VASMIR plasma engine's power source?

"Christopher M. Jones" wrote in message
om...
(John Schilling) wrote in message
...
And it's pretty clear from the design, that what the proponents really
*want* as a power source is an aneutronic magnetic-mirror fusion reactor
swapped in where the ECR heating chamber is on the viewgraphs. Don't
hold your breath...

Yup. VASIMR is really just a fusion reactor without the
fusion. We've developed all these nifty ways of containing
and externally heating plasmas to explore fusion physics.
VASIMR takes that technology and applies it directly, to
contain and heat any arbitrary plasma using an external
power source, ignoring the fusion aspect. Though, one
interesting thing about VASIMR is that it ought to provide
a pretty straightforward route to bootstrapping into fusion
propulsion. You can gain a slight performance boost by
inducing fusion reactions in the propellant plasma. This
would require using, at least partially, fusion fuels
rather than ordinary Hydrogen or other elements. But since
it's much easier to achieve conditions where fusion
reactions occur in a plasma than to achieve conditions
of energy break-even or self-sustainment, a VASIMR could
achieve a boost from fusion power with today's technology.
Moreover, improvement in the VASIMR design in the primary
areas of containment and plasma temperature as well as in
the area of fusion power naturally lead to escalating
capabilities in approaching fusion break-even and self
sustainment.

So, let's say a very powerful fission reactor (interim until fusion can
finally be developed) strapped on the ship - could the VASMIR engines
develop enough thrust to, say, liftoff from the lunar surface, or it is a
pretty much low-but-constant-thrust engine (similar to ion propulsion)?
If the thrust isn't that powerful, what future possibilites for high-thrust
engines can be developed that aren't chemical or of a NERVA type design?

(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,

http://www.nuclearspace.com/a_petrointerview.htm

Ok, we are looking to generate...there's a wide spectrum from
low to high power. At high power the big planetary mission type
scale we expect to generate tens to hundreds of Newtons of
thrust. At Isp's Specific Impulse ranging form 3000 seconds up
to 30,000sec.

Converting Isp in seconds to Veff, that's ~ 30,000 to 300,000 m/s.

The power represented by the kinetic energy in the exhaust is
0.5 * thrust * Veff and is the absolute minimum power needed from
whatever source. In reality, conversion efficiency and various
losses will make it necessary to have significantly greater output
at the source.

So if "tens" means 50 and "hundreds" means 500, the low-thrust/high
Isp regime needs 0.5 * 50 * 3e5 = 7.5 MW and the high-thrust/low Isp
regime needs 0.5 * 500 * 3e4 = 7.5 MW, again.

It appears that they're talking about reactors running in the range
of a few tens of megawatts thermal power.

"Christopher M. Jones" wrote in message
om...
(John Schilling) wrote in message
...
And it's pretty clear from the design, that what the proponents really
*want* as a power source is an aneutronic magnetic-mirror fusion reactor
swapped in where the ECR heating chamber is on the viewgraphs. Don't
hold your breath...

Yup. VASIMR is really just a fusion reactor without the
fusion. We've developed all these nifty ways of containing
and externally heating plasmas to explore fusion physics.
VASIMR takes that technology and applies it directly, to
contain and heat any arbitrary plasma using an external
power source, ignoring the fusion aspect. Though, one
interesting thing about VASIMR is that it ought to provide
a pretty straightforward route to bootstrapping into fusion
propulsion. You can gain a slight performance boost by
inducing fusion reactions in the propellant plasma. This
would require using, at least partially, fusion fuels
rather than ordinary Hydrogen or other elements. But since
it's much easier to achieve conditions where fusion
reactions occur in a plasma than to achieve conditions
of energy break-even or self-sustainment, a VASIMR could
achieve a boost from fusion power with today's technology.
Moreover, improvement in the VASIMR design in the primary
areas of containment and plasma temperature as well as in
the area of fusion power naturally lead to escalating
capabilities in approaching fusion break-even and self
sustainment.

So, let's say a very powerful fission reactor (interim until fusion can
finally be developed) strapped on the ship - could the VASMIR engines
develop enough thrust to, say, liftoff from the lunar surface, or it is a
pretty much low-but-constant-thrust engine (similar to ion propulsion)?
If the thrust isn't that powerful, what future possibilites for high-thrust
engines can be developed that aren't chemical or of a NERVA type design?

"Bob Martin" wrote in message
om...
And it's pretty clear from the design, that what the proponents really
*want* as a power source is an aneutronic magnetic-mirror fusion reactor
swapped in where the ECR heating chamber is on the viewgraphs. Don't
hold your breath...

How much thrust would that generate, do you think?

