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

Brennan, Mike (DOH) Mike.Brennan at DOH.WA.GOV
Tue Sep 30 11:05:17 CDT 2008

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


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
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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