Evaluation of Drives
In evaluating unconventional drives, it is essential to remember that they were originally proposed to overcome the limitations imposed by current conventional rocket technologies. For the purposes of this evaluation, we will take conventional rocket drives as an effective form of propulsion for only as far as the Moon. This is because flights to planets are in the order of years which when juxtaposed against the human lifespan, comes across as too long a time. In the subsequent paragraphs, we make an evaluation of unconventional drives with respect to missions ranging from interplanetary to interstellar.
In terms of feasibility, until there is a breakthrough in the technologies of the theoretical drives, we can eliminate them as they are unlikely to be used in the near future. We are then left with the promising and probable ones. Of the promising and probable drives that we have discussed above, it is noted that they all are capable of higher specific impulses than conventional rocket drive. However, most of them come with a lower thrust except for nuclear pulse propulsion, which has a remarkably higher thrust.
Ion drives possess low thrust which results in a low initial speed. The implications are that interplanetary space missions will take a long time to complete. This makes it less favourable than nuclear thermal and pulse propulsion as the main propulsion system for fast transfer missions.
In a similar manner solar sails have low initial speeds. However, they have the ability to attain very high velocities with distance travelled. A huge drawback for solar sails is that the current sail membrane structure is fragile and susceptible to damage by asteroids. On top of that, the mechanism behind solar sails makes it hard to perform nimble steering. This renders solar sails less attractive than nuclear and ion propulsion systems which are far more agile and resistant to corrosion in space.
Solar thermal propulsion and nuclear thermal propulsion both provide mid-range specific impulses and slightly better thrust than ion drives and solar sails. However, their thrust still comes across as weak compared to that of nuclear pulse propulsion. There is also significantly less interest and more opposition in the research in the two drives. It is therefore likely that nuclear pulse propulsion will win the development race against these drives.
After taking into consideration the feasibility and drawbacks of the various drives, we see nuclear pulse propulsion as the most promising drive for rockets of the near future. Due to its higher thrust, significant higher reliability over solar sails and high specific impulse, nuclear pulse propulsion is likely to completely take over chemically propelled rockets should there be worldwide endorsement of nuclear usage. Ion drives are likely to be deployed as subsidiary engines for refined control and steering. Solar sails can then be used to augment nuclear propelled crafts via the harnessing of the energy of sunlight.
In our matrix, all the existing technologies of the drives are compared against each other with parameters such as specific impulse, lifetime and thrust. This can be found here.