VASMIR plasma engine's power source?

(John Schilling) wrote in message ...
(Alex Terrell) writes:

(Allen Thomson) wrote in message . com...
(Henry Spencer) wrote

The usual assumption is that power would be from a reactor.

It just about has to be. If you look around for relevant numbers,
you can find things like,

Why?

Out as far as Mars, solar power is ligher than nuclear power for a
given power level.

If the given power level is ten kilowatts or so, yes.


Why go nuclear?

Because VASIMR wants megawatts.


You seem to be assuming that nuclear and solar power scale linearly,
that a one-megawatt solar array or nuclear reactor will have a hundred
times the mass of a ten-kilowatt solar array or nuclear reactor. This
is not the case - solar power starts to have problems above ~100 kW,
and nuclear at *less* than ~100 kW.

Probably a false assumption on my part, but why does solar have
problems above 10KW? SSPS was envisioned at 5GW, and there seems no
fundamental reason why a (collection of) low thrust electric engine(s)
couldn't pull one of those around.

Either could be made to work for a ~megawatt VASIMR, and solar might
have to be made to work if the nucleophobes have too much inflience
that year, but nuclear is clearly preferred.

Ian Stirling wrote in message ...
Alex Terrell wrote:
(Allen Thomson) wrote in message . com...
(Henry Spencer) wrote

The usual assumption is that power would be from a reactor.

It just about has to be. If you look around for relevant numbers,
you can find things like,

Why?

Out as far as Mars, solar power is ligher than nuclear power for a
given power level. Why go nuclear?

A look at http://www.entechsolar.com/SpacePaper4.pdf may be informative
for those that have not been keeping up with the state of the art (it
surprised me).
183W/Kg (at one sun) from a solar panel with silicone lenses.
I don't know what nuclear can do.

I've seen a figure of 30 tons per MW, which is about 1/6th of the
performanve you cite.

John Schilling wrote:
Sander Vesik writes:

Ian Stirling wrote:
Joseph S. Powell, III wrote:
I was just glancing at some sites mentioning VASMIR plasma engines, and
was wondering if the power source was nuclear (reactor or RTG?) or solar
or some other source?

Whatever can deliver enough power, and can be approved for launch.

RTGs deliver some 400W/Kg (of radioisotope) of thermal power (80W/
Solar arrays can achieve some 40W/Kg of electrical power.
http://www.entechsolar.com/SpacePaper4.pdf
reports on a solar array that does some 180Wk/g.

cells giving you 600W/kg exist - they use aluminium foil as substrate -
and designs using say kapon as the substrate could get up to at
least 2KW/kg. Sure, this will degrade over time in space and is distance
sensitive.


Be careful; solar *cells* are not solar *arrays*. To get power on a

right, my bad.

spacecraft, you need solar cells, interconnects, wiring harness, cover
glass, a structure to tie it all together, slew mechanism to point it
at the sun, and probably a power regulator.

But are all of these - and all of these being heavy - really necessary?

The glass covers of the solar panels are going to last much longer than
the mission time for most missions. Same applies to the large rigid
structure they are built in. In most cases, building solar panel structures
that are going to last for centuries is utter waste - anything beyond 2x
mission time really is. so instead of glass you could use slowly UV-degrading
transparent plastic, unroll a fabric of the solar panel like a sail instead
of having it be fixed on four sides, use plastics with UV-blocking materials
for parts of the structure and so on.

So the panel would look like something like this:

|| |
|| |
|| |
|+-------------------------+
|| |
|| |
|| |

where the first double line is a roll of "fabric" at the start
that will be unrolled towards the end.

over which you would then unroll the fabric, attaching it at the 'beams'
while unrolling using a cable/rope with attachments at the side of the
fabric. The panel also doesn't need to handle extra stress during unfolding

The lighter teh panel is the less power you need to unfold and turn it.


180 W/kg really would be quite good. 40 W/kg is typical.


I believe the 180W/kg figure is for an inflattible solar panel (which
would also not have a glass cover over the silicon, etc).

