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Engine Balance Overview
Engines in this mod are balanced generally in two ways - by classification and by tech level. The most important factors to consider in looking at electric engines are the power-to-thrust ratio (PTR) and the power-to-velocity ratio (PVR). These two numbers express just how much delta-v and thrust you get out of an engine per unit of electricity.
Gridded thrusters run on Xenon propellant and have lower TWRs than other electric engines. They make up with this by having superb specific impulses, going up to 19000s in one case. This makes them the best choices in the mod for pure delta-V. This is helpful due to the very high cost of Xenon - gridded thrusters make the best use possible of it.
Gridded thruster balance evolves with tech level - increasing the advancement of the engine results in considerable performance improvements. However, be warned that this also comes with a steep power curve, and the most advanced gridded engines require a lot of electricity. With their lower thrusts, making a useable craft may prove challenging.
HETs run on Argon propellant, which has a clear advantage over Xenon in that it is a lot cheaper. They also have higher thrust than Xenon engines for the same power use, though specific impulse caps out much lower. These attributes make them great comm sat or probe engines, delivering great DV for a low price and mass.
Higher tech Hall effect thrusters improve specific impulse and TWR considerably.
The MPD thrusters in the mod are extremely power-hungry engines that exist to fulfill your need for electric engines that have decent TWRs. With the best PTRs in the game, they can really shine if you don't want to wait hours for a burn to complete on a large mothership and can stomach less efficiency. They also look damn cool.
The real downside to the MPDTs is that they require absurd amounts of power even in the smallest models, which can be harder to come by. You'll probably want some nuclear reactors or gigantic solar arrays.
Running on Lithium has a price between Argon and Xenon, and has a special advantage - while Argon and Xenon can only be obtained from atmospheres, any ISRU unit can produce Lithium for MPDT fuel from Ore.
Pulsed inductive thrusters, or PITs are a specialized thruster type running on Argon. They inherit most of the HETs' Argon-based advantages in terms of cost and specific impulse/thrust balance, but come in some tech levels later and thus have improved performance. They also cap out at higher numbers - while powerful HETs do not exist past 200 kW, the largest PITs get into the megawatt range.
The PITs can also use a UI control to increase their pulse coil voltage and, which spikes heat production and power usage, but can increase specific impulse by up to a factor of two. A careful designer can take advantage of this to produce multimode ships that can operate at lower power levels but, if the power is available, cruise at high efficiency.
Lastly, the promising Variable Specific Impulse Magnetoplasmic Rockets are fairly unique. Not only can they run on either Xenon or Argon at the player's discretion, but they can dynamically vary the power distribution in the engine between different components. This variation manifests ingame as the ability to 'gear' your engine, and choose more thrust at the cost of specific impulse, or vice versa.
The highest end VASIMRs have the best overall performance in the mod - though they may not have the pure TWR advantages of the MPDTs or the ridiculous specific impulse of the highest tech gridded thrusters, they are very well rounded.