Does physics forbid such a device; a heat destroyer

[Drakkith]

There is no machine that can take thermal energy and convert it into another form of energy without losses. A simple object emitting black body radiation is not a machine.

The question is, is a theoretical method to convert heat into EM in a method similar to black body radiation but faster.

Sure. You can use energy and pump the heat from one location to another, with the end location being hotter than the source. Otherwise I don't believe you can make it any faster without expending energy.

 

[philrainey]

If one had a nuc reactor burning as hot as the surface of the sun and if there were materials that could withstand these tempertures. Could one drop a sphere into the burning mess that is in a complete vaccum. And if one had solar panels that could withstand the temperture could one have a sphere surrounded by them within the sphere in the nuc reaction receiving radiation from the hot bigger sphere surface and converting it to electricity without any heat been dumped in a cold sink?

 

[Drakkith]

If one had a nuc reactor burning as hot as the surface of the sun and if there were materials that could withstand these tempertures. Could one drop a sphere into the burning mess that is in a complete vaccum. And if one had solar panels that could withstand the temperture could one have a sphere surrounded by them within the sphere in the nuc reaction receiving radiation from the hot bigger sphere surface and converting it to electricity without any heat been dumped in a cold sink?

We don't have these materials so it isn't possible. All machines that convert energy to something else will have losses. The cold sink WILL heat up.

 

[TobyC]

First of all, let me thank everybody here for providing valuable insight and taking time to contribute to this thread, I do very much appreciate it. And now I will jump back into it,
Even if such a process followed Carnot efficiency laws, it has absolutely no bearing on the first question: can heat be converted into another form of energy and thus be effectively purge heat from a system without some proportional outside energy being used.

So, the original question is can an object be cooled by converting it's heat into another form of energy, without the need to spend energy for the conversion, and can this be done to cool it down to some lower limit. The consensus was that it could not; that it was against the rules of physics, except that this is exactly what an object does all by itself when left by it's own accord in space: a object will convert all of it's internal energy into electromagnetic energy, cooling itself off to some lower limit(in this case stasis with the background radiation).

So I guess the question as it currently stands is not: can thermal energy be removed from an object without expending energy, as this is a foregone conclusion via black-body; but if physics really does some bar it from somehow artificially increasing the rate in which this happens. One trivial way to do this is to simply increase the surface area of object, but is that the only way.

No one has ever said that it is impossible to remove heat from something without doing work, just that you need something colder for that to happen, then it happens by itself! Just put a hot thing next to a cold thing and watch the heat leave the hot thing, you can even get some useful work out of it.

Your post seemed to want to cool something down without having anything colder to dump the heat in. It is impossible to do that without putting energy in. DaleSpam came up with the neat idea of using space as your colder thing, putting the thing in contact with deep space, and if your machine is on a spaceship in deep space then that will be even easier. Like I said, the problem in that case is likely to not be cooling stuff down but keeping yourself warm.

Other statements insist that Carnot efficiency has something to do with the fundamental question I posed(I admit even I mentioned it in my OP), but as this discussion has progressed, it is now obvious that Carnot efficiency has nothing to do with it.

Carnot efficiency has everything to do with it, it tells you how much energy you have to put into take heat from a colder body and move it to a hotter body (which is what your machine will have to do if you don't have a colder reservoir handy) and importantly that energy is non-zero, it also tells you how much work you can get out if you're moving heat from a hot body to a cold body, but you don't seem to care about getting useful work out, just removing the heat. It also tells you that converting heat entirely into a 'useful' form of energy is impossible.

 

[TobyC]

If one had a nuc reactor burning as hot as the surface of the sun and if there were materials that could withstand these tempertures. Could one drop a sphere into the burning mess that is in a complete vaccum. And if one had solar panels that could withstand the temperture could one have a sphere surrounded by them within the sphere in the nuc reaction receiving radiation from the hot bigger sphere surface and converting it to electricity without any heat been dumped in a cold sink?

No that would only work while the solar panels were colder than the hot sphere. I'm not sure how solar panels work so I can't tell you exactly why it breaks down when the solar panels heat up, but once everything is at the same temperature (which will eventually happen), you can no longer get any useful energy out of the heat in the reactor.

As the panels approach the temperature of the reactor they become less and less efficient (meaning more of the energy they absorb goes into heating them up rather than producing electric current).

 

[Dale]

In thermodynamics, a heat engine is a system that performs the conversion of heat or thermal energy to mechanical work

A heat engine coverts thermal energy to mechanical work. It is simply wrong to continue to insist anything to do with thermal energy is a heat engine.

The second law of thermodynamics is about entropy. The point of talking about mechanical work is that it has no entropy. If you convert thermal energy to any other form with 0 entropy then Carnot's efficiency limit applies. It can be derived in a couple of lines directly from the second law of thermo and the definition of temperature, regardless of the actual form of the 0-entropy energy.

Especially since in this case ending up with usable work is not at all required. Even if we did want to do work, who cares, the primary concern is to dump the heat and whatever work we get out of it is icing on the cake. I repeat, the question here is how quickly and efficiently does physics allow us to remove heat from an object. Nature has already provided us with a perfect example via black-body radiation, but does physics bar anything faster and more efficient.

That isn't the question posed in the OP, but if your concern is merely to dump the heat then that is simply heat transfer, not a heat destroyer. In space the only mechanism of heat transfer is radiation. Heat transfer still requires a cold reservoir in order to follow the second law. Luckily, 2.7 K is a pretty cold cold reservoir for most purposes.

it is has been stated earlier in this thread that in order to pull heat out of mass, one must expend outside energy, this is false ... Another claim was that pulling heat out of an object and converting into electromagnetic energy violates entropy laws, this is also false

Please quote these false claims exactly, I think you are just misunderstanding or misquoting.

The question is, is a theoretical method to convert heat into EM in a method similar to black body radiation but faster.

This question has already been answered in post 25. EM radiation has entropy, so if you radiate with any spectrum other than a black-body you will reduce the entropy which will involve the second law of thermo. In the limit of a very low entropy spectrum, like a laser, you can radiate a lot of energy quickly, but Carnot's limit applies.

 

[Dale]

I'm not sure how solar panels work so I can't tell you exactly why it breaks down when the solar panels heat up

Roughly speaking there is a "band gap" between different parts of the semi-conductor. When a photon with energy higher than the band gap hits the semi-conductor then it can push an electron across the gap and generate a current.

