> Well, a solar sail is propellant-less and has constant acceleration under technically a constant amount of power I guess I should have said reaction-less and

> Well, a solar sail is propellant-less and has constant acceleration under technically a constant amount of power I guess I should have said reaction-less and not propellant-less. A solar sail is not a reaction-less drive. The momentum/energy is conserved in the system of solar sails + photons. There is no free lunch thre.