--
Sander

+++ Out of cheese error +++

(John Schilling) wrote in message ...
Why go nuclear?

Because VASIMR wants megawatts.


You seem to be assuming that nuclear and solar power scale linearly,
that a one-megawatt solar array or nuclear reactor will have a hundred
times the mass of a ten-kilowatt solar array or nuclear reactor. This
is not the case - solar power starts to have problems above ~100 kW,
and nuclear at *less* than ~100 kW.

Either could be made to work for a ~megawatt VASIMR, and solar might
have to be made to work if the nucleophobes have too much inflience
that year, but nuclear is clearly preferred.

You could maybe do a sub-megawatt VASIMR demonstration
with solar, but it would be a challenge. And then
you've got the question of where to go. Go to Mars
and you drop your power by 1/2, there's no really
good destination in the inner Solar System. Except
maybe NEOs.

(Christopher M. Jones) wrote in message om...
(John Schilling) wrote in message ...
Why go nuclear?

Because VASIMR wants megawatts.


You seem to be assuming that nuclear and solar power scale linearly,
that a one-megawatt solar array or nuclear reactor will have a hundred
times the mass of a ten-kilowatt solar array or nuclear reactor. This
is not the case - solar power starts to have problems above ~100 kW,
and nuclear at *less* than ~100 kW.

Either could be made to work for a ~megawatt VASIMR, and solar might
have to be made to work if the nucleophobes have too much inflience
that year, but nuclear is clearly preferred.

You could maybe do a sub-megawatt VASIMR demonstration
with solar, but it would be a challenge.

Why? As the other post points out, six times lighter than nuclear.

And then
you've got the question of where to go. Go to Mars
and you drop your power by 1/2, there's no really
good destination in the inner Solar System. Except
maybe NEOs.

Probably the best destination in the solar system, at least for the next 50 years.

In article ,
Sander Vesik wrote:
spacecraft, you need solar cells, interconnects, wiring harness, cover
glass, a structure to tie it all together, slew mechanism to point it
at the sun, and probably a power regulator.

But are all of these - and all of these being heavy - really necessary?

You basically can't avoid having *some form* of most of those. The slew
mechanism may be unnecessary if you point the whole spacecraft, but that's
about the only optional item.

The glass covers of the solar panels are going to last much longer than
the mission time for most missions.

Bear in mind that we're not talking about sheets of plate glass here.
These are very thin layers of glass applied directly to the cells; half a
millimeter is about the *thickest* used, and a tenth of that is not
unheard-of. The main reason for the glass, by the way, is radiation
protection for the cells.

(Do note, also, that if you make the cell mounting material thin and
flexible, you have to think about radiation shielding for the *backs*
of the cells.)

Same applies to the large rigid structure they are built in.

Large solar arrays actually are often quite flexible. Great amounts of
effort have been invested in trying to make these structures lightweight,
which includes making them no more rigid than necessary.

...so instead of glass you could use slowly UV-degrading
transparent plastic...

Um, why exactly would this be an improvement? I don't see any reason
why it would be any lighter.

unroll a fabric of the solar panel like a sail instead
of having it be fixed on four sides...

Often done already.

use plastics with UV-blocking materials for parts of the structure

Composite structures are already in use.

So the panel would look like something like this:

|| |
|| |
|| |
|+-------------------------+
|| |
|| |
|| |

where the first double line is a roll of "fabric" at the start
that will be unrolled towards the end.

Ever seen pictures of the original Hubble solar arrays? People were doing
exactly this sort of thing a quarter-century ago.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

(Alex Terrell) writes:

(John Schilling) wrote in message ...

Out as far as Mars, solar power is ligher than nuclear power for a
given power level.

If the given power level is ten kilowatts or so, yes.

[...]

