Re: Running multiple HET in parallel?

From: Malcolm Street (mstreet_at_internode.on.net)
Date: 02/18/05

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    To: sci-space-tech@moderators.isc.org
Date: Fri, 18 Feb 2005 20:04:20 +1100

    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
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