why no BIG ion engines?
Joseph J. Strout
The Isp of ion engines is so much higher than that of chemical rockets,
that it would seem one could reach orbit with MUCH less fuel (perhaps 1/10
as much?). The problem is that, while the exhaust velocity is very high,
the mass flow (mass ejected per second) is extremely low. So the resulting
force is low, much too low to lift the engine off the ground.
But why is this problem insurmountable? I'd think you could attack it two ways:
(1) increase the length of your accelerator, i.e. have multiple magnetic
coils, to increase the exhaust velocity even further
(2) build the engine as a big cluster of acceleration tubes, so that the
total mass flow is high enough (perhaps 20 kg/sec?)
I'm extremely ignorant of the physics of particle accelerators, so there's
probably a perfectly good reason why this isn't done... but I've been
scratching my head all weekend, and can't figure out what it is. A friend
(a Navy nuclear technician) and I roughly calculated that using a
nuclear-powered ion engine, we should be able to reach orbit for about
$20/kg. That was assuming a lot of things, most importantly, that our
actual delta-V would be relatively close to the ideal delta-V, and that
implies a large mass flow.
Thanks for any info...
-- Joe Strout
,------------------------------------------------------------------.
| Joseph J. Strout Department of Neuroscience, UCSD |
| jst...@ucsd.edu http://www-acs.ucsd.edu/~jstrout/ |
`------------------------------------------------------------------'
John Schilling
jst...@ucsd.edu (Joseph J. Strout) writes:
>The Isp of ion engines is so much higher than that of chemical rockets,
>that it would seem one could reach orbit with MUCH less fuel (perhaps 1/10
>as much?). The problem is that, while the exhaust velocity is very high,
>the mass flow (mass ejected per second) is extremely low. So the resulting
>force is low, much too low to lift the engine off the ground.
>But why is this problem insurmountable? I'd think you could attack it two
>ways:
>(1) increase the length of your accelerator, i.e. have multiple magnetic
>coils, to increase the exhaust velocity even further
Acceleration is done with charged grids, not magnetic coils. And due to
space charge effects, increasing the "accelerator length", i.e. the grid
spacing, decreases the achievable flow rate. The net charge of the ions
currently being accelerated, rapidly rises to the point that it prevents
the introduction of further ions into the acceleration region.
And the longer the acceleration region, the more ions will be in it at
any time, adding to the repulsive potential. This turns out to exactly
negate any gain in exhaust velocity you might have achieved with the
increased grid spacing. The maximum thrust of an ion engine is proportional
to the product of grid area and field strength, and independant of length.
>(2) build the engine as a big cluster of acceleration tubes, so that the
>total mass flow is high enough (perhaps 20 kg/sec?)
The cross-sectional area of the acceleration grids (not tubes) for a
20 kg/s flow rate would be approximately 50,000 square meters. A bit
over one acre.
>I'm extremely ignorant of the physics of particle accelerators, so there's
>probably a perfectly good reason why this isn't done... but I've been
>scratching my head all weekend, and can't figure out what it is.
Power. The limitations mentioned above can be overcome by using plasma
thrusters rather than ion thrusters, but the power input requirement for
*any* electric propulsion system capable of lifting off from the Earth's
surface would be prohibitive.
An ion or plasma thruster capable of providing one ton of thrust with a
specific impulse of 2,500 seconds, and assuming 100% efficiency, would
require over one hundred megawatts of electric power.
>A friend (a Navy nuclear technician) and I roughly calculated that using
>a nuclear-powered ion engine, we should be able to reach orbit for about
>$20/kg.
Ask your Navy friend how much a 100-MW nuclear powerplant (reactor, heat
exchangers, turbogenerator, shielding, the whole thing) weighs. It turns
out to be quite a bit more than one ton.
And until we learn to beat that figure, to build electric power generators
of some sort producing in excess of 100 MW/ton, no ion or plasma-drive
spacecraft is ever going to be able to lift off under its own power.
Sorry.
--
*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 *
Geoffrey A. Landis
In article <jstrout-ya0240800...@news.ucsd.edu> Joseph J.
Strout, jst...@ucsd.edu writes:
>The Isp of ion engines is so much higher than that of chemical rockets,
>that it would seem one could reach orbit with MUCH less fuel (perhaps 1/10
>as much?). The problem is that, while the exhaust velocity is very high,
>the mass flow (mass ejected per second) is extremely low. So the resulting
>force is low, much too low to lift the engine off the ground.
>
>But why is this problem insurmountable?
In a word: energy.
If you define the energy efficiency of an engine as the thrust per unit
of power into the engine (just as you define specific impulse, or mass
efficiency, as thrust per unit of mass-flow into the engine), you will
find that power efficiency is inversely proportional to specific impulse.
Ion engines are hugely *mass* efficient, but they are *energy*
inefficient.
