Ion engine efficiencies

What efficiency in converting elctrical power input into exhaust jet
power can be had out of ion engines? I'm finding little info on this.

On 5 Aug 2003 12:45:12 -0700, in a place far, far away,
(Scott Lowther) made the phosphor on my monitor
glow in such a way as to indicate that:

What efficiency in converting elctrical power input into exhaust jet
power can be had out of ion engines? I'm finding little info on this.

I don't know, but I would imagine it's very high. How precisely do
you need to know?

--
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interglobal space lines * 307 733-1715 (Fax) http://www.interglobal.org

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Scott Lowther wrote:
What efficiency in converting elctrical power input into exhaust jet
power can be had out of ion engines? I'm finding little info on this.

There's some data in Sutton on this. They are typically around 50%. Note
that this is much different to the question of how much kinetic energy
ends up in the vehicle. Most of the energy in ion drives ends up in the
exhaust.

On Tue, 05 Aug 2003 22:03:16 +0100, in a place far, far away, Ian
Woollard made the phosphor on my monitor
glow in such a way as to indicate that:

Scott Lowther wrote:
What efficiency in converting elctrical power input into exhaust jet
power can be had out of ion engines? I'm finding little info on this.

There's some data in Sutton on this. They are typically around 50%.

Why so poor? Where does the rest of it go--heat in the cabling, or
hysteresis losses?

--
simberg.interglobal.org * 310 372-7963 (CA) 307 739-1296 (Jackson Hole)
interglobal space lines * 307 733-1715 (Fax) http://www.interglobal.org

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On 5 Aug 2003 14:38:09 -0700, in a place far, far away,
(George William Herbert) made the phosphor on my
monitor glow in such a way as to indicate that:

Rand Simberg wrote:
Ian Woollard glowed:
Scott Lowther wrote:
What efficiency in converting elctrical power input into exhaust jet
power can be had out of ion engines? I'm finding little info on this.

There's some data in Sutton on this. They are typically around 50%.

Why so poor? Where does the rest of it go--heat in the cabling, or
hysteresis losses?

Muchly, heat in the system raising the voltage up to the
many-Kv ion accellerator voltage.

Ahhh...conversion losses. I guess I was inferring that the input
power to which Scott was referring was already high voltage (i.e. he
meant input to the thruster itself, not to the system).

--
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interglobal space lines * 307 733-1715 (Fax) http://www.interglobal.org

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(George William Herbert) writes:

Rand Simberg wrote:
Ian Woollard glowed:
Scott Lowther wrote:
What efficiency in converting elctrical power input into exhaust jet
power can be had out of ion engines? I'm finding little info on this.

There's some data in Sutton on this. They are typically around 50%.

Why so poor? Where does the rest of it go--heat in the cabling, or
hysteresis losses?

Muchly, heat in the system raising the voltage up to the
many-Kv ion accellerator voltage.

Actually, modern power processing units are quite efficient, on the
order of 95%. These are high-frequency switching power supplies,
which with modern power electronics (usu. MOSFETS) and magnetic
materials can be both very compact and very efficient.

The original ion thrusters back in the 1960s did not have benefit of
this sort of PPU, which is one of the reasons ion thrusters, electric
thrusters generally, fell out of favor in the late 1960s and didn't
come back until the late 1980s.


Some to a fraction of propellant which impacts the grids
rather than continuing on into open space.

And to beam divergence; ion thruster exhausts are not perfectly
collimated and transverse kinetic energy costs power but generates
no thrust.

Bigger loss mechanisms are ionization and neutralization. The
propellant is of course not stored as ions, and turning a neutral
Xenon atom into a positive Xenon ion requires a *minimum* of 12.13 eV,
or ~9 megajoules per kilogram of propellant. And that's assuming a
perfectly efficient ionization mechanism, which we ain't got.

What we do got, usually, is an electric discharge of some sort which
randomly smacks the Xenon atoms with moderately energetic electrons,
and that gives you *lots* of loss mechanisms. Electron hits neutral
with more energy than it needed to ionize it, and the excess is wasted.
Electron hits neutral with less energy than it needed to ionize it,
odds are the whole bit is wasted. Electron hits ion and doubly ionizes
it, that's waste. Electron recombines with ion, total waste and you
have to start over. Electron has energy left when it reaches anode,
that's wasted. Ion gets sucked into discharge cathode rather than
accelerator grids, it goes back to being neutral and everything you
put into accelerating it is wasted. Etc, etc, etc.

Takes about a hundred eV, seventy-five megajoules per kilogram, to
turn the propellant into ions.

And when you're finished with it, you have to turn it back into a
neutral, which means you have to take an electron that is sitting
comfortably in the conduction band of some part of your wiring
harness, tear it away from an electrode, and push it off towards
infinity. That again requires a minimum of a work function or so,
and in practice a lot more because we use inefficient gas discharges
to do it.

As for where the waste winds up, a mix of heat in the structure
(mostly the walls of the discharge chamber) and pretty blue to
far UV photons emitted from the plasma.


If you had a multi-Kv to Mv native DC power source
convenient you could avoid the first.

People do occasionally propose wiring solar arrays such that the
string voltage is equal to the desired drive voltage of the thruster,
and it's not a completely stupid idea. But switching power supplies
are very efficient, not impossibly heavy or expensive, and unlike
kilovoltish solar arrays are very well understood and reliable.


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Rand Simberg wrote:

On 5 Aug 2003 12:45:12 -0700, in a place far, far away,
(Scott Lowther) made the phosphor on my monitor
glow in such a way as to indicate that:

What efficiency in converting elctrical power input into exhaust jet
power can be had out of ion engines? I'm finding little info on this.

I don't know, but I would imagine it's very high. How precisely do
you need to know?

I am not so sure that is true, although I admit to not knowing the
answer to the the question. I would suspect it is lower than
50%.

I throw that number up because someone who really knows will
have a good laugh and perhaps provide some real data.

Mike Walsh

Ian Woollard wrote:

Scott Lowther wrote:
What efficiency in converting elctrical power input into exhaust jet
power can be had out of ion engines? I'm finding little info on this.

There's some data in Sutton on this. They are typically around 50%. Note
that this is much different to the question of how much kinetic energy
ends up in the vehicle. Most of the energy in ion drives ends up in the
exhaust.

And I guessed 50% without knowing. Of course, when in doubt
go down the middle.

Mike Walsh