Surprisingly, not by much!
To the first approximation an optimized plane will use 50% of its power during cruise generating lift ("not crashing into the ground"), and 50% of its power overcoming drag.[1] The tie is broken because planes expend extra more energy climbing than they save descending, and that's also part of "not crashing into the ground." :)
Not disputing you, but do you have a quick argument for this? My understanding is that passenger planes fly at high altitudes because at lower altitudes (higher pressures) they would expend most of their energy pushing air out of the way.
The way I think about it is that commercial flights spend the vast majority of the time at "cruising speed", where the engines are most efficient on a power/fuel basis. The flight already dissipates 100% of engine power output into pushing air out of the way, so what altitude does is determine what airspeed that happens at. At lower altitudes, there's more air molecules displaced per foot of travel, so the airspeed is lower.
So airplanes fly up at 30k feet because it's faster at the same power output, which also happens to be more fuel efficient, but it's not like the plane runs the engines any harder if they cruise at a lower altitude.
These ion thrusters have worse thrust/watt than an electric propellor drive, and far worse than the jet engines they use.
So there's not really any reason to use ion thrusters at cruising altitude, unless the relative silence is worth paying a lot more per-ticket.
From the Ars article (for others who were curious about this):
> Measurements showed the thrusters collectively generated five newtons for each kilowatt of power, which is actually similar to the output of jet engines. But because of many inefficiencies in the system, the overall efficiency was only about 2.5 percent—well below that of conventional aircraft.
Even then, I'd imagine that a significant portion of the noise in an airliner is wind noise rather than engine noise. Your plane is only going to get "less noisy", not "silent". Good noise canceling earphones are a far better solution (I have some specifically for plane flights, they're awesome!)
Because you would need to restart the engines to land, and that is when they are at a higher risk of failure. You could maybe throttle them right back but keep them ticking over.
Personally, I'm a little surprised we don't have them in the Bay Area. With our traffic and (mostly) year round nice weather, and great solar energy potential it seems like it could be a win.
But then again I don't know much about the technology or it's limitations.
Missiles jumped to mind. That's their whole purpose.
The point was that this current design won’t be enough energy to keep a heavy vehicle aloft, but it could be enough to propel something that can passively stay airborne.
"Excellent... when do they hit?"
"According to our calculations... ohhh about 6 weeks from now"
I was thinking how Musk could do that. Launch some tungstan rods into a highly eliptical orbit, small thruster + control pack on them to adjust their orbit when required. When it's out at say 300,000k+ fire the engine for a few seconds, adjusting the orbit so it will crash into the chosen target a few days from now. The high apogee means not a lot of power to change orbit to collision, and almost impossible to detect. It would likely appear to be an asteroid hit.
Airships (dirigibles/blimps), and to a lesser extent, sea and land vehicles.