As solar panels heat up, the electrons in the semi-conductor become more energetic, to the point where they posess enough thermal energy to jump the band gap. The problem is that with thermal energy they jump both directions. So what happens is that the energetic photon is absorbed, which pushes one electron across the band gap, but instead of generating a current a thermal electron just jumps backwards across the band gap.

This actually happens at much lower temperatures than thermal equilibrium. Keeping solar panels cool is a major design consideration wrt efficiency.

 

[Dale]

Just to put some numbers on this. Suppose that the heat source is pretty hot, producing heat at 1000ºC (1273 K). And suppose further that the device is in thermal contact with deep space, as I suggested, and so it is using deep space as the cold reservoir (2.7 K). So the Carnot efficiency is 1-Tc/Th = .998. This means that for every 1 MW of heat produced, the device could capture 998 kW as work (or other low entropy forms of energy) and would have to dump 2 kW to deep space to satisfy the second law of thermo.

You can use the law for radiative power transfer for a black body, which is \dot{Q}= \sigma (T_h^4-T_c^4) A. So to radiate 2 kW at 1273 K to a bath of 2.7 K requires an area of .014 m².

You can just scale those numbers up by however many MW you expect your power plant to produce.

The technological advances would be to use deep space as the cold reservoir while radiating at the hot temperature. That isn't something we could do now, we would use a radiator as the cold reservoir which would be at an intermediate temperature between 1273 K and 2.7 K, reducing the maximum efficiency of the engine and increasing the surface area required to radiate. But that would be the limit of what is possible according to the laws of physics as we know them, so that would be the limit of what you could get away with using "future tech" but not breaking the laws of physics.

 

[mfb]

So, the original question is can an object be cooled by converting it's heat into another form of energy, without the need to spend energy for the conversion, and can this be done to cool it down to some lower limit.

Yes, if you have access to a cooler object (like space) and transmit a fraction of this energy to the cooler object. The required fraction depends on the temperature ratio.

The question is, is a theoretical method to convert heat into EM in a method similar to black body radiation but faster.

It is possible to be faster than low-temperature radiators alone. You can use the temperature difference between "hot object" and radiators: A part of the energy has to go to the radiators, another part can be used to do something else - for example, power a laser which additionally emits energy. However, a higher temperature of the radiators would do the same (where the temperature of the hot object is the ideal value).

@DaleSpam: You already need contact to the 2.7K-bath to extract 998kW. You cannot get this and feed a 1000°C-surface with the remaining 2kW. Otherwise you could use this 1000°C-surface again, and extract .998 of the 2kW... you see the problem?

 

[Dickfore]

Anyways, the question is this: can a "heat destroyer" be made?

As I define it, this device takes simply converts heat into some other form of energy, either EM or perhaps electricity. Of course, this can already be done in many ways today, but what we're talking about is a matter of degree. The amount of power it generates is not important, nor is the efficiency, but the important part is it can do so "infinitely" i.e. you turn the device on and it brings itself to near absolute zero, I suppose a somewhat higher minimum cap is ok. Another limitation is, other than the heat, it can't be fed any other energy, except maybe for some control or other higher level stuff, but the key here is it's not like you have to feed this thing a huge amount of energy for it to work, it just "eats" the heat.

My only lead is the carnot's work, perhaps the formula making clear that close delta T's make for very little work.

Your lead is wrong. Carnot's Theorem was proven by using the second law of thermodynamics.

The machine you are describing is exactly as the one in the Kelvin formulation of the second law.

So, the answer is that it is impossible!

 

[Dale]

@DaleSpam: You already need contact to the 2.7K-bath to extract 998kW. You cannot get this and feed a 1000°C-surface with the remaining 2kW. Otherwise you could use this 1000°C-surface again, and extract .998 of the 2kW... you see the problem?

Certainly. I don't see a way around it, which is what I mentioned about the radiator at an intermediate temperature. I don't know how a heat engine could be in thermal contact with deep space rather than a radiator, but I don't know a law of physics that forbids it.

But the second law of thermo is satisfied as long as at least 2 kW/MW goes to space. So I think that anything else is fair game for "future tech". Although, maybe the "future tech" is a way of arbitrarily increasing the effective surface area of the radiator.

 

[Dale]

we would use a radiator as the cold reservoir which would be at an intermediate temperature between 1273 K and 2.7 K, reducing the maximum efficiency of the engine and increasing the surface area required to radiate.

Just out of curiosity I was playing around with this idea and optimizing the intermediate temperature such that the surface area of the radiator is minimized. For any given output power, the lower the intermediate temperature, the less energy needs to be radiated, but the less efficient the radiator. Conversely, the higher the intermediate temperature, the more efficient the radiator, but as the power plant becomes less efficient more energy needs to be radiated.

It turns out that there is a minimum at 955 K (64 m²/MW) which corresponds to a 25% efficiency on the engine. Any hotter than that and the engine becomes so inefficient that the radiator area needs to be larger, and any colder and the radiator itself becomes so inefficient that the area needs to be larger. However, there is a very broad range that is close to the minimum surface area.

 

[mfb]

Why does it grow so much above 1200K? Close to 1273K, the efficiency is ~0 and you have to dump ~33% more heat. However, the temperature is higher by 1/3, which leads to a radiation of (4/3)^4 =~ 3 times the 955K-value. Based on this, I would expect that the required radiator area does not have any minimum.

 

[Dale]

Why does it grow so much above 1200K? Close to 1273K, the efficiency is ~0 and you have to dump ~33% more heat. However, the temperature is higher by 1/3, which leads to a radiation of (4/3)^4 =~ 3 times the 955K-value. Based on this, I would expect that the required radiator area does not have any minimum.

That isn't quite how it works. Close to 1273 K you don't have to dump just 33% more heat, you have to dump an infinite amount of heat.

For example, at a radiator temperature of 1200 K the engine is terribly inefficient (~6.7%). So every 1 W of power produced requires 17.4 W of heat from the hot reservior and so you need to dump 16.4 W to the radiator. This is 448% more heat than the 955 K value (3 W), not just 33% more.

Remember, an engine is rated and designed for the power it produces, not the amount of fuel it burns. I suspect you are thinking of a constant heat input rather than a constant power output.

