>>It works with anything that has mass. The interesting part is that the 
>>efficiency is greatest when the speed of what you toss is equal and 
>>opposite to your vehicle's speed.

This is correct for non relatavistic cases. Think of it as a nother example 
of the maximum power transfer theorum. Once relativity becomes significant 
there are other considerations.

>>Iow, using a rocket to start from standstill or fly at low speed in 
>>general is extremely inefficient.

> That may well be for other reasons..

Consider - a rocket has essentially constant thrust (while some can be 
varied over a limited range and some very special ones over a wide range 
they generally work at approximately constant thrust). Work done = force x 
distance. At the moment of liftoff the rocket has most mass, and is moving 
at essentially zero speed. Work done is approximately zero!. As speed 
increases force x distance per unit time increases and work done on the 
rocket increases. Energy input to the motor per unit time is approximately 
constant. Energy actually imparted to the rocket increases as its velocity 
increases so efficiency rises with velocity. Rockets are not usually an 
expecially efficient way to go places. When it comes to going to space they 
are, so far, the only way to go places :-(.   The market value of the 
potential energy of a kilogram at geosynchronous orbit altitude is under 
$US10 per kg! The current cost of putting it there is in excess of $10,000

>>So if a photon powered spaceship would approach c then its ejecta would 
>>approach standstill, i.e. frozen photons
>
> Albert's relativity theory wouldn't agree with you at all.

Correct. but a laser could be used as a "light drive" simply by "throwing 
photons out the back".



        Russell McMahon

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