Does physics forbid such a device; a heat destroyer

[Dale]

I would love to hear a physics abiding defense against a relativistic projective.

I actually don't think that relativistic projectiles would be that effective for the reasons that rorix_bw gave above. Build a ship with redundant systems and fast repair robots; the slug goes right through and you patch the hole.

However, if you want something more exotic, then gravitational lensing could completely protect against all weapons which travel on geodesics. So that would include relativistic projectiles, lasers, and all other beam weapons. You would only be vulnerable to missiles and other self-propelled weapons.

 

[Deeviant]

I actually don't think that relativistic projectiles would be that effective for the reasons that rorix_bw gave above. Build a ship with redundant systems and fast repair robots; the slug goes right through and you patch the hole.

However, if you want something more exotic, then gravitational lensing could completely protect against all weapons which travel on geodesics. So that would include relativistic projectiles, lasers, and all other beam weapons. You would only be vulnerable to missiles and other self-propelled weapons.

A relativistic projectile would initiate a fusion reaction on contact. It would literally hit with the force of a hydrogen bomb...

Your gravitation defense is an interesting angle though. I think the biggest problem I foresee is that most defensive systems seem to amount to a huge amount of of mass (armor/material/gravitation/etc), and in space, acceleration does not come easy(I do not see an inertial-less engine in the future), so the weight would come at a huge cost of energy and the telltale energy emissions. As a somewhat tangential corollary, I see no effective way to protect a planet; a planet is the last place you would want to be in the case of intergalactic war.

 

[Deeviant]

Have you not read "The Gods Themselves" (1972) by Isaac Asimov?

You want to employ a mechanism whose conception is yet outside known science, so you need to refer your basis to an interpretation that comes from known science but is not yet known to be true or false.

The arguments against your idea are based on thermodynamics and conservation in a closed system... explore a way to employ the parallel worlds interpretation of QM to override these problems.

Your basis will then have plausible deniability for as long as it takes science to settle on one interpretation of QM...

I haven't, I'll take a look at The Gods Themselves, thanks.

 

[Dale]

A relativistic projectile would initiate a fusion reaction on contact. It would literally hit with the force of a hydrogen bomb...

Yes, the energy and force is on the order of a nuclear weapon, but the momentum is vastly different. The point is that you are focusing on the former and neglecting the latter, which tends to decrease the effectiveness.

Your gravitation defense is an interesting angle though. I think the biggest problem I foresee is that most defensive systems seem to ...

That is an engineering problem. If future tech can be allowed to solve the engineering problems of offense then "what is different in defense"? Particularly considering that we can already build bunkers to withstand a nuclear attack.

 

[rorix_bw]

Clearly the best solution is not to be stupid enough to start a war in the first place.

edit: I actually think the future of this is a swarm of smaller drone-controlled units that can somehow co-ordinate their actions. I believe it's already a given that "human" pilots are history. This is however not fun to write about.

 

[rorix_bw]

1 kilogram projectile traveling at .99 C represents 10.52 MEGATONS of energy

I noticed wrong equation. KE=1/2mv**2 is classical physics. There's a different one for relativistic, but I do not recall it. Google or the other members here will know it. The actual energy is much higher as your mass increases as you approach light speed. For reference "1 megaton TNT equivalent blast" is 4.18 petajoules.

 

[Deeviant]

I noticed wrong equation. KE=1/2mv**2 is classical physics. There's a different one for relativistic, but I do not recall it. Google or the other members here will know it. The actual energy is much higher as your mass increases as you approach light speed. For reference "1 megaton TNT equivalent blast" is 4.18 petajoules.

You right, I redid the math with the relativistic kinetic equation. It would be 1503.3 megatons for a 1 kilogram at .9999C projectile and 128.9 megatons for a 1 kilogram at .99C.

But that is only the KE. The actual amount of energy released could be more. For one, the projectile could be made of anti-matter and it's density combined with it's velocity with guarantees high amount of particle-anti particle annihilation. 1 kilogram of matter turns into 21.481 megatons of energy, of course this is assuming a 100% annihilation figure which isn't even close to the actual, however, this energy would not be solely directed in path of the projectile, but radiate outward throughout the ship(in fact much of the KE energy would do this as well). If anybody is picturing such a projectile to simply knock a neat hole right through a ship, they would be horrible mistaken, there would be little left. In fact, it would not take many of such projectiles to render Earth inhabitable.