 

[Dotini]

Question: Is the interesting process which takes place on the sun - going from very hot interior to relatively "cool" surface to very hot corona - an example of the physical process the OP has in mind?

Respectfully submitted,
Steve

 

[mfb]

Oh, you fixed the amount of usable work. Sorry, I thought you fixed the thermal input power as Deeviant does.

 

[Deeviant]

Just to put some numbers on this. Suppose that the heat source is pretty hot, producing heat at 1000ºC (1273 K). And suppose further that the device is in thermal contact with deep space, as I suggested, and so it is using deep space as the cold reservoir (2.7 K). So the Carnot efficiency is 1-Tc/Th = .998. This means that for every 1 MW of heat produced, the device could capture 998 kW as work (or other low entropy forms of energy) and would have to dump 2 kW to deep space to satisfy the second law of thermo.

You can use the law for radiative power transfer for a black body, which is \dot{Q}= \sigma (T_h^4-T_c^4) A. So to radiate 2 kW at 1273 K to a bath of 2.7 K requires an area of .014 m².

You can just scale those numbers up by however many MW you expect your power plant to produce.

The technological advances would be to use deep space as the cold reservoir while radiating at the hot temperature. That isn't something we could do now, we would use a radiator as the cold reservoir which would be at an intermediate temperature between 1273 K and 2.7 K, reducing the maximum efficiency of the engine and increasing the surface area required to radiate. But that would be the limit of what is possible according to the laws of physics as we know them, so that would be the limit of what you could get away with using "future tech" but not breaking the laws of physics.

These numbers get right at fundamentals of what a black-body cooling system would be looking at. But in the end, such a method is not what I was trying to get at. Don't get my wrong, I think using radiative cooling is super useful in space, but it is has been well hashed out in many different sci-fi worlds and it certainly isn't advanced technology, it really isn't technology at all; it's just how the universe works.

It may well be my fictional but physical law obeying spaceship relies solely on black-body emission for cooling, but I was looking for something a bit more... exotic.

 

[Drakkith]

It may well be my fictional but physical law obeying spaceship relies solely on black-body emission for cooling, but I was looking for something a bit more... exotic.

Invent something! It's fiction! Make it logically consistent with known laws as best as possible, but in the end you're going to have to break a rule or two, so go all out!

 

[Dale]

These numbers get right at fundamentals of what a black-body cooling system would be looking at. But in the end, such a method is not what I was trying to get at. Don't get my wrong, I think using radiative cooling is super useful in space, but it is has been well hashed out in many different sci-fi worlds and it certainly isn't advanced technology, it really isn't technology at all; it's just how the universe works.

It may well be my fictional but physical law obeying spaceship relies solely on black-body emission for cooling, but I was looking for something a bit more... exotic.

Honestly, it is much more important that your characters be compelling and the plot be interesting than technical details be correct.

 

[Deeviant]

Honestly, it is much more important that your characters be compelling and the plot be interesting than technical details be correct.

For most stories, yes that is certainly true. However, I am rather uninterested in human emotion and motivation; I am far more interested in the fate of humanity and future of our universe.

However, I don't want to full into a pedantic cycle of physics nit-picking, I want to look into the future.

 

[mfb]

I am highly confident that human emotions and motivations are of crucial importance for the fate and future of humanity.
I do not want to give any estimate for the influence of humanity on the future of the universe, and I do not even have a measure which would allow this.

I don't want to full into a pedantic cycle of physics nit-picking

Where is the problem with large radiators then? It is just an engineering issue, and it is usually fine to assume that those are solved in science-fiction.

 

[Deeviant]

I am highly confident that human emotions and motivations are of crucial importance for the fate and future of humanity.
I do not want to give any estimate for the influence of humanity on the future of the universe, and I do not even have a measure which would allow this.


Where is the problem with large radiators then? It is just an engineering issue, and it is usually fine to assume that those are solved in science-fiction.

The problem is that it doesn't not accomplish the goals I had in mind(I mainly wanted a device that could absorb and convert latent waste heat).

To give you a better picture of what I am working toward, picture this; In the future of spacecraft warfare, all the scenarios I have come up with is that you are completely out of luck as far as defensive systems go. I see no feasible way to create a star trek style "shield", nor any reasonable way to armor a spaceship that would be in any way capable of standing up to even the weapons we could build now, let alone weapons of the future.

Thus, I see only one defense left: stealth. The hulls would be built for stealth, and one important requirement for that would be that the hull would need to cool, specially so it does NOT emit large amounts of highly detectible EM radiation via black-body. And it must remain cool despite housing a large generator of some type inside it.

If anybody has any crazy ideas for possible defensive systems that physics may allow, I would certainly like to hear it, but I could not even think of a system that could sufficiently protect a ship against a large thermonuclear bomb. There just doesn't seem to be any good physics here to grab on too; magnetic shielding would only work against charge particles and requires too much energy anyways, a simply material based approach seems to fail against the furiosity of possible weapons, mirroring might be effective against one of the most obvious space-based weapons(laser) but would completely defeat the stealth idea and would be completely ineffective against kinetic or other more exotic weapons.

So, I would foresee that a futuristic spaceship intent on warfare would somewhat resemble a submarine, not in shape or operation, but in general philosophy. For this reason I imagine a black EM absorbing hull, some method of dumping heat that doesn't act as a beacon, a inertial/kinetic propulsion system as simple as accelerating masses to significant fractions of light (which happens to double as a weapon system).

I have however, conceptualized a few ideas which do bend the rules a bit. For instance:

"Hawking" generators/weapons: Using some unknown process normal matter is condensed and compressed past it's Schwarzschild radius, creating an non-gravitationally induced black hole. Such a small black hole will then almost immediately evaporate via hawking radiation in the form of EM radiation, funny enough this is also a black-body emission. This would allow one to convert mass directly into energy without an matter/anti-matter reaction. I may be wrong, but I assume it radiates energy equivalent to the starting mass as well as the energy introduced via the compression. Other than the obviously enormous challenge of compressing matter to such a degree, I am reasonably sure this idea is in line with accepted physics. Obviously, this would also serve as a tremendous weapon; perhaps have an anti-matter kicker that provides compression energy then resulting in a center mass finding itself within it's own Schwarzschild radius and almost immediately converting itself into extremely hard EM radiation(the temperature of a 500 metric ton black hole would be 2.454406e^17 K, found with this very nice black hole calc ap http://xaonon.dyndns.org/hawking/). Anti-matter is obviously very powerful itself and extremely possible, but has many problems, which the Hawking devices overcome; there is no ready source of anti-matter so it will never be an energy source but rather a most effective means of story and concentrating energy, and anti-matter explosions are nearly as big as you think they would be since it is very hard to get each atom to combine considering the extremely energetic reactor, in fact most anti-matter would likely be wasted with only a small amount actually being converted.

