That is, the power of a spacedrive is directly proportional to it's efficiency as a weapon.
From The Atomic Rockets of the Space Patrol, a truly excellent resource for this sort of thing:
"Jon's Law for SF authors is closely related to Niven's Kzinti Lesson. It states: 'Any interesting space drive is a weapon of mass destruction. It only matters how long you want to wait for maximum damage.' It goes on to say: 'Interesting is equal to "whatever keeps the [players] from getting bored"'."
In my last post I figured out that allowing my ships to accelerate at a base speed of 1/10th g allows them to get from planet to planet in an "interesting" amount of time. The thing is though, this forces my hand for some other things.
Let's say that a very small spaceship weighs about as much as a tank. A quick google search tells me that a M1A1 Abrams tank weighs about 60 tons. If we accelerate that for two weeks (about the minimum that we could use and still have interplanetary travel with those engines), and we could get our spacetank up to some ungodly velocity, which on impact would leave a crater just about as large as the biggest atom bomb we've yet built.
We need ships to be able to accelerate that much to be able to move about the solar system at an interesting speed. But using gigantic kinetic weapons isn't constrained to spaceships, giant space rocks work just as well. So what exactly stops our prospective alien invaders from just dropping asteroids on anything with a heat signature and taking over? That isn't just a rhetorical question, at some point we've got to have a satisfactory answer to it or we'll end up with a very short, very uninteresting game.
Ok, to start with we've got a technical restriction. You remember that bit about accelerating it for about two weeks? Yeah, it builds up a heckuva lotta momentum, but it means that whoever you're flinging your spaceship (or giant rock) at has about two weeks to do something about it. If you're trying to hit a spaceship it might be able to dodge. If you're hitting a relatively stationary target like a planet, then they might be able to deflect it before it hits. Of course, if you're willing to wait, you can start flinging your rock from the far side of Alpha Centauri, and if you can keep the engines on then you'll get it up to some insane fraction of the speed of light so the we on the receiving end would have little warning and fewer options to deal with it. That is, if we didn't see it accelerating to begin with; even from a distance we've got a shot at seeing it, and therefore doing something about it before it's too late.
There's also a practical reason to not beat a planet down with a merciless hail of asteroids. If you do, you're ruining a whole ton of useful infrastructure, from the networks of roads to the biosphere itself. The soon to be conquered inhabitants have even flagged the major deposits of gold by building mines over them. And storing the refined gold in convenient lockboxes like Ft. Knox. If you dropped a hunk of rock on Ft. Knox then you'd have to go through the work of finding and refining all that gold all over again.
There's another reason; we don't like bombing civilians. Now, our godless alien invaders might not balk at the mass slaughter of an entire alien species, but maybe they would. Maybe they'd prefer to merely conquer and occupy our nations, rather than murder us wholesale. But, at the end, I'd prefer not to rely on the benefices of an alien species. Inasmuch as it makes convenient dramatic arcs, I'd rather not drop a sudden lesson in the end about "can't we all just get along?" You can take this as a promise that the aliens will still stay evil and alien until the end of the game. But they might not be entirely amoral.
Of course, before I actually go about limiting this sort of weapon, I should really ask why. I mean, if we're fighting a war, we want to win, right? So should I really be arbitrarily setting the power of the weapons I'm using to destroy the enemy? Yes, yes I should. If the war is fought with weapons that can quickly and easily destroy the other side, then there's very little point to actually playing the game. "Let's play Global Thermonuclear War". I will now, and in the future, make decisions by nothing more than fiat that I think will make the game better.
Well, not technically by fiat. I'd much rather figure a way to explain why this doesn't work for in game reasons than just make it a rule. So, I'd rather go down the list above:
1) For small, maneuverable targets it's relatively easy to dodge this sort of thing. As long as the target can accelerate faster than the projectile. Once you've dodged, they'll have to slow down before they could try again with that weapon.
2) For capital ships I'm not really sure. Assuming that larger ships accelerate more slowly (not necessarily true until I figure out the specifics of the handwavium drive, but probably true) then you'll be able to maneuver your projectile faster than the ship can dodge, which makes for a relatively easy and cheap way to pick off capital ships. I'll have to think about it. Let me know if you come up with anything.
3) For planet cracking weapons the evil invading alien doesn't want to mess up their soon to be conquered infrastructure. So no relativistic projectiles smashing the planet entirely, no dropping huge rocks on the planet and messing up the biosphere.
4) This still leaves room open for smaller, targeted strikes. All you need is a rock large enough to not completely burn up on the way down, and maybe shape it so that it doesn't drift too far off course, and you've got a hell of a bomber. Witness "The Moon is a Harsh Mistress" by Heinlein. Again, this still needs work.
I'm not sure, but I'm thinking that I'll need to come up with some sort of defensive measure that larger ships and ground installations can use to fend off these sorts of attacks. Jury's still out.