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[Shade-o]

In the near-future, any space battle involving humans would be between powers such as the USA, Russia, China, EU, India, Japan, etc trying to control something worth investing in a new type of warfare. Right now there's nothing but communication satellites and some labs, but what can we expect in future? Lunar or Martian colonisation and land-grabs? Exploitation of minerals? A revolutionary new material or energy source that requires extraterrestrial environments? The costs inherent in not only preparing but actually fighting in such a way would mean that the stakes have never been higher.

Of course it would be interesting if something crazy was discovered, like FTL or wormholes or energy barriers. But by all accounts we can expect scientific advancements to be limited to things like "Theoretical Physics Made Both More AND Less Comprehensible" and "Algae Tanks Feed Third World!"


So for the foreseeable future, spacecraft are super fragile, weapons are massively destructive but useless at long ranges, and there are neither the means or motive to exploit space in such a way that warrants conflict.

[thobal]

I think any sort of conflict will start as a "dont tread on me" revolution type thingy.But it's not likely to go anywhere because the space-siders will have the ability to crash mountains into the Earth-siders but probably wont have the ability to be self-sufficient without occasional replacement parts.

Of course, if the space siders do become self-sufficient, I wouldnt be surprised if they launched a sneak attack to take the planet for themselves. In fact I'm not convinced that that hasnt already happened.

[Soadreqm]

About the amount of rocks needed to make a hit:

You don't need to fill the maneuver radius completely.One can leave all the gaps they want so long as no gap exceeds the size of the target(allowing a modest margin for the destruction of incoming projectiles and the spacecraft's ability to quickly dodge into gaps and target incoming).

You struck this bit out but didn't really elaborate why. Anyway, I divided the area of the two-thousand-kilometer circle with the silhouette of the 100-meter sphere, giving an extremely crude approximation for how tightly you'd have to pack the rocks to ensure that there wasn't a hole in your veil the target could slip through. Regarding the fuel weight calculations, I think the total mass of the rocks I got yesterday was something like 10000 kg for a single volley that fills a 2000 km circle. Attacking isn't exactly free, either. If the ten terajoules of energy required to dodge is too much for a spaceship to spare, then the attacker can't afford to fire.

Interstellar travel is looking like an insane science fiction fantasy right now, but I thought that was exactly what we were talking about? I don't think there are enough strategically significant sites in the solar system to bother with any space warfare. You could maybe terraform Mars, eventually. Setting up outposts elsewhere is possible, but prohibitively expensive. In a scenario like this, rocks would certainly be destructive, since practically everything worth attacking would be firmly in orbit and unable to change its direction in any meaningful way. I think warfare would focus more on terrorism, though. Tracking ballistic rocks is way too easy. Everyone can instantly see who fired what from where. If you send a saboteur or a suicide bomber, you can destroy infrastructure with greater precision and still have deniability. The mass drivers would be the nuclear option, the kill-everyone-destroy-everything-weapon that played a major part in various interplanetary dickwaving competitions, but wouldn't actually be used for fear that the enemy would use theirs.

[USEC_OFFICER]

...if these two planets are not even in the same system...

...presumably capable of crossing interstellar distances in a somewhat reasonable time... some VERY IMPRESSIVE thrusters...

That completely absurd.

Do you have any idea the amount of energy you'd need to move anything an interstellar distance? The sheer impossibility of getting from star to star in anything less than hundreds of years? The spacecraft would be huge. Even allowing for advanced electrical propulsion(500 GJ/kilogram), your 100,000,000 kilo ship will need 90,000,000,000 kilos of propellant to reach .10c.  I dont think that's accounting for relativistic effects. No, interstellar wars are impossible.

I would like to point out this following articles:

Bussard Ramjets: Why start out with your fuel, when you can just scrape it out of the void that you are travelling through? With a Bussard Ramjet, you could keep on accelerating for as long as you want (since you always have fuel.)

Magnetic Sail: The solar wind is a constant source of energy. If we tap into it with a magnetic sail, a ship could get a quick boost out of the solar system. Then we deploy the sail as a parachute when we reach the target system.

Personally, I think the Bussard Ramjet is the best chance for interstellar travel. We work out the kinks in the collection method, and we can be in Alpha Centari in, I don't know, a couple hundred years. Compared to conventional rockets, it is a massive improvement.