 

[Drakkith]

Well, you could have all ships come with a cooling system that routes the heat to a specific location where and deposited. The resulting radiation is focused into a collimated beam and emitted in a specific direction. Unless you are in the direct line of this beam then you cannot detect it. I can think of a few situations where it would be useful in the book, such as a "lucky break" in detecting an incoming attack by a patrol that by chance stumbles in line of site of this beam.

For defending against nuclear warheads, a laser of some type, perhaps UV or X-Ray could be used for destruction of the warhead before detonation. A nuke in space must be very very close to damage something, as most things in space are many many kilometers away from each other.

 

[Dale]

I have however, conceptualized a few ideas which do bend the rules a bit. For instance:

"Hawking" generators/weapons:

I don't think this is bending the rules as much as you might think: http://arxiv.org/abs/0908.1803v1

 

[Drakkith]

I don't think this is bending the rules as much as you might think: http://arxiv.org/abs/0908.1803v1

That. Is. Cool.

 

[mfb]

Thus, I see only one defense left: stealth.

In this case, I have bad news for you.

You can direct the radiated heat to some extent, but if the enemy is prepared for that the spaceship cannot really hide.

 

[Deeviant]

In this case, I have bad news for you.

You can direct the radiated heat to some extent, but if the enemy is prepared for that the spaceship cannot really hide.

That was an interesting page, with a rather large amount of thought placed into it. However, it really seemed like they came up with a conclusion and then filled in the blanks.

Really though, the only point that I agree with is the black-body radiation is going to be a problem, hence the creation of this thread in the first place. One of the points they seem to keep going back to is that a ship has to have some huge drive plume, this is not at all I how I envision my ship, they will accelerate basically by using a mass driver, which doubles as a weapon. Radar could be easily defeated, a telescope would extremely hard pressed to pick up black ship(which could also have technology to light up it's hull to mimic the background stars behind it).

If push comes to shove, I use a black-body pumped laser and lase the energy out into space in such a narrow beam, a detection systems would simply not be useful. (http://www.freepatentsonline.com/3614663.html).Still though, I want my heat destroyer =/

 

[mfb]

(which could also have technology to light up it's hull to mimic the background stars behind it)

If you know the position of the enemy (and therefore the position on the ship you have to light), which is somewhat strange as you try to make ships invisible (and probably not just the own one). As alternative, you could have light emitters everywhere. In addition, you should try to reproduce the spectrum.

Your mass driver can have a high efficiency, sure. The required high acceleration is not trivial, but might be possible. However, the exhaust is not the main point. As soon as your ship has warm parts, it also needs some blackbody radiation.

 

[Plastic]

what is heat?

radiation? -> photoelectricity.
kinetic energy of a large particle? -> you could annihilate that particle with antimatter and harvest the resulting radiation, destroying the 'heat' in the process.

 

[Dale]

The problem is that it doesn't not accomplish the goals I had in mind(I mainly wanted a device that could absorb and convert latent waste heat).

As described, you cannot absorb and convert latent waste heat to work without a cold reservoir, and even then you are limited by Carnot. However, here are three ideas:

1) They have developed a way to greatly expand their effective surface area for radiation. The bigger the surface area the closer to 2.7 K they can radiate with the radiator serving as the cold reservoir. Since it is slightly higher than 2.7 K the blackbody spectrum would be slightly different, so could still be tracked. But it could be made arbitrarily difficult by making it arbitrarily large.

2) They have developed a way to exchange heat directly with deep space at 2.7 K as the cold reservoir. Although there is no known way to do it, it does not violate the second law of thermo (but it might violate some other laws, I am not sure). In principle, there would still be energy leaving the ship, even if it were at 2.7 K, so there may still be a way to track it.

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.

 

[rorix_bw]

I see no feasible way to create a star trek style "shield", nor any reasonable way to armor a spaceship that would be in any way capable of standing up to even the weapons we could build now, let alone weapons of the future. I could not even think of a system that could sufficiently protect a ship against a large thermonuclear bomb.

You do it the same way we do it today. Shoot the missile or jam its radar.

I suggest researching modern warfare especially air and sea. This is almost all electronic now (no more dogfights!). We have both hard and soft kill systems in use today, for preventing missile hits. Modern warships are not significantly armoured in way that World War II ships were, or that tanks are.

Hard kill involves shooting the missile. There are a number of systems for this e.g. AEGIS air defence system on the US Navy "Burke" class Destroyers. This was designed for air defence (against missiles and aircraft) but is now being adapted to defend against ballistic missiles (it has hit them in tests: give it a few more years). The premise of the system is that you fit surface-to-air (SAM) missiles to a ship, add a very sophisticated air defence radar, and theoretically (if it all works, and the crew all do their jobs) you can stop missiles and aircraft from reaching you.

I haven't seen any classified data on this but you can assume it *probably* works as follows: It has 90 missile cells. Each cell holds a single long range surface-to-air missile (SAM), or 4 short range ones (the new ESSM 4-packs). It will shoot 2 of the long range missiles at each incoming air target at about >100km range and each missile *probably* hits 0.5 of the time. At about >50km range it will shoot 2 of the short range ones at any "leakers", with almost certainly higher pH. At point blank range it will use a single RAM missile vs any leaker. In tests, RAM has a 0.9 pH.

I don't know what range they are hitting ballistic missiles at. Ask me again in 10 years. Space combat would probably be extending the ranges.

Soft kill can involve electronic jamming (ECM "Electronic Counter Measures"). For example radar "decoys". These rely on the fact that a missile must somehow be able to find its target by itself (unless you want to put a pilot into it) and do this it has a radar. The radar works by giving off signals that bounce back from the target and by detecing the reflected signals, the device locates stuff. By transmitting "false echos" you can confuse a radar.

