If you had enough ion thrusters, could you take off from Earth's surface into low Earth orbit?
If not, why not?
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4 Answers
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The answer depends on how you define 'enough ion thrusters'. Per some of the discussion on this question the key at launch is, can you generate more thrust, than your vehicle masses. Else you sit still on the launch pad.
So the Shuttle uses SRB's that have awesome amounts of thrust (14.7 MN (3.3M lbs) at sea level, vs the F-1B which is in theory 8.0 MN (1.8M lbs)) but crappy ISP.
Ion thrusters (accelerate a charged particle out the back very fast, and live off the momentum gain) have awesome ISP, but crappy thrust levels.
So can you get 'enough' ion thrusters to generate enough thrust to exceed the mass of your vehicle? Today? No. Tomorrow? Who can know?
This is known as thrust to weight ratio. Since you cannot exceed 1.0 you will just sit on the pad.
It does seem like it will take a radical change in the technology to increase the thrust for this to ever work, but stranger things have happened.
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I believe the strongest engine in this class is the VASIMR VX-50 -- which produced a whopping 0.5∓0.1 N (0.11∓0.02 lbf) of thrust in a test stand for a very short time. That's less than the weight of the engine itself so I suspect we are a ways away from launching anything with an ion drive.
I think these drives also need to be operated in a vacuum.
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$\begingroup$ I don't think that a VASIMR engine really counts as an "ion engine", does it? Isn't it using thermalized plasma? (Also, that doesn't even begin to include the mass of the power plant) $\endgroup$– ikraseAug 21, 2019 at 8:29
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Ion thrusters of current designs would short out the charged grid in surface level atmosphere.
Current generation ion thrusters rely upon a difference in charges ionizing and then attracting the reaction mass. Since the electrical voltage difference between the charged grids is extremely high, and needs to remain so to operate, the presence of atmosphere could, and probably would, result in air arcing between them, and discharging the grids.
Further, as an example, the NSTAR Ion Engine masses 8.2kg, and produces a peak of 92millinewtons, which, before accounting for anything else, would be about 11mm per second per second... about 1/1100th the thrust needed to lift itself. Just the drive, power, and digital controller unit is about 25.5kg... for about 3.6mm per second per second... actual performance of the DS-1 probe, massing 373kg, was about 0.25mm per second per second... highway speed (88kph, or 24km/s) takes 97777.8 seconds or so... just over 27.1 hours (and almost 1200km).
It has been estimated that, at best, ion drives will peak at around 100 times the thrust to weight ratio they are currently at. (In part, this is because one can only accelerate the ions to just shy of light speed...) which would still leave them well below the performance shown in certain science fiction settings. After all, 1.1 N on a 25 kg package would still be only 4cm per second per second... far short of the 9.80665 Newtons per kg needed to attain the 9.80665 meters per second per second to merely hold against surface gravity...
http://en.wikipedia.org/wiki/Deep_Space_1 http://www.nasa.gov/vision/space/travelinginspace/future_propulsion.html http://www.grc.nasa.gov/WWW/ion/past/90s/nstar.htm
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$\begingroup$ Forget "electric arcs shorting out the electrical potential drop", you're also going to be straight-up flooding the acceleration chamber and possibly destroying the ionizer. It's a more general problem of "ion thrusters don't work except in a vacuum". $\endgroup$– ikraseAug 21, 2019 at 8:31
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$\begingroup$ @ikrase not true - some very low thrust versions can be used for moving air itself. $\endgroup$– aramisOct 1, 2019 at 19:16
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$\begingroup$ I'm aware of "ionocraft" or "lifters", though they have the same "not even approaching lifting the power plant" issue. $\endgroup$– ikraseOct 2, 2019 at 11:33
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$\begingroup$ I didn't claim they were useful thrusters - that's your strawman. $\endgroup$– aramisNov 1, 2019 at 18:58
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Once we have a Vactrain loop big enough.
Levitating on magnetic field, it could accelerate to nearly arbitrary speeds, before being allowed to leave the vacuum tubes, shoot through the atmosphere diagonally and head towards the orbit. The trick is enough of launch track without air and changing the horizontal acceleration vector to vertical.
Yes; plain maglev propulsion would be much more efficient in these conditions but that doesn't make using the ion drive impossible.
Until then... well, others said it.