[Shinziril]

On Bussard Ramjets:

Things started to unravel in 1978. T. A. Heppenheimer wrote an article in Journal of the British Interplanetary Society entitled "On the Infeasibility of Interstellar Ramjets." Heppenheimer applies radiative gas dynamics to ramjet design and proves that radiative losses (via bremsstrahlung and other similar synchrotron radiation-type mechanisms) from attempting to compress the ram flow for a fusion burn would exceed the fusion energy generated by nine orders of magnitude, that is, one billion times. The energy losses will probably show up as drag. This was confirmed by Dana Andrews and Robert Zubrin in 1989.

The effect of drag? What it boiled down to was that the ramjet had a maximum speed, where the relative velocity of the incoming hydrogen equaled the drive's exhaust velocity. It has a "terminal velocity", in other words.

A proton-proton fusion drive has an exhaust velocity of 12% c, so a proton-proton fusion Bussard Ramjet would have a maximum speed of 12% c. You may remember that a spacecraft with a mass ratio that equals e (that is, 2.71828...) will have a total deltaV is exactly equal to the exhaust velocity. So if a conventional fusion rocket with a mass ratio of 3 or more has a better deltaV than a Bussard Ramjet, what's the point of using a ramjet?

A bit sad, really. 

The problem with interstellar travel is that space is just really freaking big, and the amount of energy necessary to get anywhere interesting in a reasonable length of time is utterly enormous and thus difficult to acquire.  The other other problem is that any thruster capable of achieving the speeds necessary for interstellar travel in any reasonable length of time is automatically a weapon capable of biosphere destruction or planetary annihilation, due to the aforesaid amount of energy involved. 

That magnetic sail trick might work, though. 

[forsaken1111]

Ramjets always seemed a bit too magical to me. Somehow you are scooping up and compressing enough fuel to burn in a fusion flame. Okay sounds fine in theory, but you're hitting these hydrogen atoms faster and faster and must accelerate them to your ship's speed to make any use of them.

[Sir Pseudonymous]

Most combat will be waged with dumb rocks. All this talk of missiles, fighters and drones is utterly fantastic. Any war in space will quickly devolve to  two planets and whatever colonies they have throwing rocks at each other. It's not very romantic and it's not fun to write about. It's the same reason no one writes many novels or screenplays about nuclear war.

Rocks are NOT “free”, citizen.

Firstly, you must manoeuvre the Emperor’s naval vessel within reach of the asteroid belt, almost assuredly sustaining damage to the Emperor’s ship’s paint from micrometeoroids, while expending the Emperor’s fuel.

Then the tech priests must inspect the rock in question to ascertain its worthyness to do the Emperor’s bidding. Should it pass muster, the Emperor’s Servitors must use the Emperor’s auto-scrappers and melta-cutters to prepare the potential ordinance for movement. Finally, the tech priests finished, the Emperor’s officers may begin manoeuvring the Emperor’s warship to abut the asteroid at the prepared face (expending yet more of the Emperor’s fuel), and then begin boosting the stone towards the offensive planet.

After a few days of expending a prodigious amount of the Emperor’s fuel to accelerate the asteroid into an orbit more fitting to the emperor’s desires, the Emperor’s ship may then return to the planet via superluminous warp travel and await the arrival of the stone, still weeks (or months) away.

After twiddling away the Emperor’s time and eating the Emperor’s food in the wasteful pursuit of making sure that the Emperor’s enemies do not launch a deflection mission, they may finally watch the ordinance impact on the planet (assuming the Emperor’s warship does not need to attempt any last-minute course correction upon the rock, using yet more of the Emperor’s fuel).

Given a typical (class Bravo-CVII) system, we have the following:
Two months, O&M, Titan class warship: 4.2 Million Imperials.
Two months, rations, crew of same: 0.2 MI
Two months, Tech Priest pastor: 1.7 MI
Two months, Servitor parish: 0.3 MI
Paint, Titan class warship: 2.5 MI
Dihydrogen peroxide fuel: 0.9 MI
Total: 9.8 MI

Contrasting with the following:
5 warheads, magna-melta: 2.5 MI
One day, O&M, Titan class warship: 0.3 MI
One day, rations, crew of same: 0.0 MI
Dihydrogen peroxide fuel: 0.1 MI
Total: 2.9 MI

Given the same result with under one third of the cost, the Emperor will have saved a massive amount of His most sacred money and almost a full month of time, during which His warship may be bombarding an entirely different planet.

