Better Ion Engines
Jim Owens
Much of the energy expended in an ion engine is expended while ionizing the
propellant.
Why can't a self-ionizing fuel be used? Chemical reactions and nuclear
reactions ionize things. Why not ionize the fuel that way, then accelerate
the pre-ionized material electrically?
Jim Owens
Bill Brown
Jim Owens (g...@ncweb.com) wrote:
: Much of the energy expended in an ion engine is expended while ionizing the
:
Self ionizing chemical reactions?!?!?!?! I can't imagine that there are
too many reactions that produce a sufficient amount of ions as opposed to
the reaction (unusable) products. Electrical energy dumped into the
substance is wasted only as heat, and that can be minimized very well.
Ther have been operational ion thrusters used for over five years that I
know about to keep satelites in orbit. (low continuous thrust to
counteract the small drag from high atmosphere etc.)
The on the only engine I know the specs on, the power put into thrust
developemtn (accelerating the particles) is 400 times greater than the ion
production power. it is a mercury ion engine.
Hope this helps.
Bill
David Patterson
Jim Owens (g...@ncweb.com) wrote:
: Much of the energy expended in an ion engine is expended while ionizing the
: propellant.
: Why can't a self-ionizing fuel be used? Chemical reactions and nuclear
: reactions ionize things. Why not ionize the fuel that way, then accelerate
: the pre-ionized material electrically?
: Jim Owens
I don't now much about ion engines, but this seems funny to me.
Ionization energies are tens of eV (eg 13.6 eV for H). The ions can
be trivially accelerated to KeV, I would think MeV wouldn't be hard
especially if the engine was somewhat large, to keep electric fields down.
Maybe ionization sends a lot of energy into heat, but I would think
efficiencies of a few percent shouldn't be too hard.
btw, what limits the thrust of an ion engine?
Dave Patterson
Sid Zafran
David Patterson wrote:
> btw, what limits the thrust of an ion engine?
For practical purposes, the thrust of an ion engine is limited by the
power available from the host spacecraft, and by the exit plane area
available to the engine.
Henry Spencer
In article <5a6627$4...@decaxp.harvard.edu> dpat...@husc.harvard.edu (David Patterson) writes:
> btw, what limits the thrust of an ion engine?
Space charge, as it's somewhat-mysteriously called. When the flow of ions
gets dense enough, the charge on the ions themselves begins to be a major
factor in the shape of the electric fields within the thruster. In
particular, the cloud of ions near the accelerating grids begins to shield
more-distant ions from the voltages on the grids. The bottom line is some
fairly fundamental limits on ion current per unit grid area, which in turn
produces limits on thrust per unit grid area.
--
"We don't care. We don't have to. You'll buy | Henry Spencer
whatever we ship, so why bother? We're Microsoft."| he...@zoo.toronto.edu
John Schilling
dpat...@husc.harvard.edu (David Patterson) writes:
>Jim Owens (g...@ncweb.com) wrote:
>: Much of the energy expended in an ion engine is expended while ionizing the
>: propellant.
>: Why can't a self-ionizing fuel be used? Chemical reactions and nuclear
>: reactions ionize things. Why not ionize the fuel that way, then accelerate
>: the pre-ionized material electrically?
>: Jim Owens
> I don't now much about ion engines, but this seems funny to me.
>Ionization energies are tens of eV (eg 13.6 eV for H). The ions can
>be trivially accelerated to KeV, I would think MeV wouldn't be hard
>especially if the engine was somewhat large, to keep electric fields down.
Carefull; you seem to be deriving your "ion acceleration is easy" paradigm
from the word of high-energy physics, where nobody much cares how *many*
ions you accelerate. To get useful thrust, you need to push quite a few
orders of magnitude more ions per unit time than high-energy physics types
usually deal with, and that makes things quite a bit harder.
Just the power requirements are a show-stopper at the MeV level - to get
one Newton of thrust from an MeV ion accelerator would require about
a megawatt of electric power. And that's assuming 100% conversion
efficiency. Real ion engines operate at about ~1 KeV, so ~10 eV ionization
energy is not a trivial loss.