An example of such a system is "Nulka". This is a rocket that you launch from a ship, which can hover close to the sea, and gives off signals, making it look, to the small computer and basic radar in a missile, like a ship. Remember that if a missile "misses" it may not be able to turn around and re-attack: Only a small number of modern missiles can turn around for a second go (fuel limit, computer limit, maneuvering limit, G-force breaks the fins off etc) and they can be shot as they turn.

There are also various "ECM" devices that comprise transmitters mounted on the ship itself, that can emit signals, that have the same effect. e.g. "SLQ-32". The large computer on a warship is much smarter than the smaller one on a missile.

More primitive systems used "chaff" which could be described as "reflective confetti" and produced a cloud on the radar that obscured the targets. One even launched a net at the missile (that didn't really work, but in space you might use a gun that shoots a glob of expanding foam or something to catch the missile, then it can have bomb or a TASER or something in the foam cartridge - net projectors certainly have worked in the past). A basic metal grille should stop a missile, if it hits it hard enough to hurt itself ...

Finally, and this sounds crazy, but you also have to locate the target before you can shoot a weapon at it. I don't mean stealth, I mean: Say there's a CVN (aircraft carrier) attacking you, that you want to get rid of. OK, where is it? It's not going to be in sight of the coast as it can strike from over 1,000 kilometers away. (In space, it's painted black and in any case they probably parked it behind the moon and are sending drones out around the moon to attack you. "Behind the moon" is a big place.)

First, your radar and camera satellites don't cover the entire ocean. There are gaps and because their orbits are known, the carrier can sail between them: (no joke, USA used to do this to Russian RORSATS [Radar Ocean Reconnaissance SATellite) but it is harder now with more satellites (a lot harder). However, USA, Russia and China have all demonstrated the abilty to shoot satellites. In space, radar satellites aren't going to be as lopsided as they are on Earth - they will be be just extra spaceships.

Second, the carrier has a sea control zone of several hundred kilometers in any direction created by its air patrols. Your own scouts and satellites entering this zone are certainly going to come under attack well before they cross the "radar horizon" at the point at which they can detect the carrier due to line of sight over the curve of the Earth or in our case "behind the moon". If the scouts have their own radars on, this is even harder as a radar is like a searchlight: The guy you are looking for can detect you very easily by the beam from your searchlight, and will see you before you see him; it's quite easy therefore for him to shoot his rifle at the light then move to a new place - don't stand next to the guy carrying the light - the equivalent to this is a missile that homes in on active radars. There's also missiles that home in on jamming sources - it's a never ending cycle of weapon vs counter-weapon.

Even if your missiles are set for "bearing only launch" with a wide search pattern (Mr Missile, please fly 500 km in direction A then turn on your radar and look for ship targets over 100 meters in length) not only to the missiles themselves also have to survive, and not get their radars turned off, they are as likely to prang an oil tanker as they are the enemy warship. That's probably less likely in space but you never know ... one assumes you will fight for control of planets, and not randomly in deep space.

Finally, if you're in space and using nukes, note that in space, you're going to have land a nuke very close, as there is no atmosphere to create a pressure wave. There's also nothing stopping the other guy using nukes to counter your nukes. He doesn't need to destroy the missile, just damaging its electronic systems is sufficient.

Missiles can evade (modern anti-ship ones do, they are programmed to make radical maneuvers as they near the target to make it harder to hit them) but essentially they have to get within a certain distance of the target so their destination is known, and they do have a big heat source from their engine and a big searchlight on the front called a radar, so they aren't exactly hard to see.

Guns also can be effective against missiles. The reason gun-based defences are being superceeded with electronics and counter-missiles in modern warfare is due to the limited range of guns vs the speed of modern missiles. However, in space with access to lasers and railguns etc, this may be less of a factor. A gigantic "shotgun" may well be able to deal with incoming missiles.

One other thing to consider, you can stick a gun or submunition on a missile. For example a bomb-pumped X-Ray laser. This is supposed to be a nuke that explodes and directs its force forwards via rods that channel the blast; it's sort of a one use laser that doesn't require you to get close to the enemy. Today we have thing like "subroc" which is a missile that launches a torpedo (because the range of a torpedo is limited, and it does not require a separate torpedo system to launch it); many mines also don't explode, but instead launch torpedoes. You could have a single "bus" vehicle that would spit out submunitions - small anti-missile missiles - in the vague direction of any incoming missile.

Some wikis to check out

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

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

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

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

http://en.wikipedia.org/wiki/DDG_51

http://en.wikipedia.org/wiki/SLQ-32_Electronic_Warfare_Suite

BTW: I just remembered there have been nuclear tests vs warships. You have to get considerably closer than most people would imagine ... and that's with an atmophere. They have positively pressurised super-structure and decontamination wash-down systems now on warships. I'll to find the results in Google but that is something you may wish to look for.

EDIT: I also remembered the "medium problem". This is "a missile is faster than a sea ship because it moves in the air, and not the sea". But a missile is not significantly faster than an aircraft ... and only recently (e.g. Spearfish @ 70 knots) have torpedoes been faster than ships or submarines.

The Russians actually built a submarine (Akula class - NATO name, Russian "Akula" is a different class) with sufficient speed to outrun the American torpedo of the same period, and the American SR-71 "blackbird" airplane was able to outrun anti-aircraft missiles. The actual missile had similar speed to the plane, but since the missle had to close, if the aircraft saw it and altered course, it would pass behind.

 

[Deeviant]

As described, you cannot absorb and convert latent waste heat to work without a cold reservoir, and even then you are limited by Carnot. However, here are three ideas:

1) They have developed a way to greatly expand their effective surface area for radiation. The bigger the surface area the closer to 2.7 K they can radiate with the radiator serving as the cold reservoir. Since it is slightly higher than 2.7 K the blackbody spectrum would be slightly different, so could still be tracked. But it could be made arbitrarily difficult by making it arbitrarily large.

2) They have developed a way to exchange heat directly with deep space at 2.7 K as the cold reservoir. Although there is no known way to do it, it does not violate the second law of thermo (but it might violate some other laws, I am not sure). In principle, there would still be energy leaving the ship, even if it were at 2.7 K, so there may still be a way to track it.

