Abstract
Interstellar travel needs enormous amounts of energy to accelerate payload, structure, and propellant to high speeds. For a travel to a distant star all three have to be huge. Chemical propellant needs more than 90% of the whole start weight, even from an orbit around the Moon. Most of the energy is necessary to increase kinetic energy. Because kinetic energy depends on inertial mass, a reduction of effective energy could reduce the amount of energy needed. As inertial mass and gravitational mass are the same following the equivalence principle of General Relativity and gravity depends on the gravitational constant, a local variation of the gravitational constant might result in variation of the effective mass. A varying scaling between mass/energy and spacetime could also cause a new force as known form the Pioneer anomaly. Kaluza-Klein Theory is General Relativity extended to 5 dimensions (5D). The gravitational constant is substituted by a scalar field making it variable. This scalar field is predicted to change under strong dynamic electromagnetic fields. Deriving the equation of motion from this 5D-metric predicts a fifth force.