In effect, a relativistic mass-driver such as the .9999 C 1 kilo of antimatter design as outlined above would functionally be a laser that shoots planet killer level thermonuclear bombs. It is this fact, combined with that fact that a relativistic mass driver is on the lower end of the weapons theoretically possible that has led me to that classical sci-fi defensive mechanisms would be useless; armor isn't going to cut it, high maneuverability evasion of relativistic weapons doesn't seem likely at all without some sort of physics breaking propulsion engine(warp drive/higgs field blocker/etc) and there is no credible way to make a star-trek like shield with known physics(that I know of).

I will say that swarms of small expendable unmanned drones would certainly be effective, but than again, you certainly wouldn't want such a swarm to find your ship, would you...

 

[Dale]

this energy would not be solely directed in path of the projectile, but radiate outward throughout the ship

This is simply false and yet again shows that you are neglecting the momentum.

It is well known in PET imaging that matter antimatter annihilation conserves momentum. The result is that the photons released do not simply radiate outward, but are angled toward the direction of the momentum. In this case, because the momentum is so high, they would be tightly collimated, and any that did go radially would have quite low energy.

If anybody is picturing such a projectile to simply knock a neat hole right through a ship, they would be horrible mistaken, there would be little left.

And I suppose that you have done some rigorous simulations (including momentum) of such a device that would support this extremely dubious claim?

 

[Dale]

3) Military ships could be equipped with a material of very high specific heat capacity which is actively cooled down to a few μK (requiring substantial work) during "off duty" times. Then during operations that material could be used as the cold reservoir and the hull kept at 2.7 K without radiating. That would limit the time that they could be stealthed by the amount of time that it would take to heat up the material.

Btw, I ran some math on this. It turns out that the time depends linearly on the mass of heat sink material and the specific heat capacity, and inversely on the power. For a 1 kg heat sink made from a 100 J/kgK material you get 42 min at 1 kW.

Interestingly, the dependence on the starting temperature of the heat sink is very non-linear. The above calculations are for 1. μK. For 2.7 K, a factor of ~3 million change, the time is 11 min, only a factor of ~4 change.

 

[Deeviant]

This is simply false and yet again shows that you are neglecting the momentum.

It is well known in PET imaging that matter antimatter annihilation conserves momentum. The result is that the photons released do not simply radiate outward, but are angled toward the direction of the momentum. In this case, because the momentum is so high, they would be tightly collimated, and any that did go radially would have quite low energy.

And I suppose that you have done some rigorous simulations (including momentum) of such a device that would support this extremely dubious claim?

As far as my best research can dig up, matter/antimatter annihilation does indeed conserve momentum but not in the way in which you describe. Any particle/anti-particle pair will have the exact behavior you describe but photon's conserve momentum in the form of photon pairs traveling in equal but opposite vectors, but in the end, it would not much matter, simple engineering could render the point moot; for instance the projectile could burst into a web of filaments after being launched, resulting in the ship being sliced into many different pieces, violently. And that is just one idea, but there would be a great many ways to tweak the projectile to get more of a "shotgun" effect.

As far as the launcher itself, it would be trivial; on a ship that is traveling at a relativistic speed(which is not trivial, but certainly possible, especially with the fact that the ship can take a long amount of time to accelerate). The ship could also be a kill vehicle and be consisting of independent highly modular autonomous vehicles that could break apart into many different guided sections and/or launch an almost arbitrary number of filament bombs. Such a weapon would not have any difficulty at all rendering a solar system uninhabitable and/or destroying any detectible structures/ships, and moreover, such a vehicle is not fantastical in the least within the realm of hard sci-fi. It just seems like the only defense in space would be stealth(or perhaps diplomacy, but we all know how well that works in today's world).

Btw, I ran some math on this. It turns out that the time depends linearly on the mass of heat sink material and the specific heat capacity, and inversely on the power. For a 1 kg heat sink made from a 100 J/kgK material you get 42 min at 1 kW.