There's just no way you could cram energy into an ion engine fast enough
to get it to lift off the ground.
And if that energy comes from fuel-- if you just burned the fuel and
exhausted it, it would be much more efficient than burning it to power an
ion engine, then ejecting the waste overboard.
?I'd think you could attack it two ways:
>
>(1) increase the length of your accelerator, i.e. have multiple magnetic
>coils, to increase the exhaust velocity even further
>
>(2) build the engine as a big cluster of acceleration tubes, so that the
>total mass flow is high enough (perhaps 20 kg/sec?)
Neither one solves the problem-- where does the power come from?
______________________
Geoffrey A. Landis
Physicist and part-time Science Fiction writer
Ohio Aerospace Institute at NASA Lewis Research Center
http://www.sff.net/people/Geoffrey.Landis
Mike Pelletier
In article <jstrout-ya0240800...@news.ucsd.edu>,
Joseph J. Strout <jst...@ucsd.edu> wrote:
>The Isp of ion engines is so much higher than that of chemical rockets,
>that it would seem one could reach orbit with MUCH less fuel (perhaps 1/10
>as much?). The problem is that, while the exhaust velocity is very high,
>the mass flow (mass ejected per second) is extremely low. So the resulting
>force is low, much too low to lift the engine off the ground.
In other words, the force is too low compared to the mass of the engine.
>But why is this problem insurmountable? I'd think you could attack it
>two ways:
>
>(1) increase the length of your accelerator, i.e. have multiple magnetic
>coils, to increase the exhaust velocity even further
The increased length and coil material would increase the mass more
quickly than they would increase the force, and the force would still
be too low compared to the mass of the engine.
>(2) build the engine as a big cluster of acceleration tubes, so that the
>total mass flow is high enough (perhaps 20 kg/sec?)
The total mass of the cluster would increase faster than the force,
and the force ... [see above] ...
-Mike Pelletier.
Robin Colgrove
As several people have pointed out,
high-thrust ion engines are limited by the
enormous power required for reasonable
thrust. Just off the back of the napkin, I get:
F = 2E/V
Where F is the Force produced, E is the Energy
consumed and V is the Exhaust velocity.
For V = 20,000 m/s, every newton of thrust
"costs" 10kW of power and therefore to lift
a 1000kg off the Earth would take an initial
power of > 100 Megawatts; much greater than
any light, onboard power source could deliver.
But what about beam power (e.g laser launch)?
I appreciate the space-charge limits on ion
drives but are there other fundamental limits
to the thrust-to-weight ratios of electromagnetic
rockets that would preclude their use as
launchers if one could deliver enough power to them?
At the other extreme, what limits how light one
can make a particle accelerator that could
have a Ve around 0.1c for use as a sub-relativistic
interstellar drive, using a big solar-pumped laser
(a la Forward) to supply power?
It was Landis's paper on reducing the cost of
interstellar laser light sails that started me
thinking about this:
http://www.aleph.se/Trans/Tech/Space/laser.txt
thinking that for a "modest" delta-V of 0.1c,
photons were wastefully fast with very low
thrust whereas particle beams had too short a
range unless acceleration was very high.
But a laser-powered accelerator might combine
the great range of laser-power with the
higher thrust per Joule of massive particles
with Ve closer to delta-V.
That all depends of course on whether it is
possible at least in princple to make an
accelerator many orders of magnitude lighter
than the ground based devices used today.
I hunted around a little to see if someone
had worked this through already but I could
find nothing. Anybody else know?
Robin Colgrove
Jeramie Hicks
So, in conclusion, ANY electrical-based engine concept is simply not
feasible until power generators can be made lighter/cheaper/better.
Want to be a rocket scientist? Become a chemist.
- Hicks
Samuel S. Paik
In article <5ko32s$g10$1...@spock.usc.edu>, schi...@spock.usc.edu (John
Schilling) wrote:
>Power. The limitations mentioned above can be overcome by using plasma
>thrusters rather than ion thrusters, but the power input requirement for
>*any* electric propulsion system capable of lifting off from the Earth's
>surface would be prohibitive.
Could a variant of laser launch be used to leave the power source
on the ground?
Sam
--
Guy [Gavriel Kay]'s been living here in Toronto | Samuel S. Paik
for the last two years. I believe that means | pa...@webnexus.com
the new book is going to be called THE BLUE | 408-969-3258
JAYS OF AL-SKYDOME ... - Robert J. Sawyer | Speak only for self
John Schilling
pa...@webnexus.com (Samuel S. Paik) writes:
>In article <5ko32s$g10$1...@spock.usc.edu>, schi...@spock.usc.edu (John
>Schilling) wrote:
>>Power. The limitations mentioned above can be overcome by using plasma
>>thrusters rather than ion thrusters, but the power input requirement for
>>*any* electric propulsion system capable of lifting off from the Earth's
>>surface would be prohibitive.
