Too often we look for a single perfect answer to our problems. In the energy crisis people are often disappointed when hybrid batteries are found to be so environmentally unfriendly or that wind power is often incredibly harmful to local bird life or that solar cell arrays often use a lot of water in areas that are pretty arid to begin with.
Researchers looking into the future of space flight looked at combining a rocket propellant with an electric sail. An electric sail has some similarities to a solar sail in that they are both low mass propulsion systems however a solar sail uses the acceleration of photons to create slight accelerations to the apparatus while an electric sail uses an array of long, thin positively charged tethers that repel solar wind protons while attracting solar wind electrons. These arrays have all been proposed as a method of space flight that would require no additional propellant as accelerating and decelerating more mass requires more wasted energy. However an electric sail would only function in space and the gains to acceleration can sometimes be minute.
The researchers used the Hohmann transfer numbers as a starting point for interplanetary travel between Earth and its neighbors. Typical planet to planet rendezvous would include a period of time where the spacecraft was accelerating to give it a necessary velocity delta to escape Earth's orbit, then a period where it would coast to reach the orbit of the planet it's attempting to reach. Minimum flight times were calculated for this basic transfer orbit as between 0.289 years to reach Mercury and 30.613 years to reach Neptune.
The authors looked an ideal thrust-on time in comparison to minimum flight time because while shortest flight would generally be ideal you would also want to reduce your thrust time in case of your propulsion system failure. So it was better to look at a ratio between the two. They looked initially at having a secondary thrust phase after an initial coasting phase but found using the total time and thrust time ratio meant it was always optimal to have only one thrust phase followed by a coasting phase.
Their initial starting point was an object in circular orbit around the earth to which they then simulated times it might take that object to reach planets in the solar system using either a typical rocket propulsion system or the hybrid rocket and electric sail option. In the first option the rocket propellant and inertia allow it to reach the escape velocity. But in the case of the electric sail, the sail's acceleration can contribute during the thrust time towards achieving the delta velocity which of course reduces the amount of propellant the spacecraft would have to carry. I summed up their results in a slightly more clear graph.
Perhaps not surprisingly flights to Mercury and Venus were worse with the hybrid system. Though, an electric sail only system would save considerable fuel and mass and could be considered to Mercury and Venus. The major time savings were seen the farther out into the solar system. The hybrid system doesn't solve long term space flight concerns but it does add another weapon to our arsenal that perhaps future spaceflights will employ multiple methods of propulsion.
Quarta, A., Mengali, G., & Janhunen, P. (2011). Optimal interplanetary rendezvous combining electric sail and high thrust propulsion system Acta Astronautica, 68 (5-6), 603-621 DOI: 10.1016/j.actaastro.2010.01.024
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