Interestingly, the dependence on the starting temperature of the heat sink is very non-linear. The above calculations are for 1. μK. For 2.7 K, a factor of ~3 million change, the time is 11 min, only a factor of ~4 change.

So a 1 kg 100 J/kgK heat sink at 1. μK. would absorb 1 kW for 42 before reaching 2.7k? If that is true I don't understand the second part in which assumes the heatsink starting at 2.7k.

My current best design uses heat pumps to pull heat from across the ship and concentrate, then use a black-body pumped laser to lase energy in a very focused direction that I hope nobody will be listening, in addional to using heat sinks as you have suggestion with standard black-body radiator design when not in "stealth mode".

 

[Dale]

As far as my best research can dig up, matter/antimatter annihilation does indeed conserve momentum but not in the way in which you describe.

Then your best research isn't very good. Google the terms "relativistic beaming" and "relativistic aberration".

So a 1 kg 100 J/kgK heat sink at 1. μK. would absorb 1 kW for 42 before reaching 2.7k? If that is true I don't understand the second part in which assumes the heatsink starting at 2.7k.

It absorbs heat for 42 min before reaching 1275 K, at which point the engine efficiency is 0 and no more work can be extracted.

My current best design uses heat pumps to pull heat from across the ship and concentrate, then use a black-body pumped laser to lase energy in a very focused direction that I hope nobody will be listening, in addional to using heat sinks as you have suggestion with standard black-body radiator design when not in "stealth mode".

Since you are using heat pumps work is being done and the second law of thermo should be OK. I don't know much about blackbody pumped lasers. I would be surprised if they cool the blackbody at all.

 

[mfb]

As far as the launcher itself, it would be trivial; on a ship that is traveling at a relativistic speed(which is not trivial, but certainly possible, especially with the fact that the ship can take a long amount of time to accelerate).

With 10g of acceleration (too much for any humans), an acceleration to .1c requires ~4 days and ~500 million km. You would need the whole inner solar system to accelerate and break, and .1c is far away from any relativistic effects.

In addition, your system to accelerate the ship is a weapon of mass destruction on its own. And you probably want a really powerful energy source. Direct antimatter annihilation would have the capacity, but is hard to store and handle (and imagine your storage getting hit by anything!) and probably give an exhaust visible even from other stellar systems (while this is not an issue due to the delay, it means that it is easy to spot for everything in the same system). Hawking radiation might be useful but gives a lot of heat to radiate away, and it is tricky to accelerate the black hole with 10g.

My current best design uses heat pumps to pull heat from across the ship and concentrate, then use a black-body pumped laser to lase energy in a very focused direction that I hope nobody will be listening, in addional to using heat sinks as you have suggestion with standard black-body radiator design when not in "stealth mode".

Your current best design violates basic laws of physics.

 

[Dale]

With 10g of acceleration

Railguns designed with today's technology could produce ~2000 g.

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

 

[mfb]

Deeviant was talking about a ship, not a projectile. As the ship should have its own railgun, I would assume that it is too large to be launched by regular railguns.
In addition, even with an insanely oversized railgun, capable of accelerating a whole spaceship with 2000g, and a spaceship which can handle this, the acceleration track would be more than 1 million km long. At this size and in the inner solar system, the inhomogeneous gravity from the sun becomes significant. And it cannot "catch" the spaceship afterwards.

 

[Dale]

Deeviant was talking about a ship, not a projectile.

D'oh! I completely missed that. I was entirely focused on the projectile discussion.

 

[rorix_bw]

I will say that swarms of small expendable unmanned drones would certainly be effective, but than again, you certainly wouldn't want such a swarm to find your ship, would you...

That is the ship. I don't know why you need a pilot? If it isn't the ship, then you [STRIKE]just re-invented[/STRIKE] are playing carrier war in space and my original post applies.

PS: If you think relativistic projectiles will induce fusion or antimatter reactions by virtue of the energy they deliver (which they might or might not do), then even one loose hydrogen atom, or one tiny grain of space dust, would cause detonation.