You seem to be assuming that nuclear and solar power scale linearly,
that a one-megawatt solar array or nuclear reactor will have a hundred
times the mass of a ten-kilowatt solar array or nuclear reactor. This
is not the case - solar power starts to have problems above ~100 kW,
and nuclear at *less* than ~100 kW.

Probably a false assumption on my part, but why does solar have
problems above 10KW? SSPS was envisioned at 5GW, and there seems no
fundamental reason why a (collection of) low thrust electric engine(s)
couldn't pull one of those around.

I said 100 kW was the point where solar starts to have problems.

The reason for this is mostly the structural rigidity, or lack thereof,
of the solar arrays. The fact that there is no gravity does not mean
that there are no loads and one can make the structure arbitrarily weak.
There are always *some* loads, external ones such as light pressure and
internal ones such as the torque of the motors and gimbals that keep
the array pointed at the sun.

If these were steady-state loads, there would be little problem, but
some of them have transient components. And, reducing structural
mass reduces structural rigidity and damping in proportion. So when
your array-pointing motor twitches, a ripple bounces back and forth
across that gossamer structure and is loathe to just die away. If
it's still bouncing back and forth the next time the pointing motor
twitches, things start to get complicated...


Simple expansion of existing solar array designs to 100 kW levels,
gives you something that is in danger of tearing itself apart in
ordinary operation. Solving that problem, requires either the sort
of structural reinforcement that puts you on the wrong side of a
cube-square law, or Extreme Cleverness in the area of structural
dynamics. And spacecraft designers take a dim view of anyone who
proposes Extreme Cleverness for mission-critical systems.


--
*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 *

Sander Vesik writes:

John Schilling wrote:

cells giving you 600W/kg exist - they use aluminium foil as substrate -
and designs using say kapon as the substrate could get up to at
least 2KW/kg. Sure, this will degrade over time in space and is distance
sensitive.

Be careful; solar *cells* are not solar *arrays*. To get power on a

right, my bad.

spacecraft, you need solar cells, interconnects, wiring harness, cover
glass, a structure to tie it all together, slew mechanism to point it
at the sun, and probably a power regulator.

But are all of these - and all of these being heavy - really necessary?

Pretty much, yes.


The glass covers of the solar panels are going to last much longer than
the mission time for most missions. Same applies to the large rigid
structure they are built in. In most cases, building solar panel structures
that are going to last for centuries is utter waste - anything beyond 2x
mission time really is.

Believe me, nobody is building solar arrays to last for centuries.

The issue with solar array structure is not some gradual weakening or
erosion of the structure, but dynamic failure. Either you've got enough
rigidity and internal damping to keep the thing from shaking apart in
its first few hours, or you don't. If you do, you've got something
pretty close to the present state of the art in solar array design.

And the cover glass, the issue there isn't how long the glass itself
will last but whether it is thick enough to stop the deluge of energetic
electrons and protons that would kill the solar cells. If you've got
a cover glass that is half as thick as it needs to be to stop most of
the radiation, yes, your cover glass will still last decades. But
the solar cells will be dead in weeks to months depending on the
environment.

There is, at the margin, a mass vs. lifetime trade. But most of what
can be eked out of that trade already has been, and it's going to take
Extreme Cleverness to get any more substantial reductions in the mass
of the support systems without killing the arrays in very short order.

And people are working the Extreme Cleverness angle from several directions,
but that's not the sort of thing you can count on paying off.


--
*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 *

(Alex Terrell) writes:

If you go much bigger, you start to provide engine units for the solar
array, to distribute the load.

Largely pointless, because the Law of Diminishing Returns kicks in rapidly.
In the limit of a large number of thrust units, you no longer have a single
solar panel --- you have a fleet of solar-panel/engine-unit combinations
flying in _very_ tight formation, with each engine effectively pushing
only that fraction of the solar panel immediately providing power for it.
Conclusion is that using more than a few engines is largely pointless ---
one might as well stop with one main engine, and at _most_ three or four
smaller engines strategically placed to minimize panel sag when under thrust;
anything more is overkill.


-- Gordon D. Pusch

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