Re: Flight Test Vasimr?


>> Showing a reason why it's better than assorted competitors -- aside from
>> the fact that its would-be developers issue more press releases -- would
>> also be good.

> There is a good argument to be made that its low end performance isn't
> much better than other electric propulsion concepts. It requires much
> larger power sources to operate in the full range of its performance,
> but it is fully throttlable from a few dozen lbs thrust at max thrust
> with Isp typical of plasma engines, to over 30,000 secs Isp at its most
> lean, when it is using all three stages of its engine. What this means
> is that it can take less time to get out of earth orbit than any other
> electric propulsion system (less than 30 days) and accelerate to
> significant velocity on an interplanetary trajectory that is rather
> direct, then lean to a low acceleration level at extremely high Isp to
> use almost no fuel for the duration of the voyage until orbital
> insertion.

Two kinda dumb questions at first:
Why couldn't you attach two different ion engines with different
voltages to a craft? Say, the first one is "low gear" acceleration
and has isp = 1000 s, and the other one has 5000 s. I don't think the
actual thrusters weigh *that* much, it's the power source that matters.
They are already proven, and quite simple. And can the Vasimr be
efficient, if it uses hydrogen, since the ionization energy there is
big compared to mass?

There are now (lab-tested for short duration) four-grid, dual-stage
ion engines (old ones had 3 grids), which can get isp around 20000 s
with little grid erosion, high power density and small area.
An Australia-ESA-project.
http://www.esa.int/esaCP/SEMOSTG23IE_index_0.html

There's also a new player in the medium isp area, the helicon double
layer thruster developed first in Australia (?!) and recently tested
by ESA, which apparently doesn't need conventional electrodes at all:
http://www.esa.int/esaCP/SEM6HSVLWFE_index_0.html
They say isp is around the same as SMART-1 which is 1500 s, but
again power density and size is much better.

Crewed constant isp trips to Mars have to use isp less than 5000 s
anyway for reasonable acceleration times. (for 1 km/s / 12d
acceleration, 20 km/s total delta v, and 100 W/kg power density, the
best isp is 5000 s, and for lower delta v or higher acceleration,
you're lightest off with lower isp, smaller power source and more
fuel.)

>This is an advantage that a fission propulsion system does
> not have, being limited to Isps of 600-1000, and while Mars is at the
> inner limit of the range at which VASIMR is more advantageous to use
> over nuclear or chemical, the propulsion systems advantages over longer
> voyages to the jovians, kuiper belt, etc are quite clear, especially if
> we ever plan on going anywhere other than sending tiny robot probes. If
> we ever voyage to Xena, Santa, or the other KBOs, this propulsion
> system will be necessary.

One would first think that for probes beyond Mars, we might just as well
go with a constant high isp solution. What are the effects to trip times
with VASIMR then?

I know this link has been pasted before, but still, it ponders the ideas
of high isp far-out missions a little:

http://www.esa.int/gsp/ACT/propulsion/ultra_ion.htm

"Applications of the dual-stage gridded ion thruster technology include:

-Local Interstellar Explorer mission (1000kg-class with 1kW RTG power)
-Kuiper Belt Orbiter mission (similar design)
-Inner Oort Cloud rendezvous mission (similar design)
-High power NEP/SEP Hazardous Near-Earth Object Deflection mission
(100-200kW class)
-Very high power NEP human Mars mission (MW-class)
-Small low power attitude control or positioning thruster (<100W)

Technology impact studies on these applications are being performed by
ACT and will be reported in the coming months."

> In addition, it is broadly believed that the technology of VASIMR is
> the best stepping stone to controlled fusion propulsion and power
> generation, and for this reason should be considered the primary
> propulsion technology to focus next generation R&D resources on. Could
> this be why the oil industry-beholden Bush administration is apparently
> trying to kill the technology? Perhaps.

This sounds quite unbelievable. Bush brought USA back to ITER anyway.
But actually, the two-stage ion thruster is directly related to particle
beam injectors used in fusion research.
.

Relevant Pages

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    (sci.space.history)
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  • Re: Interstellar Propulsion idea using an Asteroid and a few comets!
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