How to ensure the projectile does not go BOOM inside, or close to, the launcher? (Also a crop sprayer on the front of the ship would be an effective defence while closing...)

Also I think you really need to think about the engineering "difficultes" inherent with getting up to even 1/10th of light speed. Those are not simply going to go away with bigger drives. There is the Tsolikov Rocket Equation to consider.

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

In laymans terms, top speed of a ship in space is proportional to the exhaust velocity of the drive, and the fuel system has to conserve momentum. Which means the ship is going to be like 99.99% fuel or something and you need a drive with relativistic exhaust velocity. (Which will of course explode unless you [STRIKE]concede[/STRIKE] say that relativistic mass driver weapons don't cause fusion by kinetic energy transfer)

edit: Actually i think that equation doesn't apply to relativistic speeds, upon reading further I see there is a relativistic version of it presented, however.

 

[Deeviant]

With 10g of acceleration (too much for any humans), an acceleration to .1c requires ~4 days and ~500 million km. You would need the whole inner solar system to accelerate and break, and .1c is far away from any relativistic effects.

It doesn't have to be manned(and in this case it would not be, considering it will turn itself into a projectile). Secondly, you are assuming that humans in the future will be like the humans of today, this is a bad assumption. There are no physical laws forbidding the complete resign of the human body and/or integration with technology, and even more so, we have every reason to believe that humans will continue to "improve" our design. If a human is re-engineered to live in liquid for example, the possible acceleration could be enormous as accelerating the whole tank would not put a lot of strain of the person inside, since the force would be evenly distributed as a pressure gradient in the liquid. A complete redesign of the human body using genetic code like we use software code today plus integration of technology is already part of the world I am building. Also, it does not break, it collides, the whole thing is a projectile


In addition, your system to accelerate the ship is a weapon of mass destruction on its own. And you probably want a really powerful energy source. Direct antimatter annihilation would have the capacity, but is hard to store and handle (and imagine your storage getting hit by anything!) and probably give an exhaust visible even from other stellar systems (while this is not an issue due to the delay, it means that it is easy to spot for everything in the same system). Hawking radiation might be useful but gives a lot of heat to radiate away, and it is tricky to accelerate the black hole with 10g.

There are no laws forbidding a ship traveling at sub-light speed, it is well accepted in the hard sci-fi canon, and really nothing to be gained by trying to argue against it. It is an engineering problem, I have my own ways I will theorize how they can be solve and many others have their own, but it is truly irrelevant to this discussion. The ship could start acceleration far out-system and "Buzz" by, destroying everything in sight.


Your current best design violates basic laws of physics.

It is perfectly compatible with physics. I use a heat pump, which takes energy to move heat and concentrate it into a black-body chamber, the photons are then used to pump a laser(remember I could just radiate the heat right into space via black-body if I wanted, the only thing that is different is that I want to do so in a focused beam to reduce detection) Here is a patent for a black-body pumped laser with a good description of it's operation http://www.freepatentsonline.com/3614663.html

Deeviant was talking about a ship, not a projectile. As the ship should have its own railgun, I would assume that it is too large to be launched by regular railguns.
In addition, even with an insanely oversized railgun, capable of accelerating a whole spaceship with 2000g, and a spaceship which can handle this, the acceleration track would be more than 1 million km long. At this size and in the inner solar system, the inhomogeneous gravity from the sun becomes significant. And it cannot "catch" the spaceship afterwards.

The ship accelerates itself to high relativistic speed out-system, undetectably. It then fires it's projectiles in intercept vectors when it nears or enters the system, it is almost moving at relativistic speeds the only thing the launcher would have to do is set the projectiles into intercept vectors, for added effect the entire ship can collide into a planet to sterilize it or it could split up into independent vehicles (a la MIRV) and spread the destruction

Also I think you really need to think about the engineering "difficultes" inherent with getting up to even 1/10th of light speed. Those are not simply going to go away with bigger drives. There is the Tsolikov Rocket Equation to consider.

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

Once again, these are all arguments for why it is difficult to get a ship up to relativistic speed, in hard sci-fi, it can be assumed this enormously complex but theoretically possible feat has be achieved.
.

