I've seen the X3 ion thruster named as the future for traveling between planets. Would the ideal spacecraft have only one propulsion? Or would it have multiple sources such as hydrazine and solar sails?
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$\begingroup$ Just a thought that I didn't think was worth an answer. In any vehicle, the propulsion method is the most expensive and expansive aspect of it's design. Depending on the size, if there are multiple propulsion sources, they are almost always the same kind. A car only has one engine, if it's gas you don't usually keep a backup electric motor in the trunk. Likewise, even a multi-engined ship or plane uses the same engines throughout because having multiple fuel types when one might never be used is a waste of space and creates excess weight. It's simply a matter of practicality. $\endgroup$– TitaniumTurtleAug 21, 2019 at 4:56
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1$\begingroup$ There is no ideal propulsion as long as you can't teleport. This question is highly subjective as ideal can be: efficient, fast, safe, cheap, etc... and as long as we stay away from Sci-Fi you always have to make compromises for your propulsion system. Clarify which kind of spacecraft or mission type and the question gets more answerable. (This question is interesting though as it is basically asking "Where should be put our research funding in the propulsion sector?") $\endgroup$– GittingGudAug 21, 2019 at 6:41
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2 Answers
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Selecting the ideal propulsion system for a mission is closely tight to your requirements. You are typically interested in providing a given $\Delta V$ for a vehicle using an amount of mass (both the dry mass of the propulsion system $m_d$ and propellant $m_p$) and power $P$ over a period of time $\Delta t$.
Considering these parameters you select the ideal propulsion system for you between the many different types of propulsion systems available. The main types are:
Chemical thrusters: typically these systems have a low specific impulse ($I_{sp}$), in the 100-500 seconds range. They can usually provide high levels of thrust ($T$), from 1 N in small monopropellant thrusters up to tens of MN in solid boosters. The energy required to accelerate the mass in this case is stored in the chemical bonds of the propellant, thus you do not have provide extra power to the thruster while it is burning (apart from ignition and control). Because of their high thrust capability, these systems are typically used for impulsive maneuvers where it is necessary to change the velocity in a very short time ($\Delta t$ very low), but you need a large amount of propellant for a relatively small change in orbital velocity.
Electric thrusters: these systems usually have a much higher $I_{sp}$, in the 1000-5000 seconds range, but the thrust levels that they generate is typically much lower, below 100 mN. Unlike chemical thrusters, the acceleration of mass in this case require power, so it is also necessary that you have available some power source (either using solar panels or a nuclear reactor). Because of their high specific and low thrust, these thrusters are usually employed in missions that require a large $\Delta V$ but where time is not critical, since the maneuvers usually take a long time.
Solar sails: this system uses the radiation pressure from the sun to move the vehicle, so you are avoiding using both propellant and power, since both are provided by the sun, but your dry mass is very large (because of the size of the sail). In this case the thrust levels are also very low, because of the intensity of the solar wind, so time is even less critical in the missions using these kind of systems.
Other types: there are many other concepts that were proposed in the past, that usually promise a very impressive performance, but in reality has an almost impossible technological barrier to overcome. Some examples are: photon rocket, nuclear pulse propulsion, non-rocket space-launch, between many others.
Saying all this, the best propulsion system would be something that require very low power (chemical, electrical or nuclear) to accelerate a large amount of mass to a high speed. But observing the thrust efficiency $\eta$ equation,
$$ P = \frac{g}{2 \eta} T I_{sp}, $$
where $g$ is the standard acceleration of gravity, you observe that if you want to increase both thrust and specific impulse you necessarily have to increase to power input. So the ideal propulsion is really the one that has the best balance between thrust, specific impulse and efficiency, for your case.
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There is no such thing as an ideal propulsion. The choice of a propulsion method is always, always a trade-off between various, often conflicting requirements. To name a few: price; performance; mission timeline; production/sourcing delay; state-of-the-art in propulsion; environmental & safety hazards.
For example (very hypothetical): you work at a space agency where you are given a mission to catch up with 'Oumuamua. When you work out the propulsion modes that could possibly do the job, you come to the conclusion that only nuclear-powered ion engines can do the job. That's beyond current state-of-the-art, and an environmental and safety hazard at launch (even if you launch with a liquid propellant propulsion, the vehicle can fail in the atmosphere and cause a nuclear disaster downrange). No can do, mission impossible.
