Double-Layer Ion Thruster

Mike Combs writes:

wrote:

As an aside, there has been some interesting discussion on the use of
beamed power to provide the energy for VASIMR, either through microwaves
or laser from a series of ground or orbital power stations. Regardless,
the electrical consumption of VASIMR is horrendous, but that is
typically the price you pay for high efficiency propulsion.

Hey, here's a question: Would VASIMR be a good choice for raising a SPS
built in LEO to GEO (provided you weren't in a hurry)? I understand that
with VASIMR you can trade specific impulse for thrust. Might one start
out with a high-specific impulse/low thrust regime from LEO up to the Van
Allen belts, switch over to high-thrust/low specific impulse regime
through the Van Allens, and then back again for the rest of the trip to
GEO? Or would that still be too much time spent in the Van Allens?

Orbital mechanics doesn't work that way. First, you want to burn as much
propellant as deep in a gravity well as you can for maximum performance
(Oberth effect). Second, a low acceleration burn (i.e., much less than the
local value of "gee") deep in a gravity well results in a "spiral ascent"
that incurs huge gravity losses. Both these factor drive you to a high-thrust,
low-I_sp burn out of LEO over a timescale less than one orbital period;
as a side-benefit, this implies a quick van Allen belt passage. You can
do a low-thrust, high-I_sp curcilatization burn at GEO, since the local
value of "gee" is an order of magnitude smaller there.

If you _insist_ on economizing on propellant, I suppose you _could_ slowly
raise the apogee with a series of short, low-thrust, high-I_sp perigee burns,
but this would subject the SPS to tens to hundreds of van Allen belt passages ---
which would =NOT= be good for the electronics !!!


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

"sanman" wrote:
Hmm, I remember reading about how VASIMR engines are supposed to use
microwaves to heat the plasma, and how they can produce more thrust. I
never realized that they can also vary the specific impulse and thrust
(I should have looked more closely at the acronym!)

A VASIMR is basically a failed linear fusion reactor. In a
working fusion reactor a plasma is contained magnetically and
heated mostly via fusion reactions and only brought up to
temperature via external heating (microwaves, magnetic heating,
etc.) VASIMR turns things around and says "hey, let's use
this for heating any damned thing we like to very high
temperatures using an external power supply". So it's like a
modern experimental fusion reactor except that it doesn't need
to use only fusion fuels, and, of course, it's linear and has
a magnetic nozzle. The downside is that it's not really a
full propulsion system, it's just a thruster. You have to
provide the propellant, of course, but you also have to
provide the power, and VASIMR is going to eat a lot of it. The
upside is that if you can provide the power (with a nuclear
reactor almost certainly) then you can get moderately high
thrust and high-Isp, and that's a damned decent combo.

Mike Combs wrote:

wrote:

As an aside, there has been some interesting discussion on the use of
beamed power to provide the energy for VASIMR, either through microwaves
or laser from a series of ground or orbital power stations. Regardless,
the electrical consumption of VASIMR is horrendous, but that is
typically the price you pay for high efficiency propulsion.

Hey, here's a question: Would VASIMR be a good choice for raising a SPS built
in LEO to GEO (provided you weren't in a hurry)? I understand that with VASIMR
you can trade specific impulse for thrust. Might one start out with a
high-specific impulse/low thrust regime from LEO up to the Van Allen belts,
switch over to high-thrust/low specific impulse regime through the Van Allens,
and then back again for the rest of the trip to GEO? Or would that still be
too much time spent in the Van Allens?


It depends on how serious of a radiation soak you're willing to live
with primarily (there have been serious suggestions of delivering SPS
power production components folded up inside shielded cases though the
mass penalty is mean). Note that this would not be as serious a concern
if the primary power generation method used a mechanical heat cycle
technique as opposed to photovoltaics. The described transfer would be
low thrust spiral to inner belt, high thrust through, low thrust spiral
in middle region, high thrust through outer belt, then low thrust spiral
to GEO.