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.

Ok, I think I am still managing to misunderstand you. You say we can not exchange head directly with deep space this day, but my understanding is: it happens for free via black body. The ISS is current exchanging heat directly with deep space right now as we speak, is it not? Or are you taking about some other sort of mechanism, I don't understand what you mean by "is no known way to do it".

@rorix_bw:

I think you are thinking too much in terms of today's battle scenarios. Think of a perfected electro-magnetic slug shooter a future ship may pack; let's say this thing accelerates a slug to 99.99% C. Then let's say it fires a slug at your ship the same instant a photon bounces off the ship and heads your way; the slug would only be a bit behind and that photon hitting your ship would be the first moment you could even possibly realize the ship fired a projectile at you in the first place, let alone avoid it somehow. For all intents and purposes, the thing would be like a laser that shoots bullets. Prototypes for this type of weapon technology exist today, albeit at much slower velocities(although they have to deal with air resistances and a ship obviously would not). And the thing is, I don't even think the future ship would use a railgun but something far worse that we haven't thought up of yet, my point is if beefed up versions of today's technology already defeats any kind of defense I can dream up; then defensive ship technology has only one fallback: stealth.

 

[Dale]

Ok, I think I am still managing to misunderstand you. You say we can not exchange head directly with deep space this day, but my understanding is: it happens by for free via black body. The ISS is current exchanging heat directly with deep space right now as we speak, is it not? Or are you taking about some other sort of mechanism, I don't understand what you mean by "is no known way to do it".

Sorry, I was definitely unclear there. I meant that there is no way that I know of to directly use deep space as a 2.7 K cold reservoir in a heat engine.

 

[Deeviant]

Sorry, I was definitely unclear there. I meant that there is no way that I know of to directly use deep space as a 2.7 K cold reservoir in a heat engine.

I think most space probes use the 2.7 K cold reservoir in a heat engine today. I.E. the voyager probes use a plutonium powered thermopile. Plutonium heats up one side of a peltier device and the other side is cooled via black body directly into space.

 

[rorix_bw]

Gaussian Beam

While composing a reply I was wondering if someone knows: is a gaussian beam the best you can focus a laser to? Can you actaully focus it better than this so it retains its full intensity (same beam width) over a longer distance? Wikipedia isn't clear on this, it only says that some lasers can be gaussian. I know I don't have to link to here, as many guys will know already, but I provide for convenience so you can see what I read, not what the truth is :-)

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

 

[rorix_bw]

1) Think of a perfected electro-magnetic slug shooter a future ship may pack; let's say this thing accelerates a slug to 99.99% C.
2) Then let's say it fires a slug at your ship the same instant a photon bounces off the ship and heads your way; the slug would only be a bit behind and that photon hitting your ship would be the first moment you could even possibly realize the ship fired a projectile at you in the first place
3) my point is if beefed up versions of today's technology already defeats any kind of defense I can dream up; then defensive ship technology has only one fallback: stealth

I will address the last point first. You cannot envisage a defence because modern military hardware is complex enough to no longer be obvious to one who has not studied them. Now I am not saying that it must be this way and that modern systems will work in space, but it's clear to me there's large difference between how the average person envisages defences work, and how they actually work.

For point (1), I see you changed to "impulseguns" from nuclear missiles? Well at least we're helping :-)

Thinking laterally (or again copying from modern design) you do not have to absorb the full energy of a projectile to stop it.

Whatever you make the projectile out of, someone can make armour out of, and it will most likely cause both to shatter on impact - in general, you need to be harder than something to survive striking it. Now you need to decouple the armour from the vehicle so you don't have to eat the added momentum of the surviving parts of the projectile that don't bounce off your (obviously curved) armour due to hitting at an angle.

In space we can launch a plate of armour metal, and drift along behind it. Get a drone to drive it perhaps (or maybe you can use a collapsible bracket or electromagnet). This de-coupling should provide protection against the initial salvo. You might get fragments depending on the angles of the hit and the angle your armour plate is at, but that's why you have a second layer of armour. (This system of 2 plates with a gap, with the outer plate shattering hard rounds, resembles some types of modern tank armour - the spaceship difference is that we don't need to attach the inner plate to the outer plate, so we have no momentum transferred on a hit)

As for point (2) I regard a faster than light detection system as cheating in the scenario you defined. The earliest you can fire is when you receive a reflection - with perfect "optics" both ships can in theory simultaneously detect each other and shoot at the same time. (Unless one side is invisible, which is already how it works today if the sides are mismatched).

You're going to have decide what your space combat ranges are and how fast a ship can maneuver. I am assuming no human crew on the combat units (because sci-fi, and the next generation of fighter planes built here on Earth will be pilotless) so the 9G acceleration limit on consciousness isn't relevant. (Maybe just a few techs and leaders in a command unit, everything else is robot)

If you're fighting at close ranges you need to explain how you got there past all the robofighters and missiles. If you're fighting at long ranges you probably aren't using a gun. Earth to Moon is 0.04 light seconds? Probably no chance of target dodging there, but how big is the ship and even a laser will gradually diffuse? (actually that's an interesting question ... i will pose separately)

The defence there is the same as it probably always will be, use decoys, maneuver erratically and use stealth. With so little time to identify and get a shot off, and such tiny targets, something like coming in with the sun behind you in a cloud of (mirrored?) drones might actually work. Decoys sound stupid but they do work. Really they're just another form of camoflauge, which is just another form of stealth. It's way of saying "I am not here" or "congratulations, you have more targets than guns. how lucky are you?"

Anyway stuff to think about. No-one can predict the future of spacewar but you should at least read up on current systems, since otherwise your book is going to be World War II in space.

 

[Dale]

I think most space probes use the 2.7 K cold reservoir in a heat engine today. I.E. the voyager probes use a plutonium powered thermopile. Plutonium heats up one side of a peltier device and the other side is cooled via black body directly into space.

The cold side of the junction is warmer than 2.7 K.

 

[Deeviant]

I will address the last point first. You cannot envisage a defence because modern military hardware is complex enough to no longer be obvious to one who has not studied them. Now I am not saying that it must be this way and that modern systems will work in space, but it's clear to me there's large difference between how the average person envisages defences work, and how they actually work.

