system
Given the following post, respond as if you're commenting on Reddit:
human
If there's no drag in space, what's stopping a spacecraft from accelerating to light speed?

The fuel capacity no doubt needs to increase somewhat.
But this is a petty monetary issue. 
What am I missing?
gpt
By far the biggest issue is the fact that you are underestimating just *how* much energy is needed to accelerate a massive object up to close the speed of light.

The equation used to calculate kinetic energy when talking about relativistic (close to speed of light) speeds is 

E_k = ( mc^2 / (1 - (v/c)^2 )^(1/2) ) - mc^2

Making v=c gives an infinite amount of energy, which is why it is impossible for a massive object to travel at the speed of light, but even a significant fraction of c would give energies far beyond the capabilities of any current fuel-based propulsion methods.