AW: [ RadSafe ] [RadSafe]Mission to Mars---Fission Propulsion

Tue Sep 30 16:05:31 CDT 2008

John,

thank you for the qualification. Of course you are right, provided the low energy photons do reach at least the skin.

Considering a mass shielding between the reactor and the astronaut compartment thick enough to shield sufficiently against neutrons and gammas emitted by the fuel, my guess was that only a very minor fraction of the 'soft' Bremsstrahlung photons from beta decays of the fission products would reach the skin. 

Cheers, Rainer

________________________________

Von: John R Johnson [mailto:idias at interchange.ubc.ca]
Gesendet: Di 30.09.2008 20:02
An: Facius, Rainer; radsafe at radlab.nl
Betreff: Re: [ RadSafe ] [RadSafe]Mission to Mars---Fission Propulsion

Rainer

"Soft" (low energy?) photons are not necessarily "negligible". They are
negligible only if the effective dose to people is small.

Cheers

John
***************
John R Johnson, PhD
CEO, IDIAS, Inc.
4535 West 9th Ave
604-676-3556
Vancouver, B. C.
V6R 2E2, Canada
idias at interchange.ubc.ca

----- Original Message -----
From: <Rainer.Facius at dlr.de>
To: <radsafe at radlab.nl>
Sent: Tuesday, September 30, 2008 10:02 AM
Subject: [ RadSafe ] [RadSafe]Mission to Mars---Fission Propulsion

Mike,

to my knowledge Bremsstrahlung intensities from charged particles other than
electrons are negligible. Hence only during passages through the (mainly
outer) radiation belts, Bremsstrahlung photons contribute to space radiation
doses. While operating, nuclear propulsion engines of course are intense
sources of gamma rays and neutrons. After engine shutdown, decay of fission
products by predominantly beta decay will be the source for rather soft and
hence negligible Bremsstrahlung photons. Fission product decay ceases quite
rapidly.

So, for all space flight mission scenarios other than geostationary orbits
(and of course in the vicinity of Jupiter :-), Bremsstrahlung contributes
only negligibly to space radiation exposures.

Regards, Rainer

Dr. Rainer Facius
German Aerospace Center
Institute of Aerospace Medicine
Linder Hoehe
51147 Koeln
GERMANY
Voice: +49 2203 601 3147 or 3150
FAX:   +49 2203 61970

-----Ursprüngliche Nachricht-----
Von: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] Im Auftrag
von Brennan, Mike (DOH)
Gesendet: Dienstag, 30. September 2008 18:05
An: radsafe at radlab.nl
Betreff: RE: [ RadSafe ] [RadSafe]Mission to Mars---Fission Propulsion

I suspect that the key to using nuclear propulsion in space is getting in
and out of our gravity well.  Most of the schemes for using reactors to
accelerate reaction mass can not achieve the acceleration needed to go from
Earth's surface to orbit.  Even if a nuclear rocket could develop that kind
of acceleration, I foresee non-trivial political obstacles to such a launch.
I am also a fan of having the spacecraft for the trip be rather larger than
it would be practical to launch as a single package.

I believe that a necessary first step for a manned mission to Mars is
development of a way to put packages in orbit for tens or hundreds of
dollars per kilo, rather than thousands or tens of thousands of dollars per
kilo.  It would be nice if it could be done without high accelerations, too.
Of the systems I've seen proposed (and as a science fiction fan, I've seen a
fair few), the Space Elevator concept looks the best.  Although some very
interesting work has been done, and I think it has potential for actually
coming to pass, I decline to hold my breath until it happens.

Rather than focusing on sending people to Mars, I think we should send
progressively more sophisticated robots, all the way up to robots build some
of the infrastructure that will be needed for an extended stay by people.

As for the radiation issue during the trip; I read an interesting article
that contended a big source of radiation would be Bremsstrahlung radiation
from the interaction of high energy particles and the metal ship around the
crew.  The proposed fix was to use a very large inflated spaceship for the
voyage, with small landers for the trips to and from the surface of Mars.
Selling the "balloons in Space" concept may take some work, however.

-----Original Message-----
From: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] On Behalf
Of Rainer.Facius at dlr.de
Sent: Tuesday, September 30, 2008 5:58 AM
To: JPreisig at aol.com; radsafe at radlab.nl
Subject: AW: [ RadSafe ] [RadSafe]Mission to Mars---Fission Propulsion

Joe,

as long as human astronauts (instead of chimpanzees) are manning the
spacecraft, mission duration is the NUMBER ONE factor driving the risks to
their health and hence the (counter-) measures to be taken to ensure a safe
and healthy return.

The probabilities of encountering life-threatening medical conditions
(physiological and psychological) or critical technical equipment failures
are roughly proportional to mission duration. The risk of acute early
radiation effects (and secondary risks from associated performance
decrements) due to unforeseen large solar energetic particle events adds to
these risks for untoward mission outcomes and is again roughly proportional
to mission duration (solar activity held constant). The risk for late
radiogenic cancer from (chronic) exposure to galactic cosmic radiation again
is proportional to mission duration. For reference missions to Mars of about
1000 d duration during phases of minimum solar activity (such as now),
galactic cosmic ray exposures behind conventional mass shielding can
accumulate to 1 Sv.

So, dollars spent into attempts to reduce mission duration are by far the
most cost effective in reducing the health risks of Mars travelling
astronauts.