For point (1), I see you changed to "impulseguns" from nuclear missiles? Well at least we're helping :-)

Thinking laterally (or again copying from modern design) you do not have to absorb the full energy of a projectile to stop it.

Whatever you make the projectile out of, someone can make armour out of, and it will most likely cause both to shatter on impact - in general, you need to be harder than something to survive striking it. Now you need to decouple the armour from the vehicle so you don't have to eat the added momentum of the surviving parts of the projectile that don't bounce off your (obviously curved) armour due to hitting at an angle.

In space we can launch a plate of armour metal, and drift along behind it. Get a drone to drive it perhaps (or maybe you can use a collapsible bracket or electromagnet). This de-coupling should provide protection against the initial salvo. You might get fragments depending on the angles of the hit and the angle your armour plate is at, but that's why you have a second layer of armour. (This system of 2 plates with a gap, with the outer plate shattering hard rounds, resembles some types of modern tank armour - the spaceship difference is that we don't need to attach the inner plate to the outer plate, so we have no momentum transferred on a hit)

As for point (2) I regard a faster than light detection system as cheating in the scenario you defined. The earliest you can fire is when you receive a reflection - with perfect "optics" both ships can in theory simultaneously detect each other and shoot at the same time. (Unless one side is invisible, which is already how it works today if the sides are mismatched).

You're going to have decide what your space combat ranges are and how fast a ship can maneuver. I am assuming no human crew on the combat units (because sci-fi, and the next generation of fighter planes built here on Earth will be pilotless) so the 9G acceleration limit on consciousness isn't relevant. (Maybe just a few techs and leaders in a command unit, everything else is robot)

If you're fighting at close ranges you need to explain how you got there past all the robofighters and missiles. If you're fighting at long ranges you probably aren't using a gun. Earth to Moon is 0.04 light seconds? Probably no chance of target dodging there, but how big is the ship and even a laser will gradually diffuse? (actually that's an interesting question ... i will pose separately)

The defence there is the same as it probably always will be, use decoys, maneuver erratically and use stealth. With so little time to identify and get a shot off, and such tiny targets, something like coming in with the sun behind you in a cloud of (mirrored?) drones might actually work. Decoys sound stupid but they do work. Really they're just another form of camoflauge, which is just another form of stealth. It's way of saying "I am not here" or "congratulations, you have more targets than guns. how lucky are you?"

Anyway stuff to think about. No-one can predict the future of spacewar but you should at least read up on current systems, since otherwise your book is going to be World War II in space.

To be honest, I'm really not picking up what you're putting down. There is no material that can successful absorb the energy of a .99 C projectile of any significant mass, and your idea of some sort of "decoupled" armor is laughable, you'd have to know where I am in order to put it "in line" with my attack vector; I already said I'm focused on stealth. To put it into perspective, a 1 kilogram projectile traveling at .99 C represents 10.52 MEGATONS of energy, there is NO material conceivable that will be able to withstand that much energy.

It's strange that you basically said I was wrong, then agreed with me; "The defence there is the same as it probably always will be, use decoys, maneuver erratically and use stealth". Maneuverability isn't going to happen, it takes too much energy and mass for a ship of any decent size to rapidly change velocity, decoys are only useful AFTER you have been discovered else you will just give away your general position, that leaves one thing: stealth. As for you WWII in space, that is exactly what I foresee; a scenario similar to WWII submarine warefare.

 

[Drakkith]

To be honest, I'm really not picking up what you're putting down. There is no material that can successful absorb the energy of a .99 C projectile of any significant mass, and your idea of some sort of "decoupled" armor is laughable, you'd have to know where I am in order to put it "in line" with my attack vector; I already said I'm focused on stealth. To put it into perspective, a 1 kilogram projectile traveling at .99 C represents 10.52 MEGATONS of energy, there is NO material conceivable that will be able to withstand that much energy.

Perhaps, but there is no way to accelerate a 1 kg mass to 0.9999c using a single launcher either. It require extremely hard acceleration for a very long period of time to get up to speed using a propulsion system. It takes the LHC almost an hour to accelerate protons up to their top speed, and this is using multiple stages of circular accelerators that are kilometers long, and you're talking about something many orders of magnitude larger. It isn't feasible at all.

 

[mfb]

Well, the LHC needs ~30min because the bending magnets have to be ramped up. Acceleration itself could be done within milliseconds. You will not see bending magnets at the acceleration of macroscopic objects.
However, large objects have serious issues with the maximal acceleration. Acceleration to relativistic velocities within reasonable length (not several million kilometers) is impossible today, and this is not just an engineering issue.

 

[rorix_bw]

There is no material that can successful absorb the energy of a .99 C projectile of any significant mass ... To put it into perspective, a 1 kilogram projectile traveling at .99 C represents 10.52 MEGATONS of energy, there is NO material conceivable that will be able to withstand that much energy.

I said you don't have to absorb all of it. If it breaks up when it hits something floating in space (not attached to your ship) made of the same material as it, it doesn't matter as much.

Your idea of some sort of "decoupled" armor is laughable, you'd have to know where I am in order to put it "in line" with my attack vector;

Put it all around like a cage. I found some photos down the bottom of the modern version of this. It is not decoupled from the vehicle but that is the tech advancement I was proposing for "space wars". A little drone computer to control a bunch of metal plates with engines. Or you can electromagnet them? BTW I know this doesn't stop hard rounds (it's for RPG and other "plasma jet" weapons - don't know if a physict would call it a plasma, but it is a jet of hot, vapourised metal), but that's the not point: the point is "it's a cage" and it's a cage because they can't predict which side will get shot at.

http://farm1.static.flickr.com/137/407807009_84d10d122f.jpg
http://media.defenseindustrydaily.com/images/LAND_RG-31_LROD_Armor_lg.jpg

It's strange that you basically said I was wrong, then agreed with me; "The defence there is the same as it probably always will be, use decoys, maneuver erratically and use stealth".

No you said stealth was the ONLY defence. That's not identical to what I said.

As for you WWII in space, that is exactly what I foresee; a scenario similar to WWII submarine warefare.

Getting off topic here, but I don't agree there either: WW2 submarines preferred to attack while surfaced and in most cases weren't able to fight a warship - they were typically sent after mechants instead.