The Emperor, through this – His office of Imperial outlays – hereby orders you to attend one (1) week of therapeutic accountancy training/penance. Please report to Areicon IV, Imperial City, Administatum Building CXXI, Room 1456, where you are to sit in the BLUE chair.

For the Emperor,
Bursarius Tenathis,
Purser Level XI,
Imperial Office of Outlays.

[PTTG??]

Noted.

[USEC_OFFICER]

Ramjets always seemed a bit too magical to me. Somehow you are scooping up and compressing enough fuel to burn in a fusion flame. Okay sounds fine in theory, but you're hitting these hydrogen atoms faster and faster and must accelerate them to your ship's speed to make any use of them.

That's why Bussard Ramjets only work up to a point. Once you reach a certain speed, the drag on the scoop is equal to the propulsion you get from the engines. Still, hydrogen atoms are not all that big, and you can reach some pretty high speeds using a ramjet.

On Bussard Ramjets:

A proton-proton fusion drive has an exhaust velocity of 12% c, so a proton-proton fusion Bussard Ramjet would have a maximum speed of 12% c.

A bit sad, really. 

Isn't there another, more efficient type of fusion?

FAKE-EDIT: The http://en.wikipedia.org/wiki/CNO_cycle might be a candidate for use in a Bussard Ramjet. It needs a higher temperature to start, but has a higher energy output at high temperatures than the proton-proton fusion drive.

[thobal]

Most combat will be waged with dumb rocks. All this talk of missiles, fighters and drones is utterly fantastic. Any war in space will quickly devolve to  two planets and whatever colonies they have throwing rocks at each other. It's not very romantic and it's not fun to write about. It's the same reason no one writes many novels or screenplays about nuclear war.

Rocks are NOT “free”, citizen.

Firstly, you must manoeuvre the Emperor’s naval vessel within reach of the asteroid belt, almost assuredly sustaining damage to the Emperor’s ship’s paint from micrometeoroids, while expending the Emperor’s fuel.

Then the tech priests must inspect the rock in question to ascertain its worthyness to do the Emperor’s bidding. Should it pass muster, the Emperor’s Servitors must use the Emperor’s auto-scrappers and melta-cutters to prepare the potential ordinance for movement. Finally, the tech priests finished, the Emperor’s officers may begin manoeuvring the Emperor’s warship to abut the asteroid at the prepared face (expending yet more of the Emperor’s fuel), and then begin boosting the stone towards the offensive planet.

After a few days of expending a prodigious amount of the Emperor’s fuel to accelerate the asteroid into an orbit more fitting to the emperor’s desires, the Emperor’s ship may then return to the planet via superluminous warp travel and await the arrival of the stone, still weeks (or months) away.

After twiddling away the Emperor’s time and eating the Emperor’s food in the wasteful pursuit of making sure that the Emperor’s enemies do not launch a deflection mission, they may finally watch the ordinance impact on the planet (assuming the Emperor’s warship does not need to attempt any last-minute course correction upon the rock, using yet more of the Emperor’s fuel).

Given a typical (class Bravo-CVII) system, we have the following:
Two months, O&M, Titan class warship: 4.2 Million Imperials.
Two months, rations, crew of same: 0.2 MI
Two months, Tech Priest pastor: 1.7 MI
Two months, Servitor parish: 0.3 MI
Paint, Titan class warship: 2.5 MI
Dihydrogen peroxide fuel: 0.9 MI
Total: 9.8 MI

Contrasting with the following:
5 warheads, magna-melta: 2.5 MI
One day, O&M, Titan class warship: 0.3 MI
One day, rations, crew of same: 0.0 MI
Dihydrogen peroxide fuel: 0.1 MI
Total: 2.9 MI

Given the same result with under one third of the cost, the Emperor will have saved a massive amount of His most sacred money and almost a full month of time, during which His warship may be bombarding an entirely different planet.

The Emperor, through this – His office of Imperial outlays – hereby orders you to attend one (1) week of therapeutic accountancy training/penance. Please report to Areicon IV, Imperial City, Administatum Building CXXI, Room 1456, where you are to sit in the BLUE chair.

For the Emperor,
Bursarius Tenathis,
Purser Level XI,
Imperial Office of Outlays.

Noted,

Perhaps the Emperor will provide for my transportation to the ordered study. I'm afraid that my meager non-imperial "education" does not allow me to make sense of the provided coordinates. Therefore, I must beg the Emperor's agent to provide me with more detailed travel instructions so that I may fulfill Imperial command.

For the Emperor,
Unassigned