_____

 

[Dale]

It is perfectly compatible with physics. I use a heat pump, which takes energy to move heat and concentrate it into a black-body chamber, the photons are then used to pump a laser(remember I could just radiate the heat right into space via black-body if I wanted, the only thing that is different is that I want to do so in a focused beam to reduce detection) Here is a patent for a black-body pumped laser with a good description of it's operation http://www.freepatentsonline.com/3614663.html

I didn't see anywhere in the patent that the lasing would cool the blackbody. On the contrary, they had to use a coolant to keep the lasing material cooler than the blackbody cavity.

However, you could just use mirrors and optics to radiate the heat in a focused beam. So that isn't an "in principle" problem, just a "best practice" problem.

 

[Deeviant]

I didn't see anywhere in the patent that the lasing would cool the blackbody. On the contrary, they had to use a coolant to keep the lasing material cooler than the blackbody cavity.

However, you could just use mirrors and optics to radiate the heat in a focused beam. So that isn't an "in principle" problem, just a "best practice" problem.

Yes, it wouldn't be coherent but it would at least be directional, somewhat controlled and far more efficient...

 

[Darwin123]

I didn't see anywhere in the patent that the lasing would cool the blackbody. On the contrary, they had to use a coolant to keep the lasing material cooler than the blackbody cavity.

However, you could just use mirrors and optics to radiate the heat in a focused beam. So that isn't an "in principle" problem, just a "best practice" problem.

Focusing the beam in this case may be difficult even in principle. The radiation is initially incoherent. The radiation is initially not uncollimated. There are several laws of physics that would prevent this chaotic radiation from being collimated over large distances.
Let us start with the laws thermodynamics. Initially, the radiation is incoherent and nearly isotropic. There are many degrees of freedom in this radiation. Therefore, the radiation has a large entropy content. When you collimate the beam, you are reducing the degrees of freedom. Therefore, in order to collimate the radiation completely you would have to destroy entropy. Therefore, there is no way to collimate a beam into a small area. The second law of thermodynamics places a limit on how tightly one can collimate a beam.
It turns out that even if you collimated the beam, you would need a large area to dissipate the energy. The thermal radiation would have to be collected over a large area before you collimated it. Otherwise, you would violate the second law of thermodynamics.
Now let us look at the problem from the point of imaging optics. The radiation from the hot object is incoherent. Therefore, the light coming from the hot object acts like an image. The imaging laws for lenses applies to all curved mirrors and curved lenses that are used to collimate the light. According to the laws of lenses, the image can't be shrunk to nothing. All you could do is make a smaller real image. Again, there is only a portion of the thermal radiation that can go into making a smaller image.
You could do this in terms of diffraction. The thermal radiation has a black body spectrum. Therefore, it has a spread of wavelengths. Therefore, diffraction effects prevent one from focusing or collimating the beam. There is an uncertainty relation here that may help.
A young man that I knew suggested making a death ray out of a flashlight and a large array of lenses. He suggested taking a large number of lenses to focus and collimate the beam from the flash light into a very small area. The beam would remain in a small area over a long distance. Within that area, material would be vaporized. One could then cut tanks and airplanes to pieces using this flashlight. I explained that it would not work due to the Second Law of Thermodynamics. The beam would spread out two quickly from the focal point.
He then asked me whether I ever tried it. The answer, of course, was yes. In the course of my work, I have often had a needed for a tightly focuses and tightly collimated beam of thermal radiation. I had tried many times to use lenses (refractive and reflective) to focus such radiation. It wasn't until I did some calculations using the Second Law that I gave up.
I have often wondered what would have happened if I was wrong. Suppose this young man had decided not to listen. Suppose three were a team of rebellious teenagers who didn't listen to mainstream scientists. They went ahead and made these death rays out of flashlight batteries.
Each one could split mountains in half on two Ever Ready batteries! Each one could destroy cities using one single candle!
They would also be able to cure the worlds energy woes. Using such an array of lenses, they could make perpetual motion machines that destroy entropy.
Maybe that would make a good science fiction story. Someone comes up with that hidden combination of lenses that break the Second Law of Thermodynamics.