 

[Deeviant]

Perhaps, but there is no way to accelerate a 1 kg mass to 0.9999c using a single launcher either. It require extremely hard acceleration for a very long period of time to get up to speed using a propulsion system. It takes the LHC almost an hour to accelerate protons up to their top speed, and this is using multiple stages of circular accelerators that are kilometers long, and you're talking about something many orders of magnitude larger. It isn't feasible at all.

To be honest, an engineering problem is exactly what it is. Given a few thousand years, I think it would be foolish to say that any complex engineering problem could not be solved. In the end though, it doesn't matter. It could simply be a gamma laser. Anything armoring that would defend against a laser, would make it less stealthy. The bottle line is a ship is going to have to contend with relativistic weaponry.

I said you don't have to absorb all of it. If it breaks up when it hits something floating in space (not attached to your ship) made of the same material as it, it doesn't matter as much.

Put it all around like a cage. I found some photos down the bottom of the modern version of this. It is not decoupled from the vehicle but that is the tech advancement I was proposing for "space wars". A little drone computer to control a bunch of metal plates with engines. Or you can electromagnet them? BTW I know this doesn't stop hard rounds (it's for RPG and other "plasma jet" weapons - don't know if a physict would call it a plasma, but it is a jet of hot, vapourised metal), but that's the not point: the point is "it's a cage" and it's a cage because they can't predict which side will get shot at.

As I already said, the amount of energy released by a host of possible future space weapons is far too great for a material based armor approach. Stealth IS the only defense.

 

[Drakkith]

To be honest, an engineering problem is exactly what it is. Given a few thousand years, I think it would be foolish to say that any complex engineering problem could not be solved.

An engineering problem? It is FAR more than that. To be honest I don't see any point to this thread any more. You have your answers to your initial question and most of the thread has simply been argument on non-existent weaponry and defensive measures. I don't see it going anywhere else so I am done.

 

[Aero51]

In response to the origonal post, you can actually use targeted sound waves to heat small areas to very high temperatures. This is currently being researched as a method to destroy tumors in "hard to reach" locations.

 

[Deeviant]

An engineering problem? It is FAR more than that. To be honest I don't see any point to this thread any more. You have your answers to your initial question and most of the thread has simply been argument on non-existent weaponry and defensive measures. I don't see it going anywhere else so I am done.

Right, because we have never heard of a laser before, that's totally crazy. And it would be foolish to even think a few thousand years of human advancement could yield a serviceable relativistic mass-driver, which funny enough is just about lowest tech "high-tech" weapon in common sci-fi use.

Perhaps you just don't like that fact I don't agree with you. Anyways, the thread will end when the responses do. As far as the point of the thread it is obviously useful for me to discuss these things with others and experience different viewpoints, even if I do not agree with all of them, this is how learning works.

 

[mfb]

If weapons are just an engineering problem (they are not*), what is different in defense?

*relativistic mass-drivers are somewhere between "limit of physics" and "limits of engineering". For electrostatic acceleration, you hit the physical limit of electric arcs quite soon. Pure magnetic acceleration hits similar limits in field strength and gradient. Electromagnetic interaction similar to railguns might achieve higher accelerations, but you need a direct contact between projectile and the gun, which gives serious issues with relativistic velocity. In addition, I think you will need superconductors to get a reasonable current density in the projectile. This limits the allowed temperature of the contact. It is an engineering issue, but a serious one as you have physical limits everywhere and you have to go around them.
1000km^2 of solar cells? Engineering problem. Disassembly of a planet? Engineering problem. Relativistic mass-driver? Problematic.

 

[Dale]

Yes, but the standard in hard sci-fi is just that it not violate any known physical laws. Anything else, including things between the limit of physics and the limits of engineering, is considered fair game. So a relativistic mass driver would be appropriate provided merely that conservation of energy and momentum is not violated.

However, I agree in principle with the "what is different in defense" premise. Stealth would certainly be an important defense, but not the only one.

 

[rorix_bw]

As I already said, the amount of energy released by a host of possible future space weapons is far too great for a material based armor approach. Stealth IS the only defense.

I disagree for a number of reasons, many already stated.

Now what about over-penetration?

What happens if you shoot a car with a really big, slug firing weapon, like a tank gun?

That should annihilate the car, right?

Well no. Slugs are not explosive. It will strilke the window, or door, pass straight through the car, and go out the other side. The window is weak enough that it won't be able to receive more energy from the bullet than is required to break the glass. I wouldn't like to be sitting in that car, but many of the occupants will survive the broken window glass and the vehicle will probably still be drivable.

Hunters know this too: over-penetration of targets is a consideration.

So if you're using a solid round, your target must have sufficient mass to convert its energy. For example if you instead hit a tank with that, instead of a car, it would indeed "stop" the bullet, and the impact of the bullet could physically explode the tank. (So you have 2 ammo types: explosive for unarmoured targets, and slugs for armoured).

So what if:

(a) They depressurise the ship before combat, and get into space suits. No "explosions" from explosive decompression and no fires. Yes wearing space suits INSIDE a ship doesn't sound very scifi, but you wanted realism.

(b) The spacecraft is large, but with dispersed components? It might consist of a 100 meter long "ladder frame" with the components placed on the ladder at 10 meter intervals, and (depressurised) living areas between them. No armour at all.

Now any hit from a slug weapon, that did not hit and break the frame, would either only annihilate one module, or pass through the module, depending on the contents of the module. Sure you might a hit a reactor! But the ship has two! And if not a reactor, chances are you drill through an unused crew bunk and nothing else.

At this point you realize that you need explosives or a some kind of shotgun. Then you have to work out how to build a detonator, or a shotgun mechanism that can survive "impulsegun" velocities.

It's really not as simple as you think it is. Also weapons vs defences is a constantly evolving battle and you might be in a situation where you have 2 enemies each with a difference defence, and each requires a different weapon to counter.

Also I still want to know what you're going to do about people floating the armour in a cage around the ship, or people using decoys.

 

[bahamagreen]

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...

 

[Deeviant]

Yes, but the standard in hard sci-fi is just that it not violate any known physical laws. Anything else, including things between the limit of physics and the limits of engineering, is considered fair game. So a relativistic mass driver would be appropriate provided merely that conservation of energy and momentum is not violated.

However, I agree in principle with the "what is different in defense" premise. Stealth would certainly be an important defense, but not the only one.

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