[quote=AlienBastard]
The ramming spaceship would be already accelerating towards them, and a ramming ship could keep its trajectory aligned as the no stealth thing goes both ways; every time they start changing trajectory, the ramming ship would too. Just like with a missile.

And ramming would likely be done against a bigger ship, which due to inertia likely would have a harder time changing trajectory or accelerate even if the potential speed of a bigger ship is much bigger than a smaller ship.
[/quote]

The problem is, a ramming ship requires significant velocity, the more significant the velocity the more delta-v required to adjust velocity, the longer it takes to adjust velocity in an ever-increasing recursive circle. In a proper 100% Hard Science Fiction battle. Just getting within ten thousand kilometres of your enemy is going to be considered a completely beyond effort.

Add in that. For example.

Ship A has 20% mass in thrusters with an efficiency of .8 (pretty good), Ship A is your rammer at 500 tons.
Ship B has 20% mass in thrusters with an efficiency of .8 (pretty good), Ship B is 25'000 tons. But because it has the same mass to weight in thrusters and identical efficiency, it will be capable of course adjustments exactly identical to ship A. It may weigh 24'500 tons more, but with identical thrust to weight ratio it will accelerate as quickly. It'll turn as quickly. (This is assuming both have roughly the same spherical design as well.)

Add on that if you're doing a velocity, say, twice that of the target object, the target only requires half as much effort to move OUT of your way. Missiles gain their agility by MASSIVE thrust to weight ratios (Something like 90% of a typical missile is dedicated to engine) a Starship simply could not keep its course lined up with an enemy ship while maintaining a ramming velocity, especially if under fire.

To ram an enemy ship is effectively impossible for these reasons.

[quote=catchamber]A: Which is why you make a large number of small arrays, and coordinate their efforts. Placing them close to a star would be ideal, as they could move just by adjusting their shape, and would make it difficult for invaders to target them. Proximity to a star would also make detecting relativistic rounds easier, thanks to their blueshifting against the solar wind. Stopping every round wouldn't be the goal, but stopping enough of them to maintain the steep advantages the defenders have against the invaders. This is assuming they could even get that deep into the system, which could be prevented by having a sizable quantity of arrays already present.

B: Wouldn't a strategy of total war and genocide through relativistic bombardment make you hated by almost every nearby polity? It'd make sense if it was used as a WMD, limited only to opponents unwilling to negotiate, but not as a conventional weapon. It seems much more efficient to conquer by encouraging interstellar trade and travel, then using those vectors to memetically subvert the locals. That, or by sending in self-replicating, automated warbots, which could hinder their growth long enough for you to invade in a more "traditional" manner.[/quote]

A: If your arrays are close to a star they're not just effectively worthless, they're totally worthless. They would simply never hit anything that wasn't in a relative stationary orbit. Lets say that you build an array of them one light-minute away from the sun to get the most out of them. It would take the beams from them a further seven minutes to reach earth's orbit.

B: Genocide through relativistic bombardment is the ONLY realistic and viable warfare (Again assuming 100% Hard Scifi) Interstellar Trade and Travel is completely off the table. If it takes someone 40 years to travel 10 Lightyears (And that is percieved, the actual travel time is 50 years at .2c) they're not going to want to go anywhere, and trade is equally impossible, "Let's send a shipment of computers to that colony." 

Ship arrives 50 years later to find the colony has more advanced computing already.

Interstellar Conquest simply is not realistically viable, nor is Interstellar Trade or Travel. And that is based off a society which can accelerate ships to .2c (59958.4916km/s) and another inhabited system as close as 10 Lightyears. Make it 20 Lightyears and you've got 100 years travel. 30 Lightyears = 150 years travel. Even though people on board would, according to time dilation, only experience 4/5th of the time lapsed.

For interstellar conquest, trade and travel it is ABSOLUTELY required to bypass one cardinal law. Faster than Light. Though given that NASA is re-examining the Alcubierre drive, maybe, just maybe, it's possible one day. But until that lightspeed barrier can be bypassed, ignored, moved around, etc, then colonies will always be "fire and forget"