So, let's say a very powerful fission reactor (interim until fusion can
finally be developed) strapped on the ship - could the VASMIR engines
develop enough thrust to, say, liftoff from the lunar surface, or it is a
pretty much low-but-constant-thrust engine (similar to ion propulsion)?
If the thrust isn't that powerful, what future possibilites for high-thrust
engines can be developed that aren't chemical or of a NERVA type design?

In article ,
Joseph S. Powell, III wrote:
So, let's say a very powerful fission reactor (interim until fusion can
finally be developed) strapped on the ship - could the VASMIR engines
develop enough thrust to, say, liftoff from the lunar surface...

Exceedingly unlikely. VASIMR is basically a low-thrust technology, and
even quite powerful reactors are pretty damned heavy (especially after
you figure in all the power-generation incidentals).

If the thrust isn't that powerful, what future possibilites for high-thrust
engines can be developed that aren't chemical or of a NERVA type design?

Beamed power -- e.g., laser launch -- is about the only other high-thrust
option.

The fundamental problem is that if you want exhaust velocity better than
chemical rockets *and* high thrust, the power requirement is *enormous*.
Power is directly proportional to exhaust velocity times thrust. Big
chemical rocket engines like the F-1 are already multi-gigawatt devices.

Two major problems immediately surface.

First, that power has to *come* from somewhere. Chemical energy storage
is no good, because the waste chemicals count as part of the exhaust, so
no way does this give you better-than-chemical exhaust velocity. There
basically isn't any lighter way of storing energy except nuclear. The
alternative is to get the energy from outside, but natural sources are too
thinly spread, so beamed power is almost certainly the only alternative.

Second, that power has to be employed to expel exhaust, with a minimum of
machinery mass. That's hard. The only reason multi-gigawatt chemical
rockets can be so physically small -- multi-gigawatt power plants are
massive structures covering acres of land -- is that they manage to avoid
directly *handling* most of that power. That nice property is just about
unique to thermal rockets. In particular, the machinery mass for anything
electric is huge.

So if you want high thrust and are willing to operate near (for some
definition of "near" that depends on your beaming technology) a power
station, you can consider beamed power, probably with a thermal rocket
engine at the receiving end.

But if you want it to be self-contained, then some form of nuclear-thermal
engine is really your only choice. Doesn't have to be NERVA, exactly:
Dumbo, gas-core, Orion, Medusa, pellet fusion, etc. are all options. But
barring new discoveries, there is no real alternative to nuclear-thermal
for this.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

(MattWriter) wrote:
[...]
I had a chance to see the lab testing about 18 months ago. Chang-Diaz said you
could test it with solar power, but to do anything that was worth building a
full-scale engine (travel to Mars was his example) it had to be a fission
reactor.


I seem to have lost the thread where on Monday I asked, "How close did
Chang-Diaz come to meeting his 1999 projection of a test article being
ready for being aboard a shuttle flight in early 2004?" That was in a
Pop Sci article summarizing various propulsion technologies, primarily
for deep space.

Could someone repeat the answer in this thread?

Thanks.

/dps

"Joseph S. Powell, III" wrote in message ...
"Bob Martin" wrote in message
om...
And it's pretty clear from the design, that what the proponents really
*want* as a power source is an aneutronic magnetic-mirror fusion reactor
swapped in where the ECR heating chamber is on the viewgraphs. Don't
hold your breath...

How much thrust would that generate, do you think?

So, let's say a very powerful fission reactor (interim until fusion can
finally be developed) strapped on the ship - could the VASMIR engines
develop enough thrust to, say, liftoff from the lunar surface, or it is a
pretty much low-but-constant-thrust engine (similar to ion propulsion)?

It is very similar to ion propulsion, so the short answer is no.

It does however have the ability to vary the specific impulse. If you
halve the exhaust velocity, you halve the thrust, but quarter the
power requirements. Therefore at lower specific implulses you can get
more thrust for a given power level. However, if you lower the
specific impulse too much, you might as well use chemical.

Now suppose the power source weighed next to nothing. Not impossible
if the power source is external to the vehicle, perhaps transmitted to
the vehicle by microwave. however, this is better applied to orbital
manouevering.

If the thrust isn't that powerful, what future possibilites for high-thrust
engines can be developed that aren't chemical or of a NERVA type design?

NERVA is fission based, of which there are a number of proposed
variants. The limiting factor is how hot you can run the engine before
it melts. It has been proposed to use gas cores.

There are a variety of fusion designs.

Shorter term though, tethers are a great way of exchanging momentum.
For launching from the lunar surface, try linear accelerators.

(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?

The other possibility is beamed power, which means the power source
doesn't need to be accelerated.

Alex

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
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.

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


--
*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:

(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.

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.


--
*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 *