In article <NC12e.19$45.3195@news.uchicago.edu>,
 <mmeron@cars3.uchicago.edu> wrote:
>>If you wanted to send energy to a rocket, you would be better off with a
>>laser.  ... The fuel tank would contain ammonia and nothing else.
>>The back of the rocket would have a concave mirror that concentrates the
>>laser light on one spot.  Ammonia would be sent to that spot, heat up, and
>>be ejected as exhaust.
>
>And the advantage over "something that burns" would be?  I mean other 
>than saying "I'm high tech, I'm using a laser":-)

1. You can raise the exhaust gas to higher temperatures -- giving more
energy to be converted into exhaust kinetic energy -- if you aren't
limited by the energy content of chemical fuels. 

2. You can choose exhaust gases with better gas properties -- which
convert heat more efficiently into kinetic energy -- if the gases don't
have to be combustion products.  Ammonia in particular, when you get it
hot, breaks down into nitrogen and a lot of hydrogen, giving really
excellent gas properties. 

Depending on design approach, the flying hardware may also be simpler.
(The laser is complicated, but it's on the ground, where mass doesn't
matter and maintenance is easy.)
-- 
"Think outside the box -- the box isn't our friend."    |   Henry Spencer
                                -- George Herbert       | henry@spsystems.net