Fundamental physical/technical constraints limit the specific impulse
achievable by chemical propulsion to somewhere between 400 to 500 seconds.
With solid nuclear power propulsion, specific impulses between 500 and 1000
s are achievable whereas with a gas core nuclear rocket specific impulses
between 1000 and 6000 s can be obtained, thereby reducing mission durations
by the respective factors. The technologies for solid nuclear power
propulsion - and hence for a reduction of mission duration by a factor of
two - are at hand but certainly not the limit. Los Alamos has spent quite
some efforts for advancing the gas core nuclear rocket technology - at least
theoretically - and reductions of mission durations by factors up to 10 are
conceivable thereby.

My personal prognosis is: If manned missions to Mars will occur, they will
use nuclear propulsion. Given that man's strive to extend his limits
historically has only been limited by the laws of nature, my guess is that
manned missions to Mars will take place. Whether this will generate sizeable
job opportunities for health physicists remains to be seen.

Regarding fusion propulsion, the experience of my lifetime with
announcements of fusion energy as lurking just around the corner of the next
decade makes me sceptical when or even whether we will see it working. Five
to six decades of such announcements have worn out my 'faith' into this
energy option

Thank you for your stimulating note - and good luck at your work.

Kind regards, Rainer (going back to work :-)

PS: At http://www-pub.iaea.org/MTCD/publications/PDF/Pub1197_web.pdf you can
download the IAEA publication STI/PUB/1197 (2005), The Role of Nuclear Power
and Nuclear Propulsion in the Peaceful Exploration of Space. Note, in some
instances they have specific impulses too large by a factor of ten.
Dr. Rainer Facius
German Aerospace Center
Institute of Aerospace Medicine
Linder Hoehe
51147 Koeln
GERMANY
Voice: +49 2203 601 3147 or 3150
FAX:   +49 2203 61970

-----Ursprüngliche Nachricht-----
Von: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] Im Auftrag
von JPreisig at aol.com
Gesendet: Montag, 29. September 2008 23:17
An: radsafe at radlab.nl
Betreff: [ RadSafe ] [RadSafe]Mission to Mars---Fission Propulsion

Dear Radsafe:

     This is from:    jpreisig at aol.com    .

     Hello Again Radsafers:

     Hope you all are well today and don't require a US Federal Bailout....

          I've been thinking about space travel to Mars again using fission
     propulsion.  I refer you to several articles by Freeman Dyson in the
beta
     volume of the book "Adventures in Experimental Physics" --- Bogdan
     Maglich, editor.  I hope you can find it in your library.  In one of
the articles
     Dyson indicates that one can use fission propulsion (without resorting
to
     bomb propelled rockets> to achieve exhaust velocities that are twice as
     large as the velocities that can be reached via chemical propulsion.
>From this,
     I infer that a readily achieveable fission propelled rocket can
ultimately be
     designed which can go twice as fast as chemically propelled rockets (at
     least>.  This would reduce the round trip time to Mars from 3 years to
     about 1.5 years.  This is helpful if one is living in a spaceship for
such a
     long time.  Further additional techniques used to reduce the total
travel
     time to Mars would also be desirable.

          So, considering use of Uranium to power such a trip, how could
such
     trip be safely made?  For takeoff, the fission reactor propelling the
rocket
     (or whatever> would largely contain Uranium (and not so much Cesium
     or Strontium which are produced via fission>.  So, let's assume a safe
     launch can be made.

          The rocket and astronauts (hopefully not Chimpanzees???!!!> would
     fly to Mars, land on the planet's surface, and do whatever science and
other
     tasks which need to be done.  If necessary, a chemically propelled
     lunar/Mars type lander could be used to get to Mars surface from the
     original rocket or Mother ship.  Upon completion of their time on Mars,
     the astronauts would direct their spaceship towards Earth, to return to
home.

           However, instead of returning directly to Earth, the spaceship
would
     land on the Moon, not using a lunar/Mars type lander.  The spaceship
would
     land directly on the Moon's surface, not to return to Earth anytime
soon.
     The spaceship would be left on the Lunar surface, complete with its
     reactor intact.  There's not much weather or wind storms on the Moon,
so the
     spaceship could stay there a long time without dispersal of any fission
     products or the original Uranium fuel.

          So, this leaves the astronauts on the Moon with the scientific
samples,
     stored data on computers, etc.  What happens next???  A second
     chemically propelled rocket is sent to the Moon,  and using a Lunar
lander
     the astronauts are picked up and return to Earth in the second
spacecraft.
     Mission accomplished.  I didn't say the space mission would be
inexpensive.

          In 200 years (a fair number of half-lives) the original fission
reactor
     could be picked up from the lunar surface and returned to Earth for
     processing and/or storage.

          If we ever perfect a fusion propelled rocket system, much of the
preceding
     described effort becomes unneccessary.

          Just something to think about.  I think such a fission propelled
rocket
     system could be built in the relatively near future.  Oh my, jobs for
     Health Physicists and/or Nuclear Engineers in space.

         The airplanes/jets which takeoff vertically are called Harriers.

         Another few years of relatively few (named> hurricanes and/or
         tropical storms like 2008 (so far> and I'll have to say that the
global
         warming hypothesis is fizziling out.

         Now, get back to work????

         I hope you have a wonderful week.

         Regards,   Joseph R. (Joe> Preisig, Ph.D.

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