, you are not going to be able to get very close.

 

If the spacecraft are torchships, their thrust power is several terawatts. This means the exhaust is so intense that it could be detected from Alpha Centauri.

 

The Space Shuttle's much weaker main engines could be detected past the orbit of Pluto. The Space Shuttle's manoeuvering thrusters could be seen as far as the asteroid belt. And even a puny ship using ion drive to thrust at a measly 1/1000 of a g could be spotted at one astronomical unit (i.e, the distance between the Earth and the Sun).

 

I've calculated that a spacecraft the size of a submarine with all power turned off could be detected at a range of about 150 times the distance between the Earth and the Moon (129 light-seconds or 38,800,000 kilometers).

 

This is with current off-the-shelf technology. Presumably future technology would be better.

 

Thank you for your anecdote about your uncle, it was pertinent.