VASIMR

In the latest Flight International is an article in the spacecraft
section saying that VASIMR is going to be used by NASA to move
satellites to higher orbits, and xenon gas is being considered as a
fuel. Sounds good for VASIMR in that someone has come up with a use
for the engine.

--

Christopher

Christopher wrote in
:

In the latest Flight International is an article in the spacecraft
section saying that VASIMR is going to be used by NASA to move
satellites to higher orbits, and xenon gas is being considered as a
fuel. Sounds good for VASIMR in that someone has come up with a use
for the engine.

Didn't find any article on that subject on FI's web site.

Hall effect thrusters using xenon have been in use for
years. What is the thrust and Isp, power source?

--Damon

Damon Hill wrote:
Christopher wrote in
:


In the latest Flight International is an article in the spacecraft
section saying that VASIMR is going to be used by NASA to move
satellites to higher orbits, and xenon gas is being considered as a
fuel. Sounds good for VASIMR in that someone has come up with a use
for the engine.


Didn't find any article on that subject on FI's web site.

Hall effect thrusters using xenon have been in use for
years. What is the thrust and Isp, power source?

--Damon


Here it is
http://www.flightglobal.com/Articles...et+design.html

--

ICBM Address: 45deg 31m 0.9s N 9deg 19m 24.9s E

Thanks, got it. 'Hundreds of kilowatts' from solar panels will
be impressive power levels indeed. Tens of kilowatts are the
norm for large commsats now. Hydrogen or deuterium, not xenon,
will be the reaction mass. Gridless, which is good since those
will quickly wear out at those power levels.

Isp will be 20,000 sec or thereabouts, but no thrust levels are
given. (Apparently in the 1 - 10 Newton range, a Newton being
about .22 lb/thrust, I think)

http://www.adastrarocket.com

http://en.wikipedia.org/wiki/VASIMR



--Damon

In article , Christopher says...

In the latest Flight International is an article in the spacecraft
section saying that VASIMR is going to be used by NASA to move
satellites to higher orbits, and xenon gas is being considered as a
fuel. Sounds good for VASIMR in that someone has come up with a use
for the engine.

Also sounds unlikely. I haven't heard anything about such a thing
anywhere in the usual literature, though I admit I'm a few weeks
behind on my AW&ST and Space News. I haven't heard anything about
such a thing in any of my discussions with NASA officials on their
plans for near- to mid-term use of electric propulsion systems in
spaceflight, though again it's been a few weeks. I haven't heard
about VASIMR being developed to anything remotely resembling a
flight-ready status, or about anyone committing the money to push
such a thing; quite the opposite.

Furthermore, VASIMR is a thermal-expansion rocket, albeit with a
magnetic nozzle, and thermal-expansion rockets tend to use
low-molecular-weight propellants like Hydrogen. Xenon is a classic
propellant for electrostatic thrusters, like ion engines and Hall
effect thrusters.

At a guess, either someone at NASA talked to a reporter about their
plans to use xenon-fuelled classic ion or HET plasma thrusters to
move satellites into higher orbits[1] to some idiot reporter who
said to himself, "Plasma thruster - that's that VASIMR thingy they
were talking about for that other story I wrote last month. Kewl;
I really know this stuff!"

Or, alternately, the new spinoff company that is trying to do VASIMR
work now that NASA won't fund it in-house, issued a press release
saying, "We propose to use VASIMR (which will be working Real Soon
Now) to push NASA satellites into higher orbits, and NASA hasn't
said Not A Chance In Hell Ever yet, so we're really optimistic!
Investors, please send money!"


[1] Which are themselves tenative; NASA's plans for ion thrusters
and Hall effect thrusters these days mostly have to do with deep
space work. Orbit-raising is a commercial and USAF thing.


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Damon Hill wrote:
Thanks, got it. 'Hundreds of kilowatts' from solar panels will
be impressive power levels indeed. Tens of kilowatts are the
norm for large commsats now. Hydrogen or deuterium, not xenon,
will be the reaction mass. Gridless, which is good since those
will quickly wear out at those power levels.

Isp will be 20,000 sec or thereabouts, but no thrust levels are
given. (Apparently in the 1 - 10 Newton range, a Newton being
about .22 lb/thrust, I think)

http://www.adastrarocket.com

http://en.wikipedia.org/wiki/VASIMR

Uh, what would be the propellant mass compared to power source
mass then?

They have some values on the web page. For Mars, alpha = 4 kg/kW
for a power source of 48 metric tons. (Paired with that ugly NASA
unit misnomer "mT", which actually means millitesla, unit of
magnetic flux density.)

Let's say you have a deltav of 12 km/s (to Mars and back).

With isp = 20,000, exhaust velocity is about 200 km/s, and
your mass ratio becomes 1.06, meaning that if your vehicle
weighs 106 t fuelled, 6 t of that would be fuel. We take
the alpha of 4 kg/kW and constant acceleration of
1 km/s per 12 days aka 1e-3 m/s².
Now the thrust comes to 106 N, and the power
needed is 11 megawatts. Weight of the source will be
thus 44 t. 56 t is left for payload.

With isp=5000, similar total mass, acceleration and power
efficiency criteria. Fuel will be 23 t, power 2.7 megawatts,
power source weight 11 t and thus 72 t left for payload.

High exhaust velocity is really bad for power source mass.
The exhaust velocity tends to optimize to a few times the needed
deltav. Higher acceleration favors lower isp, since
required power is P=0.5*F*v_ex where P is power, F is thrust, and
v_ex is exhaust velocity.

Very high isp is only useful for cases where you need
only very low acceleration or very high deltav (low acceleration
is a must for high deltav anyway.)

I don't know how much the variable impulse in VASIMR would
help in manned Mars trips, and their web page doesn't really
give useful info, it's not compared to hall thrusters or
such.

meiza wrote:

They have some values on the web page. For Mars, alpha = 4 kg/kW
for a power source of 48 metric tons. (Paired with that ugly NASA
unit misnomer "mT", which actually means millitesla, unit of
magnetic flux density.)

Solar might be better, but point stands


Very high isp is only useful for cases where you need
only very low acceleration or very high deltav (low acceleration
is a must for high deltav anyway.)

Which boils down to missions with long time frames, probably over 1
year.

I don't know how much the variable impulse in VASIMR would
help in manned Mars trips, and their web page doesn't really
give useful info, it's not compared to hall thrusters or
such.

All electric propulsion has the same advantages. Useles for manned
missions with forseaable, near term power units, unless you look at off
board, beamed power.