"Charles Talleyrand" writes:

What's the current state of the art in ion engines? How well
do they do, and when one adds solar panels (or some other energy source)
and such to the mix, what's the thrust/weight ratio?

The current state of the art in ion thrusters is probably the
Boeing XIPS-25, as used on the 702 series comsats. But that's
a proprietary system, and so hard data is scarce.

The NSTAR ion thruster used on the NASA DS-1 spacecraft, is still
fairly good, and there's plenty of data on that. Nominally uses
2.3 kW of electric power to produce 92 mN of thrust at a specific
impulse of 3,120 seconds, though you can tweak all of those values
as circumstances demand.

Weight of the bare thruster I have as 8.3 kg, giving a thrust/weight
ratio of 1/885. But wait(weight?), there's more. The thruster doesn't
just plug into a wall outlet and a Xenon bottle. It needs a power
processing unit which masses 15 kg and a propellant management assembly
that masses 20.2 kg, so the thrust/weight ratio of the complete system
is 1/4,640.

Assuming your solar arrays and associated hardware have a specific
power of 40 W/kg, which again is pretty much state-of-the-art, you
need another 57.5 kg of solar to get that 2.3 kW, which brings the
total mass to 101.0 kg and the overall T/W ratio to 1/10,770.


This illustrates something that often gets overlooked in cursory
analysis of advanced propulsion systems. The mass (and cost, complexity,
etc) of the thruster is usually small compared to that of the PPU and
other necessary support systems, and the mass of the propulsion system
as a whole is usually small compared to the mass of the power system.

People look at, wax enthusiastic about, and try to improve the performance
of bare thrusters, because Ion Drives! are cool and sexy and all that, but
what matters is the systems engineering that goes into boring old switching
power supplies and gas flow controllers and whatnot.


Ion engines are the current winner in the very-low-acceleration game, right?

Oh, there are propulsion sytems that offer much lower acceleration than
that :-)

I know what you mean, though, and it's not clear that Ion engines are
the winner in that regime. Their main competition is a sort of plasma
thruster called the Hall Effect Thruster or Stationary Plasma Thruster,
depending on who you talk to. Somewhat less specific impulse, but
correspondingly more thrust.

State of the art there is the Fakel SPT-140. 4.5 kW of power gives
290 mN of thrust at a specific impulse of 1770 seconds, again all
tweakable to mission demands. Bare thruster mass 6.8 kg, PPU mass
13.7 kg, propellant management I don't have good numbers for but
previous Fakel SPTs all used a 3.9 kg PMA.

So, propulsion system T/W is 1/825. Add in the 112.5 kg solar
arrays needed for that 4.5 kW, and your T/W drops to 1/4630.
More than twice the value for the NSTAR ion thruster, at slightly
more than half the specific impulse.


Most of the missions I look at, the SPT-140 is the winner. But
sometimes pure Isp is what is needed, even if your thrust drops
off the bottom of the scale, and there are applications for ion
thrusters as well.


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