>Maybe ionization sends a lot of energy into heat, but I would think
>efficiencies of a few percent shouldn't be too hard.
5% is about the best I have seen, which means 200 eV of energy per ion
produced. Combined with 1 KeV acceleration energy, this gives almost
a 20% drop in efficiency due to ion generation requirements alone.
Something better would be desirable. But chemical reactions are probably
not going to do the trick - there are quite a few that produce *some*
ionization in the reaction products, but I don't know of any that get
anywhere near the total ionization needed for a thruster system.
> btw, what limits the thrust of an ion engine?
The thrust of an ion engine itself is generally limited by space charge
effects in the acceleration region. The ion beam cannot be neutralized
until it has cleared the acceleration grids (else it wouldn't be subject
to electrostatic acceleration in the first place), so there is always a
net positive charge between the grids. This charge is proportional to
the propellant mass flow rate, and if it reaches a certain critical
value it will prevent the introduction of further ionized propellant.
Thus, the geometry of the system and the applied voltage combine to limit
the propellant mass flow rate and thrust.
In practice, however, the real limit is available power. The electric power
required to operate a 100% efficient ion engine is equal to 0.5 x Thrust
x Exhaust Velocity, so a modest ion engine with a specific impulse of 2500
seconds (~25 km/s exhaust velocity) requires at least twelve kilowatts
of electric power per Newton of thrust. Probably more like twenty.
End result is that the power supply, be it solar or nuclear, winds up
an order of magnitude larger than the thruster itself, and avaliable
power (or available mass for dedicated power supply) is the real limit
on available thrust.
--
*John Schilling * "You can have Peace, *
*Member:AIAA,NRA,ACLU,SAS,LP * or you can have Freedom. *
*University of Southern California * Don't ever count on having both *
*Aerospace Engineering Department * at the same time." *
*schi...@spock.usc.edu * - Robert A. Heinlein *
*(213)-740-5311 or 747-2527 * Finger for PGP public key *
Frank Crary
In article <5a6627$4...@decaxp.harvard.edu>,
David Patterson <dpat...@husc.harvard.edu> wrote:
>: Much of the energy expended in an ion engine is expended while ionizing the
>: propellant.
>: Why can't a self-ionizing fuel be used? Chemical reactions and nuclear
>: reactions ionize things. Why not ionize the fuel that way, then accelerate
>: the pre-ionized material electrically?
> I don't now much about ion engines, but this seems funny to me.
>Ionization energies are tens of eV (eg 13.6 eV for H).
If you pick the right element, it can be only a few eV. Hydrogen
is one of the most difficult (the most difficult?) elements to
ionize.
>...The ions can
>be trivially accelerated to KeV, I would think MeV wouldn't be hard
>especially if the engine was somewhat large, to keep electric fields down.
>Maybe ionization sends a lot of energy into heat, but I would think
>efficiencies of a few percent shouldn't be too hard.
> btw, what limits the thrust of an ion engine?
That's what keeps ion drives down below keV of potential. Thrust
depends on available, electric power, and the power requirement
for a given thrust level increases as the square of exhaust
velocity. Just a 1 keV potential would give an exhaust velocity
of around 75 km/s, but the power requirements for a useful
thrust are impractical. By the way, more than about 5 keV
or so isn't all that easy: Arcing between the grids in a
traditional ion drive (electrostatic accelerator) becomes
a potential problem at higher voltages. I'm not sure how
that affects other electric propulsion systems like Hall
effect (MPD) thrusters, but I think they are also limited to
fairly low energies. Even so, at a 30 km/s exhaust velocity,
we're talking about 250 eV of so of accelerating potential
and a few eV to ionize the reaction mass. So the ionization
is around 1% of the total power consumption (although it
would be a much larger fraction if you used hydrogen.)
Frank Crary
CU Boulder
Paul F. Dietz
dpat...@husc.harvard.edu (David Patterson) wrote:
> I don't now much about ion engines, but this seems funny to me.
