When it comes to rockets, for years there has been no choice. Newton's third law of motion dictates that every action has an equal and opposite reaction. Conventional rockets utilize propellants to quickly expand and then to push the rocket up as the propellant forces it's way out of the rocket. Ion drives work in a similar manner, but use a high energy laser or maser to free ions from a solid propellant.
Recently NASA has done work on a controversial rocket engine known as EMdrive. It is a bit of a game changer as it doesn't use a fuel.
As the basis for a new class of rockets used for travel between stars (without using propellant) the EMdrive is in the process of being fully proven as being useful. The problem is that the thrust is very low for the energy used. While this news came to light I was in the process of developing a theoretical rocket drive of my own. Based upon the combination of two verifiable theories, this new device is called a plasma inertial drive.
Although many consider the device "untested" the Thornson initial drive has been built and demonstrated to work by many enthusiasts on the subject.
Thornson demonstration device
Inertial thrust is an example of how energy itself could be used to create thrust without the constant need to refuel. The current mechanical versions may have too little thrust to be of much practical use, though.
Unique Features which May Be Included
II. Geometrically designed "cushions" to concentrate force and direct it as the thrust for the craft
III. Staged accelerators to control and drive the flow of plasma into cushion area
IV. Vectorial control of plasma thrust to steer the craft (an opposing loop of plasma may be necessary for some "emergency circumstances"... must be kept at low force)
V. A new device called a plasma "flywheel" generator, which is composed of a series of separate self contained loops of plasma which are electrically accelerated and designed to lose energy at different rates, allowing electricity to be generated from their difference in electrical potential.
Ready For "Prime-time"?
The primary use for conventional rockets currently is to launch communications satellites, with the secondary use being research into conditions in space. Neither space tourism or space industry (such as mining) have been developed in recent years. These will be the kind of uses which would dictate such rocket technology. As interstellar craft only require "re-powering" rather than refueling, the infrastructure required to open space to travel would be easier to establish and less expensive than refueling stations. This means that interstellar craft of some variety would be an eventuality as our society grows to need more resources from space.