Paul F. Dietz wrote:

Correct me if I am wrong, but I can't see anybody supporting the
development of nuclear rocket engines, given the political problems
associated with simple RTGs.

Why should this follow? RTGs are much more radioactive at launch
than are reactors.

Indeed, a point that's often overlooked.

RTGs start at peak radioactivity and then decay.

A reactor can be launched inert, with sod-all radioactivity, and then sent
critical when in a safe orbit (I recall c. 1000 miles being a figure
mentioned in a debate on this here quite a while ago).

The bigger problem with space reactors is development cost and
lack of application.

Yes. For electicity generation, compared to RTGs a reactor is much more
complicated, much more expensive to develop and probably much heavier and
bulkier.

There just hasn't been anything that's needed the sort of high long-term
power a reactor can put out. A manned mission to Mars, though...

Nuclear rockets (I include the type of HET array being suggested in this
definition) are another matter. So far there hasn't been anything that
hasn't been able to be done with chemical rockets. However (again) a
manned mission to Mars could well be such a mission; the problems of bone
loss and radiation exposure could prove to be such that a nuclear rocket
would be the only way to get there in a time that would keep the crew in
condition to actually do something when they got there, let alone back on
Earth. You'd have the weight of shielding to consider, and it could be a
trade-off between radiation from the engine and radiation from space. (ie
light shielding may allow sufficiently faster acceleration and hence
shorter journey times that you actually reduce overall radiation exposure).

I'm a great fan of Stephen Baxter, but his novel "Voyage" really doesn't do
the NERVA nuclear-thermal rocket program justice; it was both saner and
more successful than he makes out. For a start, in theory at least with
the hydrogen fuel radioactive emissions were limited to the 2% or so of
hydrogen that was deuterium. Of course the problem was that bits of engine
got spat out the back as well, but it was acceptable by '60's standards.
Of course you'd have to be more careful now.

--
Malcolm Street
Canberra, Australia
The nation's capital