>Ionization energies are tens of eV (eg 13.6 eV for H). The ions can
>be trivially accelerated to KeV, I would think MeV wouldn't be hard
>especially if the engine was somewhat large, to keep electric fields down.
>Maybe ionization sends a lot of energy into heat, but I would think
>efficiencies of a few percent shouldn't be too hard.
> btw, what limits the thrust of an ion engine?
Ion engine thrust density is limited by the space charge limit on the
current density. The space-charged limited thrust density is
proportional to the square of the voltage (and proportional the square
of the ion mass/charge ratio).
At a system level, thrust is limited by availability of electric
power. KeV or MeV ions would require far, far too much electric
power. Ion engines typically have exhaust velocities that are
*too high*, thus the interest in heavy ions to get the voltages
and exhaust velocity down while preserving thrust density and
reducing the ionization cost (too bad C60 didn't work out due to
fragmentation.)
Paul
Zakany
>>>
Why can't a self-ionizing fuel be used? Chemical reactions and nuclear
reactions ionize things. Why not ionize the fuel that way, then
accelerate
the pre-ionized material electrically?
<<<
Jim O., what reaction were you proposing? You'd need to ionize at least
10% of your reactants. It would also be helpful if your ionization
process didn't add undue complexity and weight to the thruster.
Bill B. is right. Most of the electrical power in consumed by the
accelerating grids or ion optics. Ionization and other losses account for
less than 20% of the total thruster energy, I believe.
I'm not slamming your concept, Jim, but I wanted you to be aware the
relevant problems. There's nothing quite like a high energy electron
swirling about in a magnetic field to ionize the thruster expellant!
Paul F. Dietz
fcr...@rintintin.Colorado.EDU (Frank Crary) wrote:
> the power requirement
>for a given thrust level increases as the square of exhaust
>velocity.
No, linearly in exhaust velocity. Remember, thrust per mass
flow is proportional to exhaust velocity, so the mass flow
can go down at constant thrust as exhaust velocity increases.
Paul
Morgoth
How much of the solar winds is ionized?
What about solar flares?
Working on an idea, but..
Need facts first..
Morgoth
Where can you get things ionized naturally?
I know the northern lights are ionized somehow (solar winds).
Frank Crary
In article <Pine.SUN.3.93.97010...@dwarf.nome.net>,
All of it. (Well, far over 99.9%)
>What about solar flares?
Again, essentially all of it, once the event occurs. I'm not sure
how much is ionized just before the flare, but I'm fairly sure it's
over 90%.
Frank Crary
CU Boulder
Mike Atkinson
In article <5a8skl$p...@lace.colorado.edu>, Frank Crary
<URL:mailto:fcr...@rintintin.Colorado.EDU> wrote:
>
> In article <5a6627$4...@decaxp.harvard.edu>,
> David Patterson <dpat...@husc.harvard.edu> wrote:
> >: Much of the energy expended in an ion engine is expended while ionizing the
> >: propellant.
> >: reactions ionize things. Why not ionize the fuel that way, then accelerate
> >: the pre-ionized material electrically?
>
> >Ionization energies are tens of eV (eg 13.6 eV for H).
>
> is one of the most difficult (the most difficult?) elements to
> ionize.
>
> >be trivially accelerated to KeV, I would think MeV wouldn't be hard
> >especially if the engine was somewhat large, to keep electric fields down.
> >Maybe ionization sends a lot of energy into heat, but I would think
> >efficiencies of a few percent shouldn't be too hard.
> > btw, what limits the thrust of an ion engine?
>
> for a given thrust level increases as the square of exhaust
> of around 75 km/s, but the power requirements for a useful
> thrust are impractical. By the way, more than about 5 keV
> or so isn't all that easy: Arcing between the grids in a
> traditional ion drive (electrostatic accelerator) becomes
> a potential problem at higher voltages. I'm not sure how
> that affects other electric propulsion systems like Hall
> effect (MPD) thrusters, but I think they are also limited to
> fairly low energies. Even so, at a 30 km/s exhaust velocity,
> we're talking about 250 eV of so of accelerating potential
> and a few eV to ionize the reaction mass. So the ionization
> is around 1% of the total power consumption (although it
> would be a much larger fraction if you used hydrogen.)
