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Re: 39 days to Mars possible now with nuclear-powered VASIMR.

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OK. I see your argument now. The radiated heat would come from the 
temperature of the output of the heat engine, not the high operating 
temperature of the reactor.
A discussion of this problem of high efficiency of the conversion requiring 
higher radiator mass is discussed here:

Heat Radiators.
"As an example of the severity of this problem, let us examine the case of a 
simple nuclear power plant whose energy conversion efficiency from thermal 
to electric is approximately 10 percent. The plant is to generate 100 kW of 
useful electricity. The reactor operates at approximately 800 K, and a 
radiator with emissivity equal to 0.85 would weigh about 10 kg/m2. The 
thermal power to be dissipated from the reactor would be about 1 MW. From 
the Stefan Boltzmann Law, the area of the radiator would be about 50 m2 and 
the mass approximately 500 kg. This seems quite reasonable.
However, we must assume that the electricity generated by the power plant, 
which goes into life support systems and small-scale manufacturing, would 
eventually have to be dissipated also, but at a much lower temperature 
(around 300 K). Assuming an even better, aluminum radiator of about 5 kg/m2, 
with again an emissivity of 0.85, in this case we find that the area of the 
low temperature heat rejection component is 256 m2, with a mass approaching 
1300 kg."
http://www.projectrho.com/public_html/rocket/basicdesign.php#radiators

This web site by the way provides a nice collection of the work that has 
been done on advanced space propulsion systems.

An additional problem though in regards to low efficiency conversion is that 
you need higher mass for the reactor and larger amounts of radioactive 
material that needs to be launched to orbit. For example in that blog post, 
"Nuclear powered VASIMR and plasma propulsion doable now, Page 2," 
http://exoscientist.blogspot.com/2015/10/nuclear-powered-vasimr-and-plasma.html, 
I suggested running the reactor at low power to extend the lifetime. In this 
mode the reactor weighing 2,200 kg would put out 8 megawatts using 200 kg of 
uranium fuel. If instead of the high conversion efficiency I was aiming for, 
we selected 10% conversion efficiency, we would need a 10 times larger 
reactor and 10 times more radioactive fuel. This would put the mass of the 
reactor at 22,000 kg and the uranium fuel at 2 tons.

There might be some methods to reduce the heat that needed to be radiated 
away in the high efficiency scenario. For example Stirling engines can 
operate even at low temperatures. We could use these to make use of the heat 
at low temperatures coming from the turbines of  our generator.

There is also research on lightweighting radiators such as by using 
carbon-carbon composites:

High Conductivity Carbon-Carbon Heat Pipes for Light Weight Space Power 
System Radiators.
NASA/TM—2008-215420
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080045532.pdf


  Bob Clark






-----------------------------------------------------------------------------------------------------------------------------
A mission to Europa could result in the most important scientific advance in 
human history, dwarfing even the Apollo missions,
to discover life on another world. By commercial space, launch and 
spacecraft costs can be slashed by a factor of 10 or more.
This would be a cost that could be financed privately. And at costs this low 
it can even be done at a profit:

Low cost Europa lander missions.
http://exoscientist.blogspot.com/2015/02/low-cost-europa-lander-missions.html
-----------------------------------------------------------------------------------------------------------------------------
"Thomas Koenig"  wrote in message 
news:mvlrph$69l$1@newsreader4.netcologne.de...

Robert Clark <rgregoryclark@gmSPAMBLOACKail.com> schrieb:

> Yes. You can reject more heat at higher temperatures. But for two systems
> operating at the same temperature,

What does that mean?

A thermodynamic cycle does not operate at a single temperature
(unless it does nothing).

> the one having lower waste heat, i.e.,
> better efficiency, will require smaller radiators.

---
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39 days to Mars possible now with nuclear-powered VASIMR. "Robert Clark" <rgregoryclark@gmSPAMBLOACKail.com> - 2015-08-28 08:56 -0400
  Re: 39 days to Mars possible now with nuclear-powered VASIMR. Jeff Findley <jfindley@cinci.nospam.rr.com> - 2015-08-28 19:29 -0400
    Re: 39 days to Mars possible now with nuclear-powered VASIMR. "Robert Clark" <rgregoryclark@gmSPAMBLOACKail.com> - 2015-09-09 22:10 -0400
      Re: 39 days to Mars possible now with nuclear-powered VASIMR. "Robert Clark" <rgregoryclark@gmSPAMBLOACKail.com> - 2015-10-05 01:16 -0400
        Re: 39 days to Mars possible now with nuclear-powered VASIMR. "Greg \(Strider\) Moore" <mooregr@deletethisgreenms.com> - 2015-10-05 07:07 -0400
        Re: 39 days to Mars possible now with nuclear-powered VASIMR. Thomas Koenig <tkoenig@netcologne.de> - 2015-10-05 20:57 +0000
          Re: 39 days to Mars possible now with nuclear-powered VASIMR. gilber34 <fafa@invalid.com> - 2015-10-07 00:06 -0500
            Re: 39 days to Mars possible now with nuclear-powered VASIMR. Jeff Findley <jfindley@cinci.nospam.rr.com> - 2015-10-07 06:07 -0400
              Re: 39 days to Mars possible now with nuclear-powered VASIMR. Rick Jones <rick.jones2@hp.com> - 2015-10-07 17:07 +0000
              Re: 39 days to Mars possible now with nuclear-powered VASIMR. gilber34 <fafa@invalid.com> - 2015-10-13 12:06 -0500
          Re: 39 days to Mars possible now with nuclear-powered VASIMR. "Robert Clark" <rgregoryclark@gmSPAMBLOACKail.com> - 2015-10-08 20:16 -0400
            Re: 39 days to Mars possible now with nuclear-powered VASIMR. Thomas Koenig <tkoenig@netcologne.de> - 2015-10-14 15:19 +0000
              Re: 39 days to Mars possible now with nuclear-powered VASIMR. Fred J. McCall <fjmccall@gmail.com> - 2015-10-14 08:30 -0700
                Re: 39 days to Mars possible now with nuclear-powered VASIMR. Thomas Koenig <tkoenig@netcologne.de> - 2015-10-14 16:57 +0000
                Re: 39 days to Mars possible now with nuclear-powered VASIMR. Fred J. McCall <fjmccall@gmail.com> - 2015-10-14 22:11 -0700
              Re: 39 days to Mars possible now with nuclear-powered VASIMR. "Robert Clark" <rgregoryclark@gmSPAMBLOACKail.com> - 2015-10-16 16:46 -0400
                Re: 39 days to Mars possible now with nuclear-powered VASIMR. "Robert Clark" <rgregoryclark@gmSPAMBLOACKail.com> - 2015-10-30 01:51 -0400
                Re: 39 days to Mars possible now with nuclear-powered VASIMR. Thomas Koenig <tkoenig@netcologne.de> - 2015-10-31 12:32 +0000
                Re: 39 days to Mars possible now with nuclear-powered VASIMR. "Robert Clark" <rgregoryclark@gmSPAMBLOACKail.com> - 2015-11-08 09:12 -0500
                Re: 39 days to Mars possible now with nuclear-powered VASIMR. Thomas Koenig <tkoenig@netcologne.de> - 2015-11-08 14:36 +0000
                Re: 39 days to Mars possible now with nuclear-powered VASIMR. "Robert Clark" <rgregoryclark@gmSPAMBLOACKail.com> - 2015-11-12 13:19 -0500
      Re: 39 days to Mars possible now with nuclear-powered VASIMR. "Robert Clark" <rgregoryclark@gmSPAMBLOACKail.com> - 2015-10-13 08:19 -0400

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