Propulsion in outer space today requires some form of reaction mass to be ejected from the spacecraft to generate accelerating thrust. If there were some mechanical or electrical way to obviate the need to carry this reaction mass, space travel would be revolutionized and further progress in travel to the stars could be made.
The basic law of conservation of momentum must not be violated. There may be systems of propulsion that utilize some form of reaction-less thrust that have not been discovered yet. If such a system is found, energy from a power source could be directly coupled into the kinetic energy of the spacecraft without the wasteful process of ejecting reaction mass at some relative velocity. This topic might be more in the realm of science fiction, but it would be interesting to hear people's ideas on how to make a quantum leap in spacecraft propulsion.
My own idea came to me in a dream about flying saucers and hot air balloons, over twenty years ago. The device is a simple arrangement of flywheels. Three flywheels formed of rotating steel are suspended in a set of rings at right angles to each other. The ring that carries the bearings for the flywheel is cut with spiral gears that mesh with a worm drive screw gear. The ring that runs along the perimeter of the flywheel and carries the armature coils of the motor that powers the rotation of the wheel also carry stub shafts that fit into bearings in a Y-shaped frame. The flywheels are arranged on the frame such that a motor on the worm screw drives the flywheels to tumble inward on their axis of rotation such that their angular momentum is continuously being flipped as they spin about two orthogonal axes. All motors are electric and run off ships power generated by a nuclear reactor of some sort. The centrifugal warp drive unit is mounted on bearings such that it can rotate freely about the axis of the worm screw, central to the frame, so that the reversing angular momentum of the flywheels can be allowed to spin in such a way that total angular momentum of the device can be conserved.
Energy is continuously supplied to this arrangement of flipping flywheels by the worm screw drive motor. This energy, minus losses due to bearing friction must go somewhere. It is my impression that the forced precessional motion of flywheels in this arrangement will be directed into a linear momentum vector along the axis of the worm screw which flips the flywheels. This linear momentum vector will drive the attached space vehicle forward without the necessity of ejecting reaction mass. This device is a reaction-less, or internal reaction motor. Technically, this cannot exist as it violates conservation of momentum. It is my theory that the constantly changing angular momentum of the flipping flywheels constitutes a conservation force for this unbalanced linear momentum.
It's a pity I cannot afford to machine such an object, because even if it did not work, it would look pretty interesting while it was in operation. There must be other ideas on how to get around the requirement for reaction mass in space propulsion. Perhaps some other writers have ideas on this topic.