An ion engine fuel?

For almost all practical purposes, ion engines basically have way
too much exhaust velocity and power consumption, and not nearly
enough thrust. The struggle is to find ways to *lower* the exhaust
velocity (and hence the power consumption), preferably while also
bringing thrust up.
If the vehicle had a high efficiency fission reactor onboard, could we
then not worry about the power requirement and aim for the highest
exhaust velocity possible?

dexx wrote:

If the vehicle had a high efficiency fission reactor onboard, could we
then not worry about the power requirement and aim for the highest
exhaust velocity possible?

No. The power conversion equipment doesn't get much smaller. Also, making
the efficiency of the reactor high is going to hurt, since you do
that by making the thermal cycle more efficient, which means dissipating
waste heat at a lower temperature, which means big, heavy radiators.

Paul

In article .com,
dexx wrote:
For almost all practical purposes, ion engines basically have way
too much exhaust velocity and power consumption, and not nearly
enough thrust. The struggle is to find ways to *lower* the exhaust
velocity (and hence the power consumption), preferably while also
bringing thrust up.

If the vehicle had a high efficiency fission reactor onboard, could we
then not worry about the power requirement and aim for the highest
exhaust velocity possible?

Unfortunately, while a fission reactor does make a lot of power available,
you pay for that power: a reactor plus supporting equipment is *heavy*.
There's little point in reducing propellant supply from (say) 500kg to
200kg if it means adding several tons to the power system.

Plentiful power is very welcome, but almost any realistic mission will get
better use out of that power at a lower exhaust velocity.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

"dexx" writes:

For almost all practical purposes, ion engines basically have way
too much exhaust velocity and power consumption, and not nearly
enough thrust. The struggle is to find ways to *lower* the exhaust
velocity (and hence the power consumption), preferably while also
bringing thrust up.

If the vehicle had a high efficiency fission reactor onboard, could we
then not worry about the power requirement and aim for the highest
exhaust velocity possible?

No. "Nuclear" is not a magic word that when invoked provides infinite
power for whatever purpose you have in mind. The power available from
"high efficiency fission reactors" is finite, and you have to do the
math, stay within the limits, and accept that there are many things
that even "high efficiency fission reactors", simply cannot do.

And that doesn't change even if you substitute fusion for fission.


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*John Schilling * "Anything worth doing, *
*Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" *
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For large scale missions over a very long time frame, yes. However,
even there, this is not enough to be feasible for interstellar travel.
However, this approach would work for say, a 2 decade mission to the
kuiper belt.

The other area where this is appropriate is transportation of solar
powersats from high Earth Orbit to Geostationary Orbit, where your
cargo is your power supply.

Paul F. Dietz wrote:
Christopher M. Jones wrote:

You want a high mass ion because you
maximize thrust per charge, which minimizes ionization
inefficiencies per thrust unit and maximizes total engine
power efficiency.

It also increases the thrust/area of the engine, for a given
grid spacing operating, at the space charge limit. IIRC
the thrust density at constant Isp and grid spacing goes
as the square of the mass/charge ratio.

When I think about it, it's obvious that an ion engine with
any signifcant degree of efficiency will need to operate
such that the major load in the system is from accelerating
the ions rather than the resistance of the grid. But it
still doesn't quite make sense to me intuitively, I think my
sense of electronics operating near the space charge limit
is woefully undeveloped.


Reliability and
predictability of ionization are going to be tough to
nail down even if you can produce a particle that doesn't
fragment and isn't corrosive. Milliken's oil drop
experiment comes to mind.

There's been work on 'colloid' engines, which work with
mesoscopic particles and much higher voltages. I've wondered
if it might be possible to build an engine using such
particles accelerated in a linear accelerator, designed
in such a way that it's out of tune for light ions and
electrons.

I'd imagine progress in this area ought to have expanded
substantially in recent years due to progress in devices
such as inkjet/bubble-jet print heads.


Mercury might be the best choice for applications which
rely on off-Earth resource utilization, since Xenon is
hard to come by without a terrestrial atmosphere handy,
and those are pretty rare.

How much xenon or krypton would be trapped on outer solar
system icy bodies, I wonder?

Quite a lot, I think. Krypton and Xenon gases are
routinely found as trace elements in meteorites, for
example.

If Mercury works, then other metals should be possible, except for
their melting points.

Are there metals common on the moon that could be suitable:

Aluminium?
Calcium?
Iron?
Titanium?
Magnesium?

Calcium has high moldecular mass, easy to ionise (knock off 2
electrons), and is a vapour while iron is starting to melt. It's common
on the moon, but is highly reactive - though not, I believe with other
metals.

In article ,
Christopher M. Jones wrote:
Mercury might be the best choice for applications which
rely on off-Earth resource utilization, since Xenon is
hard to come by without a terrestrial atmosphere handy...
How much xenon or krypton would be trapped on outer solar
system icy bodies, I wonder?

Quite a lot, I think. Krypton and Xenon gases are
routinely found as trace elements in meteorites, for example.

Unfortunately, the operative word is "trace". They are relatively
uncommon elements for fairly fundamental reasons. What's needed is a
mechanism that would concentrate them somewhere; none is known.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

In article . com,
Alex Terrell wrote:
If Mercury works, then other metals should be possible, except for
their melting points.

Unfortunately, what you want for ion engines is a *heavy* metal that is
easily ionized once (and preferably not too prone to double ionization)
and has a low melting point. That's not so easy to come by.

Are there metals common on the moon that could be suitable:
Aluminium?
Calcium?
Iron?
Titanium?
Magnesium?

All, unfortunately, quite light metals. The heaviest is iron, with an
atomic mass of about 56 compared to mercury's 201 or xenon's 131.

The melting points are all high enough to be a headache. Ion thrusters
don't want to run unnecessarily hot -- it hurts energy efficiency and
greatly increases materials problems.

And with the exception of iron, those are all extremely reactive metals,
complicating the materials issues further and hampering testing.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

I appreciate the problem, but no one has come up with a solution to
large scale electric thrust - and I mean something like: Shift 1,000
tons by 1km/s, with an ion velocity of 20km/s, so we need about 50 tons
of propellant, on a regular basis.

Or think assembling a 50,000 ton SSPS in High Earth Orbit, then we need
3km/s to put it in GEO. OK - with huge power available, you'd want an
ion velocity of 100km/s, but even so, you need 1,500 tons of
propellant.

Is solar powered VASIMR the best option? Or some form of Oxygen Plasma
thruster? (What will we do with all the waste oxygen mined from the
moon?)