VASIMR is not necessarily a bad choice for SPS movement from LEO,
primarily because the masses involved with SPS construction imply
reasonable transportation from earth, and the associated high total mass
to LEO. It would be simple matter of delivering water to LEO, using the
SPS to crack the water and refrigerate the products into cryogens, and
leave the oxygen behind for other uses. An inflatable tank for storing
the liquid hydrogen fuel could be reasonable. The only real difficulty
in this method is the degree of earth shadow one would face. You either
must accept high thermal cycling loads with a VASIMR engine running at
best approximately 2/3 of every orbit (an unpleasant thought considering
the extremes involved), or choose an altered setup with more overhead.

Example of an alternate configuration (possibly the most favorable), is
to have a water cracking complex be combined with the fuel/engine
complex as one contiguous device. During earth shadow, choose to burn
some of the oxygen with the hydrogen through turbomachinery to gain
enough power to run the VASIMR engine complex at a reduced capacity.
Once back in sunshine, use excess electrical capacity of the SPS beyond
that required by VASIMR to recrack the water into fuel components. This
dark time "battery" may be a separate module with it's own cryogen and
cracking equipment from the primary hydrogen propellant or the equipment
used to generate the original propellant in LEO. This could be
supplemented by laser/microwave power from ground stations (which would
be buying power during a cheap period of the day) or from an existing
SPS. The advantage of a standalone device like this is that once
delivery is complete, the option exists for returning to LEO somehow,
and uses simple and inexpensive and plentiful fuel, unlike ion
thrusters, with xenon being in finite and small supply.

I had a brief conversation with a NASA engineer who was giving a
presentation here about SPS's a year ago, who said that using SPS power
to bootstrap up was still being seriously considered, despite the rad damage.

"Gordon D. Pusch" wrote:

Orbital mechanics doesn't work that way. First, you want to burn as much
propellant as deep in a gravity well as you can for maximum performance
(Oberth effect). Second, a low acceleration burn (i.e., much less than the
local value of "gee") deep in a gravity well results in a "spiral ascent"
that incurs huge gravity losses. Both these factor drive you to a high-thrust,
low-I_sp burn out of LEO over a timescale less than one orbital period;

Yes, I was aware it would be a spiral ascent, but kind of thought we were stuck
with that due to the extremely-large-yet-flimsy structure of a SPS. But I
suppose one might launch smaller components of a SPS from LEO to GEO via
chemical rockets and then assemble the components in GEO. Or alternately,
maybe you could arrange many small chemical tugs evenly distributed over the
entire area of the SPS such that there would not be very significant flexure
stress.

I guess what I'm trying to get to is a way to capitalize on the enormous amount
of electrical power that the SPS is generating. But maybe resistojets or
arcjets would be a better choice.

--


Regards,
Mike Combs
----------------------------------------------------------------------
We should ask, critically and with appeal to the numbers, whether the
best site for a growing advancing industrial society is Earth, the
Moon, Mars, some other planet, or somewhere else entirely.
Surprisingly, the answer will be inescapable - the best site is
"somewhere else entirely."

Gerard O'Neill - "The High Frontier"

Mike Combs writes:

wrote:

As an aside, there has been some interesting discussion on the use of
beamed power to provide the energy for VASIMR, either through microwaves
or laser from a series of ground or orbital power stations. Regardless,
the electrical consumption of VASIMR is horrendous, but that is
typically the price you pay for high efficiency propulsion.

Hey, here's a question: Would VASIMR be a good choice for raising a SPS
built in LEO to GEO (provided you weren't in a hurry)? I understand that
with VASIMR you can trade specific impulse for thrust. Might one start
out with a high-specific impulse/low thrust regime from LEO up to the Van
Allen belts, switch over to high-thrust/low specific impulse regime through
the Van Allens, and then back again for the rest of the trip to GEO? Or
would that still be too much time spent in the Van Allens?

