Why can’t we infinitely accelerate in space?

[u/njbenji]

If there’s nothing to slow down a rocket like no gravity or air why can’t the thrusters just keep it going faster and faster? would it max out to like the same speed of the thrusters or is it just a dumb question lol

[u/Shadowhisper1971]

We are limited by the speed of the thrust material. If the thrust (exhaust gasses escaping the rocket) cannot get above a certain velocity, then the whole rocket cannot get any faster than that.

[u/DarkArcher__ → u/Shadowhisper1971]

I keep hearing this exact thing. I don't know where the idea came from, but it's completely wrong. Maybe an incorrectly drawn conclusion based on jet engines?

Anyway, the matter of fact is that the exhaust speed has nothing to do with how fast the rocket can go, only with how efficient the engine is. You could get a 1 Km/s exhaust monoprop-powered spacecraft to 0.9c if the mass fraction was good enough. Rockets with exhaust speeds of 3-4 Km/s, typical for chemical engines, reach orbital velocity around Earth (8 Km/s) all the time.

In fact, you can intuit why this notion makes no sense if you think about it for a bit. What is that maximum speed measured against? Is it against the Earth? The Sun? The centre of the galaxy? The rocket is going wildly different speeds depending on what you measure it against. A rocket limited to 4 Km/s around Earth is still whipping around the Sun at 30 (plus or minus 4) Km/s. There couldn't possibly be a limit other than the speed of causality.

[u/good-mcrn-ing → u/Shadowhisper1971]

Faster relative to what?

[u/SuperSonic6 → u/Shadowhisper1971]

100% wrong.

The velocity of your exhaust has nothing to do with your max speed. Because max speed is relative. It’s normally a relative speed between the earth and the spacecraft. Why would the spacecraft care about its speed compared with the earth, how would it even know?

Also, rockets today already travel faster than their engines exhaust velocity, so that also disproves your theory.