Realistic physics

In fiction we often like to talk about kilometers-long spaceships capable of going substantial fractions of light speed like it's nothing.

It's often treated as if merely being in space makes everything bigger and more powerful. But why would that be the case? In real life, doing anything in space is far more difficult and expensive than doing the same thing on planet-side. The payload has to be lifted from the surface, requiring enormous, expensive rockets. As a result, spacecraft humans can currently build have quite small and light payloads. Really big and heavy equipment stays on Earth.

If you don't have some hand-waved MacGuffin technology that makes everything in space super-powerful, then everything in space is going to be fairly light, weak, and expensive. The most efficient realistic spaceship motors, such as ion drives, output far less thrust than a chemical rocket - they are simply capable of maintaining that output for a much longer time. If your ion drive has a mere 100 pounds of thrust, that's already so far beyond current technology it's not even funny.

There is the possibility of mining the asteroid belt so that you don't have to lift everything from the planet. Or building an orbital ring or launch loop to make it cheaper to lift payloads from the planet. But you still are confronted with the difficulty of actually getting around in space. Chemical rockets produce a lot of force and last for only a handful of minutes before they've burned up their vast amount of fuel. Ion drives and photon drives can burn for years but produce tiny amounts of force.

Sure, "just" drop a 10km asteroid on a planet, and you'll do some real ecological damage. But how long is that going to take, if your thrust is measured only in pounds? It would be far easier and more cost effective to aim a bunch of nuclear weapons at key locations on the planet, than to build a motor capable of towing a 10km asteroid to where you want it in a realistic time frame.

Realistic space combat - projecting current technology a few hundred years into the future - is likely to revolve around destroying enemy spacecraft and satellites so they can't intercept your nukes. And even once you've done that, there would be land-based nuclear defenses to contend with. So it's really not so easy to strike land-based targets from space, even if you have destroyed the enemy space fleet.

Realistic physics

In fiction we often like to talk about kilometers-long spaceships capable of going substantial fractions of light speed like it's nothing.

It's often treated as if merely being in space makes everything bigger and more powerful. But why would that be the case? In real life, doing anything in space is far more difficult and expensive than doing the same thing on planet-side. The payload has to be lifted from the surface, requiring enormous, expensive rockets. As a result, spacecraft humans can currently build have quite small and light payloads. Really big and heavy equipment stays on Earth.

If you don't have some hand-waved MacGuffin technology that makes everything in space super-powerful, then everything in space is going to be fairly light, weak, and expensive. The most efficient realistic spaceship motors, such as ion drives, output far less thrust than a chemical rocket - they are simply capable of maintaining that output for a much longer time. If your ion drive has a mere 100 pounds of thrust, that's already so far beyond current technology it's not even funny.

There is the possibility of mining the asteroid belt so that you don't have to lift everything from the planet. Or building an orbital ring or launch loop to make it cheaper to lift payloads from the planet. But you still are confronted with the difficulty of actually getting around in space. Chemical rockets produce a lot of force and last for only a handful of minutes before they've burned up their vast amount of fuel. Ion drives and photon drives can burn for years but produce tiny amounts of force.

Sure, "just" drop a 10km asteroid on a planet, and you'll do some real ecological damage. But how long is that going to take, if your thrust is measured only in pounds? It would be far easier and more cost effective to aim a bunch of nuclear weapons at key locations on the planet, than to build a motor capable of towing a 10km asteroid to where you want it in a realistic time frame.

Realistic space combat - projecting current technology a few hundred years into the future - is likely to revolve around destroying enemy spacecraft and satellites so they can't intercept your nukes. And even once you've done that, there would be land-based nuclear defenses to contend with. So it's really not so easy to strike land-based targets from space, even if you have destroyed the enemy space fleet.

If you want to have realistic space battles between different stars, you may still want a MacGuffin of some sort to enable the ships to get from one star to another without spending decades or centuries. So, you can make the MacGuffin a warp drive that only works outside the gravity well of a star. (And once you drop out of warp you're back at realistic speeds, such as 20 km/s). That way, space battles once inside a system would have to be conducted with realistic technologies.