 

[Deeviant]

Focusing the beam in this case may be difficult even in principle. The radiation is initially incoherent. The radiation is initially not uncollimated. There are several laws of physics that would prevent this chaotic radiation from being collimated over large distances.
Let us start with the laws thermodynamics. Initially, the radiation is incoherent and nearly isotropic. There are many degrees of freedom in this radiation. Therefore, the radiation has a large entropy content. When you collimate the beam, you are reducing the degrees of freedom. Therefore, in order to collimate the radiation completely you would have to destroy entropy. Therefore, there is no way to collimate a beam into a small area. The second law of thermodynamics places a limit on how tightly one can collimate a beam.
It turns out that even if you collimated the beam, you would need a large area to dissipate the energy. The thermal radiation would have to be collected over a large area before you collimated it. Otherwise, you would violate the second law of thermodynamics.
Now let us look at the problem from the point of imaging optics. The radiation from the hot object is incoherent. Therefore, the light coming from the hot object acts like an image. The imaging laws for lenses applies to all curved mirrors and curved lenses that are used to collimate the light. According to the laws of lenses, the image can't be shrunk to nothing. All you could do is make a smaller real image. Again, there is only a portion of the thermal radiation that can go into making a smaller image.
You could do this in terms of diffraction. The thermal radiation has a black body spectrum. Therefore, it has a spread of wavelengths. Therefore, diffraction effects prevent one from focusing or collimating the beam. There is an uncertainty relation here that may help.
A young man that I knew suggested making a death ray out of a flashlight and a large array of lenses. He suggested taking a large number of lenses to focus and collimate the beam from the flash light into a very small area. The beam would remain in a small area over a long distance. Within that area, material would be vaporized. One could then cut tanks and airplanes to pieces using this flashlight. I explained that it would not work due to the Second Law of Thermodynamics. The beam would spread out two quickly from the focal point.
He then asked me whether I ever tried it. The answer, of course, was yes. In the course of my work, I have often had a needed for a tightly focuses and tightly collimated beam of thermal radiation. I had tried many times to use lenses (refractive and reflective) to focus such radiation. It wasn't until I did some calculations using the Second Law that I gave up.
I have often wondered what would have happened if I was wrong. Suppose this young man had decided not to listen. Suppose three were a team of rebellious teenagers who didn't listen to mainstream scientists. They went ahead and made these death rays out of flashlight batteries.
Each one could split mountains in half on two Ever Ready batteries! Each one could destroy cities using one single candle!
They would also be able to cure the worlds energy woes. Using such an array of lenses, they could make perpetual motion machines that destroy entropy.
Maybe that would make a good science fiction story. Someone comes up with that hidden combination of lenses that break the Second Law of Thermodynamics.

The point is not to create a infinitely small beam of destruction, it is to simply avoid a omni-directional profile that average black-body radiation would generate and act as a "here I am" beacon in space.

 

[Darwin123]

The point is not to create a infinitely small beam of destruction, it is to simply avoid a omni-directional profile that average black-body radiation would generate and act as a "here I am" beacon in space.

Okay. I am just pointing out a limitation of this type of stealth technology.
The distances in space are huge. If the beam isn't completely collimated, the spread of the beam increases with distance. Any deviation from being fully collimation of the beam may be a serious limitation of the stealth aspects of this technology. There also may be a lower bound on the size of the instrument that creates this type of beam.
It is your story. You can work out the details to the appropriate degree of accuracy. The writers don't worry about this type of detail in "Star Trek."

 

[Deeviant]

Okay. I am just pointing out a limitation of this type of stealth technology.
The distances in space are huge. If the beam isn't completely collimated, the spread of the beam increases with distance. Any deviation from being fully collimation of the beam may be a serious limitation of the stealth aspects of this technology. There also may be a lower bound on the size of the instrument that creates this type of beam.
It is your story. You can work out the details to the appropriate degree of accuracy. The writers don't worry about this type of detail in "Star Trek."

