system
Given the following post, respond as if you're commenting on Reddit:
human
If there is little to no friction or gravity in outer space to slow an object down, how come a space shuttle has a maximum speed? (Apparently around 28,000 kilometers per hour). Why can’t a spaceship continue to build momentum to reach up to light-speed?
gpt
They run out of fuel.

You can bring more fuel, but that increases the mass of the shuttle, which means that you get lower acceleration until you've burned that fuel up. If you ignore gravity and the atmosphere and just think about a single rocket with fuel, then it follows the "Tsiolkovsky rocket equation", which is

change of velocity = (speed of stuff ejected out of engine) x ln (1+(mass of fuel/mass of rest of rocket) )

where ln is the natural log.

What this means is that you get diminishing returns as you increase the fuel mass, because the natural log function only increases very slowly. If you increase the fuel mass from 1x of the "dry mass" of the rocket up to 100x (i.e the rocket mass is over 99% fuel), then you've only made the rocket about 6-7 times faster.

At some point, it becomes far too expensive to add more fuel to just add a bit more speed, so we end up trying for a good compromise between making a rocket go fast, and having a rocket affordable enough that you can do other missions.

Of course, you can partially get around this with multi-stage rockets, where you decrease the rocket mass as you go. But you can't do this forever, because you need to have an engine powerful enough to eject gas at a high speed without breaking, and you also need to have some mass for your cargo and passengers and equipment.

You can also try to refuel the rocket, but this also has diminishing returns. This fuel is now cargo for another rocket, and this new rocket has to carry this fuel up to a speed that matches the first rocket so it can catch up and hand over the fuel. So you end up spending a huge amount on fuel anyway.

So yeah, it's not  a situation where a space shuttle can't go beyond a certain speed because it'll break or something. It's just the limit of how efficiently you can accelerate it until you run out of fuel.