A nit: if I recall correctly, this is realistic, at least for chemically-based
propulsion systems (i.e. rockets). Someone feel free to correct me if I'm wrong.
At any rate, the speed of light would be tops... (like I said, a nit)
Nyah, don't mess with my physics, boy >
There is no "speed limit" save for that of light. Speed is all relative, after all. If you can accelerate to 10 km per second, there's no reason you can't accelerate to 20, if you have the fuel. The reason that chemical rocket-propelled ships (which the EV ships most certainly are NOT) rarely attain such speeds is that fuel is limiting. If you have a ten tonne ship, and you can explode liquid hydrogen-oxygen fuel at a velocity of four kilometers per second, then you need ten tonnes of fuel to move your ship forward at four kilometers per second. If you are to move forward at eight kilometers per second, you need twenty tonnes of fuel.
The sad fact is that the more fuel you have, the greater the mass of your ship is, and the harder it is to propell. And if you're to ever stop, you need twice as much fuel, and if you're to come back, you need four times that much.
The solution to all this is to use more fuel-efficient thrusters. Unfortunately, more fuel-efficient tends to mean less energy-efficient. An ion engine strips xenon of its electrons, and repels these electrons out of the back at 31 kilometers per second. Meaning that while ten tonnes of chemical rocket fuel can send the 10 tonne ship forward at 4 kilometers per second, ten tonnes of xenon can send it forward at 31 kilometers per second. The big drawback of an ion engine is that it can only send a very small volume of ionic xenon out at once. The more you send out at once, the less efficient you can be, and the slower it goes. An ion engine can be used to reach very great speeds with very little fuel, but it takes a very long time to accelerate.
However, as many problems as this presents in terms of fuel efficiency, it does not mean that there is a "top speed." With a sufficient quantity of fuel - massive as it may be - we can conceivably reach any speed short of the speed of light itself.
Now one rather big louse egg is that the speeds given for the ships in EV (expressed in AU per hour) often exceed that of light. Eww.
Not one among us knows much, but we must all seek wisdom.
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