Radiation shield.

"John Thingstad" wrote in message
news
On Sat, 13 Nov 2004 02:20:38 GMT, Christian Ramos
wrote:


Well utilizing the heat from the reactor directly is certainly easier.
It was the first idea and explored already back in the 50's.
Of course then it was though to be used in the atmosphere
something we would never consider today.
But a atomic reactor can generate electricity with good efficiency.
(Except the RORSAT reactor (Russian..) mentioned elsewhere)
Heating has directly you could not possibly put as much
energy into the gas.

WhileVasimr puts much more energy into the propellant, I'm interested in
doing the numbers, since you'll lose energy in the conversion to electricity
and back again to heating in Vasimr. Would you assume such a beast would
utilise a rankine or stirling cycle for electricity generation?

Then there is the fact that the gas
flow would have to be high to have a cooling effect.

Not necessarily. If the the reactor also serves the function of electricity
generation you would likely have an independant cooling system given the
propulsion system will not be constantly in use. Although balancing that
could be tricky.

P = m*v and we want m as small as possible and v as large as possible.
(I am talking about the propellant not the ship per se though the same
applies here.)
Seems to me magnetic induction of ions wins hands down.

I think you need to take into account the conversion losses. Thermal energy
available in the reactor to heat propellant versus amount of thermal energy
available to input to Vasimr after losses.


Also, given our limited understanding of plasma and magentic sciences,
are
we even sure that such a device would work well in "Outer Space" with
all
kinds of external plasma and magnetic interactions This later is a
question
I've never seen addressed so perhaps I'm being paranoid.


Knowledge of magnetic's is (or really electro-magnetics) is
decidedly where our science is the most advanced.
Plasma flow seems sufficiently well understood to make a
working motor.

It may be advanced from a relative perspective, but still along way to go.
Sure you can produce a working motor, I've can produce a working launcher
from a softdrink+bottle, but we are talking about it's viability as a
propulsion solution. If you also add in the immature technologies it
utilises such as superconductors etc, well...

(Not that you should be asking me about this..
http://www.ae.utexas.edu/design/phoenix/vasimr.html
and references should give you an idea.)

Yep. I'm familiar with the engine from tech papers. Again, it still seems a
high tech toy looking for a solution. Some references that may be of
interest to you,

The Development of the VASIMR Engine -International Conference on
Electromagnetics 1999
Rapid Mars Transits With Exhaust-Modulated Plasma Propulsion - NASA TP 3539
Accuracy Improvement in Magnetic Field Modeling for an Axisymmetric
Electromagnet - NASA/TP—2000–210194


I see it used for manned interplanetary flight.
Particularly continous acceleration means that the peaple are exposed
to
cosmic radiation and zero g for much lesser periods of time.

I always have a problem with these types of solutions of long term
acceleration. Once you get to such speeds that make Interplantary travel
viable, arent you also going to be at a speed where even the impact of
dust
particles could devastate your ship. Your ability to maneuver is also
compromised with such a solution.


Yes. Impact with debris is a problem. Maneuvering less so.

A problem inplies a solution. I dont think we are anywhere near such a
solution, so I would call it a unaddressable problem currently.

But, as I mentioned, going slow isn't safe either...
Still the area between earth and mars is mostly empty.

My comment was directed at interplanetary, in the case on intraplanetary
such long acceleration methodolgies could be useful. Although, I have a
suspicion that its more useful for unmanned activities in which case you
probably would want to use something with less failure points.

It seems to be less of a problem than in earth orbit.
For dust to sand size particles segmented shielding should do the job..

At the speeds needed for interplanetary travel, segmented shielding simply
means a bigger bang when it hits, the energy in such particles at those
speeds wont be mitigated by whipple shields with current materials.

Anything bigger... Big enough and you can see it and maneuver around it..

At those speeds and with such low acceleration forces, I suspect you would
be toast before the computer could even analyze the situation, but we are
getting too heavy into hypotheticals I think.