>Could a variant of laser launch be used to leave the power source
>on the ground?
Yes, but you'd find that even just the reciever and power-processing
hardware would weigh far more than 1 ton per hundred megawatts. If
you want to handle astronomical ammounts of energy in compact systems,
you use heat rather than electricity.
Which means rockets, and if you want to keep the power source on the
ground, laser rockets.
Jerry Boyd
>The Isp of ion engines is so much higher than that of chemical rockets,
>that it would seem one could reach orbit with MUCH less fuel (perhaps 1/10
>as much?). The problem is that, while the exhaust velocity is very high,
>the mass flow (mass ejected per second) is extremely low. So the resulting
>force is low, much too low to lift the engine off the ground.
>
>But why is this problem insurmountable? I'd think you could attack it two ways:
>
>(1) increase the length of your accelerator, i.e. have multiple magnetic
>coils, to increase the exhaust velocity even further
>
>(2) build the engine as a big cluster of acceleration tubes, so that the
>total mass flow is high enough (perhaps 20 kg/sec?)
>
>I'm extremely ignorant of the physics of particle accelerators, so there's
>probably a perfectly good reason why this isn't done... but I've been
>scratching my head all weekend, and can't figure out what it is. A friend
>(a Navy nuclear technician) and I roughly calculated that using a
>nuclear-powered ion engine, we should be able to reach orbit for about
>$20/kg. That was assuming a lot of things, most importantly, that our
>actual delta-V would be relatively close to the ideal delta-V, and that
>implies a large mass flow.
>
>Thanks for any info...
>-- Joe Strout
>
>,------------------------------------------------------------------.
>| Joseph J. Strout Department of Neuroscience, UCSD |
>| jst...@ucsd.edu http://www-acs.ucsd.edu/~jstrout/ |
>`------------------------------------------------------------------'
One thing that's going bite you pretty hard is the fact that ion
engines need a pretty good vacuum to function. The back end of your
engine has to be permeable to a large ion stream, while still keeping
out the atmosphere. If you have a neat solution, I'm sure we would all
be interested.
just $0.02 from an old hillbilly
SteveMick
John Schilling wrote:
"And until we learn to beat that figure, to build electric power
generators
of some sort producing in excess of 100 MW/ton, no ion or plasma-drive
spacecraft is ever going to be able to lift off under its own power."
Strictly speaking, this is true of course but I believe there are schemes
that use beamed energy-microwave and/or laser that do produce a plasma as
exhaust and one microwave scheme using air as reaction mass even
accelerates the plasma with an electric field created by the beamed
microwaves.
Solar thermal rockets may be able to exceed 2000sec. at an efficiency of
about 33% and a specific power of 10KW/kg or better- much better than
electric propulsion because the energy is used directly rather than being
converted into another form.
Another shameless plug for my favorite kind of rocket.
Steve Mickler
Brian Pickrell
Geoffrey A. Landis (Geoffrey...@lerc.nasa.gov) wrote:
: In article <jstrout-ya0240800...@news.ucsd.edu> Joseph J.
: Strout, jst...@ucsd.edu writes:
: >The Isp of ion engines is so much higher than that of chemical rockets,
: >that it would seem one could reach orbit with MUCH less fuel (perhaps 1/10
: >as much?). The problem is that, while the exhaust velocity is very high,
: >the mass flow (mass ejected per second) is extremely low. So the resulting
: >force is low, much too low to lift the engine off the ground.
: >
: >But why is this problem insurmountable?
: In a word: energy.
: If you define the energy efficiency of an engine as the thrust per unit
: of power into the engine (just as you define specific impulse, or mass
: efficiency, as thrust per unit of mass-flow into the engine), you will
: find that power efficiency is inversely proportional to specific impulse.
: Ion engines are hugely *mass* efficient, but they are *energy*
: inefficient.
Yes, the fact that an ion drive has a high Isp doesn't really mean
anything.
Another way to state this is that Isp is a more or less meaningless tool
for comparing rockets with ion drives. This is basically because with
rockets, the energy source is the same as the reaction mass, and it
represents most of the weight of the rocket. With an ion drive, the heavy
energy source stays along with you, and the mass of material ejected is
insignificant.
: There's just no way you could cram energy into an ion engine fast enough
: to get it to lift off the ground.
Interestingly, there is a way to give a chemical rocket lift-off
performance similar to that of an ion thruster. You simply attach 50,000
tons of bricks to the vehicle structure. With this modified vehicle, the
weight of the fuel burned becomes insignificant.
I think there's a compensating disadvantage; however, I can't quite put my
finger on what it is.
: And if that energy comes from fuel-- if you just burned the fuel and
: exhausted it, it would be much more efficient than burning it to power an
: ion engine, then ejecting the waste overboard.
