Speed of light barrier? First you are making the assumption that the speed of light is a constant - which it is not.
Constant cannot be sped up or slowed down under any condition. For this reason the speed of sound is not constant as it travels faster or slower dependent on the medium through which it travels. Even that air temp. can affect sound speed.
Researchers have been able to slow light to 38 mph
Physicists Slow Speed of Light – Harvard Gazette
I would now submit that the speed of light is not a constant.
What I do know of propulsion that the faster you go the more energy it takes and the consumption of energy is not linear. I do not think all the available universal energy could make a single shuttle craft move beyond the speed of light (which I will leave at the semi-constant of 3x10^8m/s).
Furthermore if you wanted to be in such a craft and make it a livable 4G factors - it would take you hundreds of years to accelerate to anywhere near 90%SOL. And if you hit some dust along the way it would shear your little ship to bits.
I assume you are trying to breech the SOL for travel purposes?