Star Trek is "soft sci-fi", they can make up whatever they want, and they are not at all bound to our laws of physics. I am constraining myself to what is at least possible (hard sci-fi) because I want to get to the core of what sci-fi means to me: looking into our future and taking a best guess at what we will find there. I think the story part is good too, I am actually not writing that part, I outline the "universe" and another creates the story within the confines of the world. Obviously, I want to push the limits of known physics to give as many interesting tools for examining future possibilities, which I'm pretty sure ruffles the feathers of some that tend to stick to what is tried and true(for good reason, the type of physics I'm looking for isn't suitable for too much other than what I want to use it for: getting a hint at the future).

 

[Dale]

If you want to do hard sci fi then you cannot break the second law of thermo.

 

[Deeviant]

If you want to do hard sci fi then you cannot break the second law of thermo.

I already agreed I am not going to try to use a black-body pumped laser, but merely radiate it out as directionally as possible. Where is the 2nd law being broken?

 

[Dale]

I don't think it is in the current iteration, just the earlier versions we had discussed. Personally, I like the internal heat sink idea a lot. The temporary aspect could be a useful plot device.

 

[rorix_bw]

Assuming I have understood the relativistic tsolikov rocket equation, and that the equation is correct (Wiki says citation required!): http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

S = exhaust velocity of drive (specific impulse) = 0.99c
M = initial ship mass = 18 units
N = final ship mass = 4 units
===>
maximum velocity = c * tanh (S/c * ln (M/N)) = 0.903c

[STRIKE]18/22[/STRIKE] 14/18 of the ship's mass needs to be fuel, to get from "rest" to 0.9c. That is a 100% efficient drive, that ejects particles at 0.99c and all that fuel is used.

I understand this equation holds for all possible reaction drives because of conservation of momentum, and non-reaction drives are magic and thus Not Allowed in hard sci-fi. In order to significantly change direction of movement, or decelerate to its destination, the ship would be needing to refuel along the way, or have external booster stages like the space shuttle did.

In my book there is not much difference between "impossible because it breaks a law of physics" and "unfeasible to engineer, regardless of technology".

PS: You can work out the density of interstellar vacuum and thus the amount of mass available for fuel. Sci-fi author Larry Niven proposed a "ramship" thad had a large hydrogen collecting funnel on the front. It's just not enough. I think they would need to fly through a nebula or something. At 0.90c ... I suggest this could be explosive.

edit: The c and the units in the formula cancel, so just

tanh (S * ln (M/N))

is needed. Someone please fix this if I messed it up.

 

[mfb]

Assuming [...] that the equation is correct (Wiki says citation required!): http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

In special relativity, you cannot add velocities linearly, but you can add the rapidity. Similar to the classical rocket equation, you have the acceleration at each step (where the rocket has ~0 velocity and classical formulas apply), but instead of the integral over the velocity changes you have to integrate over rapidity changes.
This is not a citation, but a good argument for the formula.

That is a 100% efficient drive, that ejects particles at 0.99c and all that fuel is used.

To get the required energy density, the ship needs antimatter/matter fuel or a hot black hole. In both cases, I do not see an option to get a cool exhaust.

I already agreed I am not going to try to use a black-body pumped laser, but merely radiate it out as directionally as possible. Where is the 2nd law being broken?

If your reactor (or heat source in general) is hot enough, your required power low enough and so on, this is just an engineering problem. As an approximation for the radiated heat, use the solid angle relative to 2pi (a simple surface can emit in 50% of the whole sphere), multiply it with the area where the radiation leaves the ship, and use the formula for black bodies to evaluate the radiated power. The radiators have to be cooler than the reactor (heat source) to be useful.

 

[rorix_bw]

I don't think I wrote that 3rd paragraph (black body pumped lasers).

I don't think relavistic velocities are feasible in hard sci-fi. I suppose it depends how hard one wants to be. Even if you break no laws directly, you still end up with a system that cannot be built without violating other laws or some physical constant like requiring more atoms than exist in the universe.

 

[Darwin123]

I already agreed I am not going to try to use a black-body pumped laser, but merely radiate it out as directionally as possible. Where is the 2nd law being broken?

Nothing. The problem is to radiate it out "as directionally as possible consistent with the second law of thermodynamics." Maybe it is not a real problem under the conditions of your story.
Go ahead. I see nothing fundamentally wrong with the concept.