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Ion thrusters are capable of providing the same amount of delta-V for far less fuel (e.g. Falcon 9 v1.1 second stage specific impulse is 340 seconds, while some ion-thrusters have a specific impulse of over 10000 seconds).

Ion thrusters have excellent life expectancy. NASA's NEXT ion drive continuously operated for over 5.5 years and after that the engine showed little signs of degradation. Rocket motors are not that reliable.

Ion thrusters have been tested and performed very well in numerous space missions, including Dawn, Hayabusa 1 and 2, Smart 1, GOCE, Deep Space 1 and more are upcoming. This concept isn't particularly new or experimental.

Generally, ion thrusters are awesome.

So why don't we use them to lift everything from LEO? They can use ~30 times less fuel, and fuel in space is very expensive. Why do we still need rockets going above LEO for unmanned spaceflight? Why develop super-complicated harsh-reentry capable reusable rocket technology when we can have spacecraft lasting for decade(s)in a stable space environment? They proved to be powerful enough to withstand air drag at very low orbit (only 255 km above the surface of Earth). What are the problems?

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    $\begingroup$ I changed the title from Taxi to Tug. Taxi service is generally interpetted to suggest earth to LEO. Tug service is generally interpretted from LEO to higher orbits as you intended. (Revert it back if you disagree with me). $\endgroup$ – geoffc Dec 12 '14 at 19:59
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While the ISP on Ion thrusters is awesome, the overall thrust is pretty low.

Thus the transit time from LEO to GEO would be quite long and slow. In some cases this matters. If it takes an extra year to get in service, that is a year of lost service while in transit.

In fact a critique of the Falcon 9's ability to do dual launches is that only the smaller electric propulsion (Ion as you like) satellites will fit, and while they WILL be dropped close to GEO/GTO orbits, they will need another 6 months to finalize the orbit on their electric thrusters.

But what about payloads that are not time sensitive? Well the other problem is that while you are slowly raising your orbit you pass through the Van Allen belts ALOT.

Passing through once, quickly, is not terrible, and survivable. But repeated passes, over the course of many months requires additional hardening and work to make it work.

Having said all that, Russia has sort of had a tug in use. The modules that delivered some of the ISS modules are technically tugs, that never got reused.

For a while there, (Cannot find the http://www.russianspaceweb.com/ reference off hand right now) they proposed a manned or cargo launcher that relied on docking to an already orbited tug to manuever to final destination. That would have been a traditional propulsion system, not ion/electric propulsion. Thus fewer missions and a shorter lifespan, but they still seriously considered it.

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    $\begingroup$ You would also have to carry prox-ops propellant and systems onboard the tug -- which would probably end up being more massive than the ion drive. $\endgroup$ – Erik Dec 13 '14 at 6:47
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    $\begingroup$ Also, if it's going to take a year to tug a satellite into the right orbit, you need an awful lot of tugs, since each one is occupied for a very long time per mission. $\endgroup$ – David Richerby Dec 13 '14 at 19:36
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A single-use, disposable ion drive can be made no larger than it needs to be, thus uses no more fuel than it needs to.

A reusable ion drive tug, first of all, has to take its payload to destination, and then come back; due to the exponential increase in fuel requirements for linear increases in delta-v, this can be more than twice as much fuel as a one-way mission.

Finally, to go anywhere interesting on an ion drive takes months or years; a tug doing a Jupiter mission is going to miss a bunch of launch windows for Mars missions.

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    $\begingroup$ Why exactly 2.7? In a one-way trip, you carry additional cargo and a full container of fuel, while the back-trip carries no cargo and has a more-than-half empty fuel container $\endgroup$ – Marius Macijauskas Dec 12 '14 at 20:11
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    $\begingroup$ Fuel fraction increase is exponential with linearly increasing delta-V requirements. 2.718 (e) is the worst case; for an ion drive the situation is better -- for a Jupiter orbit mission a round-trip ion drive vessel is going to be about 1.84 times the fueled mass of a one-way, at minimum. $\endgroup$ – Russell Borogove Dec 12 '14 at 20:27
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Tugging would require the satellites to be built for docking or berthing, which adds mass and costs. And preferably in a standardized way, which is difficult to establish in the beginning. The extra operations increases the risk. The delay of tugging can have very high opportunity costs, for example that a new communication service is delayed while competitors win market shares, which needs to be weighed against the saved launch costs.

And I wonder if there are drawbacks with having a satellite built for GEO waiting in LEO. Such as space debris collision risk, short Earth communication windows, longer and more frequent periods without solar power and with it temperature variations. Radiation problems with slowly traversing the van Allen belts has already been mentioned.

And there might be competition. The upper stage for the Falcon 9 will not be reused as in soft landed on Earth. But maybe it, or any upper stage, could be redesigned to be reused as a tug (once), coasting in the orbit of the payload it originally launched, with the rest of its fuel. If a suitable opportunity arises, it could tug a satellite from LEO to GEO, or do station keeping on a satellite in GEO, before it ends its life. I don't see how it would be profitable to refuel a tug, rather than launching a new tug together with that fuel. There seems to be no lack of used rocket engines in Earth orbit.

Now and then a valuable payload like Fobos-Grunt doesn't reach high enough orbit and is doomed to soon burn up in the atmosphere. A flexible tug ready to assist could then perform a very much more valuable service than just saving launch fuel. If it is fast and flexible enough and able to interface with almost any cargo.

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The Ion Thruster is still a very low-thrust engine which would need many of them to be used as a tug. The Thrust-to-Weight ratio of an ion engine is too low to be used in LEO effectively, so it would be too power consuming to power the Ion Thruster. Even if a particular spacecraft had the power sources available, the amount of maintenance needed to use the power would be so extreme that a craft may need a crew member or two made to just EVA everyday to repair the power source. (This is for solar) Also, if a space craft used RTGs (Radioisotope Thermoelectric Generators) like the ones Voyager and Dawn used, it would be too expensive and dangerous for the craft as the intense radiation could damage the electronics and give the crew fatal doses of radiation. That is why the Ion Thruster is only used on light weight and unmanned probe craft.

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