I was trying to understand how cost-effective it would be to use an ion engine to power a mission. But since these propulsion systems cannot be used to leave Earth's gravitational field due to their low thrust, we must use chemical rockets to get from Earth to get to orbit. So, I wanted to understand how much savings is actually possible with using liquid rocket engines first, followed by using an ion engine.
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$\begingroup$ More expensive, is likely the long trip. Simply because a device that can get there and keep itself alive and communicating is *fancy tech*(including your planned ion drive!!). More delta-v needed is definitely hoisting yourself from the surface, but that's comparatively simple engines and tech, just a lot of tons of it. Maybe better define what you mean by "expensive".. Dollar cost? $\endgroup$– CuteKItty_pleaseStopBArkingCommented Dec 15, 2021 at 12:09
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Unfortunately, your scheme of using an ion engine after Earth escape does not save all that much, since relatively speaking, the extra kick from an Earth escape trajectory to a Mars transfer is quite cheap already.
It's a difference of 390m/s of delta-v, which a LH2/LOX chemical upper stage can provide with a 91% payload fraction. So you are at most looking at a ~10% saving.
But an ion engine can't save all of that either. It's too slow to do the burn while still close to Earth, so now you're looking at a ~2900m/s to make the same transfer manoeuvre in interplanetary space. Sure, it's still a net win at a 93% payload fraction, but that's not much.
Using an ion engine during the "coasting" phase towards Mars still has its merits though. It changes the game for what planetary alignments that can be used, meaning more convenient return trips can be scheduled (the return Hohmann alignment for returning from Mars notoriously requires a long stay). It can also reduce the speed at which one encounters Mars.
Another interpretation: If you are suggesting using an ion-engine to raising an initial low orbit into an escape trajectory, that has more going for it in theory. The direct chemical impulse of ~3200m/s only has a 48% payload fraction, while the spiralling cost of ~7800m/s with an ion engine has a much better 82% payload fraction.
answered Dec 15, 2021 at 9:36
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$\begingroup$ Oh okay, that makes sense. But If ion engines do not reduce costs significantly due to their high efficiency and also produce a low thrust, why are they so popular as a propulsion system to the extent where people consider it as the future of propulsion? Is it because of the fact that it can provide acceleration throughout a space journey, which chemical rockets cannot? $\endgroup$– AryaanCommented Dec 15, 2021 at 10:07
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1$\begingroup$ Yes. And for other routes, their efficiency become much more apparent. The Dawn mission, for instance, would be very difficult to pull off without an ion engine. $\endgroup$ Commented Dec 15, 2021 at 10:19
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2$\begingroup$ @ryan ion drives do not reduce cost. What they do is increase total capability. You can go further, faster with an ion drive on a limited mass budget. Just need patience, a lot of it. and a good shovelful of money, because the ion drive and electrical systems and longterm DSN network coverage are all expensive. $\endgroup$ Commented Dec 15, 2021 at 12:16
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1$\begingroup$ @Ryan another part of the answer is that ion engines conceivably could be scaled up to have higher thrust without fundamental design changes. They likely won't ever be able to do liftoff or descent, but they totally could get 10x current thrust without hitting theoretical obstacles. $\endgroup$ Commented Dec 15, 2021 at 17:00
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2$\begingroup$ @Ryan for such a small amount of delta-v, there may not even be any mass savings, and the ion thruster itself certainly isn't cheaper. You need a trip that's long enough that the ion thruster's performance outweighs the added dry mass of the thruster, its power systems, and the solar panels to power it, and you need a way to benefit from the reduction in mass: a cheaper launch vehicle, more spacecraft on the launch vehicle, etc. And for Earth escape, you need to consider things like the radiation impact of the slow spiral out through the radiation belts. In short, it's not that simple. $\endgroup$ Commented Dec 16, 2021 at 1:23