One of the major issues with reaction engine-based rocket systems for space propulsion is the need to carry your own fuel. You can use more efficient fuels which gives you more energy per unit mass of material, but you still must carry along some fuel which adds mass to the vehicle. However, when we look at fusion engines, we note that one of the principal ingredients for a fuel source is hydrogen or hydrogen isotopes such as deuterium and tritium. An ionised hydrogen is just a proton. It is a curious occurrence that in between the stars happens to be copious amounts of hydrogen (and other materials but at lower abundance) which makes up what is known as the interstellar medium.