: ?I'd think you could attack it two ways:
: >
: >(1) increase the length of your accelerator, i.e. have multiple magnetic
: >coils, to increase the exhaust velocity even further
: >
: >(2) build the engine as a big cluster of acceleration tubes, so that the
: >total mass flow is high enough (perhaps 20 kg/sec?)
: Neither one solves the problem-- where does the power come from?
: ______________________
: Geoffrey A. Landis
: Physicist and part-time Science Fiction writer
: Ohio Aerospace Institute at NASA Lewis Research Center
: http://www.sff.net/people/Geoffrey.Landis
--
-----------------------------------------------------------------------
Brian Pickrell Viewpoints expressed in this article are not
necessarily the opinions of the author. I
agree with whatever my employer thinks.
BshpGambit
> increase the length of your accelerator, i.e. have multiple magnetic
>coils, to increase the exhaust velocity even further
This is somewhat like an idea I had; my idea is to inject the fuel at high
speed into a long screen-filled tube. The purpose of this may be to
launch a vehicle form the atmosphere if its mass flow is high enough.
Also this may be a good way to get a ship going, as I've read quite a bit
about how long it takes for a shipe to get going at a relatively good
speed with ion engines. A large pulse or a series of large pulses may
prove to aid the main engines, if employed on a ship.
Jim F. Glass x60375
In article <5ko6j0$e...@sulawesi.lerc.nasa.gov>, "Geoffrey A. Landis" <Geoffrey...@lerc.nasa.gov> writes:
|> In article <jstrout-ya0240800...@news.ucsd.edu> Joseph J.
|> Strout, jst...@ucsd.edu writes:
|> >The Isp of ion engines is so much higher than that of chemical rockets,
|> >that it would seem one could reach orbit with MUCH less fuel (perhaps 1/10
|> >as much?). The problem is that, while the exhaust velocity is very high,
|> >the mass flow (mass ejected per second) is extremely low. So the resulting
|> >force is low, much too low to lift the engine off the ground.
|> >
|> >But why is this problem insurmountable?
|>
|> In a word: energy.
|>
|> If you define the energy efficiency of an engine as the thrust per unit
|> of power into the engine (just as you define specific impulse, or mass
|> efficiency, as thrust per unit of mass-flow into the engine), you will
|> find that power efficiency is inversely proportional to specific impulse.
|> Ion engines are hugely *mass* efficient, but they are *energy*
|> inefficient.
|>
|> to get it to lift off the ground.
|>
|> exhausted it, it would be much more efficient than burning it to power an
|> ion engine, then ejecting the waste overboard.
|>
|>
|> ?I'd think you could attack it two ways:
|> >
|> >coils, to increase the exhaust velocity even further
|> >
|> >total mass flow is high enough (perhaps 20 kg/sec?)
|>
|> Neither one solves the problem-- where does the power come from?
|> ______________________
|> Geoffrey A. Landis
|> Physicist and part-time Science Fiction writer
|> Ohio Aerospace Institute at NASA Lewis Research Center
|> http://www.sff.net/people/Geoffrey.Landis
Yeah. Conventional Ion engines require roughly 5 to 20 Kw per pound (force)
of thrust.
Interestingly, this works for thermal engines (like SSME and NERVA) as well.
So a 100,000 lbf NERVA engine requires roughly 2000 megawatts thermal output
from its reactor.
Since powerplants are notoriously massy, it gets outa hand rather quickly.
Especially for electric thrusters.
That's why I am a fan of "nuclear light bulb" concepts. I once read a paper
in AIAA Journal entitled, "To Mars and Back in 30 Days via Gas-Core Nuclear
Rocket." Hooray. Now if we only knew how to build them. Me, I want to zip
to Mars for lunch and be back for supper.
"Make It So!"
Jim Glass
Closet Space Cadet
tech
> That's why I am a fan of "nuclear light bulb" concepts. I once read a
paper
> in AIAA Journal entitled, "To Mars and Back in 30 Days via Gas-Core
Nuclear
> Rocket." Hooray. Now if we only knew how to build them. Me, I want to
zip
> to Mars for lunch and be back for supper.
Scott Lowther, whom I see post here occasionally, did much work on this
concept, in the context of a manned Saturn system expedition, when he was
in school at Iowa State.
Interesting stuff.
SteveMick
Jim F. Glass wrote:
"That's why I am a fan of "nuclear light bulb" concepts. I once read a
paper
in AIAA Journal entitled, "To Mars and Back in 30 Days via Gas-Core
Nuclear
Rocket." Hooray. Now if we only knew how to build them. Me, I want to
zip
to Mars for lunch and be back for supper."
Hey if you can wait a month or so to get there' a solar thermal rocket can
get you there. Of course they don't cost much money or take a long time to
develop, so I guess it depends on how you define "waiting".
Steve