For the past hundred years, all rockets have been powered using a traditional combustion engine design. But recently, scientists have been researching and a testing an entirely new design. The ion propulsion engine.
Rockets are able to fly because of Newton’s Third Law—for every action there is an equal and opposite reaction. Gaseous hydrogen or methane is burned and expelled through the nozzle toward the ground and the equal-opposite force on the rocket propels it upward. Ion propulsion engines use the same principle, but in a completely different way.
A chamber filled with gaseous xenon is electrified—bombarded with electrons—, creating and ionized plasma of electrons and positive xenon ions. The ions are attracted to and accelerated by two electrode grids, an anode grid for the positive ions and a cathode for the electrons. The net-neutral-charge plasma is then expelled to create thrust.
There are obviously many differences between the two designs and each have their pros and cons. For example, xenon is very light and compact and the ionization process allows for a much longer engine run time and much more powerful thrust than a combustion engine, these being the main motivations for ion propulsion. However there is one considerable drawback. Ion propulsion builds thrust very gradually, whereas combustion creates a lot of thrust very quickly. For the most part, this is not a huge deal. But when it comes to making quick maneuvers to avoid obstacles or correcting trajectory, it becomes a problem. Also, it takes a lot of energy to ionize the gas, which is produced by solar panels on the rocket. But for extended trips, especially those in the future that may travel beyond the sun’s reach, solar panels can’t keep up with such a high energy demand.
For the first issue, some scientists have proposed a dual engine: ion propulsion for long term travel with a backup combustion system for short bursts of extra thrust. As to the energy problem, it’s been suggested to use nuclear energy instead of solar. Of course, both solutions have problems of their own, but for now they’re the most reasonable ones.
My personal opinion is a combination of the two: use nuclear energy to power the ionization and use any excess heat produced by the fission process for quick maneuvers. This way, there is no need for an additional complex section of the rocket for an alternate combustion engine. Also, if the key goal for now is to reach mars, the nuclear reactor could be used to power the settlement of the first group to land there, before they are able to set up solar farms.
So far, only a couple spacecraft using ion propulsion have actually been launched, all of them being space probes. But soon enough the first ion propelled rocket will be built and ready to traverse among the stars.