Does the fuel thrust in rockets have a “maximum speed” to which it can accelerate an object?

Question

If we assume we are in a perfect vacuum in space, where there’s no friction or anything to slow us down, at what point would bursts of fuel from thrusters stop increasing our speed? And would they even?

Now, I understand that by einstein, we need infinite energy to accelerate to speed of light, but i cant intuitively see how a rocket wouldnt constantly speed up in a vacuum when given constant bursts of rocket thrust. Im sure im missing something pretty simple here, but nevertheless.

In: 4

Answer 1

> at what point would bursts of fuel from thrusters stop increasing our speed?

When the fuel runs out. In discussions about rockets, fuel is often talked about in terms of “delta v”, which is the total change of speed that the engine could produce with a given amount of fuel in the absence of gravity or air resistance. This depends mainly on the mass of the rocket, the energy density of the fuel, and the efficiency of the engine.

> we need infinite energy to accelerate to speed of light, but i cant intuitively see how a rocket wouldnt constantly speed up in a vacuum when given constant bursts of rocket thrust.

As the name suggests, in relativity, the key theme is that things are relative. The rocket does not have an absolute speed – from the perspective of someone on board the rocket, it isn’t moving at all, while from the perspective of another rocket moving in the opposite direction, it’s going very fast.

But in relativity, speeds do not add together linearly. If, from my perspective, there is one rocket travelling at 90% of the speed of light in one direction, and another rocket travelling at the same speed in the opposite direction, then from the perspective of one of these rockets, the other one is still travelling at less than the speed of light.

If you travel away from earth with an incredibly powerful rocket that keeps pushing you forwards with the same force, then from the perspective of someone on earth, your acceleration will gradually slow down and you will never hit the speed of light. But that isn’t because you somehow have to work harder to attain higher speeds – it’s just because of how your reference frames relate to each other.