Too many people use the word efficiency when they actually mean efficacy. According to Wikipedia, efficiency is:
Efficiency in general, describes the extent to which time, effort or cost is well used for the intended task or purpose.
Efficacy is the capacity to produce an effect.
From this we can conclude that efficiency is a relative measure, and that efficacy is an absolute measure. The conflation of efficiency and efficacy is in particular frustrating (on-line) discussions regarding space colonization.
Quite obviously, space colonization requires the use of rockets, both to launch people and equipment from Earth, and to transport resources throughout the Solar System. However, it’s clear to anyone who studies rocketry even shallowly, that there is a wide variety of rocket types. And each different type of rocket has each own advantages and disadvantages.
Before we continue, we need to devote a few words to non-rocket space launch. Though there are many proposals for non-space launch systems, none of those have been tested in practice. The proponents of those systems often claim that their proposals will reduce the costs of space launch. This might be true in the long run, but at the short-term we need to take into account the cost of research and development of such systems.
And since it is uncertain whether these proposals will actually work, or when they will be available, it will be hard to find people who want to invest in such launch systems. On the other hand rockets are proven technology, which enables us to start with space colonization quite soon. This also makes it more likely for people to invest in space colonization.
In rocketry efficiency is indicated by the specific impulse I of a rocket, and the thrust T of the rocket. Basically thrust is the product of the specific impulse and the amount of mass exhausted by the rocket. By a given exhaust mass, a higher specific impulse would imply a higher thrust. Hence we should pick the rocket with the highest specific impulse from the catalogue.
Well, if we chose the rocket, we would pick an ion thruster. But is this also the best choice? Not entirely, though ion thrusters have a high specific impulse, they simultaneously have a very low flow of exhaust mass. So low actually, that their overall thrust is low. Because of this, ion thrusters aren’t used for launching spacecrafts from Earth, and is their use limited to deep space. Additionally because of their low thrust, ion thrusters are quite slow.
Although ion thrusters are usually advertised as very efficient, we have to realize that this claim is based on the specific impulse of this type of rockets. However, there are other ways of looking at the efficiency of a rocket. Especially, we should look at the ratio between the energy consumed by the ion thrusters, and the addition of kinetic energy.
Ion thrusters use electricity, generated either by Solar arrays or radio-isotope batteries, to ionize a propellent (usually xenon). The ionization of atoms takes a lot of energy, energy which does not increase the velocity of the spacecraft and hence kinetic energy. Consequently this would decrease the overall efficiency of an ion thruster.
The popularity of xenon as a propellent for ion thrusters, is due to the fact that xenon has a relatively low ionization energy (and hence increasing overall efficiency). However xenon is a rare noble gas, that is present in trace amounts in the Earth’s atmosphere. In order to obtain 1 liter of xenon one has to process 11.5 million liters of air. Hence it’s not hard to imagine that xenon is quite expensive.
For purely scientific space missions these disadvantages of ion thrusters are not that important. A scientific space mission might take several years, if not decades. And scientific mission do not have to return any profit. The humanization of space, however, demands enormous investments, which are only possible if there is a prospect of profit.
What alternative do we have for ion thrusters? An interesting possibility are so-called thermal rockets. Thermal rockets differ from chemical rockets, in that in the former a propellent is heated by an “external” source of energy, instead of a chemical reaction. There are several types of thermal rockets, the most important ones being nuclear and solar thermal rockets.
Nuclear thermal rockets use nuclear reactors to heat the propellent. Given the complexity of reactor technology, and additionally political concerns arising from the launch of nuclear fuels from Earth, we rule out this type of rocket for space colonization in the near future.
Solar thermal rockets (STRs), however, are much simpler. They use solar power to heat up the propellent, and instead of a reactor they need mirrors to concentrate solar heat upon the reactor. For ground launch, however, STRs will not work, so we still need another, most likely chemical rockets, to launch a STR from the ground. But once in space the STR will take over, and bring the spacecraft either to the Near Earth Asteroids or the Sun-Earth Lagrange points.
The main benefit of thermal rockets is that they can be “refueled” with propellent, also in space. Near Earth Objects contain substantial amounts of water which could be used as a propellent. Recall that spacecraft do not need to use their rockets all the time, only to reach the desired speed and to slow down at their destination.
Compared to ion thrusters thermal rockets have a smaller specific impulse, but they have a greater thrust. Consequently they have much shorter travel times than ion thrusters Further thermal rockets do not need to waste energy to ionize their propellent. And finally they don’t require expensive substances as xenon, instead they can use water or ammonia instead.