For the UK-25E (the only one I have data immediately to hand) Xenon
ion thruster, when corrected for neutraliser flow and doubly charged
ions:
Beam Voltage 2,200 V
Beam Current 2.55 A
Thrust 194 mN
Accelerator Voltage -350 V
Accelerator Current 40 mA
Discharge (anode) Voltage 30 V
Dischange Current 20 A
Keeper Voltage 8 V
Keeper Current 0.5 A
Ion Production Cost 241 W/A
Propellant Flow Rate 4.08 mg/s
Doubly Charged Ion Fraction 0.055
Assumed Neutraliser Flow 0.12 mg/s
Discharge Power 600 W
Keeper/Magnet/Accel Power 30 W
Beam Power 5,610 W
Mass Utilisation 0.80
Electrical Efficiency 0.90
Total Efficiency 0.72
Power-to-thrust 32.3 W/mN
Specific Impulse 4,621 S
So the ion production cost is 2.55 x 241 = 615 W, just under 10% of the
total power.
This thruster can range from 63 mN up to 316 mN with little change
in operating conditions, being limited at the high end by the power
supply. A quote from the report AEA-inTec-1384 "THE UK-25 ION THRUSTER"
by P.M. Latham. "Assuming grid movements are controlled,
and measures taken to reject discharge power losses, there is no physical
mechanism, such as cothode emission or extraction grid limitations,
preventing higher thrusts". (Also in JBIS December 1995)
The main effects which limit thrust (for a given diameter) seem to be
heating, grid erosion and doubly charged ions, and not space charge
as Henry Spencer reported. Higher thrust values are probably not much
use anyway as the power supply mass tends to increase linearly with
power output, and the thruster mass is already only a few percent of
the total power supply & conditioning and fuel storage & distribution
mass.
--
Mike Atkinson Mi...@ladyshot.demon.co.uk
Frank Crary
In article <32ce951f...@news.demon.co.uk>,
>Hum...I pause before answering this, okay, I'll give you the benefit
>of the doubt, here it is :
>It's all charged, how else do you think it got there?
>"The thermal energy of the ionized coronal gas is so great that the
>sun's gravitational field cannot retain the gas in a confined static
>atmosphere. "
>[Macmillan dictionary of Astronomy 2nd edition - Valerie Illingworth]
Actually, you don't need an ionized corona to have a solar wind.
It just has to be hot enough that the mean thermal velocity
of the particles is close to solar escape velocity. In the
case of the Sun, that is hot enough that the gas is also
fully ionized. But you could, for example, have a M class
star with a stellar wind that isn't fully ionized, or a
brown dwarf with a wind that was only weakly ionized. In addition,
the earliest solar wind theories (Parker wind models) assumed
that the gas was neutral. So the fact that there is a solar
wind doesn't obviously imply that it's fully ionized, although
that happens to be the case.
>>What about solar flares?
>Pretty much same again :- ...
>protons have +ve charge, an electron has -ve charge, a neutron has no
>charge, and ions are mearly high energy atoms (collection of protons,
>neutrons, and electrons) that have been stripped of electrons
>(usually, in the cold world of laboratory chemistry, ions can GAIN
>electrons and become negativly charged, but this tends not to happen
>at 6000-100,000K!) - so have a positive charge.
>The energies of individual particles is often given in eV's...
Yes, but the ionization potential of hydrogen is about 15 eV.
Only about one part in e^15 would be ionized. Well, e^-15/(1+e^-15),
but... that's still essentially neutral. Of course, flares
have energies of well over 1 eV, and the gas in flares is
probably all above 10 eV, making it at least partially ionized.
During the flare, particles can get up to GeV of energy, if
memory serves, which is more than enough to make it fully
ionized. But I'm not sure if flares regions are fully ionized
before the event, nor if there are some regions that stay
cool enough to only be partially ionized.
Frank Crary
CU Boulder