Too much time not to have to harden your electronics and solar arrays,
and if you're going to harden your electronics and solar arrays, a factor
of two or so in time spent in the Van Allen belts doesn't really matter
very much. I've done propulsion trades for the LEO-GEO transfer mission
with more sets of assumptions than I can remember, and VASIMR is never
the winner.

VASIMR can indeed trade thrust vs. specific impluse - most advanced
propulsion systems can, VASIMR over a somewhat wider range - but that
ability is usually oversold.

In particular, *no* point in VASIMR's thrust/Isp trade space qualifies
as "high thrust" by (Earth) orbital mechanics standards. You get low
thrust/high Isp, lower thrust/higher Isp, and tiny thrust/huge Isp.
"High thrust" implies the ability to do meaningful burns in less than
many orbital periods, to use semielliptical transfer orbits rather
than spirals, to exploit the Oberth effect, etc, and VASIMR at it's
beefiest isn't even close to being able to do those things. It is
constrained to the same sort of trajectories as ordinary plasma
thrusters, ion thrusters, arcjets, etc, with all the associated
disadvantages including the hefty radiation exposure through the
belts.

VASIMR also has the extra disadvantage of not existing, unlike other
sorts of plasma thrusters, ion thrusters, arcjets, etc, so you're
probably better off picking your propulsion system(s) from the
latter set.


--
*John Schilling * "Anything worth doing, *
*Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" *
*Chief Scientist & General Partner * -13th Rule of Acquisition *
*White Elephant Research, LLC * "There is no substitute *
* for success" *
*661-951-9107 or 661-275-6795 * -58th Rule of Acquisition *

"Gordon D. Pusch" wrote:

(sanman) writes:

(Gordon D. Pusch) wrote in message
...
[...]
I mentioned the fullerene fuel, with its higher molecular weight,
being able to proportionally increase the thrust. I'd imagine that
buckyonions, being concentrically layered buckyballs, would have even
higher molecular weights.

...However, unless they all have =EXACTLY= the same atomic weight, and
can be given =EXACTLY= the same charge (or at least, that they all have
exactly the same charg-to-mass ratio) they will be very poor propellants
for an ion drive, since ion drives are very picky that way...

Note for later use: The reason why the charge-to-mass ratio of the
propellant must be highly uniform in an ion drive is that ions with
the same charge to mass ratio follow the same trajectory (more or less)
through a strong electromagnetic field. Since ion drives usually contain
multiple electrodes aligned to tight tolerances, a mixture of charge-to-
mass ratios means that the various ion species follow different trajectories;
at best, one will not get a well-focused exhaust beam, and at worst,
the "off-spec" charge-to-mass ratio ions may actually strike an electrode
and cause it to melt, burning out the thruster.


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

This may be a bit late but I think we need to clarify when charge to
mass
ratio is important.
Gordon is correct that in general ions with different charge-to-mass
ratios
follow different trajectories in an electromagnetic field however in a
steady purely electrostatic field the trajectory is independent of
charge-to-mass.
It would be much easier to to separate isotopes if this was not the
case.

The standard gridded ion thruster (like on Deep Space 1) uses only
electrostatic
fields to accelerate the ions and so will operate with any mass ion.
These thrusters
usually have a set of magnets around the plasma to keep it away from the
walls but
they are not involved in accelerating the ions.
The reason for wanting a well defined charge-to-mass ratio is so that
the acceleration
voltage can be matched to the power supply and fuel mass.
The Ion thruster is not super sensitive to the charge-to-mass ratio look
at what
happens if we change from singly charged Xenon ions Xe+ to doubly
charged Xenon Xe++
(there is always a mixture of ions present in the output).

ION Energy/Ion Velocity
Xe+ Vacc ISP*g
Xe++ 2Vacc 1.414*ISP*g

For a fixed power output we will only be able to accelerate half as many
Xe++ ions per unit time and so the thrust will drop to 70% = 100% *
1.414/2
So even a change of a factor of 2 in charge-to-mass only changes the
thrust
by a factor of .7071

Ken Myrtle