Fuel requirements:

Conventional rockets accelerate by ejecting high speed propellant. As the propellant blasts out of the rocket in one direction, it pushes the spacecraft in the other - Newton's third law. We need a highly efficient energy source that can provide propulsion for a minimum mass of fuel.

One possibility: the mutual annihilation of matter and anti-matter (matter into energy according to E=mc²).

Example: suppose we want to reach a speed of 99% the speed of light:

• to reach cruising speed requires 14 x more mass in fuel than final cruising value
• to decelerate on reaching the star, we need another 14x as much fuel (196x)
• to make the return journey, requires the same => 196x196 or almost 40,000 times the mass of the payload

Ideally we need a propulsion system that needs no propellant.